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syslog-ng Open Source Edition 3.22 - Administration Guide

Preface Introduction to syslog-ng The concepts of syslog-ng Installing syslog-ng The syslog-ng OSE quick-start guide The syslog-ng OSE configuration file source: Read, receive, and collect log messages
How sources work default-network-drivers: Receive and parse common syslog messages internal: Collecting internal messages file: Collecting messages from text files wildcard-file: Collecting messages from multiple text files linux-audit: Collecting messages from Linux audit logs network: Collecting messages using the RFC3164 protocol (network() driver) nodejs: Receiving JSON messages from nodejs applications mbox: Converting local e-mail messages to log messages osquery: Collect and parse osquery result logs pipe: Collecting messages from named pipes pacct: Collecting process accounting logs on Linux program: Receiving messages from external applications python: writing server-style Python sources python-fetcher: writing fetcher-style Python sources snmptrap: Read Net-SNMP traps sun-streams: Collecting messages on Sun Solaris syslog: Collecting messages using the IETF syslog protocol (syslog() driver) system: Collecting the system-specific log messages of a platform systemd-journal: Collecting messages from the systemd-journal system log storage systemd-syslog: Collecting systemd messages using a socket tcp, tcp6, udp, udp6: Collecting messages from remote hosts using the BSD syslog protocol— OBSOLETE unix-stream, unix-dgram: Collecting messages from UNIX domain sockets stdin: Collecting messages from the standard input stream
destination: Forward, send, and store log messages
amqp: Publishing messages using AMQP collectd: sending metrics to collectd elasticsearch2: Sending messages directly to Elasticsearch version 2.0 or higher (DEPRECATED) elasticsearch-http: Sending messages to Elasticsearch HTTP Bulk API file: Storing messages in plain-text files graphite: Sending metrics to Graphite Sending logs to Graylog hdfs: Storing messages on the Hadoop Distributed File System (HDFS) Posting messages over HTTP http: Posting messages over HTTP without Java kafka: Publishing messages to Apache Kafka (Java implementation) kafka: Publishing messages to Apache Kafka (C implementation, using the librdkafka client) loggly: Using Loggly logmatic: Using mongodb: Storing messages in a MongoDB database network: Sending messages to a remote log server using the RFC3164 protocol (network() driver) osquery: Sending log messages to osquery's syslog table pipe: Sending messages to named pipes program: Sending messages to external applications pseudofile() python: writing custom Python destinations redis: Storing name-value pairs in Redis riemann: Monitoring your data with Riemann slack: Sending alerts and notifications to a Slack channel smtp: Generating SMTP messages (e-mail) from logs snmp: Sending SNMP traps Splunk: Sending log messages to Splunk sql: Storing messages in an SQL database stomp: Publishing messages using STOMP syslog: Sending messages to a remote logserver using the IETF-syslog protocol syslog-ng: Forwarding messages and tags to another syslog-ng node tcp, tcp6, udp, udp6: Sending messages to a remote log server using the legacy BSD-syslog protocol (tcp(), udp() drivers) Telegram: Sending messages to Telegram unix-stream, unix-dgram: Sending messages to UNIX domain sockets usertty: Sending messages to a user terminal: usertty() destination Write your own custom destination in Java or Python Client-side failover
log: Filter and route log messages using log paths, flags, and filters Global options of syslog-ng OSE TLS-encrypted message transfer template and rewrite: Format, modify, and manipulate log messages parser: Parse and segment structured messages db-parser: Process message content with a pattern database (patterndb) Correlating log messages Enriching log messages with external data Statistics of syslog-ng Multithreading and scaling in syslog-ng OSE Troubleshooting syslog-ng Best practices and examples The syslog-ng manual pages Creative Commons Attribution Non-commercial No Derivatives (by-nc-nd) License

Referencing earlier messages of the context

When creating the aggregated message, or in the various parameters of the grouping-by() parser, you can also refer to fields and values of earlier messages of the context by adding the @<distance-of-referenced-message-from-the-current> suffix to the macro. For example, if there are three log messages in a context, the ${HOST}@1 expression refers to the host field of the current (third) message in the context, the ${HOST}@2 expression refers to the host field of the previous (second) message in the context, ${PID}@3 to the PID of the first message, and so on. For example, the following message can be created from SSH login/logout messages: An SSH session for ${SSH_USERNAME}@1 from ${SSH_CLIENT_ADDRESS}@2 closed. Session lasted from ${DATE}@2 to ${DATE}.


