omfile: File Output Module

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

The omfile plug-in provides the core functionality of writing messages to files residing inside the local file system (which may actually be remote if methods like NFS are used). Both files named with static names as well files with names based on message content are supported by this module.

Configuration Parameters

Omfile is a built-in module that does not need to be loaded. In order to specify module parameters, use

module(load="builtin:omfile" ...parameters...)

Note that legacy directives do not affect new-style RainerScript configuration objects. See basic configuration structure doc to learn about different configuration languages in use by rsyslog.

General Notes

As can be seen in the parameters below, owner and groups can be set either by name or by direct id (uid, gid). While using a name is more convenient, using the id is more robust. There may be some situations where the OS is not able to do the name-to-id resolution, and these cases the owner information will be set to the process default. This seems to be uncommon and depends on the authentication provider and service start order. In general, using names is fine.

Module Parameters

template [templateName]

Default: RSYSLOG_FileFormat

Set the default template to be used if an action is not configured to use a specific template.

dirCreateMode [octalNumber]

Default: 0700

Sets the default dirCreateMode to be used for an action if no explicit one is specified.

fileCreateMode [default 0644] [octalNumber]

Default: 0644

Sets the default fileCreateMode to be used for an action if no explicit one is specified.

fileOwner [userName]

Default: process user

Sets the default fileOwner to be used for an action if no explicit one is specified.

fileOwnerNum [uid]

Default: process user

Sets the default fileOwnerNum to be used for an action if no explicit one is specified.

fileGroup [groupName]

Default: process user’s primary group

Sets the default fileGroup to be used for an action if no explicit one is specified.

fileGroupNum [gid]

Default: process user’s primary group

Sets the default fileGroupNum to be used for an action if no explicit one is specified.

dirOwner [userName]

Default: process user

Sets the default dirOwner to be used for an action if no explicit one is specified.

dirOwnerNum [uid]

Default: process user

Sets the default dirOwnerNum to be used for an action if no explicit one is specified.

dirGroup [groupName]

Default: process user’s primary group

Sets the default dirGroup to be used for an action if no explicit one is specified.

dirGroupNum [gid]

Default: process user’s primary group

Sets the default dirGroupNum to be used for an action if no explicit one is specified.

Action Parameters

Note that one of the parameters file or dynaFile must be specified. This selects whether a static or dynamic file (name) shall be written to.

file [fileName]

Default: none

This creates a static file output, always writing into the same file. If the file already exists, new data is appended to it. Existing data is not truncated. If the file does not already exist, it is created. Files are kept open as long as rsyslogd is active. This conflicts with external log file rotation. In order to close a file after rotation, send rsyslogd a HUP signal after the file has been rotated away.

dynaFile [templateName]

Default: none

For each message, the file name is generated based on the given template. Then, this file is opened. As with the file property, data is appended if the file already exists. If the file does not exist, a new file is created. The template given in “templateName” is just a regular rsyslog template, so all you have full control over how to format the file name.

A cache of recent files is kept. Note that this cache can consume quite some memory (especially if large buffer sizes are used). Files are kept open as long as they stay inside the cache. Files are removed from the cache when a HUP signal is sent, the closeTimeout occurs, or the cache runs out of space, in which case the least recently used entry is evicted.

template [templateName]

Default: template set via “template” module parameter

Sets the template to be used for this action.

closeTimeout [minutes]

Default: for static files: 0; for dynamic files: 10

Available since: 8.3.3

Specifies after how many minutes of inactivity a file is automatically closed. Note that this functionality is implemented based on the janitor process. See its doc to understand why and how janitor-based times are approximate.

dynaFileCacheSize [size]

Default: 10

Applies only if dynamic filenames are used. Specifies the number of DynaFiles that will be kept open. Note that this is a per-action value, so if you have multiple dynafile actions, each of them have their individual caches (which means the numbers sum up). Ideally, the cache size exactly matches the need. You can use impstats to tune this value. Note that a too-low cache size can be a very considerable performance bottleneck.

zipLevel [level]

Default: 0

if greater 0, turns on gzip compression of the output file. The higher the number, the better the compression, but also the more CPU is required for zipping.

veryRobustZip [switch]

