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In this document it is assumed that the reader has sufficient administration
skills on a UNIX-like operating system, uses the shell on a daily basis and is
familiar with troubleshooting utilities such as strace and tcpdump.

HAProxy is a single-threaded, event-driven, non-blocking daemon. This means is
uses event multiplexing to schedule all of its activities instead of relying on
the system to schedule between multiple activities. Most of the time it runs as
a single process, so the output of "ps aux" on a system will report only one
"haproxy" process, unless a soft reload is in progress and an older process is
finishing its job in parallel to the new one. It is thus always easy to trace
its activity using the strace utility.

HAProxy is designed to isolate itself into a chroot jail during startup, where
it cannot perform any file-system access at all. This is also true for the
libraries it depends on (eg: libc, libssl, etc). The immediate effect is that
a running process will not be able to reload a configuration file to apply
changes, instead a new process will be started using the updated configuration
file. Some other less obvious effects are that some timezone files or resolver
files the libc might attempt to access at run time will not be found, though
this should generally not happen as they're not needed after startup. A nice
consequence of this principle is that the HAProxy process is totally stateless,
and no cleanup is needed after it's killed, so any killing method that works
will do the right thing.

HAProxy doesn't write log files, but it relies on the standard syslog protocol
to send logs to a remote server (which is often located on the same system).

HAProxy uses its internal clock to enforce timeouts, that is derived from the
system's time but where unexpected drift is corrected. This is done by limiting
the time spent waiting in poll() for an event, and measuring the time it really
took. In practice it never waits more than one second. This explains why, when
running strace over a completely idle process, periodic calls to poll() (or any
of its variants) surrounded by two gettimeofday() calls are noticed. They are
normal, completely harmless and so cheap that the load they imply is totally
undetectable at the system scale, so there's nothing abnormal there. Example :

  16:35:40.002320 gettimeofday({1442759740, 2605}, NULL) = 0
  16:35:40.002942 epoll_wait(0, {}, 200, 1000) = 0
  16:35:41.007542 gettimeofday({1442759741, 7641}, NULL) = 0
  16:35:41.007998 gettimeofday({1442759741, 8114}, NULL) = 0
  16:35:41.008391 epoll_wait(0, {}, 200, 1000) = 0
  16:35:42.011313 gettimeofday({1442759742, 11411}, NULL) = 0

HAProxy is a TCP proxy, not a router. It deals with established connections that
have been validated by the kernel, and not with packets of any form nor with
sockets in other states (eg: no SYN_RECV nor TIME_WAIT), though their existence
may prevent it from binding a port. It relies on the system to accept incoming
connections and to initiate outgoing connections. An immediate effect of this is
that there is no relation between packets observed on the two sides of a
forwarded connection, which can be of different size, numbers and even family.
Since a connection may only be accepted from a socket in LISTEN state, all the
sockets it is listening to are necessarily visible using the "netstat" utility
to show listening sockets. Example :

  # netstat -ltnp
  Active Internet connections (only servers)
  Proto Recv-Q Send-Q Local Address   Foreign Address   State    PID/Program name
  tcp        0      0 0.0.0.0:22      0.0.0.0:*         LISTEN   1629/sshd
  tcp        0      0 0.0.0.0:80      0.0.0.0:*         LISTEN   2847/haproxy
  tcp        0      0 0.0.0.0:443     0.0.0.0:*         LISTEN   2847/haproxy

HAProxy is started by invoking the "haproxy" program with a number of arguments
passed on the command line. The actual syntax is :

  $ haproxy [<options>]*

followed by one of more letters, and possibly followed by one or multiple extra
arguments. Without any option, HAProxy displays the help page with a reminder
about supported options. Available options may vary slightly based on the
operating system. A fair number of these options overlap with an equivalent one
if the "global" section. In this case, the command line always has precedence
over the configuration file, so that the command line can be used to quickly
enforce some settings without touching the configuration files. The current
list of options is :

  -- <cfgfile>* : all the arguments following "--" are paths to configuration
    file to be loaded and processed in the declaration order. It is mostly
    useful when relying on the shell to load many files that are numerically
    ordered. See also "-f". The difference between "--" and "-f" is that one
    "-f" must be placed before each file name, while a single "--" is needed
    before all file names. Both options can be used together, the command line
    ordering still applies. When more than one file is specified, each file
    must start on a section boundary, so the first keyword of each file must be
    one of "global", "defaults", "peers", "listen", "frontend", "backend", and
    so on. A file cannot contain just a server list for example.

  -f <cfgfile> : adds <cfgfile> to the list of configuration files to be
    loaded. Configuration files are loaded and processed in their declaration
    order. This option may be specified multiple times to load multiple files.
    See also "--". The difference between "--" and "-f" is that one "-f" must
    be placed before each file name, while a single "--" is needed before all
    file names. Both options can be used together, the command line ordering
    still applies. When more than one file is specified, each file must start
    on a section boundary, so the first keyword of each file must be one of
    "global", "defaults", "peers", "listen", "frontend", "backend", and so
    on. A file cannot contain just a server list for example.

  -C <dir> : changes to directory <dir> before loading configuration
    files. This is useful when using relative paths. Warning when using
    wildcards after "--" which are in fact replaced by the shell before
    starting haproxy.

  -D : start as a daemon. The process detaches from the current terminal after
    forking, and errors are not reported anymore in the terminal. It is
    equivalent to the "daemon" keyword in the "global" section of the
    configuration. It is recommended to always force it in any init script so
    that a faulty configuration doesn't prevent the system from booting.

  -Ds : work in systemd mode. Only used by the systemd wrapper.

  -L <name> : change the local peer name to <name>, which defaults to the local
    hostname. This is used only with peers replication.

  -N <limit> : sets the default per-proxy maxconn to <limit> instead of the
    builtin default value (usually 2000). Only useful for debugging.

  -V : enable verbose mode (disables quiet mode). Reverts the effect of "-q" or
    "quiet".

  -c : only performs a check of the configuration files and exits before trying
    to bind. The exit status is zero if everything is OK, or non-zero if an
    error is encountered.

  -d : enable debug mode. This disables daemon mode, forces the process to stay
    in foreground and to show incoming and outgoing events. It is equivalent to
    the "global" section's "debug" keyword. It must never be used in an init
    script.

  -dG : disable use of getaddrinfo() to resolve host names into addresses. It
    can be used when suspecting that getaddrinfo() doesn't work as expected.
    This option was made available because many bogus implementations of
    getaddrinfo() exist on various systems and cause anomalies that are
    difficult to troubleshoot.

  -dM[<byte>] : forces memory poisonning, which means that each and every
    memory region allocated with malloc() or pool_alloc2() will be filled with
    <byte> before being passed to the caller. When <byte> is not specified, it
    defaults to 0x50 ('P'). While this slightly slows down operations, it is
    useful to reliably trigger issues resulting from missing initializations in
    the code that cause random crashes. Note that -dM0 has the effect of
    turning any malloc() into a calloc(). In any case if a bug appears or
    disappears when using this option it means there is a bug in haproxy, so
    please report it.

  -dS : disable use of the splice() system call. It is equivalent to the
    "global" section's "nosplice" keyword. This may be used when splice() is
    suspected to behave improperly or to cause performance issues, or when
    using strace to see the forwarded data (which do not appear when using
    splice()).

  -dV : disable SSL verify on the server side. It is equivalent to having
    "ssl-server-verify none" in the "global" section. This is useful when
    trying to reproduce production issues out of the production
    environment. Never use this in an init script as it degrades SSL security
    to the servers.

  -db : disable background mode and multi-process mode. The process remains in
    foreground. It is mainly used during development or during small tests, as
    Ctrl-C is enough to stop the process. Never use it in an init script.

  -de : disable the use of the "epoll" poller. It is equivalent to the "global"
    section's keyword "noepoll". It is mostly useful when suspecting a bug
    related to this poller. On systems supporting epoll, the fallback will
    generally be the "poll" poller.

  -dk : disable the use of the "kqueue" poller. It is equivalent to the
    "global" section's keyword "nokqueue". It is mostly useful when suspecting
    a bug related to this poller. On systems supporting kqueue, the fallback
    will generally be the "poll" poller.

  -dp : disable the use of the "poll" poller. It is equivalent to the "global"
    section's keyword "nopoll". It is mostly useful when suspecting a bug
    related to this poller. On systems supporting poll, the fallback will
    generally be the "select" poller, which cannot be disabled and is limited
    to 1024 file descriptors.

  -m <limit> : limit the total allocatable memory to <limit> megabytes across
    all processes. This may cause some connection refusals or some slowdowns
    depending on the amount of memory needed for normal operations. This is
    mostly used to force the processes to work in a constrained resource usage
    scenario. It is important to note that the memory is not shared between
    processes, so in a multi-process scenario, this value is first divided by
    global.nbproc before forking.

  -n <limit> : limits the per-process connection limit to <limit>. This is
    equivalent to the global section's keyword "maxconn". It has precedence
    over this keyword. This may be used to quickly force lower limits to avoid
    a service outage on systems where resource limits are too low.

  -p <file> : write all processes' pids into <file> during startup. This is
    equivalent to the "global" section's keyword "pidfile". The file is opened
    before entering the chroot jail, and after doing the chdir() implied by
    "-C". Each pid appears on its own line.

  -q : set "quiet" mode. This disables some messages during the configuration
    parsing and during startup. It can be used in combination with "-c" to
    just check if a configuration file is valid or not.

  -sf <pid>* : send the "finish" signal (SIGUSR1) to older processes after boot
    completion to ask them to finish what they are doing and to leave. <pid>
    is a list of pids to signal (one per argument). The list ends on any
    option starting with a "-". It is not a problem if the list of pids is
    empty, so that it can be built on the fly based on the result of a command
    like "pidof" or "pgrep".

  -st <pid>* : send the "terminate" signal (SIGTERM) to older processes after
    boot completion to terminate them immediately without finishing what they
    were doing. <pid> is a list of pids to signal (one per argument). The list
    is ends on any option starting with a "-". It is not a problem if the list
    of pids is empty, so that it can be built on the fly based on the result of
    a command like "pidof" or "pgrep".

  -v : report the version and build date.

  -vv : display the version, build options, libraries versions and usable
    pollers. This output is systematically requested when filing a bug report.

