syslog-ng Open Source Edition 3.20 - Administration Guide

Preface Introduction to syslog-ng The concepts of syslog-ng Installing syslog-ng The syslog-ng OSE quick-start guide The syslog-ng OSE configuration file source: Read, receive, and collect log messages
How sources work default-network-drivers: Receive and parse common syslog messages internal: Collecting internal messages file: Collecting messages from text files wildcard-file: Collecting messages from multiple text files linux-audit: Collecting messages from Linux audit logs network: Collecting messages using the RFC3164 protocol (network() driver) nodejs: Receiving JSON messages from nodejs applications mbox: Converting local e-mail messages to log messages osquery: Collect and parse osquery result logs pipe: Collecting messages from named pipes pacct: Collecting process accounting logs on Linux program: Receiving messages from external applications python: writing server-style Python sources python-fetcher: writing fetcher-style Python sources snmptrap: Read Net-SNMP traps sun-streams: Collecting messages on Sun Solaris syslog: Collecting messages using the IETF syslog protocol (syslog() driver) system: Collecting the system-specific log messages of a platform systemd-journal: Collecting messages from the systemd-journal system log storage systemd-syslog: Collecting systemd messages using a socket tcp, tcp6, udp, udp6: Collecting messages from remote hosts using the BSD syslog protocol— OBSOLETE unix-stream, unix-dgram: Collecting messages from UNIX domain sockets stdin: Collecting messages from the standard input stream
destination: Forward, send, and store log messages
amqp: Publishing messages using AMQP collectd: sending metrics to collectd elasticsearch2: Sending logs directly to Elasticsearch and Kibana 2.0 or higher file: Storing messages in plain-text files graphite: Sending metrics to Graphite Sending logs to Graylog hdfs: Storing messages on the Hadoop Distributed File System (HDFS) Posting messages over HTTP http: Posting messages over HTTP without Java kafka: Publishing messages to Apache Kafka loggly: Using Loggly logmatic: Using Logmatic.io mongodb: Storing messages in a MongoDB database network: Sending messages to a remote log server using the RFC3164 protocol (network() driver) osquery: Sending log messages to osquery's syslog table pipe: Sending messages to named pipes program: Sending messages to external applications pseudofile() python: writing custom Python destinations redis: Storing name-value pairs in Redis riemann: Monitoring your data with Riemann slack: Sending alerts and notifications to a Slack channel smtp: Generating SMTP messages (e-mail) from logs Splunk: Sending log messages to Splunk sql: Storing messages in an SQL database stomp: Publishing messages using STOMP syslog: Sending messages to a remote logserver using the IETF-syslog protocol syslog-ng: Forwarding messages and tags to another syslog-ng node tcp, tcp6, udp, udp6: Sending messages to a remote log server using the legacy BSD-syslog protocol (tcp(), udp() drivers) Telegram: Sending messages to Telegram unix-stream, unix-dgram: Sending messages to UNIX domain sockets usertty: Sending messages to a user terminal: usertty() destination Write your own custom destination in Java or Python Client-side failover
log: Filter and route log messages using log paths, flags, and filters Global options of syslog-ng OSE TLS-encrypted message transfer template and rewrite: Format, modify, and manipulate log messages parser: Parse and segment structured messages db-parser: Process message content with a pattern database (patterndb) Correlating log messages Enriching log messages with external data Statistics of syslog-ng Multithreading and scaling in syslog-ng OSE Troubleshooting syslog-ng Best practices and examples The syslog-ng manual pages Third-party contributions Creative Commons Attribution Non-commercial No Derivatives (by-nc-nd) License About us

Date-related macros

The macros related to the date of the message (for example: ${ISODATE}, ${HOUR}, and so on) have three further variants each:

  • S_ prefix, for example, ${S_DATE}: The ${S_DATE} macro represents the date found in the log message, that is, when the message was sent by the original application.

    Caution:

    To use the S_ macros, the keep-timestamp() option must be enabled (this is the default behavior of syslog-ng OSE).

  • R_ prefix, for example, ${R_DATE}: ${R_DATE} is the date when syslog-ng OSE has received the message.

  • C_ prefix, for example, ${C_DATE}: ${C_DATE} is the current date, that is when syslog-ng OSE processes the message and resolves the macro.

The ${DATE} macro equals the ${S_DATE} macro.

