doc/configuration.txt

                         ----------------------
                                 HAProxy
                          Configuration Manual
                         ----------------------
                             version 1.3.14
                             willy tarreau
                               2007/12/24


This document covers the configuration language as implemented in the version
specified above. It does not provide any hint, example or advice. For such
documentation, please refer to the Reference Manual or the Architecture Manual.


HAProxy's configuration process involves 3 major sources of parameters :

  - the arguments from the command-line, which always take precedence
  - the "global" section, which sets process-wide parameters
  - the proxies sections which can take form of "defaults", "listen",
    "frontend" and "backend".

The configuration file syntax consists in lines beginning with a keyword
referenced in this manual, optionally followed by one or several parameters
delimited by spaces. If spaces have to be entered in strings, then they must be
preceeded by a backslash ('\') to be escaped. Backslashes also have to be
escaped by doubling them.

Some parameters involve values representating time, such as timeouts. These
values are generally expressed in milliseconds (unless explicitly stated
otherwise) but may be expressed in any other unit by suffixing the unit to the
numeric value. It is important to consider this because it will not be repeated
for every keyword. Supported units are :

  - us : microseconds. 1 microsecond = 1/1000000 second
  - ms : milliseconds. 1 millisecond = 1/1000 second. This is the default.
  - s  : seconds. 1s = 1000ms
  - m  : minutes. 1m = 60s = 60000ms
  - h  : hours.   1h = 60m = 3600s = 3600000ms
  - d  : days.    1d = 24h = 1440m = 86400s = 86400000ms


1. Global parameters
--------------------

Parameters in the "global" section are process-wide and often OS-specific. They
are generally set once for all and do not need being changed once correct. Some
of them have command-line equivalents.

The following keywords are supported in the "global" section :

 * Process management and security
   - chroot
   - daemon
   - gid
   - group
   - log
   - nbproc
   - pidfile
   - uid
   - ulimit-n
   - user
   - stats
  
 * Performance tuning
   - maxconn
   - noepoll
   - nokqueue
   - nopoll
   - nosepoll
   - tune.maxpollevents
   - spread-checks
  
 * Debugging
   - debug
   - quiet


1.1) Process management and security
------------------------------------

chroot <jail dir>
  Changes current directory to <jail dir> and performs a chroot() there before
  dropping privileges. This increases the security level in case an unknown
  vulnerability would be exploited, since it would make it very hard for the
  attacker to exploit the system. This only works when the process is started
  with superuser privileges. It is important to ensure that <jail_dir> is both
  empty and unwritable to anyone.
  
daemon
  Makes the process fork into background. This is the recommended mode of
  operation. It is equivalent to the command line "-D" argument. It can be
  disabled by the command line "-db" argument.

gid <number>
  Changes the process' group ID to <number>. It is recommended that the group
  ID is dedicated to HAProxy or to a small set of similar daemons. HAProxy must
  be started with a user belonging to this group, or with superuser privileges.
  See also "group" and "uid".
  
group <group name>
  Similar to "gid" but uses the GID of group name <group name> from /etc/group.
  See also "gid" and "user".
  
log <address> <facility> [max level]
  Adds a global syslog server. Up to two global servers can be defined. They
  will receive logs for startups and exits, as well as all logs from proxies
  configured with "log global".

  <address> can be one of:

        - An IPv4 address optionally followed by a colon and an UDP port. If
          no port is specified, 514 is used by default (the standard syslog
          port).

        - A filesystem path to a UNIX domain socket, keeping in mind
          considerations for chroot (be sure the path is accessible inside
          the chroot) and uid/gid (be sure the path is appropriately
          writeable).

  <facility> must be one of the 24 standard syslog facilities :

          kern   user   mail   daemon auth   syslog lpr    news
          uucp   cron   auth2  ftp    ntp    audit  alert  cron2
          local0 local1 local2 local3 local4 local5 local6 local7

  An optional level can be specified to filter outgoing messages. By default,
  all messages are sent. If a level is specified, only messages with a severity
  at least as important as this level will be sent. 8 levels are known :

	  emerg  alert  crit   err    warning notice info  debug

nbproc <number>
  Creates <number> processes when going daemon. This requires the "daemon"
  mode. By default, only one process is created, which is the recommended mode
  of operation. For systems limited to small sets of file descriptors per
  process, it may be needed to fork multiple daemons. USING MULTIPLE PROCESSES
  IS HARDER TO DEBUG AND IS REALLY DISCOURAGED. See also "daemon".

pidfile <pidfile>
  Writes pids of all daemons into file <pidfile>. This option is equivalent to
  the "-p" command line argument. The file must be accessible to the user
  starting the process. See also "daemon".

stats socket <path> [{uid | user} <uid>] [{gid | group} <gid>] [mode <mode>]
  Creates a UNIX socket in stream mode at location <path>. Any previously
  existing socket will be backed up then replaced. Connections to this socket
  will get a CSV-formated output of the process statistics in response to the
  "show stat" command followed by a line feed. On platforms which support it,
  it is possible to restrict access to this socket by specifying numerical IDs
  after "uid" and "gid", or valid user and group names after the "user" and
  "group" keywords. It is also possible to restrict permissions on the socket
  by passing an octal value after the "mode" keyword (same syntax as chmod).
  Depending on the platform, the permissions on the socket will be inherited
  from the directory which hosts it, or from the user the process is started
  with.

stats timeout <timeout, in milliseconds>
  The default timeout on the stats socket is set to 10 seconds. It is possible
  to change this value with "stats timeout". The value must be passed in
  milliseconds, or be suffixed by a time unit among { us, ms, s, m, h, d }.

stats maxconn <connections>
  By default, the stats socket is limited to 10 concurrent connections. It is
  possible to change this value with "stats maxconn".

uid <number>
  Changes the process' user ID to <number>. It is recommended that the user ID
  is dedicated to HAProxy or to a small set of similar daemons. HAProxy must
  be started with superuser privileges in order to be able to switch to another
  one. See also "gid" and "user".

ulimit-n <number>
  Sets the maximum number of per-process file-descriptors to <number>. By
  default, it is automatically computed, so it is recommended not to use this
  option.

user <user name>
  Similar to "uid" but uses the UID of user name <user name> from /etc/passwd.
  See also "uid" and "group".


1.2) Performance tuning
-----------------------

maxconn <number>
  Sets the maximum per-process number of concurrent connections to <number>. It
  is equivalent to the command-line argument "-n". Proxies will stop accepting
  connections when this limit is reached. The "ulimit-n" parameter is
  automatically adjusted according to this value. See also "ulimit-n".

noepoll
  Disables the use of the "epoll" event polling system on Linux. It is
  equivalent to the command-line argument "-de". The next polling system
  used will generally be "poll". See also "nosepoll", and "nopoll".

nokqueue
  Disables the use of the "kqueue" event polling system on BSD. It is
  equivalent to the command-line argument "-dk". The next polling system
  used will generally be "poll". See also "nopoll".

nopoll
  Disables the use of the "poll" event polling system. It is equivalent to the
  command-line argument "-dp". The next polling system used will be "select".
  It should never be needed to disable "poll" since it's available on all
  platforms supported by HAProxy. See also "nosepoll", and "nopoll" and
  "nokqueue".

nosepoll
  Disables the use of the "speculative epoll" event polling system on Linux. It
  is equivalent to the command-line argument "-ds". The next polling system
  used will generally be "epoll". See also "nosepoll", and "nopoll".

tune.maxpollevents <number>
  Sets the maximum amount of events that can be processed at once in a call to
  the polling system. The default value is adapted to the operating system. It
  has been noticed that reducing it below 200 tends to slightly decrease
  latency at the expense of network bandwidth, and increasing it above 200
  tends to trade latency for slightly increased bandwidth.

spread-checks <0..50, in percent>
  Sometimes it is desirable to avoid sending health checks to servers at exact
  intervals, for instance when many logical servers are located on the same
  physical server. With the help of this parameter, it becomes possible to add
  some randomness in the check interval between 0 and +/- 50%. A value between
  2 and 5 seems to show good results. The default value remains at 0.