A safe way to start HAProxy from an init file consists in forcing the deamon
mode, storing existing pids to a pid file and using this pid file to notify
older processes to finish before leaving :

   haproxy -f /etc/haproxy.cfg \
           -D -p /var/run/haproxy.pid -sf $(cat /var/run/haproxy.pid)

When the configuration is split into a few specific files (eg: tcp vs http),
it is recommended to use the "-f" option :

   haproxy -f /etc/haproxy/global.cfg -f /etc/haproxy/stats.cfg \
           -f /etc/haproxy/default-tcp.cfg -f /etc/haproxy/tcp.cfg \
           -f /etc/haproxy/default-http.cfg -f /etc/haproxy/http.cfg \
           -D -p /var/run/haproxy.pid -sf $(cat /var/run/haproxy.pid)

When an unknown number of files is expected, such as customer-specific files,
it is recommended to assign them a name starting with a fixed-size sequence
number and to use "--" to load them, possibly after loading some defaults :

   haproxy -f /etc/haproxy/global.cfg -f /etc/haproxy/stats.cfg \
           -f /etc/haproxy/default-tcp.cfg -f /etc/haproxy/tcp.cfg \
           -f /etc/haproxy/default-http.cfg -f /etc/haproxy/http.cfg \
           -D -p /var/run/haproxy.pid -sf $(cat /var/run/haproxy.pid) \
           -f /etc/haproxy/default-customers.cfg -- /etc/haproxy/customers/*

Sometimes a failure to start may happen for whatever reason. Then it is
important to verify if the version of HAProxy you are invoking is the expected
version and if it supports the features you are expecting (eg: SSL, PCRE,
compression, Lua, etc). This can be verified using "haproxy -vv". Some
important information such as certain build options, the target system and
the versions of the libraries being used are reported there. It is also what
you will systematically be asked for when posting a bug report :

  $ haproxy -vv
  HA-Proxy version 1.6-dev7-a088d3-4 2015/10/08
  Copyright 2000-2015 Willy Tarreau <willy@haproxy.org>

  Build options :
    TARGET  = linux2628
    CPU     = generic
    CC      = gcc
    CFLAGS  = -pg -O0 -g -fno-strict-aliasing -Wdeclaration-after-statement \
              -DBUFSIZE=8030 -DMAXREWRITE=1030 -DSO_MARK=36 -DTCP_REPAIR=19
    OPTIONS = USE_ZLIB=1 USE_DLMALLOC=1 USE_OPENSSL=1 USE_LUA=1 USE_PCRE=1

  Default settings :
    maxconn = 2000, bufsize = 8030, maxrewrite = 1030, maxpollevents = 200

  Encrypted password support via crypt(3): yes
  Built with zlib version : 1.2.6
  Compression algorithms supported : identity("identity"), deflate("deflate"), \
                                     raw-deflate("deflate"), gzip("gzip")
  Built with OpenSSL version : OpenSSL 1.0.1o 12 Jun 2015
  Running on OpenSSL version : OpenSSL 1.0.1o 12 Jun 2015
  OpenSSL library supports TLS extensions : yes
  OpenSSL library supports SNI : yes
  OpenSSL library supports prefer-server-ciphers : yes
  Built with PCRE version : 8.12 2011-01-15
  PCRE library supports JIT : no (USE_PCRE_JIT not set)
  Built with Lua version : Lua 5.3.1
  Built with transparent proxy support using: IP_TRANSPARENT IP_FREEBIND

  Available polling systems :
        epoll : pref=300,  test result OK
         poll : pref=200,  test result OK
       select : pref=150,  test result OK
  Total: 3 (3 usable), will use epoll.

The relevant information that many non-developer users can verify here are :
  - the version : 1.6-dev7-a088d3-4 above means the code is currently at commit
    ID "a088d3" which is the 4th one after after official version "1.6-dev7".
    Version 1.6-dev7 would show as "1.6-dev7-8c1ad7". What matters here is in
    fact "1.6-dev7". This is the 7th development version of what will become
    version 1.6 in the future. A development version not suitable for use in
    production (unless you know exactly what you are doing). A stable version
    will show as a 3-numbers version, such as "1.5.14-16f863", indicating the
    14th level of fix on top of version 1.5. This is a production-ready version.

  - the release date : 2015/10/08. It is represented in the universal
    year/month/day format. Here this means August 8th, 2015. Given that stable
    releases are issued every few months (1-2 months at the beginning, sometimes
    6 months once the product becomes very stable), if you're seeing an old date
    here, it means you're probably affected by a number of bugs or security
    issues that have since been fixed and that it might be worth checking on the
    official site.

  - build options : they are relevant to people who build their packages
    themselves, they can explain why things are not behaving as expected. For
    example the development version above was built for Linux 2.6.28 or later,
    targetting a generic CPU (no CPU-specific optimizations), and lacks any
    code optimization (-O0) so it will perform poorly in terms of performance.

  - libraries versions : zlib version is reported as found in the library
    itself. In general zlib is considered a very stable product and upgrades
    are almost never needed. OpenSSL reports two versions, the version used at
    build time and the one being used, as found on the system. These ones may
    differ by the last letter but never by the numbers. The build date is also
    reported because most OpenSSL bugs are security issues and need to be taken
    seriously, so this library absolutely needs to be kept up to date. Seeing a
    4-months old version here is highly suspicious and indeed an update was
    missed. PCRE provides very fast regular expressions and is highly
    recommended. Certain of its extensions such as JIT are not present in all
    versions and still young so some people prefer not to build with them,
    which is why the biuld status is reported as well. Regarding the Lua
    scripting language, HAProxy expects version 5.3 which is very young since
    it was released a little time before HAProxy 1.6. It is important to check
    on the Lua web site if some fixes are proposed for this branch.

  - Available polling systems will affect the process's scalability when
    dealing with more than about one thousand of concurrent connections. These
    ones are only available when the correct system was indicated in the TARGET
    variable during the build. The "epoll" mechanism is highly recommended on
    Linux, and the kqueue mechanism is highly recommended on BSD. Lacking them
    will result in poll() or even select() being used, causing a high CPU usage
    when dealing with a lot of connections.

HAProxy supports a graceful and a hard stop. The hard stop is simple, when the
SIGTERM signal is sent to the haproxy process, it immediately quits and all
established connections are closed. The graceful stop is triggered when the
SIGUSR1 signal is sent to the haproxy process. It consists in only unbinding
from listening ports, but continue to process existing connections until they
close. Once the last connection is closed, the process leaves.

The hard stop method is used for the "stop" or "restart" actions of the service
management script. The graceful stop is used for the "reload" action which
tries to seamlessly reload a new configuration in a new process.

Both of these signals may be sent by the new haproxy process itself during a
reload or restart, so that they are sent at the latest possible moment and only
if absolutely required. This is what is performed by the "-st" (hard) and "-sf"
(graceful) options respectively.

To understand better how these signals are used, it is important to understand
the whole restart mechanism.

First, an existing haproxy process is running. The administrator uses a system
specific command such as "/etc/init.d/haproxy reload" to indicate he wants to
take the new configuration file into effect. What happens then is the following.
First, the service script (/etc/init.d/haproxy or equivalent) will verify that
the configuration file parses correctly using "haproxy -c". After that it will
try to start haproxy with this configuration file, using "-st" or "-sf".

Then HAProxy tries to bind to all listening ports. If some fatal errors happen
(eg: address not present on the system, permission denied), the process quits
with an error. If a socket binding fails because a port is already in use, then
the process will first send a SIGTTOU signal to all the pids specified in the
"-st" or "-sf" pid list. This is what is called the "pause" signal. It instructs
all existing haproxy processes to temporarily stop listening to their ports so
that the new process can try to bind again. During this time, the old process
continues to process existing connections. If the binding still fails (because
for example a port is shared with another daemon), then the new process sends a
SIGTTIN signal to the old processes to instruct them to resume operations just
as if nothing happened. The old processes will then restart listening to the
ports and continue to accept connections. Not that this mechanism is system
dependant and some operating systems may not support it in multi-process mode.

If the new process manages to bind correctly to all ports, then it sends either
the SIGTERM (hard stop in case of "-st") or the SIGUSR1 (graceful stop in case
of "-sf") to all processes to notify them that it is now in charge of operations
and that the old processes will have to leave, either immediately or once they
have finished their job.

It is important to note that during this timeframe, there are two small windows
of a few milliseconds each where it is possible that a few connection failures
will be noticed during high loads. Typically observed failure rates are around
1 failure during a reload operation every 10000 new connections per second,
which means that a heavily loaded site running at 30000 new connections per
second may see about 3 failed connection upon every reload. The two situations
where this happens are :

  - if the new process fails to bind due to the presence of the old process,
    it will first have to go through the SIGTTOU+SIGTTIN sequence, which
    typically lasts about one millisecond for a few tens of frontends, and
    during which some ports will not be bound to the old process and not yet
    bound to the new one. HAProxy works around this on systems that support the
    SO_REUSEPORT socket options, as it allows the new process to bind without
    first asking the old one to unbind. Most BSD systems have been supporting
    this almost forever. Linux has been supporting this in version 2.0 and
    dropped it around 2.2, but some patches were floating around by then. It
    was reintroduced in kernel 3.9, so if you are observing a connection
    failure rate above the one mentionned above, please ensure that your kernel
    is 3.9 or newer, or that relevant patches were backported to your kernel
    (less likely).

  - when the old processes close the listening ports, the kernel may not always
    redistribute any pending connection that was remaining in the socket's
    backlog. Under high loads, a SYN packet may happen just before the socket
    is closed, and will lead to an RST packet being sent to the client. In some
    critical environments where even one drop is not acceptable, these ones are
    sometimes dealt with using firewall rules to block SYN packets during the
    reload, forcing the client to retransmit. This is totally system-dependent,
    as some systems might be able to visit other listening queues and avoid
    this RST. A second case concerns the ACK from the client on a local socket
    that was in SYN_RECV state just before the close. This ACK will lead to an
    RST packet while the haproxy process is still not aware of it. This one is
    harder to get rid of, though the firewall filtering rules mentionned above
    will work well if applied one second or so before restarting the process.

For the vast majority of users, such drops will never ever happen since they
don't have enough load to trigger the race conditions. And for most high traffic
users, the failure rate is still fairly within the noise margin provided that at
least SO_REUSEPORT is properly supported on their systems.

In order to ensure that all incoming connections will successfully be served,
HAProxy computes at load time the total number of file descriptors that will be
needed during the process's life. A regular Unix process is generally granted
1024 file descriptors by default, and a privileged process can raise this limit
itself. This is one reason for starting HAProxy as root and letting it adjust
the limit. The default limit of 1024 file descriptors roughly allow about 500
concurrent connections to be processed. The computation is based on the global
maxconn parameter which limits the total number of connections per process, the
number of listeners, the number of servers which have a health check enabled,
the agent checks, the peers, the loggers and possibly a few other technical
requirements. A simple rough estimate of this number consists in simply
doubling the maxconn value and adding a few tens to get the approximate number
of file descriptors needed.

Originally HAProxy did not know how to compute this value, and it was necessary
to pass the value using the "ulimit-n" setting in the global section. This
explains why even today a lot of configurations are seen with this setting
present. Unfortunately it was often miscalculated resulting in connection
failures when approaching maxconn instead of throttling incoming connection
while waiting for the needed resources. For this reason it is important to
remove any vestigal "ulimit-n" setting that can remain from very old versions.

