omrelp: RELP Output Module

Module Name:    omrelp

Author:Rainer Gerhards <>


This module supports sending syslog messages over the reliable RELP protocol. For RELP’s advantages over plain tcp syslog, please see the documentation for imrelp (the server counterpart).


Please note that librelp is required for imrelp (it provides the core relp protocol implementation).

Action Configuration Parameters:

This module supports RainerScript configuration starting with rsyslog 7.3.10. For older versions, legacy configuration directives must be used.

Note: parameter names are case-insensitive.

  • target (mandatory) The target server to connect to.

  • template (not mandatory, default “RSYSLOG_ForwardFormat”) Defines the template to be used for the output.

  • timeout (not mandatory, default 90) Timeout for relp sessions. If set too low, valid sessions may be considered dead and tried to recover.

  • conn.timeout (not mandatory, default 10) Timeout for the socket connection.

  • windowSize (not mandatory, default 0) This is an expert parameter. It permits to override the RELP window size being used by the client. Changing the window size has both an effect on performance as well as potential message duplication in failure case. A larger window size means more performance, but also potentially more duplicated messages - and vice versa. The default 0 means that librelp’s default window size is being used, which is considered a compromise between goals reached. For your information: at the time of this writing, the librelp default window size is 128 messages, but this may change at any time. Note that there is no equivalent server parameter, as the client proposes and manages the window size in RELP protocol.

  • tls (not mandatory, values “on”,”off”, default “off”) If set to “on”, the RELP connection will be encrypted by TLS, so that the data is protected against observers. Please note that both the client and the server must have set TLS to either “on” or “off”. Other combinations lead to unpredictable results.

    Attention when using GnuTLS 2.10.x or older

    Versions older than GnuTLS 2.10.x may cause a crash (Segfault) under certain circumstances. Most likely when an imrelp inputs and an omrelp output is configured. The crash may happen when you are receiving/sending messages at the same time. Upgrade to a newer version like GnuTLS 2.12.21 to solve the problem.

  • tls.compression (not mandatory, values “on”,”off”, default “off”) The controls if the TLS stream should be compressed (zipped). While this increases CPU use, the network bandwidth should be reduced. Note that typical text-based log records usually compress rather well.

  • tls.permittedPeer peer Places access restrictions on this forwarder. Only peers which have been listed in this parameter may be connected to. This guards against rouge servers and man-in-the-middle attacks. The validation bases on the certficate the remote peer presents.

    The peer parameter lists permitted certificate fingerprints. Note that it is an array parameter, so either a single or multiple fingerprints can be listed. When a non-permitted peer is connected to, the refusal is logged together with it’s fingerprint. So if the administrator knows this was a valid request, he can simple add the fingerprint by copy and paste from the logfile to rsyslog.conf. It must be noted, though, that this situation should usually not happen after initial client setup and administrators should be alert in this case.

    Note that usually a single remote peer should be all that is ever needed. Support for multiple peers is primarily included in support of load balancing scenarios. If the connection goes to a specific server, only one specific certificate is ever expected (just like when connecting to a specific ssh server). To specify multiple fingerprints, just enclose them in braces like this:

    tls.permittedPeer=["SHA1:...1", "SHA1:....2"]

    To specify just a single peer, you can either specify the string directly or enclose it in braces.

  • tls.authMode mode Sets the mode used for mutual authentication. Supported values are either “fingerprint” or “name”. Fingerprint mode basically is what SSH does. It does not require a full PKI to be present, instead self-signed certs can be used on all peers. Even if a CA certificate is given, the validity of the peer cert is NOT verified against it. Only the certificate fingerprint counts. In “name” mode, certificate validation happens. Here, the matching is done against the certificate’s subjectAltName and, as a fallback, the subject common name. If the certificate contains multiple names, a match on any one of these names is considered good and permits the peer to talk to rsyslog.

  • tls.cacert the CA certificate that can verify the machine certs

  • tls.mycert the machine public certiificate

  • tls.myprivkey the machine private key

  • tls.prioritystring (not mandatory, string) This parameter permits to specify the so-called “priority string” to GnuTLS. This string gives complete control over all crypto parameters, including compression setting. For this reason, when the prioritystring is specified, the “tls.compression” parameter has no effect and is ignored. Full information about how to construct a priority string can be found in the GnuTLS manual. At the time of this writing, this information was contained in section 6.10 of the GnuTLS manual. Note: this is an expert parameter. Do not use if you do not exactly know what you are doing.

  • localclientip ip_address (not mandatory, string) omrelp uses ip_address as local client address while connecting to remote logserver.


The following sample sends all messages to the central server “centralserv” at port 2514 (note that that server must run imrelp on port 2514).

action(type="omrelp" target="centralserv" port="2514")

Legacy Configuration Directives:

This module uses old-style action configuration to keep consistent with the forwarding rule. So far, no additional configuration directives can be specified. To send a message via RELP, use

*.*  :omrelp:<server>:<port>;<template>

just as you use

*.*  @@<server>:<port>;<template>

to forward a message via plain tcp syslog.

Caveats/Known Bugs:

See imrelp, which documents them.

Legacy Sample:

The following sample sends all messages to the central server “centralserv” at port 2514 (note that that server must run imrelp on port 2514).

$ModLoad omrelp
*.* :omrelp:centralserv:2514

Note: to use IPv6 addresses, encode them in [::1] format.