Apache HTTP Server Version 2.5
Description: | A collection of directives that are implemented by more than one multi-processing module (MPM) |
---|---|
Status: | MPM |
Description: | Treat some errors accepting a new connection as non-fatal to the httpd process. |
---|---|
Syntax: | AcceptErrorsNonFatal ON |
Default: | OFF (ECONNREFUSED, ECONNABORTED, ECONNRESET cause the process to
exit) |
Context: | server config |
Status: | MPM |
Module: | event , worker , prefork |
Compatibility: | 2.5.1 and later |
The AcceptErrorsNonFatal
alters the servers
behavior under some rare errors that may occur while accepting a new
client connection. By default, the child process handling a request
will gracefully exit when nearly any socket error occurs during the
accept() system call. This is to ensure a potentially unhealthy
child process does not try to take on more new connections.
With AcceptErrorsNonFatal
set to "ON",
the process will not begin to exit if the accept() error is
ECONNREFUSED, ECONNABORTED, or ECONNRESET.
Description: | Directory where Apache HTTP Server attempts to switch before dumping core |
---|---|
Syntax: | CoreDumpDirectory directory |
Default: | See usage for the default setting |
Context: | server config |
Status: | MPM |
Module: | event , worker , prefork |
This controls the directory to which Apache httpd attempts to
switch before dumping core. If your operating system is configured to
create core files in the working directory of the crashing process,
CoreDumpDirectory
is necessary to change working
directory from the default ServerRoot
directory, which should not be writable by the user the server runs as.
If you want a core dump for debugging, you can use this directive to place it in a different location. This directive has no effect if your operating system is not configured to write core files to the working directory of the crashing processes.
Using this directive on Linux may allow other processes on
the system (if running with similar privileges, such as CGI
scripts) to attach to httpd children via the ptrace
system call. This may make weaken the protection from certain
security attacks. It is not recommended to use this directive
on production systems.
If Apache httpd starts as root and switches to another user, the
Linux kernel disables core dumps even if the directory is
writable for the process. Apache httpd (2.0.46 and later) reenables core dumps
on Linux 2.4 and beyond, but only if you explicitly configure a CoreDumpDirectory
.
To enable core-dumping of suid-executables on BSD-systems (such
as FreeBSD), set kern.sugid_coredump
to 1.
CoreDumpDirectory
processing only occurs for
a select set of fatal signals: SIGFPE, SIGILL, SIGABORT,
SIGSEGV, and SIGBUS.
On some operating systems, SIGQUIT also results in a core dump but
does not go through CoreDumpDirectory
or
EnableExceptionHook
processing, so the core
location is dictated entirely by the operating system.
Description: | Enables a hook that runs exception handlers after a crash |
---|---|
Syntax: | EnableExceptionHook On|Off |
Default: | EnableExceptionHook Off |
Context: | server config |
Status: | MPM |
Module: | event , worker , prefork |
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: | Specify a timeout after which a gracefully shutdown server will exit. |
---|---|
Syntax: | GracefulShutdownTimeout seconds |
Default: | GracefulShutdownTimeout 0 |
Context: | server config |
Status: | MPM |
Module: | event , worker , prefork |
The GracefulShutdownTimeout
specifies
how many seconds after receiving a "graceful-stop" signal, a
server should continue to run, handling the existing connections.
Setting this value to zero means that the server will wait indefinitely until all remaining requests have been fully served.
Description: | IP addresses and ports that the server listens to |
---|---|
Syntax: | Listen [IP-address:]portnumber [protocol] |
Context: | server config |
Status: | MPM |
Module: | event , worker , prefork , mpm_winnt , mpm_netware , mpmt_os2 |
The Listen
directive instructs Apache httpd 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 httpd.
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
The optional protocol argument is not required for most
configurations. If not specified, https
is the default for
port 443 and http
the default for all other ports. The
protocol is used to determine which module should handle a request, and
to apply protocol specific optimizations with the
AcceptFilter
directive.