Raising the number of file descriptors to accept even moderate loads is
mandatory but comes with some OS-specific adjustments. First, the select()
polling system is limited to 1024 file descriptors. In fact on Linux it used
to be capable of handling more but since certain OS ship with excessively
restrictive SELinux policies forbidding the use of select() with more than
1024 file descriptors, HAProxy now refuses to start in this case in order to
avoid any issue at run time. On all supported operating systems, poll() is
available and will not suffer from this limitation. It is automatically picked
so there is nothing ot do to get a working configuration. But poll's becomes
very slow when the number of file descriptors increases. While HAProxy does its
best to limit this performance impact (eg: via the use of the internal file
descriptor cache and batched processing), a good rule of thumb is that using
poll() with more than a thousand concurrent connections will use a lot of CPU.

For Linux systems base on kernels 2.6 and above, the epoll() system call will
be used. It's a much more scalable mechanism relying on callbacks in the kernel
that guarantee a constant wake up time regardless of the number of registered
monitored file descriptors. It is automatically used where detected, provided
that HAProxy had been built for one of the Linux flavors. Its presence and
support can be verified using "haproxy -vv".

For BSD systems which support it, kqueue() is available as an alternative. It
is much faster than poll() and even slightly faster than epoll() thanks to its
batched handling of changes. At least FreeBSD and OpenBSD support it. Just like
with Linux's epoll(), its support and availability are reported in the output
of "haproxy -vv".

Having a good poller is one thing, but it is mandatory that the process can
reach the limits. When HAProxy starts, it immediately sets the new process's
file descriptor limits and verifies if it succeeds. In case of failure, it
reports it before forking so that the administrator can see the problem. As
long as the process is started by as root, there should be no reason for this
setting to fail. However, it can fail if the process is started by an
unprivileged user. If there is a compelling reason for *not* starting haproxy
as root (eg: started by end users, or by a per-application account), then the
file descriptor limit can be raised by the system administrator for this
specific user. The effectiveness of the setting can be verified by issuing
"ulimit -n" from the user's command line. It should reflect the new limit.

Warning: when an unprivileged user's limits are changed in this user's account,
it is fairly common that these values are only considered when the user logs in
and not at all in some scripts run at system boot time nor in crontabs. This is
totally dependent on the operating system, keep in mind to check "ulimit -n"
before starting haproxy when running this way. The general advice is never to
start haproxy as an unprivileged user for production purposes. Another good
reason is that it prevents haproxy from enabling some security protections.

Once it is certain that the system will allow the haproxy process to use the
requested number of file descriptors, two new system-specific limits may be
encountered. The first one is the system-wide file descriptor limit, which is
the total number of file descriptors opened on the system, covering all
processes. When this limit is reached, accept() or socket() will typically
return ENFILE. The second one is the per-process hard limit on the number of
file descriptors, it prevents setrlimit() from being set higher. Both are very
dependent on the operating system. On Linux, the system limit is set at boot
based on the amount of memory. It can be changed with the "fs.file-max" sysctl.
And the per-process hard limit is set to 1048576 by default, but it can be
changed using the "fs.nr_open" sysctl.

File descriptor limitations may be observed on a running process when they are
set too low. The strace utility will report that accept() and socket() return
"-1 EMFILE" when the process's limits have been reached. In this case, simply
raising the "ulimit-n" value (or removing it) will solve the problem. If these
system calls return "-1 ENFILE" then it means that the kernel's limits have
been reached and that something must be done on a system-wide parameter. These
trouble must absolutely be addressed, as they result in high CPU usage (when
accept() fails) and failed connections that are generally visible to the user.
One solution also consists in lowering the global maxconn value to enforce
serialization, and possibly to disable HTTP keep-alive to force connections
to be released and reused faster.

HAProxy uses a simple and fast pool-based memory management. Since it relies on
a small number of different object types, it's much more efficient to pick new
objects from a pool which already contains objects of the appropriate size than
to call malloc() for each different size. The pools are organized as a stack or
LIFO, so that newly allocated objects are taken from recently released objects
still hot in the CPU caches. Pools of similar sizes are merged together, in
order to limit memory fragmentation.

By default, since the focus is set on performance, each released object is put
back into the pool it came from, and allocated objects are never freed since
they are expected to be reused very soon.

On the CLI, it is possible to check how memory is being used in pools thanks to
the "show pools" command :

  > show pools
  Dumping pools usage. Use SIGQUIT to flush them.
    - Pool pipe (32 bytes) : 5 allocated (160 bytes), 5 used, 3 users [SHARED]
    - Pool hlua_com (48 bytes) : 0 allocated (0 bytes), 0 used, 1 users [SHARED]
    - Pool vars (64 bytes) : 0 allocated (0 bytes), 0 used, 2 users [SHARED]
    - Pool task (112 bytes) : 5 allocated (560 bytes), 5 used, 1 users [SHARED]
    - Pool session (128 bytes) : 1 allocated (128 bytes), 1 used, 2 users [SHARED]
    - Pool http_txn (272 bytes) : 0 allocated (0 bytes), 0 used, 1 users [SHARED]
    - Pool connection (352 bytes) : 2 allocated (704 bytes), 2 used, 1 users [SHARED]
    - Pool hdr_idx (416 bytes) : 0 allocated (0 bytes), 0 used, 1 users [SHARED]
    - Pool stream (864 bytes) : 1 allocated (864 bytes), 1 used, 1 users [SHARED]
    - Pool requri (1024 bytes) : 0 allocated (0 bytes), 0 used, 1 users [SHARED]
    - Pool buffer (8064 bytes) : 3 allocated (24192 bytes), 2 used, 1 users [SHARED]
  Total: 11 pools, 26608 bytes allocated, 18544 used.

The pool name is only indicative, it's the name of the first object type using
this pool. The size in parenthesis is the object size for objects in this pool.
Object sizes are always rounded up to the closest multiple of 16 bytes. The
number of objects currently allocated and the equivalent number of bytes is
reported so that it is easy to know which pool is responsible for the highest
memory usage. The number of objects currently in use is reported as well in the
"used" field. The difference between "allocated" and "used" corresponds to the
objects that have been freed and are available for immediate use.

It is possible to limit the amount of memory allocated per process using the
"-m" command line option, followed by a number of megabytes. It covers all of
the process's addressable space, so that includes memory used by some libraries
as well as the stack, but it is a reliable limit when building a resource
constrained system. It works the same way as "ulimit -v" on systems which have
it, or "ulimit -d" for the other ones.

If a memory allocation fails due to the memory limit being reached or because
the system doesn't have any enough memory, then haproxy will first start to
free all available objects from all pools before attempting to allocate memory
again. This mechanism of releasing unused memory can be triggered by sending
the signal SIGQUIT to the haproxy process. When doing so, the pools state prior
to the flush will also be reported to stderr when the process runs in
foreground.

During a reload operation, the process switched to the graceful stop state also
automatically performs some flushes after releasing any connection so that all
possible memory is released to save it for the new process.

HAProxy normally spends most of its time in the system and a smaller part in
userland. A finely tuned 3.5 GHz CPU can sustain a rate about 80000 end-to-end
connection setups and closes per second at 100% CPU on a single core. When one
core is saturated, typical figures are :
  - 95% system, 5% user for long TCP connections or large HTTP objects
  - 85% system and 15% user for short TCP connections or small HTTP objects in
    close mode
  - 70% system and 30% user for small HTTP objects in keep-alive mode

The amount of rules processing and regular expressions will increase the user
land part. The presence of firewall rules, connection tracking, complex routing
tables in the system will instead increase the system part.

On most systems, the CPU time observed during network transfers can be cut in 4
parts :
  - the interrupt part, which concerns all the processing performed upon I/O
    receipt, before the target process is even known. Typically Rx packets are
    accounted for in interrupt. On some systems such as Linux where interrupt
    processing may be deferred to a dedicated thread, it can appear as softirq,
    and the thread is called ksoftirqd/0 (for CPU 0). The CPU taking care of
    this load is generally defined by the hardware settings, though in the case
    of softirq it is often possible to remap the processing to another CPU.
    This interrupt part will often be perceived as parasitic since it's not
    associated with any process, but it actually is some processing being done
    to prepare the work for the process.

  - the system part, which concerns all the processing done using kernel code
    called from userland. System calls are accounted as system for example. All
    synchronously delivered Tx packets will be accounted for as system time. If
    some packets have to be deferred due to queues filling up, they may then be
    processed in interrupt context later (eg: upon receipt of an ACK opening a
    TCP window).

  - the user part, which exclusively runs application code in userland. HAProxy
    runs exclusively in this part, though it makes heavy use of system calls.
    Rules processing, regular expressions, compression, encryption all add to
    the user portion of CPU consumption.

  - the idle part, which is what the CPU does when there is nothing to do. For
    example HAProxy waits for an incoming connection, or waits for some data to
    leave, meaning the system is waiting for an ACK from the client to push
    these data.

In practice regarding HAProxy's activity, it is in general reasonably accurate
(but totally inexact) to consider that interrupt/softirq are caused by Rx
processing in kernel drivers, that user-land is caused by layer 7 processing
in HAProxy, and that system time is caused by network processing on the Tx
path.

Since HAProxy runs around an event loop, it waits for new events using poll()
(or any alternative) and processes all these events as fast as possible before
going back to poll() waiting for new events. It measures the time spent waiting
in poll() compared to the time spent doing processing events. The ratio of
polling time vs total time is called the "idle" time, it's the amount of time
spent waiting for something to happen. This ratio is reported in the stats page
on the "idle" line, or "Idle_pct" on the CLI. When it's close to 100%, it means
the load is extremely low. When it's close to 0%, it means that there is
constantly some activity. While it cannot be very accurate on an overloaded
system due to other processes possibly preempting the CPU from the haproxy
process, it still provides a good estimate about how HAProxy considers it is
working : if the load is low and the idle ratio is low as well, it may indicate
that HAProxy has a lot of work to do, possibly due to very expensive rules that
have to be processed. Conversely, if HAProxy indicates the idle is close to
100% while things are slow, it means that it cannot do anything to speed things
up because it is already waiting for incoming data to process. In the example
below, haproxy is completely idle :

  $ echo "show info" | socat - /var/run/haproxy.sock | grep ^Idle
  Idle_pct: 100

When the idle ratio starts to become very low, it is important to tune the
system and place processes and interrupts correctly to save the most possible
CPU resources for all tasks. If a firewall is present, it may be worth trying
to disable it or to tune it to ensure it is not responsible for a large part
of the performance limitation. It's worth noting that unloading a stateful
firewall generally reduces both the amount of interrupt/softirq and of system
usage since such firewalls act both on the Rx and the Tx paths. On Linux,
unloading the nf_conntrack and ip_conntrack modules will show whether there is
anything to gain. If so, then the module runs with default settings and you'll
have to figure how to tune it for better performance. In general this consists
in considerably increasing the hash table size. On FreeBSD, "pfctl -d" will
disable the "pf" firewall and its stateful engine at the same time.