The values of the date-related macros are calculated using the original timezone information of the message. To convert it to a different timezone, use the time-zone() option. You can set the time-zone() option as a global option, or per destination. For sources, it applies only if the original message does not contain timezone information. Converting the timezone changes the values of the following date-related macros (macros MSEC and USEC are not changed):

  • AMPM

  • DATE

  • DAY

  • FULLDATE

  • HOUR

  • HOUR12

  • ISODATE

  • MIN

  • MONTH

  • MONTH_ABBREV

  • MONTH_NAME

  • MONTH_WEEK

  • SEC

  • STAMP

  • TZ

  • TZOFFSET

  • UNIXTIME

  • WEEK

  • WEEK_DAY

  • WEEK_DAY_ABBREV

  • WEEK_DAY_NAME

  • YEAR

  • YEAR_DAY


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Hard vs. soft macros

Hard macros contain data that is directly derived from the log message, for example, the ${MONTH} macro derives its value from the timestamp. Hard macros are read-only. Soft macros (sometimes also called name-value pairs) are either built-in macros automatically generated from the log message (for example, ${HOST}), or custom user-created macros generated by using the syslog-ng pattern database or a CSV-parser. In contrast to hard macros, soft macros are writable and can be modified within syslog-ng OSE, for example, using rewrite rules.

Hard and soft macros are rather similar and often treated as equivalent. Macros are most commonly used in filters and templates, which does not modify the value of the macro, so both soft and hard macros can be used. However, it is not possible to change the values of hard macros in rewrite rules or via any other means.

The following macros in syslog-ng OSE are hard macros and cannot be modified: BSDTAG, CONTEXT_ID, DATE, DAY, FACILITY_NUM, FACILITY, FULLDATE, HOUR, ISODATE, LEVEL_NUM, LEVEL, MIN, MONTH_ABBREV, MONTH_NAME, MONTH, MONTH_WEEK, , PRIORITY, PRI, RCPTID, SDATA, SEC, SEQNUM, SOURCEIP, STAMP, TAG, TAGS, TZOFFSET, TZ, UNIXTIME, WEEK_DAY_ABBREV, WEEK_DAY_NAME, WEEK_DAY, WEEK, YEAR_DAY, YEAR.

The following macros can be modified:FULLHOST_FROM, FULLHOST, HOST_FROM, HOST, LEGACY_MSGHDR, MESSAGE, MSG,MSGID, MSGONLY, PID, PROGRAM, SOURCE. Custom values created using rewrite rules or parsers can be modified as well, just like stored matches of regular expressions ($0 ... $255).


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Macros of syslog-ng OSE

The following macros are available in syslog-ng OSE.

Caution:

These macros are available when syslog-ng OSE successfully parses the incoming message as a syslog message, or you use some other parsing method and map the parsed values to these macros.

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, syslog-ng OSE generates a new syslog header (timestamp, host, and so on) automatically. Note that since flags(no-parse) disables message parsing, it interferes with other flags, for example, disables flags(no-multi-line).

AMPM

Description: Typically used together with the ${HOUR12} macro, ${AMPM} returns the period of the day: AM for hours before mid day and PM for hours after mid day. In reference to a 24-hour clock format, AM is between 00:00-12:00 and PM is between 12:00-24:00. 12AM is midnight. Available in syslog-ng OSE 3.4 and later.

BSDTAG

Description: Facility/priority information in the format used by the FreeBSD syslogd: a priority number followed by a letter that indicates the facility. The priority number can range from 0 to 7. The facility letter can range from A to Y, where A corresponds to facility number zero (LOG_KERN), B corresponds to facility 1 (LOG_USER), and so on.

Custom macros

Description: CSV parsers and pattern databases can also define macros from the content of the messages, for example, a pattern database rule can extract the username from a login message and create a macro that references the username. For details on using custom macros created with CSV parsers and pattern databases, see parser: Parse and segment structured messages and Using parser results in filters and templates, respectively.

DATE, C_DATE, R_DATE, S_DATE

Description: Date of the message using the BSD-syslog style timestamp format (month/day/hour/minute/second, each expressed in two digits). This is the original syslog time stamp without year information, for example: Jun 13 15:58:00.

DAY, C_DAY, R_DAY, S_DAY

Description: The day the message was sent.

FACILITY

Description: The name of the facility (for example, kern) that sent the message.

FACILITY_NUM

Description: The numerical code of the facility (for example, 0) that sent the message.

