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syslog-ng Open Source Edition 3.26 - 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 email 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 Logmatic.io 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 (email) 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(): 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) 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 Glossary

Using filters

Filters perform log routing within syslog-ng: a message passes the filter if the filter expression is true for the particular message. If a log statement includes filters, the messages are sent to the destinations only if they pass all filters of the log path. For example, a filter can select only the messages originating from a particular host. Complex filters can be created using filter functions and logical boolean expressions.

To define a filter, add a filter statement to the syslog-ng configuration file using the following syntax:

filter <identifier> { <filter_type>("<filter_expression>"); };

Then use the filter in a log path, for example:

log {
    source(s1);
    filter(<identifier>);
    destination(d1); };

You can also define the filter inline. For details, see Defining configuration objects inline.

Example: A simple filter statement

The following filter statement selects the messages that contain the word deny and come from the host example.

filter demo_filter { host("example") and match("deny" value("MESSAGE"))
};
log {
    source(s1);
    filter(demo_filter);
    destination(d1);
};

The following example does the same, but defines the filter inline.

log {
    source(s1);
    filter { host("example") and match("deny" value("MESSAGE")) };
    destination(d1);
};

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Combining filters with boolean operators

When a log statement includes multiple filter statements, syslog-ng sends a message to the destination only if all filters are true for the message. In other words, the filters are connected with the logical AND operator. In the following example, no message arrives to the destination, because the filters are exclusive (the hostname of a client cannot be example1 and example2 at the same time):

filter demo_filter1 { host("example1"); };
filter demo_filter2 { host("example2"); };
log {
    source(s1); source(s2);
    filter(demo_filter1); filter(demo_filter2);
    destination(d1); destination(d2); };

To select the messages that come from either host example1 or example2, use a single filter expression:

filter demo_filter { host("example1") or host("example2"); };
log {
    source(s1); source(s2);
    filter(demo_filter);
    destination(d1); destination(d2); };

Use the not operator to invert filters, for example, to select the messages that were not sent by host example1:

filter demo_filter { not host("example1"); };

However, to select the messages that were not sent by host example1 or example2, you have to use the and operator (that's how boolean logic works):

filter demo_filter { not host("example1") and not host("example2"); };

Alternatively, you can use parentheses to avoid this confusion:

filter demo_filter { not (host("example1") or host("example2")); };

For a complete description on filter functions, see Filter functions.

The following filter statement selects the messages that contain the word deny and come from the host example.

filter demo_filter { host("example") and match("deny" value("MESSAGE")); };

The value() parameter of the match function limits the scope of the function to the text part of the message (that is, the part returned by the ${MESSAGE} macro), or optionally to the content of any other macro. The template() parameter of the match function can be used to run a filter against a combination of macros. For details on using the match() filter function, see match().

TIP:

Filters are often used together with log path flags. For details, see Log path flags.


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Comparing macro values in filters

Starting with syslog-ng OSE version 3.2, it is also possible to compare macro values and templates as numerical and string values. String comparison is alphabetical: it determines if a string is alphabetically greater or equal to another string. Use the following syntax to compare macro values or templates. For details on macros and templates, see Customize message format using macros and templates.

filter <filter-id>
        {"<macro-or-template>" operator "<value-or-macro-or-template>"};
Example: Comparing macro values in filters

The following expression selects log messages containing a PID (that is, ${PID} macro is not empty):

filter f_pid {"${PID}" !=""};

The following expression selects log messages that do not contain a PID. Also, it uses a template as the left argument of the operator and compares the values as strings:

filter f_pid {"${HOST}${PID}" eq "${HOST}"};

The following example selects messages with priority level higher than 5.

filter f_level {"${LEVEL_NUM}" > "5"};

Note that:

  • The macro or template must be enclosed in double-quotes.

  • The $ character must be used before macros.

  • Using comparator operators can be equivalent to using filter functions, but is somewhat slower. For example, using "${HOST}" eq "myhost" is equivalent to using host("myhost" type(string)).

  • You can use any macro in the expression, including user-defined macros from parsers and results of pattern database classifications.

  • The results of filter functions are boolean values, so they cannot be compared to other values.

  • You can use boolean operators to combine comparison expressions.

The following operators are available:

Table 13: Numerical and string comparison operators
Numerical operator String operator Meaning
== eq Equals
!= ne Not equal to
> gt Greater than
< lt Less than
>= ge Greater than or equal
=< le Less than or equal

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Using wildcards, special characters, and regular expressions in filters

The host(), match(), and program() filter functions accept regular expressions as parameters. The exact type of the regular expression to use can be specified with the type() option. By default, syslog-ng OSE uses PCRE regular expressions.

In regular expressions, the asterisk (*) character means 0, 1, or any number of the previous expression. For example, in the f*ilter expression the asterisk means 0 or more f letters. This expression matches for the following strings: ilter, filter, ffilter, and so on. To achieve the wildcard functionality commonly represented by the asterisk character in other applications, use .* in your expressions, for example, f.*ilter.

Alternatively, if you do not need regular expressions, only wildcards, use type(glob) in your filter:

Example: Filtering with widcards

The following filter matches on hostnames starting with the myhost string, for example, on myhost-1, myhost-2, and so on.

filter f_wildcard {host("myhost*" type(glob));};

For details on using regular expressions in syslog-ng OSE, see Using wildcards, special characters, and regular expressions in filters.

To filter for special control characters like the carriage return (CR), use the \r escape prefix in syslog-ng OSE version 3.0 and 3.1. In syslog-ng OSE 3.2 and later, you can also use the \x escape prefix and the ASCII code of the character. For example, to filter on carriage returns, use the following filter:

filter f_carriage_return {match("\x0d" value ("MESSAGE"));};

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