Apache HTTP Server Version 2.0
Description: | A collection of directives that are implemented by more than one multi-processing module (MPM) |
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Status: | MPM |
Description: | Method that Apache uses to serialize multiple children accepting requests on network sockets |
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Syntax: | AcceptMutex Default|method |
Default: | AcceptMutex Default |
Context: | server config |
Status: | MPM |
Module: | leader , perchild , prefork , threadpool , worker |
The AcceptMutex
directives sets the method that Apache uses to serialize multiple children accepting requests on network sockets. Prior to Apache 2.0, the method was selectable only at compile time. The optimal method to use is highly architecture and platform dependent. For further details, see the performance tuning documentation.
If this directive is set to Default
, then the compile-time selected default will be used. Other possible methods are listed below. Note that not all methods are available on all platforms. If a method is specified which is not available, a message will be written to the error log listing the available methods.
flock
flock(2)
system call to lock the file defined by the LockFile
directive.fcntl
fcntl(2)
system call to lock the file defined by the LockFile
directive.posixsem
pthread
sysvsem
If you want to find out the compile time chosen default for your system, you may set your LogLevel
to debug
. Then the default AcceptMutex
will be written into the ErrorLog
.
Description: | Define the non-privileged account on BS2000 machines |
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Syntax: | BS2000Account account |
Context: | server config |
Status: | MPM |
Module: | perchild , prefork |
Compatibility: | Only available for BS2000 machines |
The BS2000Account
directive is available for BS2000 hosts only. It must be used to define the account number for the non-privileged apache server user (which was configured using the User
directive). This is required by the BS2000 POSIX subsystem (to change the underlying BS2000 task environment by performing a sub-LOGON) to prevent CGI scripts from accessing resources of the privileged account which started the server, usually SYSROOT
.
Only one BS2000Account
directive can be used.
Description: | Directory where Apache attempts to switch before dumping core |
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Syntax: | CoreDumpDirectory directory |
Default: | See usage for the default setting |
Context: | server config |
Status: | MPM |
Module: | beos , leader , mpm_winnt , perchild , prefork , threadpool , worker |
This controls the directory to which Apache attempts to switch before dumping core. The default is in the ServerRoot
directory, however since this should not be writable by the user the server runs as, core dumps won't normally get written. If you want a core dump for debugging, you can use this directive to place it in a different location.
If Apache starts as root and switches to another user, the Linux kernel disables core dumps even if the directory is writable for the process. Apache (2.0.46 and later) reenables core dumps on Linux 2.4 and beyond, but only if you explicitly configure a CoreDumpDirectory
.
Description: | Enables a hook that runs exception handlers after a crash |
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Syntax: | EnableExceptionHook On|Off |
Default: | EnableExceptionHook Off |
Context: | server config |
Status: | MPM |
Module: | leader , perchild , prefork , threadpool , worker |
Compatibility: | Available in version 2.0.49 and later |
For safety reasons this directive is only available if the server was configured with the --enable-exception-hook
option. It enables a hook that allows external modules to plug in and do something after a child crashed.
There are already two modules, mod_whatkilledus
and mod_backtrace
that make use of this hook. Please have a look at Jeff Trawick's EnableExceptionHook site for more information about these.
Description: | Group under which the server will answer requests |
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Syntax: | Group unix-group |
Default: | Group #-1 |
Context: | server config |
Status: | MPM |
Module: | beos , leader , mpmt_os2 , perchild , prefork , threadpool , worker |
Compatibility: | Only valid in global server config since Apache 2.0 |
The Group
directive sets the group under which the server will answer requests. In order to use this directive, the server must be run initially as root
. If you start the server as a non-root user, it will fail to change to the specified group, and will instead continue to run as the group of the original user. Unix-group is one of:
#
followed by a group number.Group www-group
It is recommended that you set up a new group specifically for running the server. Some admins use user nobody
, but this is not always possible or desirable.
Don't set Group
(or User
) to root
unless you know exactly what you are doing, and what the dangers are.