FILE_NAME

Description: Name of the log file (including its path) from where syslog-ng OSE received the message (only available if syslog-ng OSE received the message from a file or a wildcard-file source). If you need only the path or the filename, use the dirname and basename template functions.

FULLDATE, C_FULLDATE, R_FULLDATE, S_FULLDATE

Description: A nonstandard format for the date of the message using the same format as ${DATE}, but including the year as well, for example: 2006 Jun 13 15:58:00.

FULLHOST

Description: The name of the source host where the message originates from.

  • If the message traverses several hosts and the chain-hostnames() option is on, the first host in the chain is used.

  • If the keep-hostname() option is disabled (keep-hostname(no)), the value of the $FULLHOST macro will be the DNS hostname of the host that sent the message to syslog-ng OSE (that is, the DNS hostname of the last hop). In this case the $FULLHOST and $FULLHOST_FROM macros will have the same value.

  • If the keep-hostname() option is enabled (keep-hostname(yes)), the value of the $FULLHOST macro will be the hostname retrieved from the log message. That way the name of the original sender host can be used, even if there are log relays between the sender and the server.

    NOTE:

    The use-dns(), use-fqdn(), normalize-hostnames(), and dns-cache() options will have no effect if the keep-hostname() option is enabled (keep-hostname(yes)) and the message contains a hostname.

For details on using name resolution in syslog-ng OSE, see Using name resolution in syslog-ng.

FULLHOST_FROM

Description: The FQDN of the host that sent the message to syslog-ng as resolved by syslog-ng using DNS. If the message traverses several hosts, this is the last host in the chain.

The syslog-ng OSE application uses the following procedure to determine the value of the $FULLHOST_FROM macro:

  1. The syslog-ng OSE application takes the IP address of the host sending the message.

  2. If the use-dns() option is enabled, syslog-ng OSE attempts to resolve the IP address to a hostname. If it succeeds, the returned hostname will be the value of the $FULLHOST_FROM macro. This value will be the FQDN of the host if the use-fqdn() option is enabled, but only the hostname if use-fqdn() is disabled.

  3. If the use-dns() option is disabled, or the address resolution fails, the ${FULLHOST_FROM} macro will return the IP address of the sender host.

For details on using name resolution in syslog-ng OSE, see Using name resolution in syslog-ng.

HOUR, C_HOUR, R_HOUR, S_HOUR

Description: The hour of day the message was sent.

HOUR12, C_HOUR12, R_HOUR12, S_HOUR12

Description: The hour of day the message was sent in 12-hour clock format. See also the ${AMPM} macro. 12AM is midnight. Available in syslog-ng OSE 3.4 and later.

HOST

Description: The name of the source host where the message originates from.

  • If the message traverses several hosts and the chain-hostnames() option is on, the first host in the chain is used.

  • If the keep-hostname() option is disabled (keep-hostname(no)), the value of the $HOST macro will be the DNS hostname of the host that sent the message to syslog-ng OSE (that is, the DNS hostname of the last hop). In this case the $HOST and $HOST_FROM macros will have the same value.

  • If the keep-hostname() option is enabled (keep-hostname(yes)), the value of the $HOST macro will be the hostname retrieved from the log message. That way the name of the original sender host can be used, even if there are log relays between the sender and the server.

    NOTE:

    The use-dns(), use-fqdn(), normalize-hostnames(), and dns-cache() options will have no effect if the keep-hostname() option is enabled (keep-hostname(yes)) and the message contains a hostname.

For details on using name resolution in syslog-ng OSE, see Using name resolution in syslog-ng.

HOST_FROM

Description: The FQDN of the host that sent the message to syslog-ng as resolved by syslog-ng using DNS. If the message traverses several hosts, this is the last host in the chain.

The syslog-ng OSE application uses the following procedure to determine the value of the $HOST_FROM macro:

  1. The syslog-ng OSE application takes the IP address of the host sending the message.

  2. If the use-dns() option is enabled, syslog-ng OSE attempts to resolve the IP address to a hostname. If it succeeds, the returned hostname will be the value of the $HOST_FROM macro. This value will be the FQDN of the host if the use-fqdn() option is enabled, but only the hostname if use-fqdn() is disabled.

  3. If the use-dns() option is disabled, or the address resolution fails, the ${HOST_FROM} macro will return the IP address of the sender host.

For details on using name resolution in syslog-ng OSE, see Using name resolution in syslog-ng.