You only need to set the protocol if you are running on non-standard
ports. For example, running an https
site on port 8443:
Listen 192.170.2.1:8443 https
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 |
---|---|
Syntax: | ListenBackLog backlog |
Default: | ListenBackLog 511 |
Context: | server config |
Status: | MPM |
Module: | event , worker , prefork , mpm_winnt , mpm_netware , mpmt_os2 |
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: | Ratio between the number of CPU cores (online) and the number of listeners' buckets |
---|---|
Syntax: | ListenCoresBucketsRatio ratio |
Default: | ListenCoresBucketsRatio 0 (disabled) |
Context: | server config |
Status: | MPM |
Module: | event , worker , prefork |
Compatibility: | Available in Apache HTTP Server 2.4.17, with a kernel supporting
the socket option SO_REUSEPORT and distributing new connections
evenly across listening processes' (or threads') sockets using it (eg. Linux
3.9 and later, but not the current implementations of SO_REUSEPORT
in *BSDs. |
A ratio between the number of (online) CPU cores and the
number of listeners' buckets can be used to make Apache HTTP Server create
num_cpu_cores / ratio
listening buckets, each containing its
own Listen
-ing socket(s) on the same port(s), and
then make each child handle a single bucket (with round-robin distribution
of the buckets at children creation time).
On Linux (and also BSD) a CPU core can be turned on/off if
Hotplug
is configured, therefore ListenCoresBucketsRatio
needs to
take this parameter into account while calculating the number of buckets to create.
ListenCoresBucketsRatio
can improve the
scalability when accepting new connections is/becomes the bottleneck.
On systems with a large number of CPU cores, enabling this feature has
been tested to show significant performances improvement and shorter
responses time.
There must be at least twice the number of CPU cores than the
configured ratio for this to be active. The recommended
ratio is 8
, hence at least 16
cores should be available at runtime when this value is used.
The right ratio to obtain maximum performance needs to be calculated
for each target system, testing multiple values and observing the variations in your
key performance metrics.
This directive influences the calculation of the
MinSpareThreads
and
MaxSpareThreads
lower bound values.
The number of children processes needs to be a multiple of the number
of buckets to optimally accept connections.
Listen
ers or Apache HTTP servers on
the same IP address and portSetting the SO_REUSEPORT
option on the listening socket(s)
consequently allows multiple processes (sharing the same EUID
,
e.g. root
) to bind to the the same IP address and port,
without the binding error raised by the system in the usual case.
This also means that multiple instances of Apache httpd configured on a
same IP:port
and with a positive ListenCoresBucketsRatio
would start without an error too, and then run with incoming connections
evenly distributed accross both instances (this is NOT a recommendation or
a sensible usage in any case, but just a notice that it would prevent such
possible issues to be detected).
Within the same instance, Apache httpd will check and fail to start if
multiple Listen
directives on the exact same IP (or
hostname) and port are configured, thus avoiding the creation of some
duplicated buckets which would be useless and kill performances. However
it can't (and won't try harder to) catch all the possible overlapping cases
(like a hostname resolving to an IP used elsewhere).
Description: | Limit on the number of connections that an individual child server will handle during its life |
---|---|
Syntax: | MaxConnectionsPerChild number |
Default: | MaxConnectionsPerChild 0 |
Context: | server config |
Status: | MPM |
Module: | event , worker , prefork , mpm_winnt , mpm_netware , mpmt_os2 |
Compatibility: | Available Apache HTTP Server 2.3.9 and later. The old name
MaxRequestsPerChild is still supported. |
The MaxConnectionsPerChild
directive sets
the limit on the number of connections that an individual child
server process will handle. After
MaxConnectionsPerChild
connections, the child
process will die. If MaxConnectionsPerChild
is
0
, then the process will never expire.
Setting MaxConnectionsPerChild
to a
non-zero value limits the amount of memory that process can consume
by (accidental) memory leakage.
Description: | Maximum amount of memory that the main allocator is allowed
to hold without calling free() |
---|---|
Syntax: | MaxMemFree KBytes |
Default: | MaxMemFree 2048 |
Context: | server config |
Status: | MPM |
Module: | event , worker , prefork , mpm_winnt , mpm_netware |
The MaxMemFree
directive sets the
maximum number of free Kbytes that every allocator is allowed
to hold without calling free()
. In threaded MPMs, every
thread has its own allocator. When set
to zero, the threshold will be set to unlimited.
Description: | Maximum number of connections that will be processed simultaneously |
---|---|
Syntax: | MaxRequestWorkers number |
Default: | See usage for details |
Context: | server config |
Status: | MPM |
Module: | event , worker , prefork |
The MaxRequestWorkers
directive sets the limit
on the number of simultaneous requests that will be served. Any
connection attempts over the MaxRequestWorkers
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
),
MaxRequestWorkers
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. event
or worker
) MaxRequestWorkers
restricts
the total number of threads that will be available to serve clients.