Special note: Use of this directive in <VirtualHost>
is no longer supported. To configure your server for suexec
use SuexecUserGroup
.
Description: | IP addresses and ports that the server listens to |
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Syntax: | Listen [IP-address:]portnumber |
Context: | server config |
Status: | MPM |
Module: | beos , leader , mpm_netware , mpm_winnt , mpmt_os2 , perchild , prefork , threadpool , worker |
Compatibility: | Required directive since Apache 2.0 |
The Listen
directive instructs Apache to listen to only specific IP addresses or ports; by default it responds to requests on all IP interfaces. Listen
is now a required directive. If it is not in the config file, the server will fail to start. This is a change from previous versions of Apache.
The Listen
directive tells the server to accept incoming requests on the specified port or address-and-port combination. If only a port number is specified, the server listens to the given port on all interfaces. If an IP address is given as well as a port, the server will listen on the given port and interface.
Multiple Listen
directives may be used to specify a number of addresses and ports to listen to. The server will respond to requests from any of the listed addresses and ports.
For example, to make the server accept connections on both port 80 and port 8000, use:
Listen 80
Listen 8000
To make the server accept connections on two specified interfaces and port numbers, use
Listen 192.170.2.1:80
Listen 192.170.2.5:8000
IPv6 addresses must be surrounded in square brackets, as in the following example:
Listen [2001:db8::a00:20ff:fea7:ccea]:80
Listen
directives for the same ip address and port will result in an Address already in use
error message.
Description: | Maximum length of the queue of pending connections |
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Syntax: | ListenBacklog backlog |
Default: | ListenBacklog 511 |
Context: | server config |
Status: | MPM |
Module: | beos , leader , mpm_netware , mpm_winnt , mpmt_os2 , perchild , prefork , threadpool , worker |
The maximum length of the queue of pending connections. Generally no tuning is needed or desired, however on some systems it is desirable to increase this when under a TCP SYN flood attack. See the backlog parameter to the listen(2)
system call.
This will often be limited to a smaller number by the operating system. This varies from OS to OS. Also note that many OSes do not use exactly what is specified as the backlog, but use a number based on (but normally larger than) what is set.
Description: | Location of the accept serialization lock file |
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Syntax: | LockFile filename |
Default: | LockFile logs/accept.lock |
Context: | server config |
Status: | MPM |
Module: | leader , perchild , prefork , threadpool , worker |
The LockFile
directive sets the path to the lockfile used when Apache is used with an AcceptMutex
value of either fcntl
or flock
. This directive should normally be left at its default value. The main reason for changing it is if the logs
directory is NFS mounted, since the lockfile must be stored on a local disk. The PID of the main server process is automatically appended to the filename.
It is best to avoid putting this file in a world writable directory such as /var/tmp
because someone could create a denial of service attack and prevent the server from starting by creating a lockfile with the same name as the one the server will try to create.
Description: | Maximum number of child processes that will be created to serve requests |
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Syntax: | MaxClients number |
Default: | See usage for details |
Context: | server config |
Status: | MPM |
Module: | beos , leader , prefork , threadpool , worker |
The MaxClients
directive sets the limit on the number of simultaneous requests that will be served. Any connection attempts over the MaxClients
limit will normally be queued, up to a number based on the ListenBacklog
directive. Once a child process is freed at the end of a different request, the connection will then be serviced.
For non-threaded servers (i.e., prefork
), MaxClients
translates into the maximum number of child processes that will be launched to serve requests. The default value is 256
; to increase it, you must also raise ServerLimit
.
For threaded and hybrid servers (e.g. beos
or worker
) MaxClients
restricts the total number of threads that will be available to serve clients. The default value for beos
is 50
. For hybrid MPMs the default value is 16
(ServerLimit
) multiplied by the value of 25
(ThreadsPerChild
). Therefore, to increase MaxClients
to a value that requires more than 16 processes, you must also raise ServerLimit
.