1.3) Debugging
---------------

debug
  Enables debug mode which dumps to stdout all exchanges, and disables forking
  into background. It is the equivalent of the command-line argument "-d". It
  should never be used in a production configuration since it may prevent full
  system startup.

quiet
  Do not display any message during startup. It is equivalent to the command-
  line argument "-q".


2) Proxies
----------

Proxy configuration can be located in a set of sections :
 - defaults <name>
 - frontend <name>
 - backend  <name>
 - listen   <name>

A "defaults" section sets default parameters for all other sections following
its declaration. Those default parameters are reset by the next "defaults"
section. See below for the list of parameters which can be set in a "defaults"
section. The name is optional but its use is encouraged for better readability.

A "frontend" section describes a set of listening sockets accepting client
connections.

A "backend" section describes a set of servers to which the proxy will connect
to forward incoming connections.

A "listen" section defines a complete proxy with its frontend and backend
parts combined in one section. It is generally useful for TCP-only traffic.

All proxy names must be formed from upper and lower case letters, digits,
'-' (dash), '_' (underscore) , '.' (dot) and ':' (colon). ACL names are
case-sensitive, which means that "www" and "WWW" are two different proxies.

Historically, all proxy names could overlap, it just caused troubles in the
logs. Since the introduction of content switching, it is mandatory that two
proxies with overlapping capabilities (frontend/backend) have different names.
However, it is still permitted that a frontend and a backend share the same
name, as this configuration seems to be commonly encountered.

Right now, two major proxy modes are supported : "tcp", also known as layer 4,
and "http", also known as layer 7. In layer 4 mode, HAProxy simply forwards
bidirectionnal traffic between two sides. In layer 7 mode, HAProxy analyzes the
protocol, and can interact with it by allowing, blocking, switching, adding,
modifying, or removing arbitrary contents in requests or responses, based on
arbitrary criteria.


2.1) Quick reminder about HTTP
------------------------------

When a proxy is running in HTTP mode, both the request and the response are
fully analyzed and indexed, thus it becomes possible to build matching criteria
on almost anything found in the contents.

However, it is important to understand how HTTP requests and responses are
formed, and how HAProxy decomposes them. It will then become easier to write
correct rules and to debug existing configurations.


2.1.1) The HTTP transaction model
---------------------------------

The HTTP protocol is transaction-driven. This means that each request will lead
to one and only one response. Traditionnally, a TCP connection is established
from the client to the server, a request is sent by the client on the
connection, the server responds and the connection is closed. A new request
will involve a new connection :

  [CON1] [REQ1] ... [RESP1] [CLO1] [CON2] [REQ2] ... [RESP2] [CLO2] ...

In this mode, called the "HTTP close" mode, there are as many connection
establishments as there are HTTP transactions. Since the connection is closed
by the server after the response, the client does not need to know the content
length.

Due to the transactional nature of the protocol, it was possible to improve it
to avoid closing a connection between two subsequent transactions. In this mode
however, it is mandatory that the server indicates the content length for each
response so that the client does not wait indefinitely. For this, a special
header is used: "Content-length". This mode is called the "keep-alive" mode :

  [CON] [REQ1] ... [RESP1] [REQ2] ... [RESP2] [CLO] ...

Its advantages are a reduced latency between transactions, and less processing
power required on the server side. It is generally better than the close mode,
but not always because the clients often limit their concurrent connections to
a smaller value. HAProxy currently does not support the HTTP keep-alive mode,
but knows how to transform it to the close mode.

A last improvement in the communications is the pipelining mode. It still uses
keep-alive, but the client does not wait for the first response to send the
second request. This is useful for fetching large number of images composing a
page :

  [CON] [REQ1] [REQ2] ... [RESP1] [RESP2] [CLO] ...

This can obviously have a tremendous benefit on performance because the network
latency is eliminated between subsequent requests. Many HTTP agents do not
correctly support pipelining since there is no way to associate a response with
the corresponding request in HTTP. For this reason, it is mandatory for the
server to reply in the exact same order as the requests were received.

Right now, HAProxy only supports the first mode (HTTP close) if it needs to
process the request. This means that for each request, there will be one TCP
connection. If keep-alive or pipelining are required, HAProxy will still
support them, but will only see the first request and the first response of
each transaction. While this is generally problematic with regards to logs,
content switching or filtering, it most often causes no problem for persistence
with cookie insertion.


2.1.2) HTTP request
-------------------

First, let's consider this HTTP request :

  Line     Contents
  number 
     1     GET /serv/login.php?lang=en&profile=2 HTTP/1.1
     2     Host: www.mydomain.com
     3     User-agent: my small browser
     4     Accept: image/jpeg, image/gif
     5     Accept: image/png


2.1.2.1) The Request line
-------------------------

Line 1 is the "request line". It is always composed of 3 fields :

  - a METHOD      : GET
  - a URI         : /serv/login.php?lang=en&profile=2
  - a version tag : HTTP/1.1

All of them are delimited by what the standard calls LWS (linear white spaces),
which are commonly spaces, but can also be tabs or line feeds/carriage returns
followed by spaces/tabs. The method itself cannot contain any colon (':') and
is limited to alphabetic letters. All those various combinations make it
desirable that HAProxy performs the splitting itself rather than leaving it to
the user to write a complex or inaccurate regular expression.

The URI itself can have several forms :

  - A "relative URI" :

      /serv/login.php?lang=en&profile=2

    It is a complete URL without the host part. This is generally what is
    received by servers, reverse proxies and transparent proxies.

  - An "absolute URI", also called a "URL" :

      http://192.168.0.12:8080/serv/login.php?lang=en&profile=2

    It is composed of a "scheme" (the protocol name followed by '://'), a host
    name or address, optionally a colon (':') followed by a port number, then
    a relative URI beginning at the first slash ('/') after the address part.
    This is generally what proxies receive, but a server supporting HTTP/1.1
    must accept this form too.

  - a star ('*') : this form is only accepted in association with the OPTIONS
    method and is not relayable. It is used to inquiry a next hop's
    capabilities.
        
  - an address:port combination : 192.168.0.12:80
    This is used with the CONNECT method, which is used to establish TCP
    tunnels through HTTP proxies, generally for HTTPS, but sometimes for
    other protocols too.

In a relative URI, two sub-parts are identified. The part before the question
mark is called the "path". It is typically the relative path to static objects
on the server. The part after the question mark is called the "query string".
It is mostly used with GET requests sent to dynamic scripts and is very
specific to the language, framework or application in use.


2.1.2.2) The request headers
----------------------------

The headers start at the second line. They are composed of a name at the
beginning of the line, immediately followed by a colon (':'). Traditionally,
an LWS is added after the colon but that's not required. Then come the values.
Multiple identical headers may be folded into one single line, delimiting the
values with commas, provided that their order is respected. This is commonly
encountered in the 'Cookie:' field. A header may span over multiple lines if
the subsequent lines begin with an LWS. In the example in 2.1.2, lines 4 and 5
define a total of 3 values for the 'Accept:' header.

Contrary to a common mis-conception, header names are not case-sensitive, and
their values are not either if they refer to other header names (such as the
'Connection:' header).

The end of the headers is indicated by the first empty line. People often say
that it's a double line feed, which is not exact, even if a double line feed
is one valid form of empty line.

