rsyslog

The rocket-fast system for log processing

imuxsock: Unix Socket Input

Module Name: imuxsock
Author: Rainer Gerhards <rgerhards@adiscon.com>

Purpose

This module provides the ability to accept syslog messages from applications running on the local system via Unix sockets. Most importantly, this is the mechanism by which the syslog(3) call delivers syslog messages to rsyslogd.

Configuration Parameters

Note

Parameter names are case-insensitive.

Global Parameters

Warning

When running under systemd, many “sysSock.” parameters are ignored. See parameter descriptions and the Coexistence with systemd section for details.

  • SysSock.IgnoreTimestamp [on/off] Ignore timestamps included in the messages, applies to messages received via the system log socket.

  • SysSock.IgnoreOwnMessages [on/off] (available since 7.3.7) Ignores messages that originated from the same instance of rsyslogd. There usually is no reason to receive messages from ourselves. This setting is vital when writing messages to the Linux journal. See omjournal module documentation for a more in-depth description.

  • SysSock.Use (imuxsock) [on/off] - Listen on the default local log socket (/dev/log) or, if provided, use the log socket value assigned to the SysSock.Name parameter instead of the default. This is most useful if you run multiple instances of rsyslogd where only one shall handle the system log socket. Unless disabled by the SysSock.Unlink setting, this socket is created upon rsyslog startup and deleted upon shutdown, according to traditional syslogd behavior.

    The behavior of this parameter is different for systemd systems. For those systems, SysSock.Use still needs to be enabled, but the value of SysSock.Name is ignored and the socket provided by systemd is used instead. If this parameter is not enabled, then imuxsock will only be of use if a custom input is configured.

    See the Coexistence with systemd section for details.

  • SysSock.Name <name-of-socket> - specifies an alternate log socket to be used instead of the default system log socket, traditionally /dev/log. Unless disabled by the SysSock.Unlink setting, this socket is created upon rsyslog startup and deleted upon shutdown, according to traditional syslogd behavior.

    The behavior of this parameter is different for systemd systems. See the the Coexistence with systemd section for details.

  • SysSock.FlowControl [on/off] - specifies if flow control should be applied to the system log socket.

  • SysSock.UsePIDFromSystem [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.

  • SysSock.RateLimit.Interval [number] - specifies the rate-limiting interval in seconds. Default value is 0, which turns off rate limiting. Set it to a number of seconds (5 recommended) to activate rate-limiting. The default of 0 has been chosen as people experienced problems with this feature activated by default. Now it needs an explicit opt-in by setting this parameter.

  • SysSock.RateLimit.Burst [number] - specifies the rate-limiting burst in number of messages. Default is 200.

  • SysSock.RateLimit.Severity [numerical severity] - specifies the severity of messages that shall be rate-limited.

  • SysSock.UseSysTimeStamp [on/off] the same as the input parameter UseSysTimeStamp, but for the system log socket. See description there.

  • SysSock.Annotate <on/off> turn on annotation/trusted properties for the system log socket.

  • SysSock.ParseTrusted <on/off> if Annotation is turned on, create JSON/lumberjack properties out of the trusted properties (which can be accessed via RainerScript JSON Variables, e.g. $!pid) instead of adding them to the message.

  • SysSock.Unlink <on/off> (available since 7.3.9) if turned on (default), the system socket is unlinked and re-created when opened and also unlinked when finally closed. Note that this setting has no effect when running under systemd control (because systemd handles the socket).

  • sysSock.useSpecialParser (available since 8.9.0) The equivalent of the useSpecialParser input parameter for the system socket.

  • sysSock.parseHostname (available since 8.9.0) The equivalent of the parseHostname input parameter for the system socket.

