omfile: File Output Module — rsyslog 8.2502.0 documentation

# omfile: File Output Module

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

## Purpose

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.

## Notable Features

- Statistic Counter

## 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 parameters do not affect new-style RainerScript configuration objects. See basic configuration structure doc to learn about different configuration languages in use by rsyslog.

Note

Parameter names are case-insensitive.

### 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

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
word | RSYSLOG_FileFormat | no | `$ActionFileDefaultTemplate`

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

#### DirCreateMode

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
FileCreateMode | 0700 | no | `$DirCreateMode`

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

#### FileCreateMode

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
FileCreateMode | 0644 | no | `$FileCreateMode`

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

#### fileOwner

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
UID | process user | no | `$FileOwner`

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

#### fileOwnerNum

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
integer | process user | no | none

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

#### fileGroup

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
GID | process user’s primary group | no | `$FileGroup`

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

#### fileGroupNum

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
integer | process user’s primary group | no | none

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

#### dirOwner

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
UID | process user | no | `$DirOwner`

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

#### dirOwnerNum

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
integer | process user | no | none

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

#### dirGroup

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
GID | process user’s primary group | no | `$DirGroup`

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

#### dirGroupNum

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
integer | process user’s primary group | no | none

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

#### dynafile.donotsuspend

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
binary | on | no | none

This permits SUSPENDing dynafile actions. Traditionally, SUSPEND mode was never entered for dynafiles as it would have blocked overall processing flow. Default is not to suspend (and thus block).

#### compression.driver

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
word | zlib | no | none

New in version 8.2208.0.

For compressed operation (“zlib mode”), this permits to set the compression driver to be used. Originally, only zlib was supported and still is the default. Since 8.2208.0 zstd is also supported. It provides much better compression ratios and performance, especially with multiple zstd worker threads enabled.

Possible values are: - zlib - zstd

#### compression.zstd.workers

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
positive integer | zlib library default | no | none

New in version 8.2208.0.

In zstd mode, this enables to configure zstd-internal compression worker threads. This setting has nothing to do with rsyslog workers. The zstd library provides an enhanced worker thread pool which permits multithreaed compression of serial data streams. Rsyslog fully supports this mode for optimal performance.

Please note that for this parameter to have an effect, the zstd library must be compiled with multithreading support. As of this writing (2022), this is not the case for many frequently used distros and distro versions. In this case, you may want to custom install the zstd library with threading enabled. Note that this does not require a rsyslog rebuild.

### 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

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
string | none | no | 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. Either file or dynaFile can be used, but not both. If both are given, dynaFile will be used.

#### dynaFile

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
string | none | no | 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 you have full control over how to format the file name.

To avoid path traversal attacks, you must make sure that the template used properly escapes file paths. This is done by using the securepath parameter in the template’s property statements, or the secpath-drop or secpath-replace property options with the property replacer.

Either file or dynaFile can be used, but not both. If both are given, dynaFile will be used.

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

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
word | template set via module parameter | no | `$ActionFileDefaultTemplate`

Sets the template to be used for this action.

#### closeTimeout

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
integer | File: 0 DynaFile: 10 | no | none

New in version 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

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
integer | 10 | no | `$DynaFileCacheSize`

This parameter specifies the maximum size of the cache for dynamically-generated file names (dynafile= parameter). This setting specifies how many open file handles should be cached. If, for example, the file name is generated with the hostname in it and you have 100 different hosts, a cache size of 100 would ensure that files are opened once and then stay open. This can be a great way to increase performance. If the cache size is lower than the number of different files, the least recently used one is discarded (and the file closed).

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

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
integer | 0 | no | `$OMFileZipLevel`

If greater than 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

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
binary | off | no | none

New in version 7.3.0.

If zipLevel is greater than 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 (like gztool with: `gztool -p`). 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.

In order to avoid this degradation in compression both flushOnTXEnd and asyncWriting parameters must be set to “off” and also ioBufferSize must be raised from default “4k” value to at least “32k”. This way a reasonable compression factor is maintained, similar to a non-blocked gzip file:

    veryRobustZip="on" ioBufferSize="64k" flushOnTXEnd="off" asyncWriting="off"

Do not forget to add your desired zipLevel to this configuration line.

#### flushInterval

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
integer | 1 | no | `$OMFileFlushInterval`

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

#### asyncWriting

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
binary | off | no | `$OMFileASyncWriting`

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.

#### flushOnTXEnd

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
binary | on | no | `$OMFileFlushOnTXEnd`

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. Bear in mind that the buffer may become full only after several hours or a rsyslog shutdown (however a buffer flush can still be forced by sending rsyslogd a HUP signal). 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. Also note that the default -on- is primarily an aid to preserve the traditional syslogd behaviour.

If you are using dynamic file names (dynafiles), flushes can actually happen more frequently. In this case, a flush can also happen when the file name changes within a transaction.