Fortunately, HAProxy takes care of all these complex combinations when indexing
headers, checking values and counting them, so there is no reason to worry
about the way they could be written, but it is important not to accusate an
application of being buggy if it does unusual, valid things.

Important note:
   As suggested by RFC2616, HAProxy normalizes headers by replacing line breaks
   in the middle of headers by LWS in order to join multi-line headers. This
   is necessary for proper analysis and helps less capable HTTP parsers to work
   correctly and not to be fooled by such complex constructs.


2.1.3) HTTP response
--------------------

An HTTP response looks very much like an HTTP request. Both are called HTTP
messages. Let's consider this HTTP response :

  Line     Contents
  number 
     1     HTTP/1.1 200 OK
     2     Content-length: 350
     3     Content-Type: text/html


2.1.3.1) The Response line
--------------------------

Line 1 is the "response line". It is always composed of 3 fields :

  - a version tag : HTTP/1.1
  - a status code : 200
  - a reason      : OK

The status code is always 3-digit. The first digit indicates a general status :
 - 2xx = OK, content is following   (eg: 200, 206)
 - 3xx = OK, no content following   (eg: 302, 304)
 - 4xx = error caused by the client (eg: 401, 403, 404)
 - 5xx = error caused by the server (eg: 500, 502, 503)

Please refer to RFC2616 for the detailed meaning of all such codes. The
"reason" field is just a hint, but is not parsed by clients. Anything can be 
found there, but it's a common practise to respect the well-established
messages. It can be composed of one or multiple words, such as "OK", "Found",
or "Authentication Required".


2.1.3.2) The response headers
-----------------------------

Response headers work exactly like request headers, and as such, HAProxy uses
the same parsing function for both. Please refer to paragraph 2.1.2.2 for more
details.


2.2) Proxy keywords matrix
----------------------------

The following list of keywords is supported. Most of them may only be used in a
limited set of section types. Some of them are marked as "deprecated" because
they are inherited from an old syntax which may be confusing or functionnally
limited, and there are new recommended keywords to replace them.


keyword                 defaults   frontend   listen    backend
----------------------+----------+----------+---------+---------
acl                         -          X         X         X   
appsession                  -          -         X         X   
balance                     X          -         X         X   
bind                        -          X         X         -   
block                       -          X         X         X
capture cookie              -          X         X         -
capture request header      -          X         X         -
capture response header     -          X         X         -
clitimeout                  X          X         X         -  (deprecated)
contimeout                  X          -         X         X  (deprecated)
cookie                      X          -         X         X
default_backend             -          X         X         -
disabled                    X          X         X         X
dispatch                    -          -         X         X
enabled                     X          X         X         X
errorfile                   X          X         X         X
errorloc                    X          X         X         X
errorloc302                 X          X         X         X
errorloc303                 X          X         X         X
fullconn                    X          -         X         X
grace                       -          X         X         X
http-check disable-on-404   X          -         X         X
log                         X          X         X         X
maxconn                     X          X         X         -
mode                        X          X         X         X
monitor fail                -          X         X         -
monitor-net                 X          X         X         -
monitor-uri                 X          X         X         -
option abortonclose         X          -         X         X
option allbackups           X          -         X         X
option checkcache           X          -         X         X
option clitcpka             X          X         X         -
option contstats            X          X         X         -
option dontlognull          X          X         X         -
option forceclose           X          -         X         X
option forwardfor           X          X         X         X
option httpchk              X          -         X         X
option httpclose            X          X         X         X
option httplog              X          X         X         X
option logasap              X          X         X         -
option nolinger             X          X         X         X
option http_proxy           X          X         X         X
option persist              X          -         X         X
option redispatch           X          -         X         X
option smtpchk              X          -         X         X
option srvtcpka             X          -         X         X
option ssl-hello-chk        X          -         X         X
option tcpka                X          X         X         X
option tcplog               X          X         X         X
option tcpsplice            X          X         X         X
option transparent          X          X         X         -
redisp                      X          -         X         X
redispatch                  X          -         X         X
reqadd                      -          X         X         X
reqallow                    -          X         X         X
reqdel                      -          X         X         X
reqdeny                     -          X         X         X
reqiallow                   -          X         X         X
reqidel                     -          X         X         X
reqideny                    -          X         X         X
reqipass                    -          X         X         X
reqirep                     -          X         X         X
reqisetbe                   -          X         X         X
reqitarpit                  -          X         X         X
reqpass                     -          X         X         X
reqrep                      -          X         X         X
reqsetbe                    -          X         X         X
reqtarpit                   -          X         X         X
retries                     X          -         X         X
rspadd                      -          X         X         X
rspdel                      -          X         X         X
rspdeny                     -          X         X         X
rspidel                     -          X         X         X
rspideny                    -          X         X         X
rspirep                     -          X         X         X
rsprep                      -          X         X         X
server                      -          -         X         X
source                      X          -         X         X
srvtimeout                  X          -         X         X  (deprecated)
stats auth                  X          -         X         X
stats enable                X          -         X         X
stats realm                 X          -         X         X
stats refresh               X          -         X         X
stats scope                 X          -         X         X
stats uri                   X          -         X         X
stats hide-version          X          -         X         X
timeout appsession          X          -         X         X
timeout client              X          X         X         -
timeout clitimeout          X          X         X         -  (deprecated)
timeout connect             X          -         X         X
timeout contimeout          X          -         X         X  (deprecated)
timeout queue               X          -         X         X
timeout server              X          -         X         X
timeout srvtimeout          X          -         X         X  (deprecated)
timeout tarpit              X          X         X         -
transparent                 X          X         X         -
use_backend                 -          X         X         -
usesrc                      X          -         X         X
----------------------+----------+----------+---------+---------
keyword                 defaults   frontend   listen    backend


2.2.1) Alphabetically sorted keywords reference
-----------------------------------------------

This section provides a description of each keyword and its usage.


acl <aclname> <criterion> [flags] [operator] <value> ...
  Declare or complete an access list.
  May be used in sections :   defaults | frontend | listen | backend
                                 no    |    yes   |   yes  |   yes
  Example:
        acl invalid_src  src          0.0.0.0/7 224.0.0.0/3
        acl invalid_src  src_port     0:1023
        acl local_dst    hdr(host) -i localhost

  See section 2.3 about ACL usage.


appsession <cookie> len <length> timeout <holdtime>
  Define session stickiness on an existing application cookie.
  May be used in sections :   defaults | frontend | listen | backend
                                 no    |    no    |   yes  |   yes
  Arguments :
    <cookie>   this is the name of the cookie used by the application and which
               HAProxy will have to learn for each new session.

    <length>   this is the number of characters that will be memorized and
               checked in each cookie value.

    <holdtime> this is the time after which the cookie will be removed from
               memory if unused. If no unit is specified, this time is in
               milliseconds.

  When an application cookie is defined in a backend, HAProxy will check when
  the server sets such a cookie, and will store its value in a table, and
  associate it with the server's identifier. Up to <length> characters from
  the value will be retained. On each connection, haproxy will look for this
  cookie both in the "Cookie:" headers, and as a URL parameter in the query
  string. If a known value is found, the client will be directed to the server
  associated with this value. Otherwise, the load balancing algorithm is
  applied. Cookies are automatically removed from memory when they have been
  unused for a duration longer than <holdtime>.

  The definition of an application cookie is limited to one per backend.