Default: on

Available since: 7.3.0

if zipLevel is greater 0, then this setting controls if extra headers are written to make the resulting file extra hardened against malfunction. If set to off, data appended to previously unclean closed files may not be accessible without extra tools. Note that this risk is usually expected to be bearable, and thus “off” is the default mode. The extra headers considerably degrade compression, files with this option set to “on” may be four to five times as large as files processed in “off” mode.

flushInterval [interval]

Default: 1

Defines, in seconds, the interval after which unwritten data is flushed.

asyncWriting [switch]

Default: off

if turned on, the files will be written in asynchronous mode via a separate thread. In that case, double buffers will be used so that one buffer can be filled while the other buffer is being written. Note that in order to enable FlushInterval, AsyncWriting must be set to “on”. Otherwise, the flush interval will be ignored. Also note that when FlushOnTXEnd is “on” but AsyncWriting is off, output will only be written when the buffer is full. This may take several hours, or even require a rsyslog shutdown. However, a buffer flush can be forced in that case by sending rsyslogd a HUP signal.

flushOnTXEnd [switch]

Default: on

Omfile has the capability to write output using a buffered writer. Disk writes are only done when the buffer is full. So if an error happens during that write, data is potentially lost. In cases where this is unacceptable, set FlushOnTXEnd to on. Then, data is written at the end of each transaction (for pre-v5 this means after each log message) and the usual error recovery thus can handle write errors without data loss. Note that this option severely reduces the effect of zip compression and should be switched to off for that use case. Note that the default -on- is primarily an aid to preserve the traditional syslogd behaviour.

ioBufferSize [size]

Default: 4 KiB

size of the buffer used to writing output data. The larger the buffer, the potentially better performance is. The default of 4k is quite conservative, it is useful to go up to 64k, and 128K if you used gzip compression (then, even higher sizes may make sense)

dirOwner [userName]

Default: system default

Set the file owner for directories newly created. Please note that this setting does not affect the owner of directories already existing. The parameter is a user name, for which the userid is obtained by rsyslogd during startup processing. Interim changes to the user mapping are not detected.

dirOwnerNum [uid]

Default: system default

Available since: 7.5.8, 8.1.4

Set the file owner for directories newly created. Please note that this setting does not affect the owner of directories already existing. The parameter is a numerical ID, which is used regardless of whether the user actually exists. This can be useful if the user mapping is not available to rsyslog during startup.

dirGroup [groupName]

Default: system default

Set the group for directories newly created. Please note that this setting does not affect the group of directories already existing. The parameter is a group name, for which the groupid is obtained by rsyslogd on during startup processing. Interim changes to the user mapping are not detected.

dirGroupNum [gid]

Default: system default

Set the group for directories newly created. Please note that this setting does not affect the group of directories already existing. The parameter is a numerical ID, which is used regardless of whether the group actually exists. This can be useful if the group mapping is not available to rsyslog during startup.

fileOwner [userName]

Default: system default

Set the file owner for files newly created. Please note that this setting does not affect the owner of files already existing. The parameter is a user name, for which the userid is obtained by rsyslogd during startup processing. Interim changes to the user mapping are not detected.

fileOwnerNum [uid]

Default: system default

Available since: 7.5.8, 8.1.4

Set the file owner for files newly created. Please note that this setting does not affect the owner of files already existing. The parameter is a numerical ID, which which is used regardless of whether the user actually exists. This can be useful if the user mapping is not available to rsyslog during startup.

fileGroup [groupName]

Default: system default

Set the group for files newly created. Please note that this setting does not affect the group of files already existing. The parameter is a group name, for which the groupid is obtained by rsyslogd during startup processing. Interim changes to the user mapping are not detected.

fileGroupNum [gid]

Default: system default

Available since: 7.5.8, 8.1.4

Set the group for files newly created. Please note that this setting does not affect the group of files already existing. The parameter is a numerical ID, which is used regardless of whether the group actually exists. This can be useful if the group mapping is not available to rsyslog during startup.

fileCreateMode [octalNumber]

Default: equally-named module parameter

The FileCreateMode directive allows to specify the creation mode with which rsyslogd creates new files. If not specified, the value 0644 is used (which retains backward-compatibility with earlier releases). The value given must always be a 4-digit octal number, with the initial digit being zero. Please note that the actual permission depend on rsyslogd’s process umask. If in doubt, use “$umask 0000” right at the beginning of the configuration file to remove any restrictions.

dirCreateMode [octalNumber]

Default: equally-named module parameter

This is the same as FileCreateMode, but for directories automatically generated.

failOnChOwnFailure [switch]

Default: on

This option modifies behaviour of file creation. If different owners or groups are specified for new files or directories and rsyslogd fails to set these new owners or groups, it will log an error and NOT write to the file in question if that option is set to “on”. If it is set to “off”, the error will be ignored and processing continues. Keep in mind, that the files in this case may be (in)accessible by people who should not have permission. The default is “on”.

createDirs [switch]

Default: on

create directories on an as-needed basis

sync [switch]

Default: off

enables file syncing capability of omfile.