#### ioBufferSize

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
size | 4 KiB | no | `$OMFileIOBufferSize`

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

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
UID | system default | no | `$DirOwner`

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

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
integer | system default | no | `$DirOwnerNum`

New in version 7.5.8.

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

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
GID | system default | no | `$DirGroup`

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

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
integer | system default | no | `$DirGroupNum`

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

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
UID | system default | no | `$FileOwner`

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

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
integer | system default | no | `$FileOwnerNum`

New in version 7.5.8.

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 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

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
GID | system default | no | `$FileGroup`

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

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
integer | system default | no | `$FileGroupNum`

New in version 7.5.8.

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

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
string | equally-named module parameter | no | `$FileCreateMode`

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

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
string | equally-named module parameter | no | `$DirCreateMode`

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

#### failOnChOwnFailure

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
binary | on | no | `$FailOnCHOwnFailure`

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

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
binary | on | no | `$CreateDirs`

Create directories on an as-needed basis

#### sync

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
binary | off | no | `$ActionFileEnableSync`

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

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
word | no signature provider | no | none

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

Currently there is one signature provider available: “ksi_ls12”.

Previous signature providers “gt” and “ksi” are deprecated.

#### cry.Provider

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
word | no crypto provider | no | none

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

Currently, there are two providers called “gcry” and “ossl”.

#### rotation.sizeLimit

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
size | 0 (disabled) | no | $outchannel (partly)

This permits to set a size limit on the output file. When the limit is reached, rotation of the file is tried. The rotation script needs to be configured via rotation.sizeLimitCommand.

Please note that the size limit is not exact. Some excess bytes are permitted to prevent messages from being split across two files. Also, a full batch of messages is not terminated in between. As such, in practice, the size of the output file can grow some KiB larger than configured.

Also avoid to configure a too-low limit, especially for busy files. Calling the rotation script is relatively performance intense. As such, it could negatively affect overall rsyslog performance.

#### rotation.sizeLimitCommand

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
binary | (empty) | no | $outchannel (partly)

This permits to configure the script to be called whe a size limit on the output file is reached. The actual size limit needs to be configured via rotation.sizeLimit.

## Statistic Counter

This plugin maintains statistics for each dynafile cache. Dynafile cache performance is critical for overall system performance, so reviewing these counters on a busy system (especially one experiencing performance problems) is advisable. The statistic is named “dynafile cache”, followed by the template name used for this dynafile action.

The following properties are maintained for each dynafile:

- request - total number of requests made to obtain a dynafile

- level0 - requests for the current active file, so no real cache lookup needed to be done. These are extremely good.

- missed - cache misses, where the required file did not reside in cache. Even with a perfect cache, there will be at least one miss per file. That happens when the file is being accessed for the first time and brought into cache. So “missed” will always be at least as large as the number of different files processed.

- evicted - the number of times a file needed to be evicted from the cache as it ran out of space. These can simply happen when date-based files are used, and the previous date files are being removed from the cache as time progresses. It is better, though, to set an appropriate “closeTimeout” (counter described below), so that files are removed from the cache after they become no longer accessed. It is bad if active files need to be evicted from the cache. This is a very costly operation as an evict requires to close the file (thus a full flush, no matter of its buffer state) and a later access requires a re-open – and the eviction of another file, as the cache obviously has run out of free entries. If this happens frequently, it can severely affect performance. So a high eviction rate is a sign that the dynafile cache size should be increased. If it is already very high, it is recommended to re-think about the design of the file store, at least if the eviction process causes real performance problems.

- maxused - the maximum number of cache entries ever used. This can be used to trim the cache down to a value that’s actually useful but does not waste resources. Note that when date-based files are used and rsyslog is run for an extended period of time, the cache gradually fills up to the max configured value as older files are migrated out of it. This will make “maxused” questionable after some time. Frequently enough purging the cache can prevent this (usually, once a day is sufficient).

- closetimeouts - available since 8.3.3 – tells how often a file was closed due to timeout settings (“closeTimeout” action parameter). These are cases where dynafiles or static files have been closed by rsyslog due to inactivity. Note that if no “closeTimeout” is specified for the action, this counter always is zero. A high or low number in itself doesn’t mean anything good or bad. It totally depends on the use case, so no general advise can be given.

## 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 paths!). 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.

- If `zipLevel` is greater than 0 and `veryRobustZip` is set to off, data appended to previously unclean closed files will not be accessible with `gunzip` if rsyslog writes again in the same file. Nonetheless, data is still there and can be correctly accessed with other tools like gztool (v>=1.1) with: `gztool -p`.

## Examples

### Example 1

The following command writes all syslog messages into a file.

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

See also

Help with configuring/using `Rsyslog`:

- Mailing list - best route for general questions

- GitHub: rsyslog source project - detailed questions, reporting issues that are believed to be bugs with `Rsyslog`

See also

Contributing to `Rsyslog`:

- Source project: rsyslog project README.

- Documentation: rsyslog-doc project README

Copyright 2008-2023 Rainer Gerhards (Großrinderfeld), and Others.