Extended Logging Target

After creating one or more extended logging targets with the log extended EXTLOGNAME command, the target(s) must be configured for the desired logging output.

Each extended log target supports emitting log messages in one of the following formats:

  • rfc5424 - RFC 5424 - modern syslog with ISO 8601 timestamps, time zone and structured data (key/value pairs) support

  • rfc3164 - RFC 3164 - legacy BSD syslog, timestamps with 1 second granularity

  • local-syslog - same as RFC 3164, but without the hostname field

  • journald - systemd’s native journald protocol. This protocol also supports structured data (key/value pairs).


The output location is configured with the following subcommands:

destination none

Disable the target while retaining its remaining configuration.

destination syslog [supports-rfc5424]

Send log messages to the system’s standard log destination (/dev/log). This does not use the C library’s syslog() function, instead writing directly to /dev/log.

On NetBSD and FreeBSD, the RFC5424 format is automatically used when the OS version is recent enough (5.0 for NetBSD, 12.0 for FreeBSD). Unfortunately, support for this format cannot be autodetected otherwise, and particularly on Linux systems must be enabled manually.

destination journald

Send log messages to systemd’s journald.

destination <stdout|stderr|fd <(0-63)|envvar WORD>>              [format FORMAT]

Send log messages to one of the daemon’s file descriptors. The fd (0-63) and fd envvar WORD variants are intended to work with the shell’s command 3>something and bash’s command {ENVVAR}>something I/O redirection specifiers.

Only file descriptors open at a daemon’s startup time can be used for this; accidental misuse of a file descriptor that has been opened by FRR itself is prevented.

Using FIFOs with this option will work but is unsupported and can cause daemons to hang or crash depending on reader behavior.

Format defaults to RFC5424 if none is specified.


When starting FRR daemons from custom shell scripts, make sure not to leak / leave extraneous file descriptors open. FRR daemons do not close these.

destination file PATH              [create [{user WORD|group WORD|mode PERMS}]|no-create]              [format FORMAT]

Log to a regular file. File permissions can be specified when FRR creates the file itself.

Format defaults to RFC5424 if none is specified.


FRR will never change permissions or ownership on an existing log file. In many cases, FRR will also not have permissions to set user and group arbitrarily.

destination unix PATH [format FORMAT]

Connect to a UNIX domain socket and send log messages there. This will autodetect SOCK_STREAM, SOCK_SEQPACKET and SOCK_DGRAM and adjust behavior appropriately.


priority PRIORITY

Select minimum priority of messages to send to this target. Defaults to debugging.

facility FACILITY

Select syslog facility for messages on this target. Defaults to daemon. The log facility [FACILITY] command does not affect extended targets.

timestamp precision (0-9)

Set desired number of sub-second timestamp digits. This only has an effect for RFC5424 and journald format targets; the RFC3164 and local-syslogd formats do not support any sub-second digits.

timestamp local-time

Use the local system timezone for timestamps rather than UTC (the default.)

RFC5424 and journald formats include zone information (Z or +-NN:NN suffix in ISO8601). RFC3164 and local-syslogd offer no way of identifying the time zone used, care must be taken that this option and the receiver are configured identically, or the timestamp is replaced at the receiver.


FRR includes a timestamp in journald messages, but journald always provides its own timestamp.

structured-data <code-location|version|unique-id|error-category|format-args>

Select additional key/value data to be included for the RFC5424 and journald formats. Refer to the next section for details.

unique-id and error-category are enabled by default.


Log messages can grow in size significantly when enabling additional data.

Structured data

When using the RFC5424 or journald formats, FRR can provide additional metadata for log messages as key/value pairs. The following information can be added in this way:


5424 group

5424 item(s)

journald field


always active




Thread ID

always active




Multi-instance number





XXXXX-XXXXX unique message identifier





Integer error category number





Source code file name





Source code line number





Source code function name





Message printf format arguments (n = 1..16)





FRR version information (IETF format)

The information added by version is [origin enterpriseId="50145" software="FRRouting" swVersion="..."] and is the same for all log messages. (Hence makes little sense to include in most scenarios.) 50145 is the FRRouting IANA Enterprise Number.

Crashlogs / backtraces do not include any additional information since it cannot safely be retrieved from a crash handler. However, all of the above destinations will deliver crashlogs.

Restart and Reconfiguration caveats

FRR uses “add-delete” semantics when reconfiguring log targets of any type (including both extended targets mentioned here as well as the global log stdout LEVEL and log syslog [LEVEL] variants.) This means that when changing logging configuration, log messages from threads executing in parallel may be duplicated for a brief window of time.

For the unix, syslog and journald extended destinations, messages can be lost when the receiver is restarted without the use of socket activation (i.e. keeping the receiver socket open.) FRR does not buffer log messages for later delivery, meaning anything logged while the receiver is unavailable is lost. Since systemd provides socket activation for journald, no messages will be lost on the journald target.