When referencing an earlier message of the context, always enclose the field name between braces, for example, ${PID}@3. The reference will not work if you omit the braces.


To use a literal @ character in a template, use @@.

Example: Referencing values from an earlier message

The following action can be used to log the length of an SSH session (the time difference between a login and a logout message in the context):

    value('value name="MESSAGE" An SSH session for ${SSH_USERNAME}@1 from ${SSH_CLIENT_ADDRESS}@2 closed. Session lasted from ${DATE}@2 to ${DATE}')

For another example, see The grouping-by() parser in syslog-ng blog post

If you do not know in which message of the context contains the information you need, you can use the grep template function. For details, see grep.

Example: Using the grep template function

The following example selects the message of the context that has a username name-value pair with the root value, and returns the value of the auth_method name-value pair.

$(grep ("${username}" == "root") ${auth_method})

To perform calculations on fields that have numerical values, see Numerical operations.

Options of grouping-by parsers

The grouping-by has the following options.

Synopsis: aggregate()

Description: Specifies the message that syslog-ng OSE generates when the context is closed. This option is mandatory.

Note that the aggregate() option has access to every message of the context, and has the following options:

  • inherit-mode: This attribute controls which name-value pairs and tags are propagated to the newly generated message.

    • context: syslog-ng OSE collects every name-value pair from each message stored in the context, and includes them in the generated message. If a name-value pair appears in multiple messages of the context, the value in the latest message will be used. Note that tags are not merged, the generated message will inherit the tags assigned to the last message of the context.

    • last-message: Only the name-value pairs appearing in the last message are copied. If the context contains only a single message, then it is the message that triggered the action.

    • none: An empty message is created, without inheriting any tags or name-value pairs.

    The default value of inherit-mode() is context.

    For details on the message context, see Correlating messages using the grouping-by() parser.

  • tags: Adds the specified tag to the list of tags.

  • value: Adds a name-value pair to the generated message. You can include text, macros, template functions, and you can also reference every message of the context. For details on accessing other messages of the context, see Referencing earlier messages of the context.

Synopsis: having()

Description: Specifies a filter: syslog-ng OSE generates the aggregate message only if the result of the filter expression is true. Note that the having() filter has access to every message of the context. For details on accessing other messages of the context, see Referencing earlier messages of the context.

Synopsis: inject-mode()

Description: By default, the aggregated message that syslog-ng OSE generates is injected into the same place where the grouping-by() statement is referenced in the log path. To post the generated message into the internal() source instead, use the inject-mode() option in the definition of the parser.

Example: Sending triggered messages to the internal() source

To send the generated messages to the internal source, use the inject-mode("internal") option:

parser p_grouping-by {grouping-by(

To inject the generated messages where the parser is referenced, use the inject-mode("pass-through") option:

parser p_grouping-by {grouping-by(

You can configure the generated message in the aggregate() option (see aggregate()). You can create an entire message, use macros and values extracted from the original message, and so on.

Synopsis: key()

Description: Specifies the key as a template (that is, the name of a name-value pair) that every message must have to be added to the context. The value of the key must be the same for every message of the context. For example, this can be a session-id parsed from firewall messages, and so on.

This is a mandatory option.


Messages that do not have a key will all belong to the same context.


If the value of the key is static (for example, key("PROGRAM") instead of key("$PROGRAM")), all messages will belong to the same context.

Synopsis: scope()

Description: Specifies which messages belong to the same context. The following values are available:

  • process: Only messages that are generated by the same process of a client belong to the same context, that is, messages that have identical ${HOST}, ${PROGRAM} and ${PID} values.