  Example :
        appsession JSESSIONID len 52 timeout 3h

  See also : "cookie", "capture cookie" and "balance".


balance <algorithm> [ <arguments> ]
  Define the load balancing algorithm to be used in a backend.
  May be used in sections :   defaults | frontend | listen | backend
                                 yes   |    no    |   yes  |   yes
  Arguments :
    <algorithm> is the algorithm used to select a server when doing load
                balancing. This only applies when no persistence information
                is available, or when a connection is redispatched to another
                server. <algorithm> may be one of the following :

      roundrobin  Each server is used in turns, according to their weights.
                  This is the smoothest and fairest algorithm when the server's
                  processing time remains equally distributed. This algorithm
                  is dynamic, which means that server weights may be adjusted
                  on the fly for slow starts for instance.

      source      The source IP address is hashed and divided by the total
                  weight of the running servers to designate which server will
                  receive the request. This ensures that the same client IP
                  address will always reach the same server as long as no
                  server goes down or up. If the hash result changes due to the
                  number of running servers changing, many clients will be
                  directed to a different server. This algorithm is generally
                  used in TCP mode where no cookie may be inserted. It may also
                  be used on the Internet to provide a best-effort stickyness
                  to clients which refuse session cookies. This algorithm is
                  static, which means that changing a server's weight on the
                  fly will have no effect.

      uri         The left part of the URI (before the question mark) is hashed
                  and divided by the total weight of the running servers. The
                  result designates which server will receive the request. This
                  ensures that a same URI will always be directed to the same
                  server as long as no server goes up or down. This is used
                  with proxy caches and anti-virus proxies in order to maximize
                  the cache hit rate. Note that this algorithm may only be used
                  in an HTTP backend. This algorithm is static, which means
                  that changing a server's weight on the fly will have no
                  effect.

      url_param   The URL parameter specified in argument will be looked up in
                  the query string of each HTTP request. If it is found
                  followed by an equal sign ('=') and a value, then the value
                  is hashed and divided by the total weight of the running
                  servers. The result designates which server will receive the
                  request. This is used to track user identifiers in requests
                  and ensure that a same user ID will always be sent to the
                  same server as long as no server goes up or down. If no value
                  is found or if the parameter is not found, then a round robin
                  algorithm is applied. Note that this algorithm may only be
                  used in an HTTP backend. This algorithm is static, which
                  means that changing a server's weight on the fly will have no
                  effect.

    <arguments> is an optional list of arguments which may be needed by some
                algorithms. Right now, only the "url_param" algorithm supports
                a mandatory argument.

  The definition of the load balancing algorithm is mandatory for a backend
  and limited to one per backend.

  Examples :
        balance roundrobin
        balance url_param userid

  See also : "dispatch", "cookie", "appsession", "transparent" and "http_proxy".


bind [<address>]:<port> [, ...]
  Define one or several listening addresses and/or ports in a frontend.
  May be used in sections :   defaults | frontend | listen | backend
                                  no   |    yes   |   yes  |   no
  Arguments :
    <address> is optional and can be a host name, an IPv4 address, an IPv6
              address, or '*'. It designates the address the frontend will
              listen on. If unset, all IPv4 addresses of the system will be
              listened on. The same will apply for '*' or the system's special
              address "0.0.0.0".

    <port>    is the TCP port number the proxy will listen on. The port is
              mandatory. Note that in the case of an IPv6 address, the port is
              always the number after the last colon (':').

  It is possible to specify a list of address:port combinations delimited by
  commas. The frontend will then listen on all of these addresses. There is no
  fixed limit to the number of addresses and ports which can be listened on in
  a frontend, as well as there is no limit to the number of "bind" statements
  in a frontend.

  Example :
        listen http_proxy
            bind :80,:443
            bind 10.0.0.1:10080,10.0.0.1:10443

  See also : "source".


block { if | unless } <condition>
  Block a layer 7 request if/unless a condition is matched
  May be used in sections :   defaults | frontend | listen | backend
                                 no    |    yes   |   yes  |   yes

  The HTTP request will be blocked very early in the layer 7 processing
  if/unless <condition> is matched. A 403 error will be returned if the request
  is blocked. The condition has to reference ACLs (see section 2.3). This is
  typically used to deny access to certain sensible resources if some
  conditions are met or not met. There is no fixed limit to the number of
  "block" statements per instance.

  Example:
        acl invalid_src  src          0.0.0.0/7 224.0.0.0/3
        acl invalid_src  src_port     0:1023
        acl local_dst    hdr(host) -i localhost
        block if invalid_src || local_dst

  See section 2.3 about ACL usage.


capture cookie <name> len <length>
  Capture and log a cookie in the request and in the response.
  May be used in sections :   defaults | frontend | listen | backend
                                  no   |    yes   |   yes  |   no
  Arguments :
    <name>    is the beginning of the name of the cookie to capture. In order
              to match the exact name, simply suffix the name with an equal
              sign ('='). The full name will appear in the logs, which is
              useful with application servers which adjust both the cookie name
              and value (eg: ASPSESSIONXXXXX).

    <length>  is the maximum number of characters to report in the logs, which
              include the cookie name, the equal sign and the value, all in the
              standard "name=value" form. The string will be truncated on the
              right if it exceeds <length>.

  Only the first cookie is captured. Both the "cookie" request headers and the
  "set-cookie" response headers are monitored. This is particularly useful to
  check for application bugs causing session crossing or stealing between
  users, because generally the user's cookies can only change on a login page.

  When the cookie was not presented by the client, the associated log column
  will report "-". When a request does not cause a cookie to be assigned by the
  server, a "-" is reported in the response column.

  The capture is performed in the frontend only because it is necessary that
  the log format does not change for a given frontend depending on the
  backends. This may change in the future. Note that there can be only one
  "capture cookie" statement in a frontend. The maximum capture length is
  configured in the souces by default to 64 characters. It is not possible to
  specify a capture in a "defaults" section.

  Example:
        capture cookie ASPSESSION len 32

  See also : "capture request header", "capture response header" as well as
            section 2.4 about logging.


capture request header <name> len <length>
  Capture and log the first occurrence of the specified request header.
  May be used in sections :   defaults | frontend | listen | backend
                                  no   |    yes   |   yes  |   no
  Arguments :
    <name>    is the name of the header to capture. The header names are not
              case-sensitive, but it is a common practise to write them as they
              appear in the requests, with the first letter of each word in
              upper case. The header name will not appear in the logs, only the
              value is reported, but the position in the logs is respected.

    <length>  is the maximum number of characters to extract from the value and
              report in the logs. The string will be truncated on the right if
              it exceeds <length>.

  Only the first value of the first occurrence of the header is captured. The
  value will be added to the logs between braces ('{}'). If multiple headers
  are captured, they will be delimited by a vertical bar ('|') and will appear
  in the same order they were declared in the configuration. Common uses for
  request header captures include the "Host" field in virtual hosting
  environments, the "Content-length" when uploads are supported, "User-agent"
  to quickly differenciate between real users and robots, and "X-Forwarded-For"
  in proxied environments to find where the request came from.

  There is no limit to the number of captured request headers, but each capture
  is limited to 64 characters. In order to keep log format consistent for a
  same frontend, header captures can only be declared in a frontend. It is not
  possible to specify a capture in a "defaults" section.

  Example:
        capture request header Host len 15
        capture request header X-Forwarded-For len 15
        capture request header Referrer len 15

  See also : "capture cookie", "capture response header" as well as section 2.4
             about logging.


capture response header <name> len <length>
  Capture and log the first occurrence of the specified response header.
  May be used in sections :   defaults | frontend | listen | backend
                                  no   |    yes   |   yes  |   no
  Arguments :
    <name>    is the name of the header to capture. The header names are not
              case-sensitive, but it is a common practise to write them as they
              appear in the response, with the first letter of each word in
              upper case. The header name will not appear in the logs, only the
              value is reported, but the position in the logs is respected.

    <length>  is the maximum number of characters to extract from the value and
              report in the logs. The string will be truncated on the right if
              it exceeds <length>.

