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 with 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);
filter {
source(s_network);
};
destination(d_file1);
log {
filter {
source(s_network);
};
destination(d_file2);
};
};
You can use if {}, elif {}, and else {} blocks to configure conditional expressions.
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.
An alternative, less straightforward way to implement conditional evaluation is to use junctions. For details on junctions and channels, see 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.)
NOTE: Certain parsers can also act as filters:
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 syslog-ng PE, 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"); };
};
Note that syslog-ng PE has several parsers that you can use to parse non-compliant messages. You can even write a custom syslog-ng parser in Python. For details, see parser: Parse and segment structured messages.
NOTE: Junctions differ from embedded log statements, because embedded log statements are like branches: they split the flow of messages into separate paths, and the different paths do not meet again. Messages processed on different embedded log statements cannot be combined together for further processing. However, junctions split the messages to channels, then combine the channels together.
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.
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.
Table 12: 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, syslog-ng PE 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 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 (syslog-ng PE 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 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 syslog-ng 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, syslog-ng will drop messages if the destination queues are full. If enabled, syslog-ng will only drop messages if the destination queues/window sizes are improperly sized. For details, see Managing incoming and outgoing messages with flow-control. |
|
|
Caution:
The final, fallback, and catchall flags apply only for the top-level log paths, they have no effect on embedded log paths. |
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 syslog-ng 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 syslog-ng 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, syslog-ng PE treats the message as 'processed'. Since the first log path includes the final flag, syslog-ng PE will not pass the message to the second log path (the one with the fallback flag). As a result, syslog-ng PE 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 syslog-ng PE 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, syslog-ng PE would check if the message complies with the second if condition, that is, whether or not the message contains the string bar .)
