Log paths

• 1: Embedded log statements
• 1.1: Using embedded log statements
• 2: if-else-elif: Conditional expressions
• 3: Junctions and channels
• 4: Log path flags

   log {
        source(s1); source(s2); ...
        optional_element(filter1|parser1|rewrite1);
        optional_element(filter2|parser2|rewrite2);
        ...
        destination(d1); destination(d2); ...
        flags(flag1[, flag2...]);
    };

log top-level {
    source(s_local);

    log inner-1 {
        filter(f_inner_1);
        destination(d_local_1);
    };

    log inner-2 {
        filter(f_inner_2);
        destination(d_local_2);
    };
};

   source s_localhost {
        network(
            ip(127.0.0.1)
            port(1999)
        );
    };
    destination d_tcp {
        network("10.1.2.3"
            port(1999)
            localport(999)
        );
    };
    log {
        source(s_localhost);
        destination(d_tcp);
    };

1 - Embedded log statements

Starting from version 3.0, AxoSyslog can handle embedded log statements (also called log pipes). Embedded log statements are useful for creating complex, multi-level log paths with several destinations and use filters, parsers, and rewrite rules.

For example, if you want to filter your incoming messages based on the facility parameter, and then use further filters to send messages arriving from different hosts to different destinations, you would use embedded log statements.

Embedded log statements include sources — and usually filters, parsers, rewrite rules, or destinations — and other log statements that can include filters, parsers, rewrite rules, and destinations. The following rules apply to embedded log statements:

• Only the beginning (also called top-level) log statement can include sources.

• Embedded log statements can include multiple log statements on the same level (that is, a top-level log statement can include two or more log statements).

• Embedded log statements can include several levels of log statements (that is, a top-level log statement can include a log statement that includes another log statement, and so on).

• After an embedded log statement, you can write either another log statement, or the flags() option of the original log statement. You cannot use filters or other configuration objects. This also means that flags (except for the flow-control flag) apply to the entire log statement, you cannot use them only for the embedded log statement.

• Embedded log statements that are on the same level receive the same messages from the higher-level log statement. For example, if the top-level log statement includes a filter, the lower-level log statements receive only the messages that pass the filter.

Embedded log filters can be used to optimize the processing of log messages, for example, to re-use the results of filtering and rewriting operations.

1.1 - Using embedded log statements

Embedded log statements (for details, see Embedded log statements) re-use the results of processing messages (for example, the results of filtering or rewriting) to create complex log paths. Embedded log statements use the same syntax as regular log statements, but they cannot contain additional sources. To define embedded log statements, use the following syntax:

   log {
        source(s1); source(s2); ...
    
        optional_element(filter1|parser1|rewrite1);
        optional_element(filter2|parser2|rewrite2);
        ...
        destination(d1); destination(d2); ...
    
        #embedded log statement
        log {
            optional_element(filter1|parser1|rewrite1);
            optional_element(filter2|parser2|rewrite2);
            ...
            destination(d1); destination(d2); ...
    
            #another embedded log statement
            log {
                optional_element(filter1|parser1|rewrite1);
                optional_element(filter2|parser2|rewrite2);
                ...
                destination(d1); destination(d2); ...
            };
        };
        #set flags after the embedded log statements
        flags(flag1[, flag2...]);
    };

Example: Using embedded log paths

The following log path sends every message to the configured destinations: both the d_file1 and the d_file2 destinations receive every message of the source.

   log {
        source(s_localhost);
        destination(d_file1);
        destination(d_file2);
    };

The next example is equivalent to the one above, but uses an embedded log statement.

   log {
        source(s_localhost);
        destination(d_file1);
        log {
            destination(d_file2);
        };
    };

The following example uses two filters:

• messages coming from the host 192.168.1.1 are sent to the d_file1 destination, and

• messages coming from the host 192.168.1.1 and containing the string example are sent to the d_file2 destination.

   log {
        source(s_localhost);
        filter {
            host(192.168.1.1);
        };
        destination(d_file1);
        log {
            message("example");
            destination(d_file2);
        };
    };

The following example collects logs from multiple source groups and uses the source() filter in the embedded log statement to select messages of the s_network source group.

   log {
        source(s_localhost);
        source(s_network);
        destination(d_file1);
        log {
            filter {
                source(s_network);
            };
        destination(d_file2);
        };
    };

2 - if-else-elif: Conditional expressions

You can use if {}, elif {}, and else {} blocks to configure conditional expressions.

