Modifying messages using rewrite rules
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.
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NOTE:
The order of filters, rewriting rules, and parsers in the log statement is important, as they are processed sequentially. |
Replacing message parts
To replace a part of the log message, you have to:
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define a string or regular expression to find the text to replace
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define a string to replace the original text (macros can be used as well)
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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 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:
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.
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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 $0)", value("MESSAGE"));};Setting message fields to specific values
To set a field of the message to a specific value, you have to:
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define the string to include in the message, and
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select the field where it should be included.
You can set the value of available macros, for example HOST, MESSAGE, PROGRAM, or any user-defined macros created using parsers (for details, see parser: Parse and segment structured messages and db-parser: Process message content with a pattern database (patterndb)). Hard macros cannot be modified. For details on the hard and soft macros, see Hard vs. soft macros). Note that the rewrite operation completely replaces any previous value of that field. Use the following syntax:
Declaration:
rewrite <name_of_the_rule> {
set("<string to include>", value(<field name>));
};Example: Setting message fields to a particular value
The following example sets the HOST field of the message to myhost.
rewrite r_rewrite_set{
set("myhost", value("HOST"));
};
The following example appends the "suffix" string to the MESSAGE field:
rewrite r_rewrite_set{
set("$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");
};
frac-digits()
| Type: | number |
| Default: | 0 |
Description: The syslog-ng application can store fractions of a second in the timestamps according to the ISO8601 format. The frac-digits() parameter specifies the number of digits stored. The digits storing the fractions are padded by zeros if the original timestamp of the message specifies only seconds. Fractions can always be stored for the time the message was received. Note that syslog-ng can add the fractions to non-ISO8601 timestamps as well.
local-time-zone()
| Type: | name of the timezone, or the timezone offset |
| Default: | The local timezone. |
Description: Sets the timezone used when expanding filename and tablename templates.
The timezone can be specified by using the name, for example, time-zone("Europe/Budapest")), or as the timezone offset in +/-HH:MM format, for example, +01:00). On Linux and UNIX platforms, the valid timezone names are listed under the /usr/share/zoneinfo directory.
on-error()
| Accepted values: |
drop-message|drop-property|fallback-to-string| silently-drop-message|silently-drop-property|silently-fallback-to-string |
| Default: | Use the global setting (which defaults to drop-message) |
Description: Controls what happens when type-casting fails and syslog-ng OSE cannot convert some data to the specified type. By default, syslog-ng OSE drops the entire message and logs the error. Currently the value-pairs() option uses the settings of on-error().
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drop-message: Drop the entire message and log an error message to the internal() source. This is the default behavior of syslog-ng OSE.
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drop-property: Omit the affected property (macro, template, or message-field) from the log message and log an error message to the internal() source.
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fallback-to-string: Convert the property to string and log an error message to the internal() source.
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silently-drop-message: Drop the entire message silently, without logging the error.
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silently-drop-property: Omit the affected property (macro, template, or message-field) silently, without logging the error.
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silently-fallback-to-string: Convert the property to string silently, without logging the error.
send-time-zone()
| Accepted values: | name of the timezone, or the timezone offset |
| Default: | local timezone |
Description: Specifies the time zone associated with the messages sent by syslog-ng, if not specified otherwise in the message or in the destination driver. For details, see Timezones and daylight saving.
The timezone can be specified by using the name, for example, time-zone("Europe/Budapest")), or as the timezone offset in +/-HH:MM format, for example, +01:00). On Linux and UNIX platforms, the valid timezone names are listed under the /usr/share/zoneinfo directory.
time-zone()
| Type: | name of the timezone, or the timezone offset |
| Default: | unspecified |
Description: Convert timestamps to the timezone specified by this option. If this option is not set, then the original timezone information in the message is used. Converting the timezone changes the values of all date-related macros derived from the timestamp, for example, HOUR. For the complete list of such macros, see Date-related macros.
The timezone can be specified by using the name, for example, time-zone("Europe/Budapest")), or as the timezone offset in +/-HH:MM format, for example, +01:00). On Linux and UNIX platforms, the valid timezone names are listed under the /usr/share/zoneinfo directory.
ts-format()
| Type: | rfc3164, bsd, rfc3339, iso |
| Default: | rfc3164 |
Description: Override the global timestamp format (set in the global ts-format() parameter) for the specific destination. For details, see ts-format().
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NOTE:
This option applies only to file and file-like destinations. Destinations that use specific protocols (for example, network(), or syslog()) ignore this option. For protocol-like destinations, use a template locally in the destination, or use the proto-template option. |
Unsetting message fields
You can unset macros or fields of the message, including any user-defined macros created using parsers (for details, see parser: Parse and segment structured messages and db-parser: Process message content with a pattern database (patterndb)). Hard macros cannot be modified. For details on hard and soft macros, see Hard vs. 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.*"));
};