If it is observed that a lot of time is spent in interrupt/softirq, it is
important to ensure that they don't run on the same CPU. Most systems tend to
pin the tasks on the CPU where they receive the network traffic because for
certain workloads it improves things. But with heavily network-bound workloads
it is the opposite as the haproxy process will have to fight against its kernel
counterpart. Pinning haproxy to one CPU core and the interrupts to another one,
all sharing the same L3 cache tends to sensibly increase network performance
because in practice the amount of work for haproxy and the network stack are
quite close, so they can almost fill an entire CPU each. On Linux this is done
using taskset (for haproxy) or using cpu-map (from the haproxy config), and the
interrupts are assigned under /proc/irq. Many network interfaces support
multiple queues and multiple interrupts. In general it helps to spread them
across a small number of CPU cores provided they all share the same L3 cache.
Please always stop irq_balance which always does the worst possible thing on
such workloads.

For CPU-bound workloads consisting in a lot of SSL traffic or a lot of
compression, it may be worth using multiple processes dedicated to certain
tasks, though there is no universal rule here and experimentation will have to
be performed.

In order to increase the CPU capacity, it is possible to make HAProxy run as
several processes, using the "nbproc" directive in the global section. There
are some limitations though :
  - health checks are run per process, so the target servers will get as many
    checks as there are running processes ;
  - maxconn values and queues are per-process so the correct value must be set
    to avoid overloading the servers ;
  - outgoing connections should avoid using port ranges to avoid conflicts
  - stick-tables are per process and are not shared between processes ;
  - each peers section may only run on a single process at a time ;
  - the CLI operations will only act on a single process at a time.

With this in mind, it appears that the easiest setup often consists in having
one first layer running on multiple processes and in charge for the heavy
processing, passing the traffic to a second layer running in a single process.
This mechanism is suited to SSL and compression which are the two CPU-heavy
features. Instances can easily be chained over UNIX sockets (which are cheaper
than TCP sockets and which do not waste ports), and the proxy protocol which is
useful to pass client information to the next stage. When doing so, it is
generally a good idea to bind all the single-process tasks to process number 1
and extra tasks to next processes, as this will make it easier to generate
similar configurations for different machines.

On Linux versions 3.9 and above, running HAProxy in multi-process mode is much
more efficient when each process uses a distinct listening socket on the same
IP:port ; this will make the kernel evenly distribute the load across all
processes instead of waking them all up. Please check the "process" option of
the "bind" keyword lines in the configuration manual for more information.

For logging, HAProxy always relies on a syslog server since it does not perform
any file-system access. The standard way of using it is to send logs over UDP
to the log server (by default on port 514). Very commonly this is configured to
127.0.0.1 where the local syslog daemon is running, but it's also used over the
network to log to a central server. The central server provides additional
benefits especially in active-active scenarios where it is desirable to keep
the logs merged in arrival order. HAProxy may also make use of a UNIX socket to
send its logs to the local syslog daemon, but it is not recommended at all,
because if the syslog server is restarted while haproxy runs, the socket will
be replaced and new logs will be lost. Since HAProxy will be isolated inside a
chroot jail, it will not have the ability to reconnect to the new socket. It
has also been observed in field that the log buffers in use on UNIX sockets are
very small and lead to lost messages even at very light loads. But this can be
fine for testing however.

It is recommended to add the following directive to the "global" section to
make HAProxy log to the local daemon using facility "local0" :

      log 127.0.0.1:514 local0

and then to add the following one to each "defaults" section or to each frontend
and backend section :

      log global

This way, all logs will be centralized through the global definition of where
the log server is.

Some syslog daemons do not listen to UDP traffic by default, so depending on
the daemon being used, the syntax to enable this will vary :

  - on sysklogd, you need to pass argument "-r" on the daemon's command line
    so that it listens to a UDP socket for "remote" logs ; note that there is
    no way to limit it to address 127.0.0.1 so it will also receive logs from
    remote systems ;

  - on rsyslogd, the following lines must be added to the configuration file :

      $ModLoad imudp
      $UDPServerAddress *
      $UDPServerRun 514

  - on syslog-ng, a new source can be created the following way, it then needs
    to be added as a valid source in one of the "log" directives :

      source s_udp {
        udp(ip(127.0.0.1) port(514));
      };

Please consult your syslog daemon's manual for more information. If no logs are
seen in the system's log files, please consider the following tests :

  - restart haproxy. Each frontend and backend logs one line indicating it's
    starting. If these logs are received, it means logs are working.

  - run "strace -tt -s100 -etrace=sendmsg -p <haproxy's pid>" and perform some
    activity that you expect to be logged. You should see the log messages
    being sent using sendmsg() there. If they don't appear, restart using
    strace on top of haproxy. If you still see no logs, it definitely means
    that something is wrong in your configuration.

  - run tcpdump to watch for port 514, for example on the loopback interface if
    the traffic is being sent locally : "tcpdump -As0 -ni lo port 514". If the
    packets are seen there, it's the proof they're sent then the syslogd daemon
    needs to be troubleshooted.

While traffic logs are sent from the frontends (where the incoming connections
are accepted), backends also need to be able to send logs in order to report a
server state change consecutive to a health check. Please consult HAProxy's
configuration manual for more information regarding all possible log settings.

It is convenient to chose a facility that is not used by other deamons. HAProxy
examples often suggest "local0" for traffic logs and "local1" for admin logs
because they're never seen in field. A single facility would be enough as well.
Having separate logs is convenient for log analysis, but it's also important to
remember that logs may sometimes convey confidential information, and as such
they must not be mixed with other logs that may accidently be handed out to
unauthorized people.

For in-field troubleshooting without impacting the server's capacity too much,
it is recommended to make use of the "halog" utility provided with HAProxy.
This is sort of a grep-like utility designed to process HAProxy log files at
a very fast data rate. Typical figures range between 1 and 2 GB of logs per
second. It is capable of extracting only certain logs (eg: search for some
classes of HTTP status codes, connection termination status, search by response
time ranges, look for errors only), count lines, limit the output to a number
of lines, and perform some more advanced statistics such as sorting servers
by response time or error counts, sorting URLs by time or count, sorting client
addresses by access count, and so on. It is pretty convenient to quickly spot
anomalies such as a bot looping on the site, and block them.

It is possible to query HAProxy about its status. The most commonly used
mechanism is the HTTP statistics page. This page also exposes an alternative
CSV output format for monitoring tools. The same format is provided on the
Unix socket.

The statistics may be consulted either from the unix socket or from the HTTP
page. Both means provide a CSV format whose fields follow. The first line
begins with a sharp ('#') and has one word per comma-delimited field which
represents the title of the column. All other lines starting at the second one
use a classical CSV format using a comma as the delimiter, and the double quote
('"') as an optional text delimiter, but only if the enclosed text is ambiguous
(if it contains a quote or a comma). The double-quote character ('"') in the
text is doubled ('""'), which is the format that most tools recognize. Please
do not insert any column before these ones in order not to break tools which
use hard-coded column positions.

In brackets after each field name are the types which may have a value for
that field. The types are L (Listeners), F (Frontends), B (Backends), and
S (Servers).

  0. pxname [LFBS]: proxy name
  1. svname [LFBS]: service name (FRONTEND for frontend, BACKEND for backend,
     any name for server/listener)
  2. qcur [..BS]: current queued requests. For the backend this reports the
     number queued without a server assigned.
  3. qmax [..BS]: max value of qcur
  4. scur [LFBS]: current sessions
  5. smax [LFBS]: max sessions
  6. slim [LFBS]: configured session limit
  7. stot [LFBS]: cumulative number of connections
  8. bin [LFBS]: bytes in
  9. bout [LFBS]: bytes out
 10. dreq [LFB.]: requests denied because of security concerns.
     - For tcp this is because of a matched tcp-request content rule.
     - For http this is because of a matched http-request or tarpit rule.
 11. dresp [LFBS]: responses denied because of security concerns.
     - For http this is because of a matched http-request rule, or
       "option checkcache".
 12. ereq [LF..]: request errors. Some of the possible causes are:
     - early termination from the client, before the request has been sent.
     - read error from the client
     - client timeout
     - client closed connection
     - various bad requests from the client.
     - request was tarpitted.
 13. econ [..BS]: number of requests that encountered an error trying to
     connect to a backend server. The backend stat is the sum of the stat
     for all servers of that backend, plus any connection errors not
     associated with a particular server (such as the backend having no
     active servers).
 14. eresp [..BS]: response errors. srv_abrt will be counted here also.
     Some other errors are:
     - write error on the client socket (won't be counted for the server stat)
     - failure applying filters to the response.
 15. wretr [..BS]: number of times a connection to a server was retried.
 16. wredis [..BS]: number of times a request was redispatched to another
     server. The server value counts the number of times that server was
     switched away from.
 17. status [LFBS]: status (UP/DOWN/NOLB/MAINT/MAINT(via)...)
 18. weight [..BS]: total weight (backend), server weight (server)
 19. act [..BS]: number of active servers (backend), server is active (server)
 20. bck [..BS]: number of backup servers (backend), server is backup (server)
 21. chkfail [...S]: number of failed checks. (Only counts checks failed when
     the server is up.)
 22. chkdown [..BS]: number of UP->DOWN transitions. The backend counter counts
     transitions to the whole backend being down, rather than the sum of the
     counters for each server.
 23. lastchg [..BS]: number of seconds since the last UP<->DOWN transition
 24. downtime [..BS]: total downtime (in seconds). The value for the backend
     is the downtime for the whole backend, not the sum of the server downtime.
 25. qlimit [...S]: configured maxqueue for the server, or nothing in the
     value is 0 (default, meaning no limit)
 26. pid [LFBS]: process id (0 for first instance, 1 for second, ...)
 27. iid [LFBS]: unique proxy id
 28. sid [L..S]: server id (unique inside a proxy)
 29. throttle [...S]: current throttle percentage for the server, when
     slowstart is active, or no value if not in slowstart.
 30. lbtot [..BS]: total number of times a server was selected, either for new
     sessions, or when re-dispatching. The server counter is the number
     of times that server was selected.
 31. tracked [...S]: id of proxy/server if tracking is enabled.
 32. type [LFBS]: (0=frontend, 1=backend, 2=server, 3=socket/listener)
 33. rate [.FBS]: number of sessions per second over last elapsed second
 34. rate_lim [.F..]: configured limit on new sessions per second
 35. rate_max [.FBS]: max number of new sessions per second
 36. check_status [...S]: status of last health check, one of:
        UNK     -> unknown
        INI     -> initializing
        SOCKERR -> socket error
        L4OK    -> check passed on layer 4, no upper layers testing enabled
        L4TOUT  -> layer 1-4 timeout
        L4CON   -> layer 1-4 connection problem, for example
                   "Connection refused" (tcp rst) or "No route to host" (icmp)
        L6OK    -> check passed on layer 6
        L6TOUT  -> layer 6 (SSL) timeout
        L6RSP   -> layer 6 invalid response - protocol error
        L7OK    -> check passed on layer 7
        L7OKC   -> check conditionally passed on layer 7, for example 404 with
                   disable-on-404
        L7TOUT  -> layer 7 (HTTP/SMTP) timeout
        L7RSP   -> layer 7 invalid response - protocol error
        L7STS   -> layer 7 response error, for example HTTP 5xx
 37. check_code [...S]: layer5-7 code, if available
 38. check_duration [...S]: time in ms took to finish last health check
 39. hrsp_1xx [.FBS]: http responses with 1xx code
 40. hrsp_2xx [.FBS]: http responses with 2xx code
 41. hrsp_3xx [.FBS]: http responses with 3xx code
 42. hrsp_4xx [.FBS]: http responses with 4xx code
 43. hrsp_5xx [.FBS]: http responses with 5xx code
 44. hrsp_other [.FBS]: http responses with other codes (protocol error)
 45. hanafail [...S]: failed health checks details
 46. req_rate [.F..]: HTTP requests per second over last elapsed second
 47. req_rate_max [.F..]: max number of HTTP requests per second observed
 48. req_tot [.F..]: total number of HTTP requests received
 49. cli_abrt [..BS]: number of data transfers aborted by the client
 50. srv_abrt [..BS]: number of data transfers aborted by the server
     (inc. in eresp)
 51. comp_in [.FB.]: number of HTTP response bytes fed to the compressor
 52. comp_out [.FB.]: number of HTTP response bytes emitted by the compressor
 53. comp_byp [.FB.]: number of bytes that bypassed the HTTP compressor
     (CPU/BW limit)
 54. comp_rsp [.FB.]: number of HTTP responses that were compressed
 55. lastsess [..BS]: number of seconds since last session assigned to
     server/backend
 56. last_chk [...S]: last health check contents or textual error
 57. last_agt [...S]: last agent check contents or textual error
 58. qtime [..BS]: the average queue time in ms over the 1024 last requests
 59. ctime [..BS]: the average connect time in ms over the 1024 last requests
 60. rtime [..BS]: the average response time in ms over the 1024 last requests
     (0 for TCP)
 61. ttime [..BS]: the average total session time in ms over the 1024 last
     requests