Conditional expressions’ format

Conditional expressions have two formats:

• Explicit filter expression:

      if (message('foo')) {
          parser { date-parser(); };
      } else {
          ...
      };
  

  This format only uses the filter expression in if(). If if does not contain 'foo', the else branch is taken.

  The else{} branch can be empty, you can use it to send the message to the default branch.

• Condition embedded in the log path:

      if {
          filter { message('foo')); };
          parser { date-parser(); };
      } else {
          ...
      };
  

  This format considers all filters and all parsers as the condition, combined. If the message contains 'foo' and the date-parser() fails, the else branch is taken. Similarly, if the message does not contain 'foo', the else branch is taken.

Using the if {} and else {} blocks in your configuration

You can copy-paste the following example and use it as a template for using the if {} and else {} blocks in your configuration.

Example for using the if {} and else {} blocks in your configuration

The following configuration can be used as a template for using the if {} and else {} blocks:

   log{
      source { example-msg-generator(num(1) template("...,STRING-TO-MATCH,..."));};
      source { example-msg-generator(num(1) template("...,NO-MATCH,..."));};
     
    if (message("STRING-TO-MATCH")) 
       {   
        destination { file(/dev/stdout template("matched: $MSG\n") persist-name("1")); };
       }
    else    
       {
        destination { file(/dev/stdout template("unmatched: $MSG\n") persist-name("2")); };
       };
    };

The configuration results in the following console printout:

   matched: ...,STRING-TO-MATCH,...
    unmatched: ...,NO-MATCH,...

An alternative, less straightforward way to implement conditional evaluation is to use junctions. For details on junctions and channels, see Junctions and channels.

3 - Junctions and channels

Junctions make it possible to send the messages to different channels, process the messages differently on each channel, and then join every channel together again. You can define any number of channels in a junction: every channel receives a copy of every message that reaches the junction. Every channel can process the messages differently, and at the end of the junction, the processed messages of every channel return to the junction again, where further processing is possible.

A junction includes one or more channels. A channel usually includes at least one filter, though that is not enforced. Otherwise, channels are identical to log statements, and can include any kind of objects, for example, parsers, rewrite rules, destinations, and so on. (For details on using channels, as well as on using channels outside junctions, see Using channels in configuration objects.)

You can also use log-path flags in the channels of the junction. Within the junction, a message is processed by every channel, in the order the channels appear in the configuration file. Typically if your channels have filters, you also set the flags(final) option for the channel. However, note that the log-path flags of the channel apply only within the junction, for example, if you set the final flag for a channel, then the subsequent channels of the junction will not receive the message, but this does not affect any other log path or junction of the configuration. The only exception is the flow-control flag: if you enable flow-control in a junction, it affects the entire log path. For details on log-path flags, see Log path flags.

   junction {
        channel { <other-syslog-ng-objects> <log-path-flags>};
        channel { <other-syslog-ng-objects> <log-path-flags>};
        ...
    };

Example: Using junctions

For example, suppose that you have a single network source that receives log messages from different devices, and some devices send messages that are not RFC-compliant (some routers are notorious for that). To solve this problem in earlier versions of AxoSyslog, you had to create two different network sources using different IP addresses or ports: one that received the RFC-compliant messages, and one that received the improperly formatted messages (for example, using the flags(no-parse) option). Using junctions this becomes much more simple: you can use a single network source to receive every message, then use a junction and two channels. The first channel processes the RFC-compliant messages, the second everything else. At the end, every message is stored in a single file. The filters used in the example can be host() filters (if you have a list of the IP addresses of the devices sending non-compliant messages), but that depends on your environment.

   log {
        source {
            syslog(
                ip(10.1.2.3)
                transport("tcp")
                flags(no-parse)
            );
        };
        junction {
            channel {
                filter(f_compliant_hosts);
                parser {
                    syslog-parser();
                };
            };
            channel {
                filter(f_noncompliant_hosts);
            };
        };
        destination {
            file("/var/log/messages");
        };
    };

Since every channel receives every message that reaches the junction, use the flags(final) option in the channels to avoid the unnecessary processing the messages multiple times:

   log {
        source {
            syslog(
                ip(10.1.2.3)
                transport("tcp")
                flags(no-parse)
            );
        };
        junction {
            channel {
                filter(f_compliant_hosts);
                parser {
                    syslog-parser();
                };
                flags(final);
            };
            channel {
                filter(f_noncompliant_hosts);
                flags(final);
            };
        };
        destination {
            file("/var/log/messages");
        };
    };

An alternative, more straightforward way to implement conditional evaluation is to configure conditional expressions using if {}, elif {}, and else {} blocks. For details, see if-else-elif: Conditional expressions.

