Replace 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. You can also rewrite the structured-data fields of messages complying to the RFC5424 (IETF-syslog) message format.

See also the equivalent FilterX function, regexp_subst(), or str_replace() for literal strings.

Substitution rules use the following syntax:

Declaration

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.

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"));
};

<!-- DISCLAIMER: This file is based on the syslog-ng Open Source Edition documentation https://github.com/balabit/syslog-ng-ose-guides/commit/2f4a52ee61d1ea9ad27cb4f3168b95408fddfdf2 and is used under the terms of The syslog-ng Open Source Edition Documentation License. The file has been modified by Axoflow. -->

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"));};

Options

The subst() rewrite rule has the following options.

<!-- This file is under the copyright of Axoflow, and licensed under Apache License 2.0, except for using the Axoflow and AxoSyslog trademarks. -->

condition()

Description: Applies the rewrite rule only to the messages that match the specified filter expression. Messages that don’t match the filter pass through the rule unmodified, and continue to the next element of the log path. You can use any filter expression here, and you can reference an existing filter with the filter() function. For details, see Conditional rewrites.

flags()

Description: The flags of the regular expression. The available flags depend on the type() of the regular expression. For case-insensitive searches, use flags(ignore-case). To replace every occurrence of the pattern, use flags(global). Note that the store-matches flag is automatically enabled in rewrite rules. For details, see Regular expressions.

internal()

Description: Marks this pipeline element as internal. Elements marked as internal() are treated as an implementation detail, so for example statistics of the given pipe are available only on higher stats level. This option is mainly useful for developers or when writing SCL blocks and integrations.

type()

Description: Sets how AxoSyslog interprets the search pattern: as a Perl Compatible Regular Expression (pcre, used by default), a literal string search (string), or a glob pattern without regular expression support (glob). For details, see Options of regular expressions.

<!-- This file is under the copyright of Axoflow, and licensed under Apache License 2.0, except for using the Axoflow and AxoSyslog trademarks. -->

value()

Description: Selects the field of the message that the rewrite rule modifies. If you don’t set it, the rule operates on the MESSAGE field.

Write the name of the field without the $ prefix, for example, value("HOST"), not value("$HOST"). The $ prefix is only needed in templates, and AxoSyslog logs a warning if you use it here. You cannot set a hard macro in the value() option, AxoSyslog rejects the configuration with an error.