Input Parameters

  • ruleset [name] Binds specified ruleset to this input. If not set, the default ruleset is bound. (available since 8.17.0)
  • IgnoreTimestamp [on/off] Ignore timestamps included in messages received from the input being defined.
  • IgnoreOwnMessages [on/off] (available since 7.3.7) Ignore messages that originated from the same instance of rsyslogd. There usually is no reason to receive messages from ourselves. This setting is vital when writing messages to the Linux journal. See omjournal module documentation for a more in-depth description.
  • FlowControl [on/off] - specifies if flow control should be applied to the input being defined.
  • RateLimit.Interval [number] - specifies the rate-limiting interval in seconds. Default value is 0, which turns off rate limiting. Set it to a number of seconds (5 recommended) to activate rate-limiting. The default of 0 has been chosen as people experienced problems with this feature activated by default. Now it needs an explicit opt-in by setting this parameter.
  • RateLimit.Burst [number] - specifies the rate-limiting burst in number of messages. Default is 200.
  • RateLimit.Severity [numerical severity] - specifies the severity of messages that shall be rate-limited.
  • UsePIDFromSystem [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.
  • UseSysTimeStamp [on/off] instructs imuxsock to obtain message time from the system (via control messages) instead of using time recorded inside the message. This may be most useful in combination with systemd. Note: this option was introduced with version 5.9.1. Due to the usefulness of it, we decided to enable it by default. As such, 5.9.1 and above behave slightly different than previous versions. However, we do not see how this could negatively affect existing environments.
  • CreatePath [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). 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.
  • Socket <name-of-socket> adds additional unix socket, default none – former -a option
  • HostName <hostname> permits to override the hostname that shall be used inside messages taken from the input that is being defined.
  • Annotate <on/off> turn on annotation/trusted properties for the input that is being defined.
  • ParseTrusted <on/off> equivalent to the SysSock.ParseTrusted module parameter, but applies to the input that is being defined.
  • Unlink <on/off> (available since 7.3.9) if turned on (default), the socket is unlinked and re-created when opened and also unlinked when finally closed. Set it to off if you handle socket creation yourself. Note that handling socket creation oneself has the advantage that a limited amount of messages may be queued by the OS if rsyslog is not running.
  • useSpecialParser <on/off> (available since 8.9.0) If turned on (the default and the way it was up until 8.8.0) a special parser is used that parses the format that is usually used on the system log socket (the one syslog(3) creates). If set to “off”, the regular parser chain is used, in which case the format on the log socket can be arbitrary. Note that when the special parser is used, rsyslog is able to inject a more precise timestamp into the message (it is obtained from the log socket). If the regular parser chain is used, this is not possible.
  • parseHostname <on/off> (available since 8.9.0) Normally, the local log sockets do not contain hostnames. With this directive, the parser chain can be instructed to not expect them (setting “off”, the default). If set to on, parsers will expect hostnames just like in regular formats. Note: this option only has an effect if useSpecialParsers is set to “off”.

Input rate limiting

New in version 5.7.1.

rsyslog supports (optional) input rate limiting to guard against the problems of a wild running logging process. If more than SysSock.RateLimit.Interval * SysSock.RateLimit.Burst log messages are emitted from the same process, those messages with SysSock.RateLimit.Severity 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 once 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 RateLimit… 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.

Trusted (syslog) properties

New in version 5.9.4.

rsyslog can annotate messages from system log sockets (via imuxsock) with so-called Trusted syslog properties, (or just “Trusted Properties” for short). These are message properties not provided by the logging client application itself, but rather obtained from the system. As such, they can not be faked by the user application and are trusted in this sense. This feature is based on a similar idea introduced in systemd.

This feature requires a recent enough Linux Kernel and access to the /proc file system. In other words, this may not work on all platforms and may not work fully when privileges are dropped (depending on how they are dropped). Note that trusted properties can be very useful, but also typically cause the message to grow rather large. Also, the format of log messages is changed by adding the trusted properties at the end. For these reasons, the feature is not enabled by default. If you want to use it, you must turn it on (via SysSock.Annotate and Annotate).

Flow-control of Unix log sockets

If processing queues fill up, the unix socket reader is blocked for a short while to help prevent overrunning the queues. If the queues are overrun, this may cause excessive disk-io and impact performance.

While turning on flow control for many systems does not hurt, it can lead to a very unresponsive system and as such is disabled by default.

This means that log records are placed as quickly as possible into the processing queues. If you would like to have flow control, you need to enable it via the SysSock.FlowControl and FlowControl config directives. Just make sure you have thought about the implications and have tested the change on a non-production system first.

Control over application timestamps

Application 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 has done for many years. Alternate behaviour may be desirable if gateway-like processes send messages via the local log slot. In that case, it can be enabled via the SysSock.IgnoreTimestamp and IgnoreTimestamp config directives.

Coexistence with systemd

Rsyslog should by default be configured for systemd support on all platforms that usually run systemd (which means most Linux distributions, but not, for example, Solaris).

Rsyslog is able to coexist with systemd with minimal changes on the part of the local system administrator. While the systemd journal now assumes full control of the the local /dev/log system log socket, systemd provides access to logging data via the /run/systemd/journal/syslog log socket. This log socket is provided by the syslog.socket file that is shipped with systemd.

New in version 8.32.0: rsyslog emits an informational message noting the system log socket provided by systemd.

The imuxsock module can still be used in this setup and provides superior performance over imjournal, the alternative journal input module.