  Only the first value of the first occurrence of the header is captured. The
  result will be added to the logs between braces ('{}') after the captured
  request headers. If multiple headers are captured, they will be delimited by
  a vertical bar ('|') and will appear in the same order they were declared in
  the configuration. Common uses for response header captures include the
  "Content-length" header which indicates how many bytes are expected to be
  returned, the "Location" header to track redirections.

  There is no limit to the number of captured response headers, but each
  capture is limited to 64 characters. In order to keep log format consistent
  for a same frontend, header captures can only be declared in a frontend. It
  is not possible to specify a capture in a "defaults" section.

  Example:
        capture response header Content-length len 9
        capture response header Location len 15

  See also : "capture cookie", "capture request header" as well as section 2.4
             about logging.


clitimeout <timeout>
  Set the maximum inactivity time on the client side.
  May be used in sections :   defaults | frontend | listen | backend
                                 yes   |    yes   |   yes  |   no
  Arguments :
    <timeout> is the timeout value is specified in milliseconds by default, but
              can be in any other unit if the number is suffixed by the unit,
              as explained at the top of this document.

  The inactivity timeout applies when the client is expected to acknowledge or
  send data. In HTTP mode, this timeout is particularly important to consider
  during the first phase, when the client sends the request, and during the
  response while it is reading data sent by the server. The value is specified
  in milliseconds by default, but can be in any other unit if the number is
  suffixed by the unit, as specified at the top of this document. In TCP mode
  (and to a lesser extent, in HTTP mode), it is highly recommended that the
  client timeout remains equal to the server timeout in order to avoid complex
  situations to debug. It is a good practise to cover one or several TCP packet
  losses by specifying timeouts that are slightly above multiples of 3 seconds
  (eg: 4 or 5 seconds).

  This parameter is specific to frontends, but can be specified once for all in
  "defaults" sections. This is in fact one of the easiest solutions not to
  forget about it. An unspecified timeout results in an infinite timeout, which
  is not recommended. Such a usage is accepted and works but reports a warning
  during startup because it may results in accumulation of expired sessions in
  the system if the system's timeouts are not configured either.

  This parameter is provided for compatibility but is currently deprecated.
  Please use "timeout client" instead.

  See also : "timeout client", "timeout server", "srvtimeout".


contimeout <timeout>
  Set the maximum time to wait for a connection attempt to a server to succeed.
  May be used in sections :   defaults | frontend | listen | backend
                                 yes   |    no    |   yes  |   yes
  Arguments :
    <timeout> is the timeout value is specified in milliseconds by default, but
              can be in any other unit if the number is suffixed by the unit,
              as explained at the top of this document.

  If the server is located on the same LAN as haproxy, the connection should be
  immediate (less than a few milliseconds). Anyway, it is a good practise to
  cover one or several TCP packet losses by specifying timeouts that are 
  slightly above multiples of 3 seconds (eg: 4 or 5 seconds). By default, the
  connect timeout also presets the queue timeout to the same value if this one
  has not been specified. Historically, the contimeout was also used to set the
  tarpit timeout in a listen section, which is not possible in a pure frontend.

  This parameter is specific to backends, but can be specified once for all in
  "defaults" sections. This is in fact one of the easiest solutions not to
  forget about it. An unspecified timeout results in an infinite timeout, which
  is not recommended. Such a usage is accepted and works but reports a warning
  during startup because it may results in accumulation of failed sessions in
  the system if the system's timeouts are not configured either.

  This parameter is provided for backwards compatibility but is currently
  deprecated. Please use "timeout connect", "timeout queue" or "timeout tarpit"
  instead.

  See also : "timeout connect", "timeout queue", "timeout tarpit",
             "timeout server", "contimeout".


cookie <name> [ rewrite|insert|prefix ] [ indirect ] [ nocache ] [ postonly ]
  Enable cookie-based persistence in a backend.
  May be used in sections :   defaults | frontend | listen | backend
                                 yes   |    no    |   yes  |   yes
  Arguments :
    <name>    is the name of the cookie which will be monitored, modified or
              inserted in order to bring persistence. This cookie is sent to
              the client via a "Set-Cookie" header in the response, and is
              brought back by the client in a "Cookie" header in all requests.
              Special care should be taken to choose a name which does not
              conflict with any likely application cookie. Also, if the same
              backends are subject to be used by the same clients (eg:
              HTTP/HTTPS), care should be taken to use different cookie names
              between all backends if persistence between them is not desired.

    rewrite   This keyword indicates that the cookie will be provided by the
              server and that haproxy will have to modify its value to set the
              server's identifier in it. This mode is handy when the management
              of complex combinations of "Set-cookie" and "Cache-control"
              headers is left to the application. The application can then
              decide whether or not it is appropriate to emit a persistence
              cookie. Since all responses should be monitored, this mode only
              works in HTTP close mode. Unless the application behaviour is
              very complex and/or broken, it is advised not to start with this
              mode for new deployments. This keyword is incompatible with
              "insert" and "prefix".

    insert    This keyword indicates that the persistence cookie will have to
              be inserted by haproxy in the responses. If the server emits a
              cookie with the same name, it will be replaced anyway. For this
              reason, this mode can be used to upgrade existing configurations
              running in the "rewrite" mode. The cookie will only be a session
              cookie and will not be stored on the client's disk. Due to
              caching effects, it is generally wise to add the "indirect" and
              "nocache" or "postonly" keywords (see below). The "insert"
              keyword is not compatible with "rewrite" and "prefix".

    prefix    This keyword indicates that instead of relying on a dedicated
              cookie for the persistence, an existing one will be completed.
              This may be needed in some specific environments where the client
              does not support more than one single cookie and the application
              already needs it. In this case, whenever the server sets a cookie
              named <name>, it will be prefixed with the server's identifier
              and a delimiter. The prefix will be removed from all client
              requests so that the server still finds the cookie it emitted.
              Since all requests and responses are subject to being modified,
              this mode requires the HTTP close mode. The "prefix" keyword is
              not compatible with "rewrite" and "insert".

    indirect  When this option is specified in insert mode, cookies will only
              be added when the server was not reached after a direct access,
              which means that only when a server is elected after applying a
              load-balancing algorithm, or after a redispatch, then the cookie
              will be inserted. If the client has all the required information
              to connect to the same server next time, no further cookie will
              be inserted. In all cases, when the "indirect" option is used in
              insert mode, the cookie is always removed from the requests
              transmitted to the server. The persistence mechanism then becomes
              totally transparent from the application point of view.

    nocache   This option is recommended in conjunction with the insert mode
              when there is a cache between the client and HAProxy, as it
              ensures that a cacheable response will be tagged non-cacheable if
              a cookie needs to be inserted. This is important because if all
              persistence cookies are added on a cacheable home page for
              instance, then all customers will then fetch the page from an
              outer cache and will all share the same persistence cookie,
              leading to one server receiving much more traffic than others.
              See also the "insert" and "postonly" options.

    postonly  This option ensures that cookie insertion will only be performed
              on responses to POST requests. It is an alternative to the
              "nocache" option, because POST responses are not cacheable, so
              this ensures that the persistence cookie will never get cached.
              Since most sites do not need any sort of persistence before the
              first POST which generally is a login request, this is a very
              efficient method to optimize caching without risking to find a
              persistence cookie in the cache.
              See also the "insert" and "nocache" options.

  There can be only one persistence cookie per HTTP backend, and it can be
  declared in a defaults section. The value of the cookie will be the value
  indicated after the "cookie" keyword in a "server" statement. If no cookie
  is declared for a given server, the cookie is not set.