Description: | Maximum amount of memory that the main allocator is allowed to hold without calling free() |
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Syntax: | MaxMemFree KBytes |
Default: | MaxMemFree 0 |
Context: | server config |
Status: | MPM |
Module: | beos , leader , mpm_netware , prefork , threadpool , worker , mpm_winnt |
The MaxMemFree
directive sets the maximum number of free Kbytes that the main allocator is allowed to hold without calling free()
. When not set, or when set to zero, the threshold will be set to unlimited.
Description: | Limit on the number of requests that an individual child server will handle during its life |
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Syntax: | MaxRequestsPerChild number |
Default: | MaxRequestsPerChild 10000 |
Context: | server config |
Status: | MPM |
Module: | leader , mpm_netware , mpm_winnt , mpmt_os2 , perchild , prefork , threadpool , worker |
The MaxRequestsPerChild
directive sets the limit on the number of requests that an individual child server process will handle. After MaxRequestsPerChild
requests, the child process will die. If MaxRequestsPerChild
is 0
, then the process will never expire.
The default value for mpm_netware
and mpm_winnt
is 0
.
Setting MaxRequestsPerChild
to a non-zero limit has two beneficial effects:
For KeepAlive
requests, only the first request is counted towards this limit. In effect, it changes the behavior to limit the number of connections per child.
Description: | Maximum number of idle threads |
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Syntax: | MaxSpareThreads number |
Default: | See usage for details |
Context: | server config |
Status: | MPM |
Module: | beos , leader , mpm_netware , mpmt_os2 , perchild , threadpool , worker |
Maximum number of idle threads. Different MPMs deal with this directive differently.
For perchild
the default is MaxSpareThreads 10
. This MPM monitors the number of idle threads on a per-child basis. If there are too many idle threads in that child, the server will begin to kill threads within that child.
For worker
, leader
and threadpool
the default is MaxSpareThreads 250
. These MPMs deal with idle threads on a server-wide basis. If there are too many idle threads in the server then child processes are killed until the number of idle threads is less than this number.
For mpm_netware
the default is MaxSpareThreads 100
. Since this MPM runs a single-process, the spare thread count is also server-wide.
beos
and mpmt_os2
work similar to mpm_netware
. The default for beos
is MaxSpareThreads 50
. For mpmt_os2
the default value is 10
.
The range of the MaxSpareThreads
value is restricted. Apache will correct the given value automatically according to the following rules:
perchild
requires MaxSpareThreads
to be less or equal than ThreadLimit
.mpm_netware
wants the value to be greater than MinSpareThreads
.leader
, threadpool
and worker
the value must be greater or equal than the sum of MinSpareThreads
and ThreadsPerChild
.Description: | Minimum number of idle threads available to handle request spikes |
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Syntax: | MinSpareThreads number |
Default: | See usage for details |
Context: | server config |
Status: | MPM |
Module: | beos , leader , mpm_netware , mpmt_os2 , perchild , threadpool , worker |
Minimum number of idle threads to handle request spikes. Different MPMs deal with this directive differently.
perchild
uses a default of MinSpareThreads 5
and monitors the number of idle threads on a per-child basis. If there aren't enough idle threads in that child, the server will begin to create new threads within that child. Thus, if you set NumServers
to 10
and a MinSpareThreads
value of 5
, you'll have at least 50 idle threads on your system.
worker
, leader
and threadpool
use a default of MinSpareThreads 75
and deal with idle threads on a server-wide basis. If there aren't enough idle threads in the server then child processes are created until the number of idle threads is greater than number.
mpm_netware
uses a default of MinSpareThreads 10
and, since it is a single-process MPM, tracks this on a server-wide bases.
beos
and mpmt_os2
work similar to mpm_netware
. The default for beos
is MinSpareThreads 1
. For mpmt_os2
the default value is 5
.
Description: | File where the server records the process ID of the daemon |
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Syntax: | PidFile filename |
Default: | PidFile logs/httpd.pid |
Context: | server config |
Status: | MPM |
Module: | beos , leader , mpm_winnt , mpmt_os2 , perchild , prefork , threadpool , worker |
The PidFile
directive sets the file to which the server records the process id of the daemon. If the filename is not absolute then it is assumed to be relative to the ServerRoot
.