ISODATE, C_ISODATE, R_ISODATE, S_ISODATE

Description: Date of the message in the ISO 8601 compatible standard timestamp format (yyyy-mm-ddThh:mm:ss+-ZONE), for example: 2006-06-13T15:58:00.123+01:00. If possible, it is recommended to use ${ISODATE} for timestamping. Note that syslog-ng can produce fractions of a second (for example milliseconds) in the timestamp by using the frac-digits() global or per-destination option.

LEVEL_NUM

Description: The priority (also called severity) of the message, represented as a numeric value, for example, 3. For the textual representation of this value, use the ${LEVEL} macro. See PRIORITY or LEVEL for details.

LOGHOST

Description: The hostname of the computer running syslog-ng OSE — it returns the same result as the hostname command.

MESSAGE

Description: Text contents of the log message without the program name and pid. The program name and the pid together are available in the ${MSGHDR} macro, and separately in the ${PROGRAM} and ${PID} macros.

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, syslog-ng OSE generates a new syslog header (timestamp, host, and so on) automatically. Note that since flags(no-parse) disables message parsing, it interferes with other flags, for example, disables flags(no-multi-line).

The ${MSG} macro is an alias of the ${MESSAGE} macro: using ${MSG} in syslog-ng OSE is equivalent to ${MESSAGE}.

Note that before syslog-ng version 3.0, the ${MESSAGE} macro included the program name and the pid. In syslog-ng 3.0, the ${MESSAGE} macro became equivalent with the ${MSGONLY} macro.

MIN, C_MIN, R_MIN, S_MIN

Description: The minute the message was sent.

MONTH, C_MONTH, R_MONTH, S_MONTH

Description: The month the message was sent as a decimal value, prefixed with a zero if smaller than 10.

MONTH_ABBREV, C_MONTH_ABBREV, R_MONTH_ABBREV, S_MONTH_ABBREV

Description: The English abbreviation of the month name (3 letters).

MONTH_NAME, C_MONTH_NAME, R_MONTH_NAME, S_MONTH_NAME

Description: The English name of the month name.

MONTH_WEEK, C_MONTH_WEEK, R_MONTH_WEEK, S_MONTH_WEEK

Description: The number of the week in the given month (0-5). The week with numerical value 1 is the first week containing a Monday. The days of month before the first Monday are considered week 0. For example, if a 31-day month begins on a Sunday, then the 1st of the month is week 0, and the end of the month (the 30th and 31st) is week 5.

MSEC, C_MSEC, R_MSEC, S_MSEC

Description: The millisecond the message was sent.

Available in syslog-ng OSE version 3.4 and later.

MSG

The ${MSG} macro is an alias of the ${MESSAGE} macro, using ${MSG} in syslog-ng OSE is equivalent to ${MESSAGE}. For details on this macro, see MESSAGE.

MSGHDR

Description: The name and the PID of the program that sent the log message in PROGRAM[PID]: format. Includes a trailing whitespace. Note that the macro returns an empty value if both the PROGRAM and PID fields of the message are empty.

MSGID

Description: A string specifying the type of the message in IETF-syslog (RFC5424-formatted) messages. For example, a firewall might use the ${MSGID} "TCPIN" for incoming TCP traffic and the ${MSGID} "TCPOUT" for outgoing TCP traffic. By default, syslog-ng OSE does not specify this value, but uses a dash (-) character instead. If an incoming message includes the ${MSGID} value, it is retained and relayed without modification.

MSGONLY

Description: Message contents without the program name or pid. Starting with syslog-ng OSE 3.0, the following macros are equivalent: ${MSGONLY}, ${MSG}, ${MESSAGE}. For consistency, use the ${MESSAGE} macro. For details, see MESSAGE.

PID

Description: The PID of the program sending the message.

PRI

Description: The priority and facility encoded as a 2 or 3 digit decimal number as it is present in syslog messages.

PRIORITY or LEVEL

Description: The priority (also called severity) of the message, for example, error. For the textual representation of this value, use the ${LEVEL} macro. See PRIORITY or LEVEL for details.

PROGRAM

Description: The name of the program sending the message. Note that the content of the ${PROGRAM} variable may not be completely trusted as it is provided by the client program that constructed the message.

RAWMSG

Description: The original message as received from the client. Note that this macro is available only in 3.16 and later, and only if syslog-ng received the message using the default-network-drivers-ng() source, or the source receiving the message has the store-raw-message flag set.