  Examples :
        cookie JSESSIONID prefix
        cookie SRV insert indirect nocache
        cookie SRV insert postonly indirect

  See also : "appsession", "balance source", "capture cookie", "server".


default_backend <backend>
  Specify the backend to use when no "use_backend" rule has been matched.
  May be used in sections :   defaults | frontend | listen | backend
                                 yes   |    yes   |   yes  |   no
  Arguments :
    <backend> is the name of the backend to use.

  When doing content-switching between frontend and backends using the
  "use_backend" keyword, it is often useful to indicate which backend will be
  used when no rule has matched. It generally is the dynamic backend which
  will catch all undetermined requests.

  The "default_backend" keyword is also supported in TCP mode frontends to
  facilitate the ordering of configurations in frontends and backends,
  eventhough it does not make much more sense in case of TCP due to the fact
  that use_backend currently does not work in TCP mode.

  Example :

        use_backend     dynamic  if  url_dyn
        use_backend     static   if  url_css url_img extension_img
        default_backend dynamic


disabled
  Disable a proxy, frontend or backend.
  May be used in sections :   defaults | frontend | listen | backend
                                 yes   |    yes   |   yes  |   yes
  Arguments : none

  The "disabled" keyword is used to disable an instance, mainly in order to
  liberate a listening port or to temporarily disable a service. The instance
  will still be created and its configuration will be checked, but it will be
  created in the "stopped" state and will appear as such in the statistics. It
  will not receive any traffic nor will it send any health-checks or logs. It
  is possible to disable many instances at once by adding the "disabled"
  keyword in a "defaults" section.

  See also : "enabled"


enabled
  Enable a proxy, frontend or backend.
  May be used in sections :   defaults | frontend | listen | backend
                                 yes   |    yes   |   yes  |   yes
  Arguments : none

  The "enabled" keyword is used to explicitly enable an instance, when the
  defaults has been set to "disabled". This is very rarely used.

  See also : "disabled"


errorfile <code> <file>
  Return a file contents instead of errors generated by HAProxy
  May be used in sections :   defaults | frontend | listen | backend
                                 yes   |    yes   |   yes  |   yes
  Arguments :
    <code>    is the HTTP status code. Currently, HAProxy is capable of
              generating codes 400, 403, 408, 500, 502, 503, and 504.

    <file>    designates a file containing the full HTTP response. It is
              recommended to follow the common practise of appending ".http" to
              the filename so that people do not confuse the response with HTML
              error pages.

  It is important to understand that this keyword is not meant to rewrite
  errors returned by the server, but errors detected and returned by HAProxy.
  This is why the list of supported errors is limited to a small set.

  The files are returned verbatim on the TCP socket. This allows any trick such
  as redirections to another URL or site, as well as tricks to clean cookies,
  force enable or disable caching, etc... The package provides default error
  files returning the same contents as default errors.

  The files are read at the same time as the configuration and kept in memory.
  For this reason, the errors continue to be returned even when the process is
  chrooted, and no file change is considered while the process is running. A
  simple method for developping those files consists in associating them to the
  403 status code and interrogating a blocked URL.

  See also : "errorloc", "errorloc302", "errorloc303"


http-check disable-on-404
  Enable a maintenance mode upon HTTP/404 response to health-checks
  May be used in sections :   defaults | frontend | listen | backend
                                 no    |    no    |   yes  |   yes

  Arguments : none

  When this option is set, a server which returns an HTTP code 404 will be
  excluded from further load-balancing, but will still receive persistent
  connections. This provides a very convenient method for Web administrators
  to perform a graceful shutdown of their servers. It is also important to note
  that a server which is detected as failed while it was in this mode will not
  generate an alert, just a notice. If the server responds 2xx or 3xx again, it
  will immediately be reinserted into the farm. The status on the stats page
  reports "NOLB" for a server in this mode. It is important to note that this
  option only works in conjunction with the "httpchk" option.


monitor fail [if | unless] <condition>
  Add a condition to report a failure to a monitor request.
  May be used in sections :   defaults | frontend | listen | backend
                                 no    |    yes   |   yes  |   no

  Arguments :
    if <cond>     the monitor request will fail if the condition is satisfied,
                  and will succeed otherwise. The condition should describe a
                  combinated test which must induce a failure if all conditions
                  are met, for instance a low number of servers both in a
                  backend and its backup.

    unless <cond> the monitor request will succeed only if the condition is
                  satisfied, and will fail otherwise. Such a condition may be
                  based on a test on the presence of a minimum number of active
                  servers in a list of backends.

  This statement adds a condition which can force the response to a monitor
  request to report a failure. By default, when an external component queries
  the URI dedicated to monitoring, a 200 response is returned. When one of the
  conditions above is met, haproxy will return 503 instead of 200. This is
  very useful to report a site failure to an external component which may base
  routing advertisements between multiple sites on the availability reported by
  haproxy. In this case, one would rely on an ACL involving the "nbsrv"
  criterion.

  Example:
     frontend www
        acl site_dead nbsrv(dynamic) lt 2
        acl site_dead nbsrv(static)  lt 2
        monitor-uri   /site_alive
        monitor fail  if site_dead


option contstats
  Enable continuous traffic statistics updates
  May be used in sections :   defaults | frontend | listen | backend
                                 yes   |    yes   |   yes  |   no
  Arguments : none

  By default, counters used for statistics calculation are incremented
  only when a session finishes. It works quite well when serving small
  objects, but with big ones (for example large images or archives) or
  with A/V streaming, a graph generated from haproxy counters looks like
  a hedgehog. With this option enabled counters get incremented continuously,
  during a whole session. Recounting touches a hotpath directly so
  it is not enabled by default, as it has small performance impact (~0.5%).


timeout client <timeout>
timeout clitimeout <timeout> (deprecated)
  Set the maximum inactivity time on the client side.
  May be used in sections :   defaults | frontend | listen | backend
                                 yes   |    yes   |   yes  |   no
  Arguments :
    <timeout> is the timeout value is specified in milliseconds by default, but
              can be in any other unit if the number is suffixed by the unit,
              as explained at the top of this document.

  The inactivity timeout applies when the client is expected to acknowledge or
  send data. In HTTP mode, this timeout is particularly important to consider
  during the first phase, when the client sends the request, and during the
  response while it is reading data sent by the server. The value is specified
  in milliseconds by default, but can be in any other unit if the number is
  suffixed by the unit, as specified at the top of this document. In TCP mode
  (and to a lesser extent, in HTTP mode), it is highly recommended that the
  client timeout remains equal to the server timeout in order to avoid complex
  situations to debug. It is a good practise to cover one or several TCP packet
  losses by specifying timeouts that are slightly above multiples of 3 seconds
  (eg: 4 or 5 seconds).

  This parameter is specific to frontends, but can be specified once for all in
  "defaults" sections. This is in fact one of the easiest solutions not to
  forget about it. An unspecified timeout results in an infinite timeout, which
  is not recommended. Such a usage is accepted and works but reports a warning
  during startup because it may results in accumulation of expired sessions in
  the system if the system's timeouts are not configured either.

  This parameter replaces the old, deprecated "clitimeout". It is recommended
  to use it to write new configurations. The form "timeout clitimeout" is
  provided only by backwards compatibility but its use is strongly discouraged.

  See also : "clitimeout", "timeout server".


timeout connect <timeout>
timeout contimeout <timeout> (deprecated)
  Set the maximum time to wait for a connection attempt to a server to succeed.
  May be used in sections :   defaults | frontend | listen | backend
                                 yes   |    no    |   yes  |   yes
  Arguments :
    <timeout> is the timeout value is specified in milliseconds by default, but
              can be in any other unit if the number is suffixed by the unit,
              as explained at the top of this document.