PidFile /var/run/apache.pid
It is often useful to be able to send the server a signal, so that it closes and then re-opens its ErrorLog
and TransferLog
, and re-reads its configuration files. This is done by sending a SIGHUP (kill -1) signal to the process id listed in the PidFile
.
The PidFile
is subject to the same warnings about log file placement and security.
As of Apache 2 it is recommended to use only the apachectl
script for (re-)starting or stopping the server.
Description: | TCP receive buffer size |
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Syntax: | ReceiveBufferSize bytes |
Default: | ReceiveBufferSize 0 |
Context: | server config |
Status: | MPM |
Module: | beos , leader , mpm_netware , mpm_winnt , mpmt_os2 , perchild , prefork , threadpool , worker |
The server will set the TCP receive buffer size to the number of bytes specified.
If set to the value of 0
, the server will use the OS default.
Description: | Location of the file used to store coordination data for the child processes |
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Syntax: | ScoreBoardFile file-path |
Default: | ScoreBoardFile logs/apache_status |
Context: | server config |
Status: | MPM |
Module: | beos , leader , mpm_winnt , perchild , prefork , threadpool , worker |
Apache uses a scoreboard to communicate between its parent and child processes. Some architectures require a file to facilitate this communication. If the file is left unspecified, Apache first attempts to create the scoreboard entirely in memory (using anonymous shared memory) and, failing that, will attempt to create the file on disk (using file-based shared memory). Specifying this directive causes Apache to always create the file on the disk.
ScoreBoardFile /var/run/apache_status
File-based shared memory is useful for third-party applications that require direct access to the scoreboard.
If you use a ScoreBoardFile
then you may see improved speed by placing it on a RAM disk. But be careful that you heed the same warnings about log file placement and security.
Description: | TCP buffer size |
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Syntax: | SendBufferSize bytes |
Default: | SendBufferSize 0 |
Context: | server config |
Status: | MPM |
Module: | beos , leader , mpm_netware , mpm_winnt , mpmt_os2 , perchild , prefork , threadpool , worker |
The server will set the TCP send buffer size to the number of bytes specified. Very useful to increase past standard OS defaults on high speed high latency (i.e., 100ms or so, such as transcontinental fast pipes).
If set to the value of 0
, the server will use the OS default.
Description: | Upper limit on configurable number of processes |
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Syntax: | ServerLimit number |
Default: | See usage for details |
Context: | server config |
Status: | MPM |
Module: | leader , perchild , prefork , threadpool , worker |
For the prefork
MPM, this directive sets the maximum configured value for MaxClients
for the lifetime of the Apache process. For the worker
MPM, this directive in combination with ThreadLimit
sets the maximum configured value for MaxClients
for the lifetime of the Apache process. Any attempts to change this directive during a restart will be ignored, but MaxClients
can be modified during a restart.
Special care must be taken when using this directive. If ServerLimit
is set to a value much higher than necessary, extra, unused shared memory will be allocated. If both ServerLimit
and MaxClients
are set to values higher than the system can handle, Apache may not start or the system may become unstable.
With the prefork
MPM, use this directive only if you need to set MaxClients
higher than 256 (default). Do not set the value of this directive any higher than what you might want to set MaxClients
to.
With worker
, leader
and threadpool
use this directive only if your MaxClients
and ThreadsPerChild
settings require more than 16 server processes (default). Do not set the value of this directive any higher than the number of server processes required by what you may want for MaxClients
and ThreadsPerChild
.
With the perchild
MPM, use this directive only if you need to set NumServers
higher than 8 (default).
There is a hard limit of ServerLimit 20000
compiled into the server. This is intended to avoid nasty effects caused by typos.
Description: | Number of child server processes created at startup |
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Syntax: | StartServers number |
Default: | See usage for details |
Context: | server config |
Status: | MPM |
Module: | leader , mpmt_os2 , prefork , threadpool , worker |
The StartServers
directive sets the number of child server processes created on startup. As the number of processes is dynamically controlled depending on the load, there is usually little reason to adjust this parameter.
