Filter functions

• 1: facility()
• 2: filter()
• 3: host()
• 4: in-list()
• 5: level() or priority()
• 6: match()
• 7: message()
• 8: netmask()
• 9: netmask6()
• 10: program()
• 11: rate-limit()
• 11.1: Options of rate-limit() filter
• 12: source()
• 13: tags()

1 - facility()

Description: Match messages having one of the listed facility codes.

The facility() filter accepts both the name and the numerical code of the facility or the importance level. Facility codes 0-23 are predefined and can be referenced by their usual name. Facility codes above 24 are not defined.

You can use the facility filter the following ways:

• Use a single facility name, for example, facility(user)

• Use a single facility code, for example, facility(1)

• Use a facility range (works only with facility names), for example, facility(local0..local5)

The AxoSyslog application recognizes the following facilities: (Note that some of these facilities are available only on specific platforms.)

syslog Message Facilities recognized by the facility() filter

2 - filter()

Description: Call another filter rule and evaluate its value. For example:

   filter demo_filter { host("example") and match("deny" value("MESSAGE")) };
    filter inverted_demo_filter { not filter(demo_filter) }

3 - host()

Description: Match messages by using a regular expression against the hostname field of log messages. Note that you can filter only on the actual content of the HOST field of the message (or what it was rewritten to). That is, AxoSyslog will compare the filter expression to the content of the ${HOST} macro. This means that for the IP address of a host will not match, even if the IP address and the hostname field refers to the same host. To filter on IP addresses, use the netmask() filter.

   filter demo_filter { host("example") };

4 - in-list()

Description: Matches the value of the specified field to a list stored in a file, allowing you to do simple, file-based black- and whitelisting. The file must be a plain-text file, containing one entry per line.

The AxoSyslog application loads the entire file, and compares the value of the specified field (for example, ${PROGRAM}) to entries in the file. When you use the in-list() filter, note the following points:

• Comparing the values is case-sensitive.
• Only exact matches are supported, partial and substring matches are not.
• If you modify the list file, reload the configuration of AxoSyslog for the changes to take effect.
• Make sure that there is an empty line in the file after the last entry. If the last entry is the last line, it will be excluded from the list.

Available in AxoSyslog 3.5 and later.

Example: Selecting messages using the in-list() filter

Create a text file that contains the programs (as in the ${PROGRAM} field of their log messages) you want to select. For example, you want to forward only the logs of a few applications from a host: kernel, sshd, and sudo. Create the /etc/syslog-ng/programlist.list file with the following contents. Note the empty line after the last entry.

kernel
sshd
sudo

The following filter selects only the messages of the listed applications:

   filter f_whitelist { in-list("/etc/syslog-ng/programlist.list", value("PROGRAM")); };

Create the appropriate sources and destinations for your environment, then create a log path that uses the previous filter to select only the log messages of the applications you need:

   log {
        source(s_all);
        filter(f_whitelist);
        destination(d_logserver); };

To create a blacklist filter, simply negate the in-list() filter:

   filter f_blacklist { not in-list("/etc/syslog-ng/programlist.list", value("PROGRAM")); };

5 - level() or priority()

Description: The level() filter selects messages corresponding to a single importance level, or a level-range. To select messages of a specific level, use the name of the level as a filter parameter, for example, use the following to select warning messages:

   level(warning)

To select a range of levels, include the beginning and the ending level in the filter, separated with two dots (..). For example, to select every message of error or higher level, use the following filter:

   level(err..emerg)

The level() filter accepts the following levels: emerg, alert, crit, err, warning, notice, info, debug.

6 - match()

Description: Match a regular expression to the headers and the message itself (that is, the values returned by the MSGHDR and MSG macros). Note that in AxoSyslog version 2.1 and earlier, the match() filter was applied only to the text of the message, excluding the headers. This functionality has been moved to the message() filter.

To limit the scope of the match to a specific part of the message (identified with a macro), use the match(regexp value("MACRO")) syntax. Do not include the $ sign in the parameter of the value() option.