  If the server is located on the same LAN as haproxy, the connection should be
  immediate (less than a few milliseconds). Anyway, it is a good practise to
  cover one or several TCP packet losses by specifying timeouts that are 
  slightly above multiples of 3 seconds (eg: 4 or 5 seconds). By default, the
  connect timeout also presets the queue timeout to the same value if this one
  has not been specified.

  This parameter is specific to backends, but can be specified once for all in
  "defaults" sections. This is in fact one of the easiest solutions not to
  forget about it. An unspecified timeout results in an infinite timeout, which
  is not recommended. Such a usage is accepted and works but reports a warning
  during startup because it may results in accumulation of failed sessions in
  the system if the system's timeouts are not configured either.

  This parameter replaces the old, deprecated "contimeout". It is recommended
  to use it to write new configurations. The form "timeout contimeout" is
  provided only by backwards compatibility but its use is strongly discouraged.

  See also : "timeout queue", "timeout server", "contimeout".


2.3) Using ACLs
---------------

The use of Access Control Lists (ACL) provides a flexible solution to perform
content switching and generally to take decisions based on content extracted
from the request, the response or any environmental status. The principle is
simple :

  - define test criteria with sets of values
  - perform actions only if a set of tests is valid

The actions generally consist in blocking the request, or selecting a backend.

In order to define a test, the "acl" keyword is used. The syntax is :

   acl <aclname> <criterion> [flags] [operator] <value> ...

This creates a new ACL <aclname> or completes an existing one with new tests.
Those tests apply to the portion of request/response specified in <criterion>
and may be adjusted with optional flags [flags]. Some criteria also support
an operator which may be specified before the set of values. The values are
of the type supported by the criterion, and are separated by spaces.

ACL names must be formed from upper and lower case letters, digits, '-' (dash),
'_' (underscore) , '.' (dot) and ':' (colon). ACL names are case-sensitive,
which means that "my_acl" and "My_Acl" are two different ACLs.

There is no enforced limit to the number of ACLs. The unused ones do not affect
performance, they just consume a small amount of memory.

The following ACL flags are currently supported :

   -i : ignore case during matching.
   -- : force end of flags. Useful when a string looks like one of the flags.

Supported types of values are :

  - integers or integer ranges
  - strings
  - regular expressions
  - IP addresses and networks


2.3.1) Matching integers
------------------------

Matching integers is special in that ranges and operators are permitted. Note
that integer matching only applies to positive values. A range is a value
expressed with a lower and an upper bound separated with a colon, both of which
may be omitted.

For instance, "1024:65535" is a valid range to represent a range of
unprivileged ports, and "1024:" would also work. "0:1023" is a valid
representation of privileged ports, and ":1023" would also work.

For an easier usage, comparison operators are also supported. Note that using
operators with ranges does not make much sense and is strongly discouraged.
Similarly, it does not make much sense to perform order comparisons with a set
of values.

Available operators for integer matching are :

  eq : true if the tested value equals at least one value
  ge : true if the tested value is greater than or equal to at least one value
  gt : true if the tested value is greater than at least one value
  le : true if the tested value is less than or equal to at least one value
  lt : true if the tested value is less than at least one value

For instance, the following ACL matches any negative Content-Length header :

  acl negative-length hdr_val(content-length) lt 0


2.3.2) Matching strings
-----------------------

String matching applies to verbatim strings as they are passed, with the
exception of the backslash ("\") which makes it possible to escape some
characters such as the space. If the "-i" flag is passed before the first
string, then the matching will be performed ignoring the case. In order
to match the string "-i", either set it second, or pass the "--" flag
before the first string. Same applies of course to match the string "--".


2.3.3) Matching regular expressions (regexes)
---------------------------------------------

Just like with string matching, regex matching applies to verbatim strings as
they are passed, with the exception of the backslash ("\") which makes it
possible to escape some characters such as the space. If the "-i" flag is
passed before the first regex, then the matching will be performed ignoring
the case. In order to match the string "-i", either set it second, or pass
the "--" flag before the first string. Same principle applies of course to
match the string "--".


2.3.4) Matching IPv4 addresses
------------------------------

IPv4 addresses values can be specified either as plain addresses or with a
netmask appended, in which case the IPv4 address matches whenever it is
within the network. Plain addresses may also be replaced with a resolvable
host name, but this practise is generally discouraged as it makes it more
difficult to read and debug configurations. If hostnames are used, you should
at least ensure that they are present in /etc/hosts so that the configuration
does not depend on any random DNS match at the moment the configuration is
parsed.


2.3.5) Available matching criteria
----------------------------------

2.3.5.1) Matching at Layer 4 and below
--------------------------------------

A first set of criteria applies to information which does not require any
analysis of the request or response contents. Those generally include TCP/IP
addresses and ports, as well as internal values independant on the stream.

always_false
  This one never matches. All values and flags are ignored. It may be used as
  a temporary replacement for another one when adjusting configurations.

always_true
  This one always matches. All values and flags are ignored. It may be used as
  a temporary replacement for another one when adjusting configurations.

src <ip_address>
  Applies to the client's IPv4 address. It is usually used to limit access to
  certain resources such as statistics. Note that it is the TCP-level source
  address which is used, and not the address of a client behind a proxy.

src_port <integer>
  Applies to the client's TCP source port. This has a very limited usage.

dst <ip_address>
  Applies to the local IPv4 address the client connected to. It can be used to
  switch to a different backend for some alternative addresses.

dst_port <integer>
  Applies to the local port the client connected to. It can be used to switch
  to a different backend for some alternative ports.

dst_conn <integer>
  Applies to the number of currently established connections on the frontend,
  including the one being evaluated. It can be used to either return a sorry
  page before hard-blocking, or to use a specific backend to drain new requests
  when the farm is considered saturated.

nbsrv <integer>
nbsrv(backend) <integer>
  Returns true when the number of usable servers of either the current backend
  or the named backend matches the values or ranges specified. This is used to
  switch to an alternate backend when the number of servers is too low to
  to handle some load. It is useful to report a failure when combined with
  "monitor fail".


2.3.5.2) Matching at Layer 7
----------------------------

A second set of criteria applies to information which can be found at the
application layer (layer 7). Those require that a full HTTP request has been
read, and are only evaluated then. They may require slightly more CPU resources
than the layer 4 ones, but not much since the request and response are indexed.

method <string>
  Applies to the method in the HTTP request, eg: "GET". Some predefined ACL
  already check for most common methods.

req_ver <string>
  Applies to the version string in the HTTP request, eg: "1.0". Some predefined
  ACL already check for versions 1.0 and 1.1.

path <string>
  Returns true when the path part of the request, which starts at the first
  slash and ends before the question mark, equals one of the strings. It may be
  used to match known files, such as /favicon.ico.

path_beg <string>
  Returns true when the path begins with one of the strings. This can be used
  to send certain directory names to alternative backends.

path_end <string>
  Returns true when the path ends with one of the strings. This may be used to
  control file name extension.

path_sub <string>
  Returns true when the path contains one of the strings. It can be used to
  detect particular patterns in paths, such as "../" for example. See also
  "path_dir".

path_dir <string>
  Returns true when one of the strings is found isolated or delimited with
  slashes in the path. This is used to perform filename or directory name
  matching without the risk of wrong match due to colliding prefixes. See also
  "url_dir" and "path_sub".

path_dom <string>
  Returns true when one of the strings is found isolated or delimited with dots
  in the path. This may be used to perform domain name matching in proxy
  requests. See also "path_sub" and "url_dom".

path_reg <regex>
  Returns true when the path matches one of the regular expressions. It can be
  used any time, but it is important to remember that regex matching is slower
  than other methods. See also "url_reg" and all "path_" criteria.

url <string>
  Applies to the whole URL passed in the request. The only real use is to match
  "*", for which there already is a predefined ACL.

