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syslog-ng Premium Edition 7.0.19 - Administration Guide

Preface Introduction to syslog-ng The concepts of syslog-ng Installing syslog-ng The syslog-ng PE quick-start guide The syslog-ng PE configuration file Collecting log messages — sources and source drivers
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) office365: Fetching logs from Office 365 osquery: Collect and parse osquery result logs pipe: Collecting messages from named pipes program: Receiving messages from external applications python: writing server-style Python sources python-fetcher: writing fetcher-style Python sources snmptrap: Read Net-SNMP traps 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 udp-balancer: Receiving UDP messages at very high rate unix-stream, unix-dgram: Collecting messages from UNIX domain sockets windowsevent: Collecting Windows event logs
Sending and storing log messages — destinations and destination drivers
elasticsearch2: Sending messages directly to Elasticsearch version 2.0 or higher (DEPRECATED) elasticsearch-http: Sending messages to Elasticsearch HTTP Event Collector file: Storing messages in plain-text files hdfs: Storing messages on the Hadoop Distributed File System (HDFS) http: Posting messages over HTTP kafka: Publishing messages to Apache Kafka logstore: Storing messages in encrypted files mongodb: Storing messages in a MongoDB database network: Sending messages to a remote log server using the RFC3164 protocol (network() driver) pipe: Sending messages to named pipes program: Sending messages to external applications python: writing custom Python destinations sentinel: Sending logs to the Microsoft Azure Sentinel cloud smtp: Generating SMTP messages (email) from logs splunk-hec: Sending messages to Splunk HTTP Event Collector sql: Storing messages in an SQL database stackdriver: Sending logs to the Google Stackdriver cloud syslog: Sending messages to a remote logserver using the IETF-syslog protocol syslog-ng(): Forward logs 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) unix-stream, unix-dgram: Sending messages to UNIX domain sockets usertty: Sending messages to a user terminal — usertty() destination Client-side failover
Routing messages: log paths, flags, and filters Global options of syslog-ng PE TLS-encrypted message transfer Advanced Log Transfer Protocol Reliability and minimizing the loss of log messages Manipulating messages parser: Parse and segment structured messages Processing message content with a pattern database Correlating log messages Enriching log messages with external data Monitoring statistics and metrics of syslog-ng Multithreading and scaling in syslog-ng PE Troubleshooting syslog-ng Best practices and examples The syslog-ng manual pages Glossary

Modifying the on-the-wire message format

Macros, templates, and template functions allow you to fully customize the format of the message. This flexibility makes it possible to use syslog-ng PE in some unexpected way if needed, for example, to emulate simple, plain-text protocols. The following example shows you how to send LPUSH commands to a Redis server.

The following template is a valid LPUSH command in accordance with the Redis protocol, and puts the $MESSAGE into a separate list for every $PROGRAM:

template t_redis_lpush {
    template("*3\r\n$$5\r\nLPUSH\r\n$$$(length ${PROGRAM})\r\n${PROGRAM}\r\n$$$(length ${MESSAGE})\r\n${MESSAGE}\r\n");

If you use this template in a network() destination, syslog-ng PE formats the message according to the template, and sends it to the Redis server.

destination d_redis_tcp {
    network("" port(6379) template(t_redis_lpush));

Modifying messages using rewrite rules

The syslog-ng application can rewrite parts of the messages using rewrite rules. Rewrite rules are global objects similar to parsers and filters and can be used in log paths. The syslog-ng application has two methods to rewrite parts of the log messages: substituting (setting) a part of the message to a fix value, and a general search-and-replace mode.

Substitution completely replaces a specific part of the message that is referenced using a built-in or user-defined macro.

General rewriting searches for a string in the entire message (or only a part of the message specified by a macro) and replaces it with another string. Optionally, this replacement string can be a template that contains macros.

Rewriting messages is often used in conjunction with message parsing parser: Parse and segment structured messages.

Rewrite rules are similar to filters: they must be defined in the syslog-ng configuration file and used in the log statement. You can also define the rewrite rule inline in the log path.


The order of filters, rewriting rules, and parsers in the log statement is important, as they are processed sequentially.

Replacing message parts

To replace a part of the log message, you have to:

  • define a string or regular expression to find the text to replace

  • define a string to replace the original text (macros can be used as well)

  • select the field of the message that the rewrite rule should process

Substitution rules can operate on any soft macros, for example, MESSAGE, PROGRAM, or any user-defined macros created using parsers. Hard macros cannot be modified. For details on the hard and soft macros, see Hard versus soft macros). You can also rewrite the structured-data fields of messages complying to the RFC5424 (IETF-syslog) message format. Substitution rules use the following syntax:

rewrite <name_of_the_rule> {
    subst("<string or regular expression to find>",
        "<replacement string>", value(<field name>), flags() );

The type() and flags() options are optional. The type() specifies the type of regular expression to use, while the flags() are the flags of the regular expressions. For details on regular expressions, see Regular expressions.

A single substitution rule can include multiple substitutions that are applied sequentially to the message. Note that rewriting rules must be included in the log statement to have any effect.


For case-insensitive searches, add the flags(ignore-case) option. To replace every occurrence of the string, add flags(global) option. Note that the store-matches flag is automatically enabled in rewrite rules.

Example: Using substitution rules

The following example replaces the IP in the text of the message with the string IP-Address.

rewrite r_rewrite_subst{subst("IP", "IP-Address", value("MESSAGE"));};

To replace every occurrence, use:

rewrite r_rewrite_subst{
    subst("IP", "IP-Address", value("MESSAGE"), flags("global"));

Multiple substitution rules are applied sequentially. The following rules replace the first occurrence of the string IP with the string IP-Addresses.

rewrite r_rewrite_subst{
    subst("IP", "IP-Address", value("MESSAGE"));
    subst("Address", "Addresses", value("MESSAGE"));
Example: Anonymizing IP addresses

The following example replaces every IPv4 address in the MESSAGE part with its SHA-1 hash:

rewrite pseudonymize_ip_addresses_in_message {subst ("((([0-9]|[1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5])[.]){3}([0-9]|[1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5]))", "$(sha1 $0)", value("MESSAGE"));};

Setting message fields to specific values

To set a field of the message to a specific value, you have to:

  • define the string to include in the message, and

  • select the field where it should be included.

You can set the value of available macros, for example, HOST, MESSAGE, PROGRAM, or any user-defined macros created using parsers (for details, see parser: Parse and segment structured messages and Processing message content with a pattern database). Hard macros cannot be modified. For details on the hard and soft macros, see Hard versus soft macros). Note that the rewrite operation completely replaces any previous value of that field. Use the following syntax:

rewrite <name_of_the_rule> {
    set("<string to include>", value(<field name>));
Example: Setting message fields to a particular value

The following example sets the HOST field of the message to myhost.

rewrite r_rewrite_set{set("myhost", value("HOST"));};

The following example appends the "suffix" string to the MESSAGE field:

rewrite r_rewrite_set{set("$MESSAGE suffix", value("MESSAGE"));};

For details on rewriting SDATA fields, see Creating custom SDATA fields.

You can also use the following options in rewrite rules that use the set() operator.

rewrite <name_of_the_rule> {
    set("<string to include>", value(<field name>), on-error("fallback-to-string");
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