RCPTID

Description: When the use-rcptid global option is set to yes, syslog-ng OSE automatically assigns a unique reception ID to every received message. You can access this ID and use it in templates via the ${RCPTID} macro. The reception ID is a monotonously increasing 48-bit integer number, that can never be zero (if the counter overflows, it restarts with 1).

RUNID

Description: An ID that changes its value every time syslog-ng OSE is restarted, but not when reloaded.

SDATA, .SDATA.SDID.SDNAME

Description: The syslog-ng application automatically parses the STRUCTURED-DATA part of IETF-syslog messages, which can be referenced in macros. The ${SDATA} macro references the entire STRUCTURED-DATA part of the message, while structured data elements can be referenced using the ${.SDATA.SDID.SDNAME} macro.

NOTE:

When using STRUCTURED-DATA macros, consider the following:

  • When referencing an element of the structured data, the macro must begin with the dot (.) character. For example, ${.SDATA.timeQuality.isSynced}.

  • The SDID and SDNAME parts of the macro names are case sensitive: ${.SDATA.timeQuality.isSynced} is not the same as ${.SDATA.TIMEQUALITY.ISSYNCED}.

Example: Using SDATA macros

For example, if a log message contains the following structured data: [exampleSDID@0 iut="3" eventSource="Application" eventID="1011"][examplePriority@0 class="high"] you can use macros like: ${.SDATA.exampleSDID@0.eventSource} — this would return the Application string in this case.

SEC, C_SEC, R_SEC, S_SEC

Description: The second the message was sent.

SEQNUM

Description: The ${SEQNUM} macro contains a sequence number for the log message. The value of the macro depends on the scenario, and can be one of the following:

  • If syslog-ng OSE receives a message via the IETF-syslog protocol that includes a sequence ID, this ID is automatically available in the ${SEQNUM} macro.

  • If the message is a Cisco IOS log message using the extended timestamp format, then syslog-ng OSE stores the sequence number from the message in this macro. If you forward this message the IETF-syslog protocol, syslog-ng OSE includes the sequence number received from the Cisco device in the ${.SDATA.meta.sequenceId} part of the message.

    NOTE:

    To enable sequence numbering of log messages on Cisco devices, use the following command on the device (available in IOS 10.0 and later): service sequence-numbers. For details, see the manual of your Cisco device.

  • For locally generated messages (that is, for messages that are received from a local source, and not from the network), syslog-ng OSE calculates a sequence number when sending the message to a destination (it is not calculated for relayed messages).

    • The sequence number is not global, but per-destination. Essentially, it counts the number of messages sent to the destination.

    • This sequence number increases by one for every message sent to the destination. It not lost when syslog-ng OSE is reloaded, but it is reset when syslog-ng OSE is restarted.

    • This sequence number is added to every message that uses the IETF-syslog protocol (${.SDATA.meta.sequenceId}), and can be added to BSD-syslog messages using the ${SEQNUM} macro.

NOTE:

If you need a sequence number for every log message that syslog-ng OSE receives, use the RCPTID macro.

SOURCE

Description: The identifier of the source statement in the syslog-ng OSE configuration file that received the message. For example, if syslog-ng OSE received the log message from the source s_local { internal(); }; source statement, the value of the ${SOURCE} macro is s_local. This macro is mainly useful for debugging and troubleshooting purposes.

SOURCEIP

Description: IP address of the host that sent the message to syslog-ng. (That is, the IP address of the host in the ${FULLHOST_FROM} macro.) Please note that when a message traverses several relays, this macro contains the IP of the last relay.

STAMP, R_STAMP, S_STAMP

Description: A timestamp formatted according to the ts-format() global or per-destination option.

SYSUPTIME

Description: The time elapsed since the syslog-ng OSE instance was started (that is, the uptime of the syslog-ng OSE process). The value of this macro is an integer containing the time in 1/100th of the second.

Available in syslog-ng OSE version 3.4 and later.

TAG

Description: The priority and facility encoded as a 2 digit hexadecimal number.

TAGS

Description: A comma-separated list of the tags assigned to the message.

NOTE:

Note that the tags are not part of the log message and are not automatically transferred from a client to the server. For example, if a client uses a pattern database to tag the messages, the tags are not transferred to the server. A way of transferring the tags is to explicitly add them to the log messages using a template and the ${TAGS} macro, or to add them to the structured metadata part of messages when using the IETF-syslog message format.