The stats socket is not enabled by default. In order to enable it, it is
necessary to add one line in the global section of the haproxy configuration.
A second line is recommended to set a larger timeout, always appreciated when
issuing commands by hand :

    global
        stats socket /var/run/haproxy.sock mode 600 level admin
        stats timeout 2m

It is also possible to add multiple instances of the stats socket by repeating
the line, and make them listen to a TCP port instead of a UNIX socket. This is
never done by default because this is dangerous, but can be handy in some
situations :

    global
        stats socket /var/run/haproxy.sock mode 600 level admin
        stats socket ipv4@192.168.0.1:9999 level admin
        stats timeout 2m

To access the socket, an external utility such as "socat" is required. Socat is
a swiss-army knife to connect anything to anything. We use it to connect
terminals to the socket, or a couple of stdin/stdout pipes to it for scripts.
The two main syntaxes we'll use are the following :

    # socat /var/run/haproxy.sock stdio
    # socat /var/run/haproxy.sock readline

The first one is used with scripts. It is possible to send the output of a
script to haproxy, and pass haproxy's output to another script. That's useful
for retrieving counters or attack traces for example.

The second one is only useful for issuing commands by hand. It has the benefit
that the terminal is handled by the readline library which supports line
editing and history, which is very convenient when issuing repeated commands
(eg: watch a counter).

The socket supports two operation modes :
  - interactive
  - non-interactive

The non-interactive mode is the default when socat connects to the socket. In
this mode, a single line may be sent. It is processed as a whole, responses are
sent back, and the connection closes after the end of the response. This is the
mode that scripts and monitoring tools use. It is possible to send multiple
commands in this mode, they need to be delimited by a semi-colon (';'). For
example :

    # echo "show info;show stat;show table" | socat /var/run/haproxy stdio

If a command needs to use a semi-colon (eg: in a value), it must be preceeded
by a backslash ('\').

The interactive mode displays a prompt ('>') and waits for commands to be
entered on the line, then processes them, and displays the prompt again to wait
for a new command. This mode is entered via the "prompt" command which must be
sent on the first line in non-interactive mode. The mode is a flip switch, if
"prompt" is sent in interactive mode, it is disabled and the connection closes
after processing the last command of the same line.

For this reason, when debugging by hand, it's quite common to start with the
"prompt" command :

   # socat /var/run/haproxy readline
   prompt
   > show info
   ...
   >

Since multiple commands may be issued at once, haproxy uses the empty line as a
delimiter to mark an end of output for each command, and takes care of ensuring
that no command can emit an empty line on output. A script can thus easily
parse the output even when multiple commands were pipelined on a single line.

It is important to understand that when multiple haproxy processes are started
on the same sockets, any process may pick up the request and will output its
own stats.

The list of commands currently supported on the stats socket is provided below.
If an unknown command is sent, haproxy displays the usage message which reminds
all supported commands. Some commands support a more complex syntax, generally
it will explain what part of the command is invalid when this happens.

Add an entry into the acl <acl>. <acl> is the #<id> or the <file> returned by
"show acl". This command does not verify if the entry already exists. This
command cannot be used if the reference <acl> is a file also used with a map.
In this case, you must use the command "add map" in place of "add acl".

Add an entry into the map <map> to associate the value <value> to the key
<key>. This command does not verify if the entry already exists. It is
mainly used to fill a map after a clear operation. Note that if the reference
<map> is a file and is shared with a map, this map will contain also a new
pattern entry.

Clear the max values of the statistics counters in each proxy (frontend &
backend) and in each server. The cumulated counters are not affected. This
can be used to get clean counters after an incident, without having to
restart nor to clear traffic counters. This command is restricted and can
only be issued on sockets configured for levels "operator" or "admin".

Clear all statistics counters in each proxy (frontend & backend) and in each
server. This has the same effect as restarting. This command is restricted
and can only be issued on sockets configured for level "admin".

Remove all entries from the acl <acl>. <acl> is the #<id> or the <file>
returned by "show acl". Note that if the reference <acl> is a file and is
shared with a map, this map will be also cleared.

Remove all entries from the map <map>. <map> is the #<id> or the <file>
returned by "show map". Note that if the reference <map> is a file and is
shared with a acl, this acl will be also cleared.

Remove entries from the stick-table <table>.

This is typically used to unblock some users complaining they have been
abusively denied access to a service, but this can also be used to clear some
stickiness entries matching a server that is going to be replaced (see "show
table" below for details).  Note that sometimes, removal of an entry will be
refused because it is currently tracked by a session. Retrying a few seconds
later after the session ends is usual enough.

In the case where no options arguments are given all entries will be removed.

When the "data." form is used entries matching a filter applied using the
stored data (see "stick-table" in section 4.2) are removed.  A stored data
type must be specified in <type>, and this data type must be stored in the
table otherwise an error is reported. The data is compared according to
<operator> with the 64-bit integer <value>.  Operators are the same as with
the ACLs :

  - eq : match entries whose data is equal to this value
  - ne : match entries whose data is not equal to this value
  - le : match entries whose data is less than or equal to this value
  - ge : match entries whose data is greater than or equal to this value
  - lt : match entries whose data is less than this value
  - gt : match entries whose data is greater than this value

When the key form is used the entry <key> is removed.  The key must be of the
same type as the table, which currently is limited to IPv4, IPv6, integer and
string.

    $ echo "show table http_proxy" | socat stdio /tmp/sock1
>>> # table: http_proxy, type: ip, size:204800, used:2
>>> 0x80e6a4c: key=127.0.0.1 use=0 exp=3594729 gpc0=0 conn_rate(30000)=1 \
      bytes_out_rate(60000)=187
>>> 0x80e6a80: key=127.0.0.2 use=0 exp=3594740 gpc0=1 conn_rate(30000)=10 \
      bytes_out_rate(60000)=191

    $ echo "clear table http_proxy key 127.0.0.1" | socat stdio /tmp/sock1

    $ echo "show table http_proxy" | socat stdio /tmp/sock1
>>> # table: http_proxy, type: ip, size:204800, used:1
>>> 0x80e6a80: key=127.0.0.2 use=0 exp=3594740 gpc0=1 conn_rate(30000)=10 \
      bytes_out_rate(60000)=191
    $ echo "clear table http_proxy data.gpc0 eq 1" | socat stdio /tmp/sock1
    $ echo "show table http_proxy" | socat stdio /tmp/sock1
>>> # table: http_proxy, type: ip, size:204800, used:1

Delete all the acl entries from the acl <acl> corresponding to the key <key>.
<acl> is the #<id> or the <file> returned by "show acl". If the <ref> is used,
this command delete only the listed reference. The reference can be found with
listing the content of the acl. Note that if the reference <acl> is a file and
is shared with a map, the entry will be also deleted in the map.

Delete all the map entries from the map <map> corresponding to the key <key>.
<map> is the #<id> or the <file> returned by "show map". If the <ref> is used,
this command delete only the listed reference. The reference can be found with
listing the content of the map. Note that if the reference <map> is a file and
is shared with a acl, the entry will be also deleted in the map.

Mark the auxiliary agent check as temporarily stopped.

In the case where an agent check is being run as a auxiliary check, due
to the agent-check parameter of a server directive, new checks are only
initialised when the agent is in the enabled. Thus, disable agent will
prevent any new agent checks from begin initiated until the agent
re-enabled using enable agent.

When an agent is disabled the processing of an auxiliary agent check that
was initiated while the agent was set as enabled is as follows: All
results that would alter the weight, specifically "drain" or a weight
returned by the agent, are ignored. The processing of agent check is
otherwise unchanged.

The motivation for this feature is to allow the weight changing effects
of the agent checks to be paused to allow the weight of a server to be
configured using set weight without being overridden by the agent.

This command is restricted and can only be issued on sockets configured for
level "admin".

Mark the frontend as temporarily stopped. This corresponds to the mode which
is used during a soft restart : the frontend releases the port but can be
enabled again if needed. This should be used with care as some non-Linux OSes
are unable to enable it back. This is intended to be used in environments
where stopping a proxy is not even imaginable but a misconfigured proxy must
be fixed. That way it's possible to release the port and bind it into another
process to restore operations. The frontend will appear with status "STOP"
on the stats page.

The frontend may be specified either by its name or by its numeric ID,
prefixed with a sharp ('#').

This command is restricted and can only be issued on sockets configured for
level "admin".

Mark the primary health check as temporarily stopped. This will disable
sending of health checks, and the last health check result will be ignored.
The server will be in unchecked state and considered UP unless an auxiliary
agent check forces it down.

This command is restricted and can only be issued on sockets configured for
level "admin".

Mark the server DOWN for maintenance. In this mode, no more checks will be
performed on the server until it leaves maintenance.
If the server is tracked by other servers, those servers will be set to DOWN
during the maintenance.

In the statistics page, a server DOWN for maintenance will appear with a
"MAINT" status, its tracking servers with the "MAINT(via)" one.

Both the backend and the server may be specified either by their name or by
their numeric ID, prefixed with a sharp ('#').

This command is restricted and can only be issued on sockets configured for
level "admin".

Resume auxiliary agent check that was temporarily stopped.

See "disable agent" for details of the effect of temporarily starting
and stopping an auxiliary agent.

This command is restricted and can only be issued on sockets configured for
level "admin".