The default value differs from MPM to MPM. For leader
, threadpool
and worker
the default is StartServers 3
. For prefork
defaults to 5
and for mpmt_os2
to 2
.
Description: | Number of threads created on startup |
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Syntax: | StartThreads number |
Default: | See usage for details |
Context: | server config |
Status: | MPM |
Module: | beos , mpm_netware , perchild |
Number of threads created on startup. As the number of threads is dynamically controlled depending on the load, there is usually little reason to adjust this parameter.
For perchild
the default is StartThreads 5
and this directive tracks the number of threads per process at startup.
For mpm_netware
the default is StartThreads 50
and, since there is only a single process, this is the total number of threads created at startup to serve requests.
For beos
the default is StartThreads 10
. It also reflects the total number of threads created at startup to serve requests.
Description: | Sets the upper limit on the configurable number of threads per child process |
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Syntax: | ThreadLimit number |
Default: | See usage for details |
Context: | server config |
Status: | MPM |
Module: | leader , mpm_winnt , perchild , threadpool , worker |
Compatibility: | Available for mpm_winnt in Apache 2.0.41 and later |
This directive sets the maximum configured value for ThreadsPerChild
for the lifetime of the Apache process. Any attempts to change this directive during a restart will be ignored, but ThreadsPerChild
can be modified during a restart up to the value of this directive.
Special care must be taken when using this directive. If ThreadLimit
is set to a value much higher than ThreadsPerChild
, extra unused shared memory will be allocated. If both ThreadLimit
and ThreadsPerChild
are set to values higher than the system can handle, Apache may not start or the system may become unstable. Do not set the value of this directive any higher than your greatest predicted setting of ThreadsPerChild
for the current run of Apache.
The default value for ThreadLimit
is 1920
when used with mpm_winnt
and 64
when used with the others.
There is a hard limit of ThreadLimit 20000
(or ThreadLimit 15000
with mpm_winnt
) compiled into the server. This is intended to avoid nasty effects caused by typos.
Description: | Number of threads created by each child process |
---|---|
Syntax: | ThreadsPerChild number |
Default: | See usage for details |
Context: | server config |
Status: | MPM |
Module: | leader , mpm_winnt , threadpool , worker |
This directive sets the number of threads created by each child process. The child creates these threads at startup and never creates more. If using an MPM like mpm_winnt
, where there is only one child process, this number should be high enough to handle the entire load of the server. If using an MPM like worker
, where there are multiple child processes, the total number of threads should be high enough to handle the common load on the server.
The default value for ThreadsPerChild
is 64
when used with mpm_winnt
and 25
when used with the others.
Description: | The userid under which the server will answer requests |
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Syntax: | User unix-userid |
Default: | User #-1 |
Context: | server config |
Status: | MPM |
Module: | leader , perchild , prefork , threadpool , worker |
Compatibility: | Only valid in global server config since Apache 2.0 |
The User
directive sets the user ID as which the server will answer requests. In order to use this directive, the server must be run initially as root
. If you start the server as a non-root user, it will fail to change to the lesser privileged user, and will instead continue to run as that original user. If you do start the server as root
, then it is normal for the parent process to remain running as root. Unix-userid is one of:
The user should have no privileges that result in it being able to access files that are not intended to be visible to the outside world, and similarly, the user should not be able to execute code that is not meant for HTTP requests. It is recommended that you set up a new user and group specifically for running the server. Some admins use user nobody
, but this is not always desirable, since the nobody
user can have other uses on the system.
Don't set User
(or Group
) to root
unless you know exactly what you are doing, and what the dangers are.
With the perchild
MPM, which is intended to server virtual hosts run under different user IDs, the User
directive defines the user ID for the main server and the fallback for <VirtualHost>
sections without an AssignUserID
directive.
Special note: Use of this directive in <VirtualHost>
is no longer supported. To configure your server for suexec
use SuexecUserGroup
.