For hybrid MPMs the default value is 16
(ServerLimit
) multiplied by the value of
25
(ThreadsPerChild
). Therefore, to increase MaxRequestWorkers
to a value that requires more than 16 processes,
you must also raise ServerLimit
.
MaxRequestWorkers
was called
MaxClients
before version 2.3.13. The old name is still
supported.
Description: | Maximum number of idle threads |
---|---|
Syntax: | MaxSpareThreads number |
Default: | See usage for details |
Context: | server config |
Status: | MPM |
Module: | event , worker , mpm_netware , mpmt_os2 |
Maximum number of idle threads. Different MPMs deal with this directive differently.
For worker
and event
, 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. Additional processes/threads
might be created if ListenCoresBucketsRatio
is enabled.
For mpm_netware
the default is
MaxSpareThreads 100
. Since this MPM runs a
single-process, the spare thread count is also server-wide.
mpmt_os2
works
similar to mpm_netware
. For
mpmt_os2
the default value is 10
.
The range of the MaxSpareThreads
value
is restricted. Apache httpd will correct the given value automatically
according to the following rules:
mpm_netware
wants the value to be greater than
MinSpareThreads
.worker
and event
, the value
must be greater or equal to the sum of
MinSpareThreads
and
ThreadsPerChild
.Description: | Minimum number of idle threads available to handle request spikes |
---|---|
Syntax: | MinSpareThreads number |
Default: | See usage for details |
Context: | server config |
Status: | MPM |
Module: | event , worker , mpm_netware , mpmt_os2 |
Minimum number of idle threads to handle request spikes. Different MPMs deal with this directive differently.
worker
and event
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. Additional processes/threads
might be created if ListenCoresBucketsRatio
is enabled.
mpm_netware
uses a default of
MinSpareThreads 10
and, since it is a single-process
MPM, tracks this on a server-wide bases.
mpmt_os2
works
similar to mpm_netware
. For
mpmt_os2
the default value is 5
.
Description: | File where the server records the process ID of the daemon |
---|---|
Syntax: | PidFile filename |
Default: | PidFile httpd.pid |
Context: | server config |
Status: | MPM |
Module: | event , worker , prefork , mpm_winnt , mpmt_os2 |
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
DefaultRuntimeDir
.
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 HTTP Server 2, we recommended that you only use the apachectl
script, or the init script that your OS provides,
for (re-)starting or stopping the server.
Description: | TCP receive buffer size |
---|---|
Syntax: | ReceiveBufferSize bytes |
Default: | ReceiveBufferSize 0 |
Context: | server config |
Status: | MPM |
Module: | event , worker , prefork , mpm_winnt , mpm_netware , mpmt_os2 |
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 |
---|---|
Syntax: | ScoreBoardFile file-path |
Default: | ScoreBoardFile apache_runtime_status |
Context: | server config |
Status: | MPM |
Module: | event , worker , prefork , mpm_winnt |
Apache HTTP Server 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 httpd 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 httpd to always create the file on the disk.
If file-path is not an absolute path, the location specified
will be relative to the value of
DefaultRuntimeDir
.
ScoreBoardFile /var/run/apache_runtime_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 |
---|---|
Syntax: | SendBufferSize bytes |
Default: | SendBufferSize 0 |
Context: | server config |
Status: | MPM |
Module: | event , worker , prefork , mpm_winnt , mpm_netware , mpmt_os2 |
Sets the server's TCP send buffer size to the number of bytes specified. It is often useful to set this past the OS's standard default value on high speed, high latency connections (i.e., 100ms or so, such as transcontinental fast pipes).
If set to the value of 0
, the server will use the
default value provided by your OS.
Further configuration of your operating system may be required to elicit better performance on high speed, high latency connections.
On some operating systems, changes in TCP behavior resulting
from a larger SendBufferSize
may not be seen unless
EnableSendfile
is set to OFF. This
interaction applies only to static files.
Description: | Upper limit on configurable number of processes |
---|---|
Syntax: | ServerLimit number |
Default: | See usage for details |
Context: | server config |
Status: | MPM |
Module: | event , worker , prefork |
For the prefork
MPM, this directive sets the
maximum configured value for MaxRequestWorkers
for the lifetime of the
Apache httpd process. For the worker
and event
MPMs, this directive in combination with ThreadLimit
sets
the maximum configured value for MaxRequestWorkers
for the lifetime of the
Apache httpd process. For the event
MPM, this directive
also defines how many old server processes may keep running and finish processing
open connections.