Resume a frontend which was temporarily stopped. It is possible that some of
the listening ports won't be able to bind anymore (eg: if another process
took them since the 'disable frontend' operation). If this happens, an error
is displayed. Some operating systems might not be able to resume a frontend
which was disabled.

The frontend may be specified either by its name or by its numeric ID,
prefixed with a sharp ('#').

This command is restricted and can only be issued on sockets configured for
level "admin".

Resume a primary health check that was temporarily stopped. This will enable
sending of health checks again. Please see "disable health" for details.

This command is restricted and can only be issued on sockets configured for
level "admin".

If the server was previously marked as DOWN for maintenance, this marks the
server UP and checks are re-enabled.

Both the backend and the server may be specified either by their name or by
their numeric ID, prefixed with a sharp ('#').

This command is restricted and can only be issued on sockets configured for
level "admin".

Lookup the value <value> in the map <map> or in the ACL <acl>. <map> or <acl>
are the #<id> or the <file> returned by "show map" or "show acl". This command
returns all the matching patterns associated with this map. This is useful for
debugging maps and ACLs. The output format is composed by one line par
matching type. Each line is composed by space-delimited series of words.

The first two words are:

   <match method>:   The match method applied. It can be "found", "bool",
                     "int", "ip", "bin", "len", "str", "beg", "sub", "dir",
                     "dom", "end" or "reg".

   <match result>:   The result. Can be "match" or "no-match".

The following words are returned only if the pattern matches an entry.

   <index type>:     "tree" or "list". The internal lookup algorithm.

   <case>:           "case-insensitive" or "case-sensitive". The
                     interpretation of the case.

   <entry matched>:  match="<entry>". Return the matched pattern. It is
                     useful with regular expressions.

The two last word are used to show the returned value and its type. With the
"acl" case, the pattern doesn't exist.

   return=nothing:        No return because there are no "map".
   return="<value>":      The value returned in the string format.
   return=cannot-display: The value cannot be converted as string.

   type="<type>":         The type of the returned sample.

Report the current weight and the initial weight of server <server> in
backend <backend> or an error if either doesn't exist. The initial weight is
the one that appears in the configuration file. Both are normally equal
unless the current weight has been changed. Both the backend and the server
may be specified either by their name or by their numeric ID, prefixed with a
sharp ('#').

Print the list of known keywords and their basic usage. The same help screen
is also displayed for unknown commands.

Toggle the prompt at the beginning of the line and enter or leave interactive
mode. In interactive mode, the connection is not closed after a command
completes. Instead, the prompt will appear again, indicating the user that
the interpreter is waiting for a new command. The prompt consists in a right
angle bracket followed by a space "> ". This mode is particularly convenient
when one wants to periodically check information such as stats or errors.
It is also a good idea to enter interactive mode before issuing a "help"
command.

Close the connection when in interactive mode.

Modify the value corresponding to each key <key> in a map <map>. <map> is the
#<id> or <file> returned by "show map". If the <ref> is used in place of
<key>, only the entry pointed by <ref> is changed. The new value is <value>.

Dynamically change the specified frontend's maxconn setting. Any positive
value is allowed including zero, but setting values larger than the global
maxconn does not make much sense. If the limit is increased and connections
were pending, they will immediately be accepted. If it is lowered to a value
below the current number of connections, new connections acceptation will be
delayed until the threshold is reached. The frontend might be specified by
either its name or its numeric ID prefixed with a sharp ('#').

Dynamically change the global maxconn setting within the range defined by the
initial global maxconn setting. If it is increased and connections were
pending, they will immediately be accepted. If it is lowered to a value below
the current number of connections, new connections acceptation will be
delayed until the threshold is reached. A value of zero restores the initial
setting.

Change the process-wide connection rate limit, which is set by the global
'maxconnrate' setting. A value of zero disables the limitation. This limit
applies to all frontends and the change has an immediate effect. The value
is passed in number of connections per second.

Change the maximum input compression rate, which is set by the global
'maxcomprate' setting. A value of zero disables the limitation. The value is
passed in number of kilobytes per second. The value is available in the "show
info" on the line "CompressBpsRateLim" in bytes.

Change the process-wide session rate limit, which is set by the global
'maxsessrate' setting. A value of zero disables the limitation. This limit
applies to all frontends and the change has an immediate effect. The value
is passed in number of sessions per second.

Change the process-wide SSL session rate limit, which is set by the global
'maxsslrate' setting. A value of zero disables the limitation. This limit
applies to all frontends and the change has an immediate effect. The value
is passed in number of sessions per second sent to the SSL stack. It applies
before the handshake in order to protect the stack against handshake abuses.

Replace the current IP address of a server by the one provided.

Force a server's agent to a new state. This can be useful to immediately
switch a server's state regardless of some slow agent checks for example.
Note that the change is propagated to tracking servers if any.

Force a server's health to a new state. This can be useful to immediately
switch a server's state regardless of some slow health checks for example.
Note that the change is propagated to tracking servers if any.

Force a server's administrative state to a new state. This can be useful to
disable load balancing and/or any traffic to a server. Setting the state to
"ready" puts the server in normal mode, and the command is the equivalent of
the "enable server" command. Setting the state to "maint" disables any traffic
to the server as well as any health checks. This is the equivalent of the
"disable server" command. Setting the mode to "drain" only removes the server
from load balancing but still allows it to be checked and to accept new
persistent connections. Changes are propagated to tracking servers if any.

Change a server's weight to the value passed in argument. This is the exact
equivalent of the "set weight" command below.

This command is used to update an OCSP Response for a certificate (see "crt"
on "bind" lines). Same controls are performed as during the initial loading of
the response. The <response> must be passed as a base64 encoded string of the
DER encoded response from the OCSP server.

openssl ocsp -issuer issuer.pem -cert server.pem \
             -host ocsp.issuer.com:80 -respout resp.der
echo "set ssl ocsp-response $(base64 -w 10000 resp.der)" | \
             socat stdio /var/run/haproxy.stat

Set the next TLS key for the <id> listener to <tlskey>. This key becomes the
ultimate key, while the penultimate one is used for encryption (others just
decrypt). The oldest TLS key present is overwritten. <id> is either a numeric
#<id> or <file> returned by "show tls-keys". <tlskey> is a base64 encoded 48
bit TLS ticket key (ex. openssl rand -base64 48).

Create or update a stick-table entry in the table. If the key is not present,
an entry is inserted. See stick-table in section 4.2 to find all possible
values for <data_type>. The most likely use consists in dynamically entering
entries for source IP addresses, with a flag in gpc0 to dynamically block an
IP address or affect its quality of service. It is possible to pass multiple
data_types in a single call.

Change the CLI interface timeout for current connection. This can be useful
during long debugging sessions where the user needs to constantly inspect
some indicators without being disconnected. The delay is passed in seconds.

Change a server's weight to the value passed in argument. If the value ends
with the '%' sign, then the new weight will be relative to the initially
configured weight.  Absolute weights are permitted between 0 and 256.
Relative weights must be positive with the resulting absolute weight is
capped at 256.  Servers which are part of a farm running a static
load-balancing algorithm have stricter limitations because the weight
cannot change once set. Thus for these servers, the only accepted values
are 0 and 100% (or 0 and the initial weight). Changes take effect
immediately, though certain LB algorithms require a certain amount of
requests to consider changes. A typical usage of this command is to
disable a server during an update by setting its weight to zero, then to
enable it again after the update by setting it back to 100%. This command
is restricted and can only be issued on sockets configured for level
"admin". Both the backend and the server may be specified either by their
name or by their numeric ID, prefixed with a sharp ('#').

Dump last known request and response errors collected by frontends and
backends. If <iid> is specified, the limit the dump to errors concerning
either frontend or backend whose ID is <iid>. This command is restricted
and can only be issued on sockets configured for levels "operator" or
"admin".

The errors which may be collected are the last request and response errors
caused by protocol violations, often due to invalid characters in header
names. The report precisely indicates what exact character violated the
protocol. Other important information such as the exact date the error was
detected, frontend and backend names, the server name (when known), the
internal session ID and the source address which has initiated the session
are reported too.

All characters are returned, and non-printable characters are encoded. The
most common ones (\t = 9, \n = 10, \r = 13 and \e = 27) are encoded as one
letter following a backslash. The backslash itself is encoded as '\\' to
avoid confusion. Other non-printable characters are encoded '\xNN' where
NN is the two-digits hexadecimal representation of the character's ASCII
code.

Lines are prefixed with the position of their first character, starting at 0
for the beginning of the buffer. At most one input line is printed per line,
and large lines will be broken into multiple consecutive output lines so that
the output never goes beyond 79 characters wide. It is easy to detect if a
line was broken, because it will not end with '\n' and the next line's offset
will be followed by a '+' sign, indicating it is a continuation of previous
line.

    $ echo "show errors" | socat stdio /tmp/sock1
>>> [04/Mar/2009:15:46:56.081] backend http-in (#2) : invalid response
      src 127.0.0.1, session #54, frontend fe-eth0 (#1), server s2 (#1)
      response length 213 bytes, error at position 23:

      00000  HTTP/1.0 200 OK\r\n
      00017  header/bizarre:blah\r\n
      00038  Location: blah\r\n
      00054  Long-line: this is a very long line which should b
      00104+ e broken into multiple lines on the output buffer,
      00154+  otherwise it would be too large to print in a ter
      00204+ minal\r\n
      00211  \r\n

In the example above, we see that the backend "http-in" which has internal
ID 2 has blocked an invalid response from its server s2 which has internal
ID 1. The request was on session 54 initiated by source 127.0.0.1 and
received by frontend fe-eth0 whose ID is 1. The total response length was
213 bytes when the error was detected, and the error was at byte 23. This
is the slash ('/') in header name "header/bizarre", which is not a valid
HTTP character for a header name.

Dump the list of backends available in the running process

Dump info about haproxy status on current process.

Dump info about map converters. Without argument, the list of all available
maps is returned. If a <map> is specified, its contents are dumped. <map> is
the #<id> or <file>. The first column is a unique identifier. It can be used
as reference for the operation "del map" and "set map". The second column is
the pattern and the third column is the sample if available. The data returned
are not directly a list of available maps, but are the list of all patterns
composing any map. Many of these patterns can be shared with ACL.

Dump info about acl converters. Without argument, the list of all available
acls is returned. If a <acl> is specified, its contents are dumped. <acl> if
the #<id> or <file>. The dump format is the same than the map even for the
sample value. The data returned are not a list of available ACL, but are the
list of all patterns composing any ACL. Many of these patterns can be shared
with maps.

Dump the status of internal memory pools. This is useful to track memory
usage when suspecting a memory leak for example. It does exactly the same
as the SIGQUIT when running in foreground except that it does not flush
the pools.

Dump the state of the servers found in the running configuration. A backend
name or identifier may be provided to limit the output to this backend only.

The dump has the following format:
 - first line contains the format version (1 in this specification);
 - second line contains the column headers, prefixed by a sharp ('#');
 - third line and next ones contain data;
 - each line starting by a sharp ('#') is considered as a comment.