When enabled, rsyslog does a sync to the data file as well as the directory it resides after processing each batch. There currently is no way to sync only after each n-th batch.

Enabling sync causes a severe performance hit. Actually, it slows omfile so much down, that the probability of loosing messages increases. In short, you should enable syncing only if you know exactly what you do, and fully understand how the rest of the engine works, and have tuned the rest of the engine to lossless operations.

sig.provider [providerName]

Default: no signature provider

Selects a signature provider for log signing. By selecting a provider, the signature feature is turned on.

Currently, there are two providers available “gt” and “ksi”.

cry.provider [providerName]

Default: no crypto provider

Selects a crypto provider for log encryption. By selecting a provider, the encryption feature is turned on.

Currently, there only is one provider called “gcry”.

Caveats/Known Bugs

  • people often report problems that dynafiles are not properly created. The common cause for this problem is SELinux rules, which do not permit the create of those files (check generated file names and pathes!). The same happens for generic permission issues (this is often a problem under Ubuntu where permissions are dropped by default)

  • One needs to be careful with log rotation if signatures and/or encryption are being used. These create side-files, which form a set and must be kept together. For signatures, the ”.sigstate” file must NOT be rotated away if signature chains are to be build across multiple files. This is because .sigstate contains just global information for the whole file set. However, all other files need to be rotated together. The proper sequence is to

    1. move all files inside the file set
    2. only AFTER this is completely done, HUP rsyslog

    This sequence will ensure that all files inside the set are atomically closed and in sync. HUPing only after a subset of files have been moved results in inconsistencies and will most probably render the file set unusable.

Example

The following command writes all syslog messages into a file.

action(type="omfile" dirCreateMode="0700" FileCreateMode="0644"
       File="/var/log/messages")

Legacy Configuration Directives

Note that the legacy configuration parameters do not affect new-style action definitions via the action() object. This is by design. To set default for action() objects, use module parameters in the

module(load="builtin:omfile" ...)

object.

Read about the importance of order in legacy configuration to understand how to use these configuration directives. Legacy directives should NOT be used when writing new configuration files.

  • $DynaFileCacheSize equivalent to the “dynaFileCacheSize” parameter
  • $OMFileZipLevel equivalent to the “zipLevel” parameter
  • $OMFileFlushInterval equivalent to the “flushInterval” parameter
  • $OMFileASyncWriting equivalent to the “asyncWriting” parameter
  • $OMFileFlushOnTXEnd equivalent to the “flushOnTXEnd” parameter
  • $OMFileIOBufferSize equivalent to the “IOBufferSize” parameter
  • $DirOwner equivalent to the “dirOwner” parameter
  • $DirGroup equivalent to the “dirGroup” parameter
  • $FileOwner equivalent to the “fileOwner” parameter
  • $FileGroup equivalent to the “fileGroup” parameter
  • $DirCreateMode equivalent to the “dirCreateMode” parameter
  • $FileCreateMode equivalent to the “fileCreateMode” parameter
  • $FailOnCHOwnFailure equivalent to the “failOnChOwnFailure” parameter
  • $F$OMFileForceCHOwn equivalent to the “ForceChOwn” parameter
  • $CreateDirs equivalent to the “createDirs” parameter
  • $ActionFileEnableSync equivalent to the “enableSync” parameter
  • $ActionFileDefaultTemplate equivalent to the “template” module parameter
  • $ResetConfigVariables Resets all configuration variables to their default value.

Legacy Sample

The following command writes all syslog messages into a file.

$DirCreateMode 0700
$FileCreateMode 0644
*.* /var/log/messages

This documentation is part of the rsyslog project. Copyright © 2008-2014 by Rainer Gerhards and Adiscon. Released under the GNU GPL version 3 or higher.