Starting with 3.2, it is possible to apply a rewrite rule to a message only if certain conditions are met. The condition() option effectively embeds a filter expression into the rewrite rule: the message is modified only if the message passes the filter. If the condition is not met, the message is passed to the next element of the log path (that is, the element following the rewrite rule in the log statement, for example, the destination). Any filter expression normally used in filters can be used as a rewrite condition. Existing filter statements can be referenced using the filter() function within the condition. For details on filters, see Filters.

TIP: Using conditions in rewrite rules can simplify your syslog-ng OSE configuration file, as you do not need to create separate log paths to modify certain messages.

How conditional rewriting works

Purpose:

The following procedure summarizes how conditional rewrite rules (rewrite rules that have the condition() parameter set) work. The following configuration snippet is used to illustrate the procedure:

rewrite r_rewrite_set{
    set(
        "myhost",
        value("HOST")
        condition(program("myapplication"))
    );
};
log {
    source(s1);
    rewrite(r_rewrite_set);
    destination(d1);
};

Steps:

The log path receives a message from the source (s1).
The rewrite rule (r_rewrite_set) evaluates the condition. If the message matches the condition (the PROGRAM field of the message is "myapplication"), syslog-ng OSE rewrites the log message (sets the value of the HOST field to "myhost"), otherwise it is not modified.
The next element of the log path processes the message (d1).

Example: Using conditional rewriting

The following example sets the HOST field of the message to myhost only if the message was sent by the myapplication program.

rewrite r_rewrite_set{set("myhost", value("HOST") condition(program("myapplication")));};

The following example is identical to the previous one, except that the condition references an existing filter template.

filter f_rewritefilter {program("myapplication");};
rewrite r_rewrite_set{set("myhost", value("HOST") condition(filter(f_rewritefilter)));};

Adding and deleting tags

template and rewrite: Format, modify, and manipulate log messages > Modifying messages using rewrite rules > Adding and deleting tags

To add or delete a tag, you can use rewrite rules. To add a tag, use the following syntax:

rewrite <name_of_the_rule> {
    set-tag("<tag-to-add>");
};

To delete a tag, use the following syntax:

rewrite <name_of_the_rule> {
    clear-tag("<tag-to-delete>");
};

You cannot use macros in the tags.

Rewrite the timezone of a message

template and rewrite: Format, modify, and manipulate log messages > Modifying messages using rewrite rules > Rewrite the timezone of a message

Starting with version 3.24 of the syslog-ng Open Source Edition (syslog-ng OSE) application, you can manipulate the timezone information of messages using rewrite rules. You can:

Set a timezone to a specific value
Fix a timezone if it was improperly parsed
Assuming the sender is sending messages in near-real-time, syslog-ng OSE can guess the timezone

By default, these operations modify the date-related macros of the message that correspond to the date the message was sent (that is, the S_ macros). You can modify the dates when syslog-ng OSE has received the messages (that is, the R_ macros), but this is rarely needed. To do so, include the time-stamp(recvd) option in the operation, for example:

rewrite { fix-time-zone("EST5EDT" time-stamp(recvd)); };

fix-time-zone()

Use the fix-time-zone() operation to correct the timezone of a message if it was parsed incorrectly for some reason, or if the client did not include any timezone information in the message. You can specify the new timezone as the name of a timezone, or as a template string. For example, use the following rewrite rule to set the timezone to EST5EDT:

rewrite { fix-time-zone("EST5EDT"); };

If you have lots of clients that do not send timezone information in the log messages, you can create a database file that stores the timezone of the clients, and feed this data to syslog-ng OSE using the add-contextual-data() feature. For details, see Adding metadata from an external file.

guess-time-zone()

Use the guess-time-zone() operation attempts to set the timezone of the message automatically, using heuristics on the timestamps. Normally the syslog-ng OSE application performs this operation automatically when it parses the incoming message. Using this operation in a rewrite rule can be useful if you cannot parse the incoming message for some reason (and use the flags(no-parse) option in your source, but you want to set the timezone automatically later (for example, after you have preprocessed the message).

Using this operation is identical to using the flags(guess-timezone) flag in the source.

set-time-zone()

Use the set-time-zone() operation to set the timezone of the message to a specific value, that is to convert an existing timezone to a different one. This operation is identical to setting the time-zone() option in a destination or as a global option, but can be applied selectively to the messages using conditions.

Anonymizing credit card numbers

template and rewrite: Format, modify, and manipulate log messages > Modifying messages using rewrite rules > Anonymizing credit card numbers

Log messages of banking and e-commerce applications might include credit card numbers (Primary Account Number or PAN). According to privacy best practices and the requirements of the Payment Card Industry Data Security Standards (PCI-DSS), PAN must be rendered unreadable. The syslog-ng OSE application uses a regular expression to detect credit card numbers, and provides two ways to accomplish this: you can either mask the credit card numbers, or replace them with a hash. To mask the credit card numbers, use the credit-card-mask() or the credit-card-hash() rewrite rules in a log path.

Declaration:

@include "scl/rewrite/cc-mask.conf"

rewrite {
    credit-card-mask(value("<message-field-to-process>"));
};

By default, these rewrite rules process the MESSAGE part of the log message.

credit-card-hash()

Synopsis:

credit-card-hash(value("<message-field-to-process>"))

Description: Process the specified message field (by default, ${MESSAGE}), and replace any credit card numbers (Primary Account Number or PAN) with a 16-character-long hash. This hash is generated by calculating the SHA-1 hash of the credit card number, selecting the first 64 bits of this hash, and representing this 64 bits in 16 characters.

credit-card-mask()

Synopsis:

credit-card-mask(value("<message-field-to-process>"))

Description: Process the specified message field (by default, ${MESSAGE}), and replace the 7-12th character of any credit card numbers (Primary Account Number or PAN) with asterisks (*). For example, syslog-ng OSE replaces the number 5542043004559005 with 554204******9005.

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