Since multiple versions of the ouptput may co-exist, below is the list of
fields and their order per file format version :
 1:
   be_id:                       Backend unique id.
   be_name:                     Backend label.
   srv_id:                      Server unique id (in the backend).
   srv_name:                    Server label.
   srv_addr:                    Server IP address.
   srv_op_state:                Server operational state (UP/DOWN/...).
                                In source code: SRV_ST_*.
   srv_admin_state:             Server administrative state (MAINT/DRAIN/...).
                                In source code: SRV_ADMF_*.
   srv_uweight:                 User visible server's weight.
   srv_iweight:                 Server's initial weight.
   srv_time_since_last_change:  Time since last operational change.
   srv_check_status:            Last health check status.
   srv_check_result:            Last check result (FAILED/PASSED/...).
                                In source code: CHK_RES_*.
   srv_check_health:            Checks rise / fall current counter.
   srv_check_state:             State of the check (ENABLED/PAUSED/...).
                                In source code: CHK_ST_*.
   srv_agent_state:             State of the agent check (ENABLED/PAUSED/...).
                                In source code: CHK_ST_*.
   bk_f_forced_id:              Flag to know if the backend ID is forced by
                                configuration.
   srv_f_forced_id:             Flag to know if the server's ID is forced by
                                configuration.

Dump all known sessions. Avoid doing this on slow connections as this can
be huge. This command is restricted and can only be issued on sockets
configured for levels "operator" or "admin".

Display a lot of internal information about the specified session identifier.
This identifier is the first field at the beginning of the lines in the dumps
of "show sess" (it corresponds to the session pointer). Those information are
useless to most users but may be used by haproxy developers to troubleshoot a
complex bug. The output format is intentionally not documented so that it can
freely evolve depending on demands. You may find a description of all fields
returned in src/dumpstats.c

The special id "all" dumps the states of all sessions, which must be avoided
as much as possible as it is highly CPU intensive and can take a lot of time.

Dump statistics in the CSV format. By passing <id>, <type> and <sid>, it is
possible to dump only selected items :
  - <iid> is a proxy ID, -1 to dump everything
  - <type> selects the type of dumpable objects : 1 for frontends, 2 for
     backends, 4 for servers, -1 for everything. These values can be ORed,
     for example:
        1 + 2     = 3   -> frontend + backend.
        1 + 2 + 4 = 7   -> frontend + backend + server.
  - <sid> is a server ID, -1 to dump everything from the selected proxy.

    $ echo "show info;show stat" | socat stdio unix-connect:/tmp/sock1
>>> Name: HAProxy
    Version: 1.4-dev2-49
    Release_date: 2009/09/23
    Nbproc: 1
    Process_num: 1
    (...)

    # pxname,svname,qcur,qmax,scur,smax,slim,stot,bin,bout,dreq,  (...)
    stats,FRONTEND,,,0,0,1000,0,0,0,0,0,0,,,,,OPEN,,,,,,,,,1,1,0, (...)
    stats,BACKEND,0,0,0,0,1000,0,0,0,0,0,,0,0,0,0,UP,0,0,0,,0,250,(...)
    (...)
    www1,BACKEND,0,0,0,0,1000,0,0,0,0,0,,0,0,0,0,UP,1,1,0,,0,250, (...)

    $

Here, two commands have been issued at once. That way it's easy to find
which process the stats apply to in multi-process mode. Notice the empty
line after the information output which marks the end of the first block.
A similar empty line appears at the end of the second block (stats) so that
the reader knows the output has not been truncated.

Dump statistics for the given resolvers section, or all resolvers sections
if no section is supplied.

For each name server, the following counters are reported:
  sent: number of DNS requests sent to this server
  valid: number of DNS valid responses received from this server
  update: number of DNS responses used to update the server's IP address
  cname: number of CNAME responses
  cname_error: CNAME errors encountered with this server
  any_err: number of empty response (IE: server does not support ANY type)
  nx: non existent domain response received from this server
  timeout: how many time this server did not answer in time
  refused: number of requests refused by this server
  other: any other DNS errors
  invalid: invalid DNS response (from a protocol point of view)
  too_big: too big response
  outdated: number of response arrived too late (after an other name server)

Dump general information on all known stick-tables. Their name is returned
(the name of the proxy which holds them), their type (currently zero, always
IP), their size in maximum possible number of entries, and the number of
entries currently in use.

    $ echo "show table" | socat stdio /tmp/sock1
>>> # table: front_pub, type: ip, size:204800, used:171454
>>> # table: back_rdp, type: ip, size:204800, used:0

Dump contents of stick-table <name>. In this mode, a first line of generic
information about the table is reported as with "show table", then all
entries are dumped. Since this can be quite heavy, it is possible to specify
a filter in order to specify what entries to display.

When the "data." form is used the filter applies to the stored data (see
"stick-table" in section 4.2).  A stored data type must be specified
in <type>, and this data type must be stored in the table otherwise an
error is reported. The data is compared according to <operator> with the
64-bit integer <value>.  Operators are the same as with the ACLs :

  - eq : match entries whose data is equal to this value
  - ne : match entries whose data is not equal to this value
  - le : match entries whose data is less than or equal to this value
  - ge : match entries whose data is greater than or equal to this value
  - lt : match entries whose data is less than this value
  - gt : match entries whose data is greater than this value


When the key form is used the entry <key> is shown.  The key must be of the
same type as the table, which currently is limited to IPv4, IPv6, integer,
and string.

    $ echo "show table http_proxy" | socat stdio /tmp/sock1
>>> # table: http_proxy, type: ip, size:204800, used:2
>>> 0x80e6a4c: key=127.0.0.1 use=0 exp=3594729 gpc0=0 conn_rate(30000)=1  \
      bytes_out_rate(60000)=187
>>> 0x80e6a80: key=127.0.0.2 use=0 exp=3594740 gpc0=1 conn_rate(30000)=10 \
      bytes_out_rate(60000)=191

    $ echo "show table http_proxy data.gpc0 gt 0" | socat stdio /tmp/sock1
>>> # table: http_proxy, type: ip, size:204800, used:2
>>> 0x80e6a80: key=127.0.0.2 use=0 exp=3594740 gpc0=1 conn_rate(30000)=10 \
      bytes_out_rate(60000)=191

    $ echo "show table http_proxy data.conn_rate gt 5" | \
        socat stdio /tmp/sock1
>>> # table: http_proxy, type: ip, size:204800, used:2
>>> 0x80e6a80: key=127.0.0.2 use=0 exp=3594740 gpc0=1 conn_rate(30000)=10 \
      bytes_out_rate(60000)=191

    $ echo "show table http_proxy key 127.0.0.2" | \
        socat stdio /tmp/sock1
>>> # table: http_proxy, type: ip, size:204800, used:2
>>> 0x80e6a80: key=127.0.0.2 use=0 exp=3594740 gpc0=1 conn_rate(30000)=10 \
      bytes_out_rate(60000)=191

When the data criterion applies to a dynamic value dependent on time such as
a bytes rate, the value is dynamically computed during the evaluation of the
entry in order to decide whether it has to be dumped or not. This means that
such a filter could match for some time then not match anymore because as
time goes, the average event rate drops.

It is possible to use this to extract lists of IP addresses abusing the
service, in order to monitor them or even blacklist them in a firewall.

$ echo "show table http_proxy data.gpc0 gt 0" \
  | socat stdio /tmp/sock1 \
  | fgrep 'key=' | cut -d' ' -f2 | cut -d= -f2 > abusers-ip.txt
  ( or | awk '/key/{ print a[split($2,a,"=")]; }' )

Dump all loaded TLS ticket keys. The TLS ticket key reference ID and the
file from which the keys have been loaded is shown. Both of those can be
used to update the TLS keys using "set ssl tls-key".

Completely delete the specified frontend. All the ports it was bound to will
be released. It will not be possible to enable the frontend anymore after
this operation. This is intended to be used in environments where stopping a
proxy is not even imaginable but a misconfigured proxy must be fixed. That
way it's possible to release the port and bind it into another process to
restore operations. The frontend will not appear at all on the stats page
once it is terminated.

The frontend may be specified either by its name or by its numeric ID,
prefixed with a sharp ('#').

This command is restricted and can only be issued on sockets configured for
level "admin".

Immediately terminate the session matching the specified session identifier.
This identifier is the first field at the beginning of the lines in the dumps
of "show sess" (it corresponds to the session pointer). This can be used to
terminate a long-running session without waiting for a timeout or when an
endless transfer is ongoing. Such terminated sessions are reported with a 'K'
flag in the logs.

Immediately terminate all the sessions attached to the specified server. This
can be used to terminate long-running sessions after a server is put into
maintenance mode, for instance. Such terminated sessions are reported with a
'K' flag in the logs.

It is very common that two HAProxy nodes constituting a cluster share exactly
the same configuration modulo a few addresses. Instead of having to maintain a
duplicate configuration for each node, which will inevitably diverge, it is
possible to include environment variables in the configuration. Thus multiple
configuration may share the exact same file with only a few different system
wide environment variables. This started in version 1.5 where only addresses
were allowed to include environment variables, and 1.6 goes further by
supporting environment variables everywhere. The syntax is the same as in the
UNIX shell, a variable starts with a dollar sign ('$'), followed by an opening
curly brace ('{'), then the variable name followed by the closing brace ('}').
Except for addresses, environment variables are only interpreted in arguments
surrounded with double quotes (this was necessary not to break existing setups
using regular expressions involving the dollar symbol).

Environment variables also make it convenient to write configurations which are
expected to work on various sites where only the address changes. It can also
permit to remove passwords from some configs. Example below where the the file
"site1.env" file is sourced by the init script upon startup :

  $ cat site1.env
  LISTEN=192.168.1.1
  CACHE_PFX=192.168.11
  SERVER_PFX=192.168.22
  LOGGER=192.168.33.1
  STATSLP=admin:pa$$w0rd
  ABUSERS=/etc/haproxy/abuse.lst
  TIMEOUT=10s

  $ cat haproxy.cfg
  global
      log "${LOGGER}:514" local0

  defaults
      mode http
      timeout client "${TIMEOUT}"
      timeout server "${TIMEOUT}"
      timeout connect 5s

  frontend public
      bind "${LISTEN}:80"
      http-request reject if { src -f "${ABUSERS}" }
      stats uri /stats
      stats auth "${STATSLP}"
      use_backend cache if { path_end .jpg .css .ico }
      default_backend server

  backend cache
      server cache1 "${CACHE_PFX}.1:18080" check
      server cache2 "${CACHE_PFX}.2:18080" check

  backend server
      server cache1 "${SERVER_PFX}.1:8080" check
      server cache2 "${SERVER_PFX}.2:8080" check

Once in a while, someone reports that after a system reboot, the haproxy
service wasn't started, and that once they start it by hand it works. Most
often, these people are running a clustered IP address mechanism such as
keepalived, to assign the service IP address to the master node only, and while
it used to work when they used to bind haproxy to address 0.0.0.0, it stopped
working after they bound it to the virtual IP address. What happens here is
that when the service starts, the virtual IP address is not yet owned by the
local node, so when HAProxy wants to bind to it, the system rejects this
because it is not a local IP address. The fix doesn't consist in delaying the
haproxy service startup (since it wouldn't stand a restart), but instead to
properly configure the system to allow binding to non-local addresses. This is
easily done on Linux by setting the net.ipv4.ip_nonlocal_bind sysctl to 1. This
is also needed in order to transparently intercept the IP traffic that passes
through HAProxy for a specific target address.