The value() parameter accepts both built-in macros and user-defined ones created with a parser or using a pattern database. For details on macros and parsers, see Templates and macros, Parsing messages with comma-separated and similar values, and Using parser results in filters and templates.

Starting with version 3.22, the match() filter can work on templates as well. This means that you can a match against an expression combined of macros, instead of a single macro. Note that when using a template, you must reference macros with the $ sign (unlike when using the value() parameter). For example:

   match("^my-regular-expression" template("${HOST}|${PROGRAM}${PID}|${MESSAGE}"));

Using a template with a single macro is equivalent with using the value() parameter. For example, the following two lines are equivalent:

   match("^my-regular-expression" value("MESSAGE"));
    match("^my-regular-expression" template("${MESSAGE}"));

7 - message()

Description: Match a regular expression to the text of the log message, excluding the headers (that is, the value returned by the MSG macros). Note that in syslog-ng version 2.1 and earlier, this functionality was performed by the match() filter.

8 - netmask()

Description: Select only messages sent by a host whose IP address belongs to the specified IPv4 subnet. Note that this filter checks the IP address of the last-hop relay (the host that actually sent the message to AxoSyslog), not the contents of the HOST field of the message. You can use both the dot-decimal and the CIDR notation to specify the netmask. For example, 192.168.5.0/255.255.255.0 or 192.168.5.0/24. To filter IPv6 addresses, see netmask6().

9 - netmask6()

Description: Select only messages sent by a host whose IP address belongs to the specified IPv6 subnet. Note that this filter checks the IP address of the last-hop relay (the host that actually sent the message to AxoSyslog), not the contents of the HOST field of the message. You can use both the regular and the compressed format to specify the IP address, for example, 1080:0:0:0:8:800:200C:417A or 1080::8:800:200C:417A. If you do not specify the address, localhost is used.

Use the netmask (also called prefix) to specify how many of the leftmost bits of the address comprise the netmask (values 1-128 are valid). For example, the following specify a 60-bit prefix: 12AB:0000:0000:CD30:0000:0000:0000:0000/60 or 12AB::CD30:0:0:0:0/60. Note that if you set an IP address and a prefix, AxoSyslog will ignore the bits of the address after the prefix. To filter IPv4 addresses, see netmask().

The netmask6() filter is available in AxoSyslog 3.7 and later.

10 - program()

Description: Match messages by using a regular expression against the program name field of log messages.

11 - rate-limit()

Description: Limits messages rate based on arbitrary keys in each message. The key will be resolved using the key() option. Each resolution will be allowed to have the number of messages each second, set by the rate() option. For example if key($HOST) and rate(5000) are set, and there are 2 hosts sending messages to AxoSyslog, a total of 10000 messages will be allowed by the rate-limit() filter, 5000 from the first and 5000 from the second host. If key() was not set instead, then 5000 messages would be allowed each second, regardless of their content.

Example: Using the rate-limit() filter

The following example depicts the scenario described in the description part of this section.

   filter f_rate_limit {
                    rate-limit(
                    key("$HOST")
                    rate(5000)
                    );
                };

11.1 - Options of rate-limit() filter

The rate-limit() filter has the following options.

key()

Description: The resolved template, that will be used to create unique rate-limit token buckets. In AxoSyslog version 4.4 and earlier, the name of this option was template().

rate()

Description: The number of messages for each unique macro resolution, that will be let through (matched) by the filter each second.

12 - source()

Description: Select messages of a source statement. This filter can be used in embedded log statements if the parent statement contains multiple source groups — only messages originating from the selected source group are sent to the destination of the embedded log statement.

13 - tags()

Description: Select messages labeled with the specified tag. Every message automatically has the tag of its source in .source.<id_of_the_source_statement> format. This option is available only in AxoSyslog 3.1 and later.

Example: Adding tags and filtering messages with tags

   source s_tcp {
        network(ip(192.168.1.1) port(1514) tags("tcp", "router"));
    };

Use the tags() option of the filters to select only specific messages:

   filter f_tcp {
        tags(".source.s_tcp");
    };
    
    filter f_router {
        tags("router");
    };

   filter f_tag_filter {tags(".classifier.system");};