Any attempts to change this directive during a restart will be ignored, but
MaxRequestWorkers
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 MaxRequestWorkers
are set to values
higher than the system can handle, Apache httpd may not start or the
system may become unstable.
With the prefork
MPM, use this directive only
if you need to set MaxRequestWorkers
higher than 256 (default).
Do not set the value of this directive any higher than what you
might want to set MaxRequestWorkers
to.
With worker
, use this directive only if your
MaxRequestWorkers
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 MaxRequestWorkers
and ThreadsPerChild
.
With event
, increase this directive if the process
number defined by your MaxRequestWorkers
and ThreadsPerChild
settings, plus the
number of gracefully shutting down processes, is more than 16 server
processes (default).
There is a hard limit of ServerLimit 20000
compiled
into the server (for the prefork
MPM 200000). This is
intended to avoid nasty effects caused by typos. To increase it
even further past this limit, you will need to modify the value of
MAX_SERVER_LIMIT in the mpm source file and rebuild the server.
Description: | Number of child server processes created at startup |
---|---|
Syntax: | StartServers number |
Default: | See usage for details |
Context: | server config |
Status: | MPM |
Module: | event , worker , prefork , mpmt_os2 |
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, (see
MinSpareThreads
,
MaxSpareThreads
,
MinSpareServers
, MaxSpareServers
)
there is usually little reason to adjust this parameter.
The default value differs from MPM to MPM. worker
and
event
default to StartServers 3
;
prefork
defaults to 5
; mpmt_os2
defaults to 2
.
Description: | Number of threads created on startup |
---|---|
Syntax: | StartThreads number |
Default: | See usage for details |
Context: | server config |
Status: | MPM |
Module: | mpm_netware |
Number of threads created on startup. As the
number of threads is dynamically controlled depending on the
load, (see
MinSpareThreads
,
MaxSpareThreads
,
MinSpareServers
, MaxSpareServers
)
there is usually little reason to adjust this
parameter.
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.
Description: | Sets the upper limit on the configurable number of threads per child process |
---|---|
Syntax: | ThreadLimit number |
Default: | See usage for details |
Context: | server config |
Status: | MPM |
Module: | event , worker , mpm_winnt |
This directive sets the maximum configured value for ThreadsPerChild
for the lifetime
of the Apache httpd 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 httpd 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 httpd.
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 100000
with event
,
ThreadLimit 15000
with mpm_winnt
)
compiled into the server. This is intended to avoid nasty effects
caused by typos. To increase it even further past this limit, you
will need to modify the value of MAX_THREAD_LIMIT in the mpm
source file and rebuild the server.
Description: | Number of threads created by each child process |
---|---|
Syntax: | ThreadsPerChild number |
Default: | See usage for details |
Context: | server config |
Status: | MPM |
Module: | event , worker , mpm_winnt |
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.
The value of ThreadsPerChild
can not exceed the
value of ThreadLimit
. If a
higher value is configured, it will be automatically reduced at start-up
and a warning will be logged. The relationship between these 2 directives
is explained in ThreadLimit
.
Description: | The size in bytes of the stack used by threads handling client connections |
---|---|
Syntax: | ThreadStackSize size |
Default: | 65536 on NetWare; varies on other operating systems |
Context: | server config |
Status: | MPM |
Module: | event , worker , mpm_winnt , mpm_netware , mpmt_os2 |
The ThreadStackSize
directive sets the
size of the stack (for autodata) of threads which handle client
connections and call modules to help process those connections.
In most cases the operating system default for stack size is
reasonable, but there are some conditions where it may need to be
adjusted:
ThreadStackSize
to a
value higher than the operating system default. This type of
adjustment is necessary only if the provider of the third-party
module specifies that it is required, or if diagnosis of an Apache httpd
crash indicates that the thread stack size was too small.ThreadStackSize
is
set to a value lower than the operating system default. This type
of adjustment should only be made in a test environment which allows
the full set of web server processing can be exercised, as there
may be infrequent requests which require more stack to process.
The minimum required stack size strongly depends on the modules
used, but any change in the web server configuration can invalidate
the current ThreadStackSize
setting.ulimit -s
(8MB if unlimited) is used as the default stack
size.ThreadStackSize
unless a high number of threads per child process is needed. On some
platforms (including Linux), a setting of 128000 is already too low and
causes crashes with some common modules.