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11.2.1. 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 Section 11.1.4, Hard vs. 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 Section 11.3, 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 11.18. 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 11.19. 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)", value("MSG"));};