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Parsing syslog messages

By default, AxoSyslog parses every message using the syslog-parser as a syslog message, and fills the macros with values of the message. The syslog-parser does not discard messages: the message cannot be parsed as a syslog message, the entire message (including its header) is stored in the ${MSG} macro. If you do not want to parse the message as a syslog message, use the flags(no-parse) option of the source.

You can also use the syslog-parser to explicitly parse a message, or a part of a message as a syslog message (for example, after rewriting the beginning of a message that does not comply with the syslog standards).

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");
        };
    };

Note that by default, the syslog-parser attempts to parse the message as an RFC3164-formatted (BSD-syslog) message. To parse the message as an RFC5424-formatted message, use the flags(syslog-protocol) option in the parser.

   syslog-parser(flags(syslog-protocol));

Parsing errors

AxoSyslog 4.7 and newer automatically adds the following tags if it encounters errors when parsing syslog messages.

  • message.utf8_sanitized
  • message.parse_error
  • syslog.missing_pri
  • syslog.missing_timestamp
  • syslog.invalid_hostname
  • syslog.unexpected_framing
  • syslog.rfc3164_missing_header
  • syslog.rfc5424_unquoted_sdata_value

1 - Options of syslog-parser() parsers

The syslog-parser() has the following options:

default-facility()
Type:facility string
Default:kern

Description: This parameter assigns a facility value to the messages received from the file source if the message does not specify one.

default-priority()
Type:priority string
Default:

Description: This parameter assigns an emergency level to the messages received from the file source if the message does not specify one. For example, default-priority(warning).

drop-invalid()
Type:yes or no
Values:`yes
Default:no

Description: This option determines how the syslog-parser() affects messages when parsing fails.

If you set drop-invalid() to yes, syslog-parser() will drop the message if the parsing fails.

If you set drop-invalid() to no, the parsing error triggers syslog-parser() to rewrite and extend the original log message with the following additional information:

  • It prepends the following message to the contents of the $MESSAGE field: Error processing log message.
  • It sets the contents of the $PROGRAM field to syslog-ng.
  • It sets the contents of the facility field to syslog.
  • It sets the contents of the severity field to error.

Example: enabling the drop-invalid() option

   parser p_syslog {  syslog-parser(drop-invalid(yes)); };

flags()
Type:assume-utf8, empty-lines, expect-hostname, kernel, no-hostname, no-multi-line, no-parse, sanitize-utf8, store-legacy-msghdr, store-raw-message, syslog-protocol, validate-utf8
Default:empty set

Description: Specifies the log parsing options of the source.

  • assume-utf8: The assume-utf8 flag assumes that the incoming messages are UTF-8 encoded, but does not verify the encoding. If you explicitly want to validate the UTF-8 encoding of the incoming message, use the validate-utf8 flag.

  • empty-lines: Use the empty-lines flag to keep the empty lines of the messages. By default, AxoSyslog removes empty lines automatically.

  • expect-hostname: If the expect-hostname flag is enabled, AxoSyslog will assume that the log message contains a hostname and parse the message accordingly. This is the default behavior for TCP sources. Note that pipe sources use the no-hostname flag by default.

  • guess-timezone: Attempt to guess the timezone of the message if this information is not available in the message. Works when the incoming message stream is close to real time, and the timezone information is missing from the timestamp.

  • kernel: The kernel flag makes the source default to the LOG_KERN | LOG_NOTICE priority if not specified otherwise.

  • no-header: The no-header flag triggers AxoSyslog to parse only the PRI field of incoming messages, and put the rest of the message contents into $MSG.

    Its functionality is similar to that of the no-parse flag, except the no-header flag does not skip the PRI field.

    Example: using the no-header flag with the syslog-parser() parser

    The following example illustrates using the no-header flag with the syslog-parser() parser:

        parser p_syslog {
      syslog-parser(
        flags(no-header)
      );
    };

  • no-hostname: Enable the no-hostname flag if the log message does not include the hostname of the sender host. That way AxoSyslog assumes that the first part of the message header is ${PROGRAM} instead of ${HOST}. For example:

        source s_dell {
        network(
            port(2000)
            flags(no-hostname)
        );
    };

  • no-multi-line: The no-multi-line flag disables line-breaking in the messages: the entire message is converted to a single line. Note that this happens only if the underlying transport method actually supports multi-line messages. Currently the file() and pipe() drivers support multi-line messages.

  • no-parse: By default, AxoSyslog parses incoming messages as syslog messages. The no-parse flag completely disables syslog message parsing and processes the complete line as the message part of a syslog message. The AxoSyslog application will generate a new syslog header (timestamp, host, and so on) automatically and put the entire incoming message into the MESSAGE part of the syslog message (available using the ${MESSAGE} macro). This flag is useful for parsing messages not complying to the syslog format.

    If you are using the flags(no-parse) option, then syslog message parsing is completely disabled, and the entire incoming message is treated as the ${MESSAGE} part of a syslog message. In this case, AxoSyslog generates a new syslog header (timestamp, host, and so on) automatically. Note that even though flags(no-parse) disables message parsing, some flags can still be used, for example, the no-multi-line flag.

  • dont-store-legacy-msghdr: By default, AxoSyslog stores the original incoming header of the log message. This is useful if the original format of a non-syslog-compliant message must be retained (AxoSyslog automatically corrects minor header errors, for example, adds a whitespace before msg in the following message: Jan 22 10:06:11 host program:msg). If you do not want to store the original header of the message, enable the dont-store-legacy-msghdr flag.

  • sanitize-utf8: When using the sanitize-utf8 flag, AxoSyslog converts non-UTF-8 input to an escaped form, which is valid UTF-8.

    Prior to version 4.6, this flag worked only when parsing RFC3164 messages. Starting with version 4.6, it works also for RFC5424 and raw messages.

  • store-raw-message: Save the original message as received from the client in the ${RAWMSG} macro. You can forward this raw message in its original form to another AxoSyslog node using the syslog-ng() destination, or to a SIEM system, ensuring that the SIEM can process it. Available only in 3.16 and later.

  • syslog-protocol: The syslog-protocol flag specifies that incoming messages are expected to be formatted according to the new IETF syslog protocol standard (RFC5424), but without the frame header. Note that this flag is not needed for the syslog driver, which handles only messages that have a frame header.

  • validate-utf8: The validate-utf8 flag enables encoding-verification for messages.

    Prior to version 4.6, this flag worked only when parsing RFC3164 messages. Starting with version 4.6, it works also for RFC5424 and raw messages.

    For RFC5424-formatted messages, if the BOM character is missing, but the message is otherwise UTF-8 compliant, AxoSyslog automatically adds the BOM character to the message.

    The byte order mark (BOM) is a Unicode character used to signal the byte-order of the message text.

sdata-prefix()
Type:string
Default:.SDATA.

Available in AxoSyslog 4.1 and later.

Description: Adds a specific string before the names of the parsed SDATA fields to store the name-value pairs created from the SDATA fields separately. Note that unless the value of sdata-prefix starts with .SDATA., using this option excludes the parsed fields from the sdata and rfc5424 scopes of the value pairs.

template()
Synopsis:template("${<macroname>}")

Description: The macro that contains the part of the message that the parser will process. It can also be a macro created by a previous parser of the log path. By default, the parser processes the entire message (${MESSAGE}).