  • program: Messages that are generated by the same application of a client belong to the same context, that is, messages that have identical ${HOST} and ${PROGRAM} values.

  • host: Every message generated by a client belongs to the same context, only the ${HOST} value of the messages must be identical.

  • global: Every message belongs to the same context. This is the default value.

Synopsis: sort-key()

Description: Allows sorting of the elements before they are aggregated into a context. Use this when entries are not received in order. This works similarly to the SQL ORDER BY keyword.


  • Sorting is done by syslog-ng OSE when the context is about to be closed by trigger() or timeout(), but before syslog-ng OSE evaluates the having() option.
  • syslog-ng OSE can slow down if you specify several sort-key macro or template options, for example, sort-key("${3}${4}").
Synopsis: timeout([seconds])

Description: Specifies the maximum time to wait for all messages of the context to arrive. If no new message is added to the context during this period, the context is assumed to be complete and syslog-ng OSE generates and sends the triggered message (specified in the aggregate() option), and clears the context. If a new message is added to the context, the timeout period is restarted.

This option is mandatory, and its value must be equal to or greater than 1.

Synopsis: trigger()

Description: A filter that specifies the final message of the context. If the filter matches the incoming message, syslog-ng OSE generates and sends the triggered message (specified in the aggregate() option), and clears the context.

Synopsis: where()

Description: Specifies a filter condition. Messages not matching the filter will not be added to the context. Note that the where() filter has access only to the current message.

Enriching log messages with external data

To properly interpret the events that the log messages describe, you must be able to handle log messages as part of a system of events, instead of individual information chunks. The syslog-ng OSE application allows you to import data from external sources to include in the log messages, thus extending, enriching, and complementing the data found in the log message.

The syslog-ng OSE application currently provides the following possibilities to enrich log messages.

Adding metadata from an external file

In syslog-ng OSE version 3.8 and later, you can use an external database file to add additional metadata to your log messages. For example, you can create a database (or export it from an existing tool) that contains a list of hostnames or IP addresses, and the department of your organization that the host belongs to, the role of the host (mailserver, webserver, and so on), or similar contextual information.

The database file is a simple text file in comma-separated value (CSV) format, where each line contains the following information:

  • A selector or ID that appears in the log messages, or the name of a filter that matches the messages, for example, the hostname.

  • The name of the name-value pair that syslog-ng OSE adds to matching log messages.

  • The value of the name-value pairs. Starting with syslog-ng OSE version 3.22, the value of the name-value pair can be a template or a template function, for example, "selector3,name,$(echo $HOST_FROM)";

For example, the following csv-file contains three lines identified with the IP address, and adds the host-role field to the log message.,host-role,webserver,host-role,firewall,host-role,mailserver
The database file:

The database file must comply with the RFC4180 CSV format, with the following exceptions and limitations:

  • The values of the CSV-file cannot contain line-breaks

To add multiple name-value pairs to a message, include a separate line in the database for each name-value pair, for example:,host-role,webserver,contact-person,"John Doe",contact-email,

Technically, add-contextual-data() is a parser in syslog-ng OSE so you have to define it as a parser object.

parser p_add_context_data {

You can also add data to messages that do not have a matching selector entry in the database using the default-selector() option.

If you modify the database file, you have to reload syslog-ng OSE for the changes to take effect. If reloading syslog-ng OSE or the database file fails for some reason, syslog-ng OSE will keep using the last working database file.

Example: Adding metadata from a CSV file

The following example defines uses a CSV database to add the role of the host based on its IP address, and prefixes the added name-value pairs with .metadata. The destination includes a template that simply appends the added name-value pairs to the end of the log message.

@include "scl.conf"

source s_network {

destination d_local {
    template("$MSG Additional metadata:[${}]")};

parser p_add_context_data {
    add-contextual-data(selector("$SOURCEIP"), database("context-info-db.csv"), default-selector("unknown"), prefix(".metadata."));

log {
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