Note

It must be noted, however, that the journal tends to drop messages when it becomes busy instead of forwarding them to the system log socket. This is because the journal uses an async log socket interface for forwarding instead of the traditional synchronous one.

Handling of sockets

What follows is a brief description of the process rsyslog takes to determine what system socket to use, which sockets rsyslog should listen on, whether the sockets should be created and how rsyslog should handle the sockets when shutting down.

Step 1: Select name of system socket

  1. If the user has not explicitly chosen to set SysSock.Use="off" then the default listener socket (aka, “system log socket” or simply “system socket”) name is set to /dev/log. Otherwise, if the user has explicitly set SysSock.Use="off", then rsyslog will not listen on /dev/log OR any socket defined by the SysSock.Name parameter and the rest of this section does not apply.
  2. If the user has specified sysSock.Name="/path/to/custom/socket" (and not explicitly set SysSock.Use="off"), then the default listener socket name is overwritten with /path/to/custom/socket.
  3. Otherwise, if rsyslog is running under systemd AND /run/systemd/journal/syslog exists, (AND the user has not explicitly set SysSock.Use="off") then the default listener socket name is overwritten with /run/systemd/journal/syslog.

Step 2: Listen on specified sockets

Note

This is true for all sockets, be it system socket or not. But if SysSock.Use="off", the system socket will not be listened on.

rsyslog evaluates the list of sockets it has been asked to activate:

  • the system log socket (if still enabled after completion of the last section)
  • any custom inputs defined by the user

and then checks to see if it has been passed in via systemd (name is checked). If it was passed in via systemd, the socket is used as-is (e.g., not recreated upon rsyslog startup), otherwise if not passed in via systemd the log socket is unlinked, created and opened.

Step 3: Shutdown log sockets

Note

This is true for all sockets, be it system socket or not.

Upon shutdown, rsyslog processes each socket it is listening on and evaluates it. If the socket was originally passed in via systemd (name is checked), then rsyslog does nothing with the socket (systemd maintains the socket).

If the socket was not passed in via systemd AND the configuration permits rsyslog to do so (the default setting), rsyslog will unlink/remove the log socket. If not permitted to do so (the user specified otherwise), then rsyslog will not unlink the log socket and will leave that cleanup step to the user or application that created the socket to remove it.

Statistic Counter

This plugin maintains a global statistics with the following properties:

  • submitted - total number of messages submitted for processing since startup
  • ratelimit.discarded - number of messages discarded due to rate limiting
  • ratelimit.numratelimiters - number of currently active rate limiters (smal data structures used for the rate limiting logic)

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.
  • When running under systemd, many “sysSock.” parameters are ignored. See parameter descriptions and the Coexistence with systemd section for details.
  • On systems where systemd is used this module is often not loaded by default. See the Coexistence with systemd section for details.
  • Application timestamps are ignored by default. See the Control over application timestamps section for details.

Examples

Minimum setup

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

module(load="imuxsock")

This only needs to be done once.

Enable flow control

 module(load="imuxsock" # needs to be done just once
        SysSock.FlowControl="on") # enable flow control (use if needed)

Enable trusted properties

As noted in the Trusted (syslog) properties section, trusted properties are disabled by default. If you want to use them, you must turn the feature on via SysSock.Annotate for the system log socket and Annotate for inputs.

Append to end of message

The following sample is used to activate message annotation and thus trusted properties on the system log socket. These trusted properties are appended to the end of each message.

module(load="imuxsock" # needs to be done just once
       SysSock.Annotate="on")

Store in JSON message properties

The following sample is similiar to the first one, but enables parsing of trusted properties, which places the results into JSON/lumberjack variables.

module(load="imuxsock"
       SysSock.Annotate="on" SysSock.ParseTrusted="on")

Read log data from jails

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

module(load="imuxsock") # needs to be done just once
input(type="imuxsock"
      HostName="jail1.example.net"
      Socket="/jail/1/dev/log") input(type="imuxsock"
      HostName="jail2.example.net" Socket="/jail/2/dev/log")

Read from socket on temporary file system

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 daemon, it needs to create the socket directories, because it otherwise can not open the socket and thus not listen to openssh messages.

module(load="imuxsock") # needs to be done just once
input(type="imuxsock"
      Socket="/var/run/sshd/dev/log"
      CreatePath="on")

Disable rate limiting

The following sample is used to turn off input rate limiting on the system log socket.

module(load="imuxsock" # needs to be done just once
       SysSock.RateLimit.Interval="0") # turn off rate limiting