url_beg <string>
  Returns true when the URL begins with one of the strings. This can be used to
  check whether a URL begins with a slash or with a protocol scheme.

url_end <string>
  Returns true when the URL ends with one of the strings. It has very limited
  use. "path_end" should be used instead for filename matching.

url_sub <string>
  Returns true when the URL contains one of the strings. It can be used to
  detect particular patterns in query strings for example. See also "path_sub".

url_dir <string>
  Returns true when one of the strings is found isolated or delimited with
  slashes in the URL. This is used to perform filename or directory name
  matching without the risk of wrong match due to colliding prefixes. See also
  "path_dir" and "url_sub".

url_dom <string>
  Returns true when one of the strings is found isolated or delimited with dots
  in the URL. This is used to perform domain name matching without the risk of
  wrong match due to colliding prefixes. See also "url_sub".

url_reg <regex>
  Returns true when the URL matches one of the regular expressions. It can be
  used any time, but it is important to remember that regex matching is slower
  than other methods. See also "path_reg" and all "url_" criteria.

url_ip <ip_address>
  Applies to the IP address specified in the absolute URI in an HTTP request.
  It can be used to prevent access to certain resources such as local network.
  It is useful with option 'http_proxy'.

url_port <integer>
  Applies to the port specified in the absolute URI in an HTTP request. It can
  be used to prevent access to certain resources. It is useful with option
  'http_proxy'. Note that if the port is not specified in the request, port 80
  is assumed.

hdr <string> 
hdr(header) <string>
  Note: all the "hdr*" matching criteria either apply to all headers, or to a
  particular header whose name is passed between parenthesis and without any
  space. The header name is not case-sensitive. The header matching complies
  with RFC2616, and treats as separate headers all values delimited by commas.

  The "hdr" criteria returns true if any of the headers matching the criteria
  match any of the strings. This can be used to check exact for values. For
  instance, checking that "connection: close" is set :

     hdr(Connection) -i close

hdr_beg <string>
hdr_beg(header) <string>
  Returns true when one of the headers begins with one of the strings. See
  "hdr" for more information on header matching.

hdr_end <string>
hdr_end(header) <string>
  Returns true when one of the headers ends with one of the strings. See "hdr"
  for more information on header matching.

hdr_sub <string>
hdr_sub(header) <string>
  Returns true when one of the headers contains one of the strings. See "hdr"
  for more information on header matching.

hdr_dir <string>
hdr_dir(header) <string>
  Returns true when one of the headers contains one of the strings either
  isolated or delimited by slashes. This is used to perform filename or
  directory name matching, and may be used with Referer. See "hdr" for more
  information on header matching.

hdr_dom <string>
hdr_dom(header) <string>
  Returns true when one of the headers contains one of the strings either
  isolated or delimited by dots. This is used to perform domain name matching,
  and may be used with the Host header. See "hdr" for more information on
  header matching.

hdr_reg <regex>
hdr_reg(header) <regex>
  Returns true when one of the headers matches of the regular expressions. It
  can be used at any time, but it is important to remember that regex matching
  is slower than other methods. See also other "hdr_" criteria, as well as
  "hdr" for more information on header matching.

hdr_val <integer>
hdr_val(header) <integer>
  Returns true when one of the headers starts with a number which matches the
  values or ranges specified. This may be used to limit content-length to
  acceptable values for example. See "hdr" for more information on header
  matching.

hdr_cnt <integer>
hdr_cnt(header) <integer>
  Returns true when the number of occurrence of the specified header matches
  the values or ranges specified. It is important to remember that one header
  line may count as several headers if it has several values. This is used to
  detect presence, absence or abuse of a specific header, as well as to block
  request smugling attacks by rejecting requests which contain more than one
  of certain headers. See "hdr" for more information on header matching.


2.3.6) Pre-defined ACLs
-----------------------

Some predefined ACLs are hard-coded so that they do not have to be declared in
every frontend which needs them. They all have their names in upper case in
order to avoid confusion. Their equivalence is provided below. Please note that
only the first three ones are not layer 7 based.

ACL name          Equivalent to                Usage
---------------+-----------------------------+---------------------------------
TRUE             always_true  1                always match
FALSE            always_false 0                never match
LOCALHOST        src 127.0.0.1/8               match connection from local host
HTTP_1.0         req_ver 1.0                   match HTTP version 1.0
HTTP_1.1         req_ver 1.1                   match HTTP version 1.1
METH_CONNECT     method  CONNECT               match HTTP CONNECT method
METH_GET         method  GET HEAD              match HTTP GET or HEAD method
METH_HEAD        method  HEAD                  match HTTP HEAD method
METH_OPTIONS     method  OPTIONS               match HTTP OPTIONS method
METH_POST        method  POST                  match HTTP POST method
METH_TRACE       method  TRACE                 match HTTP TRACE method
HTTP_URL_ABS     url_reg ^[^/:]*://            match absolute URL with scheme
HTTP_URL_SLASH   url_beg /                     match URL begining with "/"
HTTP_URL_STAR    url     *                     match URL equal to "*"
HTTP_CONTENT     hdr_val(content-length) gt 0  match an existing content-length
---------------+-----------------------------+---------------------------------


2.3.7) Using ACLs to form conditions
------------------------------------

Some actions are only performed upon a valid condition. A condition is a
combination of ACLs with operators. 3 operators are supported :

  - AND (implicit)
  - OR  (explicit with the "or" keyword or the "||" operator)
  - Negation with the exclamation mark ("!")

A condition is formed as a disjonctive form :

   [!]acl1 [!]acl2 ... [!]acln  { or [!]acl1 [!]acl2 ... [!]acln } ...

Such conditions are generally used after an "if" or "unless" statement,
indicating when the condition will trigger the action.

For instance, to block HTTP requests to the "*" URL with methods other than
"OPTIONS", as well as POST requests without content-length, and GET or HEAD
requests with a content-length greater than 0, and finally every request which
is not either GET/HEAD/POST/OPTIONS !

   acl missing_cl hdr_cnt(Content-length) eq 0
   block if HTTP_URL_STAR !METH_OPTIONS || METH_POST missing_cl
   block if METH_GET HTTP_CONTENT
   block unless METH_GET or METH_POST or METH_OPTIONS

To select a different backend for requests to static contents on the "www" site
and to every request on the "img", "video", "download" and "ftp" hosts :

   acl url_static  path_beg         /static /images /img /css
   acl url_static  path_end         .gif .png .jpg .css .js
   acl host_www    hdr_beg(host) -i www
   acl host_static hdr_beg(host) -i img. video. download. ftp.

   # now use backend "static" for all static-only hosts, and for static urls
   # of host "www". Use backend "www" for the rest.
   use_backend static if host_static or host_www url_static
   use_backend www    if host_www

See below for the detailed help on the "block" and "use_backend" keywords.


2.4) Server options
-------------------

slowstart <start_time_in_ms>
  The 'slowstart' parameter for a server accepts a value in milliseconds which
  indicates after how long a server which has just come back up will run at
  full speed. Just as with every other time-based parameter, it can be entered
  in any other explicit unit among { us, ms, s, m, h, d }. The speed grows
  linearly from 0 to 100% during this time. The limitation applies to two
  parameters :

  - maxconn: the number of connections accepted by the server will grow from 1
    to 100% of the usual dynamic limit defined by (minconn,maxconn,fullconn).

  - weight: when the backend uses a dynamic weighted algorithm, the weight
    grows linearly from 1 to 100%. In this case, the weight is updated at every
    health-check. For this reason, it is important that the 'inter' parameter
    is smaller than the 'slowstart', in order to maximize the number of steps.

  The slowstart never applies when haproxy starts, otherwise it would cause
  trouble to running servers. It only applies when a server has been previously
  seen as failed.


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