When sent as structured metadata, it is possible to reference to the list of tags on the central server, and for example, to add them to a database column.

TZ, C_TZ, R_TZ, S_TZ

Description: An alias of the ${TZOFFSET} macro.

TZOFFSET, C_TZOFFSET, R_TZOFFSET, S_TZOFFSET

Description: The time-zone as hour offset from GMT, for example: -07:00. In syslog-ng 1.6.x this used to be -0700 but as ${ISODATE} requires the colon it was added to ${TZOFFSET} as well.

UNIXTIME, C_UNIXTIME, R_UNIXTIME, S_UNIXTIME

Description: Standard UNIX timestamp, represented as the number of seconds since 1970-01-01T00:00:00.

.TLS.X509

Description: When using a transport that uses TLS, these macros contain information about the peer's certificate. That way, you can use information from the client certificate in filenames, database values, or as other metadata. If you clients have their own certificates, then these values are unique per client, but unchangeable by the client. The following macros are available in syslog-ng OSE version 3.9 and later.

  • .TLS.X509_CN: The Common Name of the certificate.

  • .TLS.X509_O: The value of the Organization field.

  • .TLS.X509_OU: The value of the Organization Unit field.

UNIQID

Description: A globally unique ID generated from the HOSTID and the RCPTID in the format of HOSTID@RCPTID. For details, see use-uniqid() and RCPTID.

Available in syslog-ng OSE version 3.7 and later.

USEC, C_USEC, R_USEC, S_USEC

Description: The microsecond the message was sent.

Available in syslog-ng OSE version 3.4 and later.

YEAR, C_YEAR, R_YEAR, S_YEAR

Description: The year the message was sent.

WEEK, C_WEEK, R_WEEK, S_WEEK

Description: The week number of the year, prefixed with a zero for the first nine week of the year. (The first Monday in the year marks the first week.)

WEEK_DAY_ABBREV, C_WEEK_DAY_ABBREV, R_WEEK_DAY_ABBREV, S_WEEK_DAY_ABBREV

Description: The 3-letter English abbreviation of the name of the day the message was sent, for example Thu.

WEEK_DAY, C_WEEK_DAY, R_WEEK_DAY, S_WEEK_DAY

Description: The day of the week as a numerical value (1-7).

WEEKDAY, C_WEEKDAY, R_WEEKDAY, S_WEEKDAY

Description: These macros are deprecated, use ${WEEK_DAY_ABBREV}, ${R_WEEK_DAY_ABBREV}, ${S_WEEK_DAY_ABBREV} instead. The 3-letter name of the day of week the message was sent, for example Thu.

WEEK_DAY_NAME, C_WEEK_DAY_NAME, R_WEEK_DAY_NAME, S_WEEK_DAY_NAME

Description: The English name of the day.


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Using template functions

A template function is a transformation: it modifies the way macros or name-value pairs are expanded. Template functions can be used in template definitions, or when macros are used in the configuration of syslog-ng OSE. Template functions use the following syntax:

$(function-name parameter1 parameter2 parameter3 ...)

For example, the $(echo) template function simply returns the value of the macro it receives as a parameter, thus $(echo ${HOST}) is equivalent to ${HOST}.

The parameters of template functions are separated by a whitespace character. A template function can have maximum 64 parameters. If you want to use a longer string or multiple macros as a single parameter, enclose the parameter in double-quotes or apostrophes. For example:

$(echo "${HOST} ${PROGRAM} ${PID}")

Template functions can be nested into each other, so the parameter of a template function can be another template function, like:

$(echo $(echo ${HOST}))

For details on the available template functions, see the descriptions of the individual template functions in Template functions of syslog-ng OSE.

You can define your own template function as a regular configuration object (for example, to reuse the same function in different places in your configuration).

Declaration:
template-function <name-of-the-template-function> "<template-expression-using-strings-macros-template-functions>";
Example: Using custom template functions

The following template function can be used to reformat the message. It adds the length of the message to the message template.

template-function my-template-function "${ISODATE} ${HOST} message-length=$(length "${MSG}") ${MESSAGE}";
destination d_file {
    file("/tmp/mylogs.log" template("$(my-template-function)\n"));
};

You can also refer to existing templates in your template function.

template my-custom-header-template "${ISODATE} ${HOST_FROM} ${MSGHDR}";
template-function my-template-function "$(my-custom-header-template) message-length=$(length "${MESSAGE}") ${MESSAGE}";

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