Unix Socket Input

Module Name: imuxsock

Author: Rainer Gerhards <rgerhards@adiscon.com>

Description:

Provides the ability to accept syslog messages via local Unix sockets. Most importantly, this is the mechanism by which the syslog(3) call delivers syslog messages to rsyslogd. So you need to have this module loaded to read the system log socket and be able to process log messages from applications running on the local system.

Application-provided timestamps are ignored by default. This is needed, as some programs (e.g. sshd) log with inconsistent timezone information, what messes up the local logs (which by default don't even contain time zone information). This seems to be consistent with what sysklogd did for the past four years. Alternate behaviour may be desirable if gateway-like processes send messages via the local log slot - in this case, it can be enabled via the $InputUnixListenSocketIgnoreMsgTimestamp and $SystemLogSocketIgnoreMsgTimestamp config directives

There is input rate limiting available, (since 5.7.1) to guard you against the problems of a wild running logging process. If more than $SystemLogRateLimitInterval * $SystemLogRateLimitBurst log messages are emitted from the same process, those messages with $SystemLogRateLimitSeverity or lower will be dropped. It is not possible to recover anything about these messages, but imuxsock will tell you how many it has dropped one the interval has expired AND the next message is logged. Rate-limiting depends on SCM_CREDENTIALS. If the platform does not support this socket option, rate limiting is turned off. If multiple sockets are configured, rate limiting works independently on each of them (that should be what you usually expect). The same functionality is available for additional log sockets, in which case the config statements just use the prefix $IMUXSockRateLimit... but otherwise works exactly the same. When working with severities, please keep in mind that higher severity numbers mean lower severity and configure things accordingly. To turn off rate limiting, set the interval to zero.

Unix log sockets can be flow-controlled. That is, if processing queues fill up, the unix socket reader is blocked for a short while. This may be useful to prevent overruning the queues (which may cause exessive disk-io where it actually would not be needed). However, flow-controlling a log socket (and especially the system log socket) can lead to a very unresponsive system. As such, flow control is disabled by default. That means any log records are places as quickly as possible into the processing queues. If you would like to have flow control, you need to enable it via the $SystemLogSocketFlowControl and $InputUnixListenSocketFlowControl config directives. Just make sure you thought about the implications. Note that for many systems, turning on flow control does not hurt.

Configuration Directives:

$InputUnixListenSocketIgnoreMsgTimestamp [on/off]
Ignore timestamps included in the message. Applies to the next socket being added.
$InputUnixListenSocketFlowControl [on/off] - specifies if flow control should be applied to the next socket.
$IMUXSockRateLimitInterval [number] - specifies the rate-limiting interval in seconds. Default value is 5 seconds. Set it to 0 to turn rate limiting off.
$IMUXSockRateLimitBurst [number] - specifies the rate-limiting burst in number of messages. Default is 200.
$IMUXSockRateLimitSeverity [numerical severity] - specifies the severity of messages that shall be rate-limited.
$InputUnixListenSocketUsePIDFromSystem [on/off] - specifies if the pid being logged shall be obtained from the log socket itself. If so, the TAG part of the message is rewritten. It is recommended to turn this option on, but the default is "off" to keep compatible with earlier versions of rsyslog. This option was introduced in 5.7.0.
$SystemLogSocketIgnoreMsgTimestamp [on/off]
Ignore timestamps included in the messages, applies to messages received via the system log socket.
$OmitLocalLogging (imuxsock) [on/off] -- former -o option
$SystemLogSocketName <name-of-socket> -- former -p option
$SystemLogFlowControl [on/off] - specifies if flow control should be applied to the system log socket.
$SystemLogUsePIDFromSystem [on/off] - specifies if the pid being logged shall be obtained from the log socket itself. If so, the TAG part of the message is rewritten. It is recommended to turn this option on, but the default is "off" to keep compatible with earlier versions of rsyslog. This option was introduced in 5.7.0.
$SystemLogRateLimitInterval [number] - specifies the rate-limiting interval in seconds. Default value is 5 seconds. Set it to 0 to turn rate limiting off.
$SystemLogRateLimitBurst [number] - specifies the rate-limiting burst in number of messages. Default is 200.
$SystemLogRateLimitSeverity [numerical severity] - specifies the severity of messages that shall be rate-limited.
$InputUnixListenSocketCreatePath [on/off] - create directories in the socket path if they do not already exist. They are created with 0755 permissions with the owner being the process under which rsyslogd runs. The default is not to create directories. Keep in mind, though, that rsyslogd always creates the socket itself if it does not exist (just not the directories by default).
Note that this statement affects the next $AddUnixListenSocket directive that follows in sequence in the configuration file. It never works on the system log socket (where it is deemed unnecessary). Also note that it is automatically being reset to "off" after the $AddUnixListenSocket directive, so if you would have it active for two additional listen sockets, you need to specify it in front of each one. This option is primarily considered useful for defining additional sockets that reside on non-permanent file systems. As rsyslogd probably starts up before the daemons that create these sockets, it is a vehicle to enable rsyslogd to listen to those sockets even though their directories do not yet exist. [available since 4.7.0 and 5.3.0]
$AddUnixListenSocket <name-of-socket> adds additional unix socket, default none -- former -a option
$InputUnixListenSocketHostName <hostname> permits to override the hostname that shall be used inside messages taken from the next $AddUnixListenSocket socket. Note that the hostname must be specified before the $AddUnixListenSocket configuration directive, and it will only affect the next one and then automatically be reset. This functionality is provided so that the local hostname can be overridden in cases where that is desired.

Caveats/Known Bugs:

There is a compile-time limit of 50 concurrent sockets. If you need more, you need to change the array size in imuxsock.c.
This documentation is sparse and incomplete.

Sample:

The following sample is the minimum setup required to accept syslog messages from applications running on the local system.

The following sample is a configuration where rsyslogd pulls logs from two jails, and assigns different hostnames to each of the jails:

The following sample is a configuration where rsyslogd reads the openssh log messages via a separate socket, but this socket is created on a temporary file system. As rsyslogd starts up before the sshd, it needs to create the socket directories, because it otherwise can not open the socket and thus not listen to openssh messages. Note that it is vital not to place any other socket between the $InputUnixListenSocketCreatePath and the $InputUnixListenSocketHostName.

The following sample is used to turn off input rate limiting on the system log socket. $ModLoad imuxsock # needs to be done just once $SystemLogRateLimitInterval 0 # turn off rate limiting

[rsyslog.conf overview] [manual index] [rsyslog site]