Multi-process configurations involving source port ranges may apparently seem
to work but they will cause some random failures under high loads because more
than one process may try to use the same source port to connect to the same
server, which is not possible. The system will report an error and a retry will
happen, picking another port. A high value in the "retries" parameter may hide
the effect to a certain extent but this also comes with increased CPU usage and
processing time. Logs will also report a certain number of retries. For this
reason, port ranges should be avoided in multi-process configurations.

Since HAProxy uses SO_REUSEPORT and supports having multiple independant
processes bound to the same IP:port, during troubleshooting it can happen that
an old process was not stopped before a new one was started. This provides
absurd test results which tend to indicate that any change to the configuration
is ignored. The reason is that in fact even the new process is restarted with a
new configuration, the old one also gets some incoming connections and
processes them, returning unexpected results. When in doubt, just stop the new
process and try again. If it still works, it very likely means that an old
process remains alive and has to be stopped. Linux's "netstat -lntp" is of good
help here.

When adding entries to an ACL from the command line (eg: when blacklisting a
source address), it is important to keep in mind that these entries are not
synchronized to the file and that if someone reloads the configuration, these
updates will be lost. While this is often the desired effect (for blacklisting)
it may not necessarily match expectations when the change was made as a fix for
a problem. See the "add acl" action of the CLI interface.

When HAProxy is started with the "-d" option, it will stay in the foreground
and will print one line per event, such as an incoming connection, the end of a
connection, and for each request or response header line seen. This debug
output is emitted before the contents are processed, so they don't consider the
local modifications. The main use is to show the request and response without
having to run a network sniffer. The output is less readable when multiple
connections are handled in parallel, though the "debug2ansi" and "debug2html"
scripts found in the examples/ directory definitely help here by coloring the
output.

If a request or response is rejected because HAProxy finds it is malformed, the
best thing to do is to connect to the CLI and issue "show errors", which will
report the last captured faulty request and response for each frontend and
backend, with all the necessary information to indicate precisely the first
character of the input stream that was rejected. This is sometimes needed to
prove to customers or to developers that a bug is present in their code. In
this case it is often possible to relax the checks (but still keep the
captures) using "option accept-invalid-http-request" or its equivalent for
responses coming from the server "option accept-invalid-http-response". Please
see the configuration manual for more details.

> show errors
Total events captured on [13/Oct/2015:13:43:47.169] : 1

[13/Oct/2015:13:43:40.918] frontend HAProxyLocalStats (#2): invalid request
  backend <NONE> (#-1), server <NONE> (#-1), event #0
  src 127.0.0.1:51981, session #0, session flags 0x00000080
  HTTP msg state 26, msg flags 0x00000000, tx flags 0x00000000
  HTTP chunk len 0 bytes, HTTP body len 0 bytes
  buffer flags 0x00808002, out 0 bytes, total 31 bytes
  pending 31 bytes, wrapping at 8040, error at position 13:

  00000  GET /invalid request HTTP/1.1\r\n

The output of "show info" on the CLI provides a number of useful information
regarding the maximum connection rate ever reached, maximum SSL key rate ever
reached, and in general all information which can help to explain temporary
issues regarding CPU or memory usage. Example :

  > show info
  Name: HAProxy
  Version: 1.6-dev7-e32d18-17
  Release_date: 2015/10/12
  Nbproc: 1
  Process_num: 1
  Pid: 7949
  Uptime: 0d 0h02m39s
  Uptime_sec: 159
  Memmax_MB: 0
  Ulimit-n: 120032
  Maxsock: 120032
  Maxconn: 60000
  Hard_maxconn: 60000
  CurrConns: 0
  CumConns: 3
  CumReq: 3
  MaxSslConns: 0
  CurrSslConns: 0
  CumSslConns: 0
  Maxpipes: 0
  PipesUsed: 0
  PipesFree: 0
  ConnRate: 0
  ConnRateLimit: 0
  MaxConnRate: 1
  SessRate: 0
  SessRateLimit: 0
  MaxSessRate: 1
  SslRate: 0
  SslRateLimit: 0
  MaxSslRate: 0
  SslFrontendKeyRate: 0
  SslFrontendMaxKeyRate: 0
  SslFrontendSessionReuse_pct: 0
  SslBackendKeyRate: 0
  SslBackendMaxKeyRate: 0
  SslCacheLookups: 0
  SslCacheMisses: 0
  CompressBpsIn: 0
  CompressBpsOut: 0
  CompressBpsRateLim: 0
  ZlibMemUsage: 0
  MaxZlibMemUsage: 0
  Tasks: 5
  Run_queue: 1
  Idle_pct: 100
  node: wtap
  description:

When an issue seems to randomly appear on a new version of HAProxy (eg: every
second request is aborted, occasional crash, etc), it is worth trying to enable
memory poisonning so that each call to malloc() is immediately followed by the
filling of the memory area with a configurable byte. By default this byte is
0x50 (ASCII for 'P'), but any other byte can be used, including zero (which
will have the same effect as a calloc() and which may make issues disappear).
Memory poisonning is enabled on the command line using the "-dM" option. It
slightly hurts performance and is not recommended for use in production. If
an issue happens all the time with it or never happens when poisoonning uses
byte zero, it clearly means you've found a bug and you definitely need to
report it. Otherwise if there's no clear change, the problem it is not related.

When debugging some latency issues, it is important to use both strace and
tcpdump on the local machine, and another tcpdump on the remote system. The
reason for this is that there are delays everywhere in the processing chain and
it is important to know which one is causing latency to know where to act. In
practice, the local tcpdump will indicate when the input data come in. Strace
will indicate when haproxy receives these data (using recv/recvfrom). Warning,
openssl uses read()/write() syscalls instead of recv()/send(). Strace will also
show when haproxy sends the data, and tcpdump will show when the system sends
these data to the interface. Then the external tcpdump will show when the data
sent are really received (since the local one only shows when the packets are
queued). The benefit of sniffing on the local system is that strace and tcpdump
will use the same reference clock. Strace should be used with "-tts200" to get
complete timestamps and report large enough chunks of data to read them.
Tcpdump should be used with "-nvvttSs0" to report full packets, real sequence
numbers and complete timestamps.

In practice, received data are almost always immediately received by haproxy
(unless the machine has a saturated CPU or these data are invalid and not
delivered). If these data are received but not sent, it generally is because
the output buffer is saturated (ie: recipient doesn't consume the data fast
enough). This can be confirmed by seeing that the polling doesn't notify of
the ability to write on the output file descriptor for some time (it's often
easier to spot in the strace output when the data finally leave and then roll
back to see when the write event was notified). It generally matches an ACK
received from the recipient, and detected by tcpdump. Once the data are sent,
they may spend some time in the system doing nothing. Here again, the TCP
congestion window may be limited and not allow these data to leave, waiting for
an ACK to open the window. If the traffic is idle and the data take 40 ms or
200 ms to leave, it's a different issue (which is not an issue), it's the fact
that the Nagle algorithm prevents empty packets from leaving immediately, in
hope that they will be merged with subsequent data. HAProxy automatically
disables Nagle in pure TCP mode and in tunnels. However it definitely remains
enabled when forwarding an HTTP body (and this contributes to the performance
improvement there by reducing the number of packets). Some HTTP non-compliant
applications may be sensitive to the latency when delivering incomplete HTTP
response messages. In this case you will have to enable "option http-no-delay"
to disable Nagle in order to work around their design, keeping in mind that any
other proxy in the chain may similarly be impacted. If tcpdump reports that data
leave immediately but the other end doesn't see them quickly, it can mean there
is a congestionned WAN link, a congestionned LAN with flow control enabled and
preventing the data from leaving, or more commonly that HAProxy is in fact
running in a virtual machine and that for whatever reason the hypervisor has
decided that the data didn't need to be sent immediately. In virtualized
environments, latency issues are almost always caused by the virtualization
layer, so in order to save time, it's worth first comparing tcpdump in the VM
and on the external components. Any difference has to be credited to the
hypervisor and its accompanying drivers.

When some TCP SACK segments are seen in tcpdump traces (using -vv), it always
means that the side sending them has got the proof of a lost packet. While not
seeing them doesn't mean there are no losses, seeing them definitely means the
network is lossy. Losses are normal on a network, but at a rate where SACKs are
not noticeable at the naked eye. If they appear a lot in the traces, it is
worth investigating exactly what happens and where the packets are lost. HTTP
doesn't cope well with TCP losses, which introduce huge latencies.

The "netstat -i" command will report statistics per interface. An interface
where the Rx-Ovr counter grows indicates that the system doesn't have enough
resources to receive all incoming packets and that they're lost before being
processed by the network driver. Rx-Drp indicates that some received packets
were lost in the network stack because the application doesn't process them
fast enough. This can happen during some attacks as well. Tx-Drp means that
the output queues were full and packets had to be dropped. When using TCP it
should be very rare, but will possibly indicte a saturated outgoing link.

HAProxy is designed to run with very limited privileges. The standard way to
use it is to isolate it into a chroot jail and to drop its privileges to a
non-root user without any permissions inside this jail so that if any future
vulnerability were to be discovered, its compromise would not affect the rest
of the system.

In order to perfom a chroot, it first needs to be started as a root user. It is
pointless to build hand-made chroots to start the process there, these ones are
painful to build, are never properly maintained and always contain way more
bugs than the main file-system. And in case of compromise, the intruder can use
the purposely built file-system. Unfortunately many administrators confuse
"start as root" and "run as root", resulting in the uid change to be done prior
to starting haproxy, and reducing the effective security restrictions.

HAProxy will need to be started as root in order to :
  - adjust the file descriptor limits
  - bind to privileged port numbers
  - bind to a specific network interface
  - transparently listen to a foreign address
  - isolate itself inside the chroot jail
  - drop to another non-privileged UID

HAProxy may require to be run as root in order to :
  - bind to an interface for outgoing connections
  - bind to privileged source ports for outgoing connections
  - transparently bind to a foreing address for outgoing connections

Most users will never need the "run as root" case. But the "start as root"
covers most usages.

A safe configuration will have :

  - a chroot statement pointing to an empty location without any access
    permissions. This can be prepared this way on the UNIX command line :

      # mkdir /var/empty && chmod 0 /var/empty || echo "Failed"

    and referenced like this in the HAProxy configuration's global section :

      chroot /var/empty

  - both a uid/user and gid/group statements in the global section :

      user haproxy
      group haproxy

  - a stats socket whose mode, uid and gid are set to match the user and/or
    group allowed to access the CLI so that nobody may access it :

      stats socket /var/run/haproxy.stat uid hatop gid hatop mode 600