4 - Log path flags

Flags influence the behavior of syslog-ng, and the way it processes messages. The following flags may be used in the log paths, as described in Log paths.

Log statement flags

catchall

This flag means that the source of the message is ignored, only the filters of the log path are taken into account when matching messages. A log statement using the catchall flag processes every message that arrives to any of the defined sources.

drop-unmatched

This flag means that the message is dropped along a log path when it does not match a filter or is discarded by a parser. Without using the drop-unmatched flag, AxoSyslog would continue to process the message along alternative paths.

fallback

This flag makes a log statement ‘fallback’. Fallback log statements process messages that were not processed by other, ’non-fallback’ log statements.

Processed means that every filter of a log path matched the message. Note that in the case of embedded log paths, the message is considered to be processed if it matches the filters of the outer log path, even if it does not match the filters of the embedded log path. For details, see Example: Using log path flags.

final

This flag means that the processing of log messages processed by the log statement ends here, other log statements appearing later in the configuration file will not process the messages processed by the log statement labeled as ‘final’. Note that this does not necessarily mean that matching messages will be stored only once, as there can be matching log statements processed before the current one (AxoSyslog evaluates log statements in the order they appear in the configuration file).

Processed means that every filter of a log path matched the message. Note that in the case of embedded log paths, the message is considered to be processed if it matches the filters of the outer log path, even if it does not match the filters of the embedded log path. For details, see Example: Using log path flags.

flow-control

Enables flow-control to the log path, meaning that AxoSyslog will stop reading messages from the sources of this log statement if the destinations are not able to process the messages at the required speed. If disabled, AxoSyslog will drop messages if the destination queues are full. If enabled, AxoSyslog will only drop messages if the destination queues/window sizes are improperly sized. For details, see Managing incoming and outgoing messages with flow-control.

Example: Using log path flags

Let’s suppose that you have two hosts (myhost_A and myhost_B) that run two applications each (application_A and application_B), and you collect the log messages to a central AxoSyslog server. On the server, you create two log paths:

• one that processes only the messages sent by myhost_A, and

• one that processes only the messages sent by application_A.

This means that messages sent by application_A running on myhost_A will be processed by both log paths, and the messages of application_B running on myhost_B will not be processed at all.

• If you add the final flag to the first log path, then only this log path will process the messages of myhost_A, so the second log path will receive only the messages of application_A running on myhost_B.

• If you create a third log path that includes the fallback flag, it will process the messages not processed by the first two log paths, in this case, the messages of application_B running on myhost_B.

• Adding a fourth log path with the catchall flag would process every message received by the AxoSyslog server.

      log { source(s_localhost); destination(d_file); flags(catchall); };
  

The following example shows a scenario that can result in message loss. Do NOT use such a configuration, unless you know exactly what you are doing. The problem is if a message matches the filters in the first part of the first log path, AxoSyslog treats the message as ‘processed’. Since the first log path includes the final flag, AxoSyslog will not pass the message to the second log path (the one with the fallback flag). As a result, AxoSyslog drops messages that do not match the filter of the embedded log path.

   # Do not use such a configuration, unless you know exactly what you are doing.
    log {
        source(s_network);
        # Filters in the external log path.
        # If a message matches this filter, it is treated as 'processed'
        filter(f_program);
        filter(f_message);
        log {
            # Filter in the embedded log path.
            # If a message does not match this filter, it is lost, it will not be processed by the 'fallback' log path
            filter(f_host);
            destination(d_file1);
        };
        flags(final);
    };
    
    log {
        source(s_network);
        destination(d_file2);
        flags(fallback);
    };

Example: Using the drop-unmatched flag

In the following example, if a log message arrives whose $MSG part does not contain the string foo, then AxoSyslog will discard the message and will not check compliance with the second if condition.

   ...
    if {
        filter { message('foo') };
        flags(drop-unmatched)
    };
    if {
        filter { message('bar') };
    };
    ...

(Without the drop-unmatched flag, AxoSyslog would check if the message complies with the second if condition, that is, whether or not the message contains the string bar .)