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.
NOTE: The order of filters, rewriting rules, and parsers in the log statement is important, as they are processed sequentially.
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:
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.
TIP: 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 $0)", value("MESSAGE"));};
To set a field of the message to a specific value, you have to:
You can set the value of available macros, for example, HOST, MESSAGE, PROGRAM, or any user-defined macros created using parsers (for details, see parser: Parse and segment structured messages and Processing message content with a pattern database). Hard macros cannot be modified. For details on the hard and soft macros, see Hard versus soft macros). Note that the rewrite operation completely replaces any previous value of that field. Use the following syntax:
Declaration
rewrite <name_of_the_rule> {
set("<string to include>", value(<field name>));
};
Example: Setting message fields to a particular value
The following example sets the HOST field of the message to myhost.
rewrite r_rewrite_set{set("myhost", value("HOST"));};
The following example appends the "suffix" string to the MESSAGE field:
rewrite r_rewrite_set{set("$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");
};
You can unset a macro or a field of the message, including 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 hard and soft macros, see Hard versus soft macros). Note that the unset operation completely deletes any previous value of the field that you apply it on. Use the following syntax:
Declaration
rewrite <name_of_the_rule> {
unset(value("<field name>"));
};
Example: Unsetting a message field
The following example unsets the HOST field of the message.
rewrite r_rewrite_unset{unset(value("HOST"));};
To unset a group of fields, you can use the groupunset() rewrite rule.
Declaration
rewrite <name_of_the_rule> {
groupunset(values("<expression-for-field-names>"));
};
Example: Unsetting a group of fields
The following rule clears all SDATA fields:
rewrite r_rewrite_unset_SDATA{ groupunset(values(".SDATA.*"));};
