# omrelp: RELP Output Module

Module Name: | omrelp
Author: | Rainer Gerhards <rgerhards@adiscon.com>

## Purpose

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).

Setup

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

## Configuration Parameters

Note

Parameter names are case-insensitive.

### Module Parameters

#### tls.tlslib

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
word | none | no | none

Added in version 8.1903.0.

Permits to specify the TLS library used by librelp. All relp protocol operations or actually performed by librelp and not rsyslog itself. This value specified is directly passed down to librelp. Depending on librelp version and build parameters, supported tls libraries differ (or TLS may not be supported at all). In this case rsyslog emits an error message.

Usually, the following options should be available: “openssl”, “gnutls”.

Note that “gnutls” is the current default for historic reasons. We actually recommend to use “openssl”. It provides better error messages and accepts a wider range of certificate types.

If you have problems with the default setting, we recommend to switch to “openssl”.

### Action Parameters

#### Target

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
word | none | yes | none

The target server to connect to.

#### Port

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
word | 514 | no | none

Name or numerical value of TCP port to use when connecting to target.

#### Template

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
word | RSYSLOG_ForwardFormat | no | none

Defines the template to be used for the output.

#### Timeout

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
int | 90 | no | none

Timeout for relp sessions. If set too low, valid sessions may be considered dead and tried to recover.

#### Conn.Timeout

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
int | 10 | no | none

Timeout for the socket connection.

#### RebindInterval

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
int | 0 | no | none

Permits to specify an interval at which the current connection is broken and re-established. This setting is primarily an aid to load balancers. After the configured number of messages has been transmitted, the current connection is terminated and a new one started. This usually is perceived as a ``new connection’’ by load balancers, which in turn forward messages to another physical target system.

#### KeepAlive

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
binary | off | no | none

Enable or disable keep-alive packets at the TCP socket layer. By default keep-alive is disabled.

#### KeepAlive.Probes

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
integer | 0 | no | none

The number of keepalive probes to send before considering the connection dead. The default, 0, uses the operating system defaults. This only has an effect if keep-alive is enabled and may not be available on all platforms.

#### KeepAlive.Interval

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
integer | 0 | no | none

The interval between subsequent keepalive probes. The default, 0, uses the operating system defaults. This only has an effect if keep-alive is enabled and may not be available on all platforms.

#### KeepAlive.Time

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
integer | 0 | no | none

The idle time before the first keepalive probe is sent. The default, 0, uses the operating system defaults. This only has an effect if keep-alive is enabled and may not be available on all platforms.

#### WindowSize

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
int | 0 | no | none

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

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
binary | off | no | none

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

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
binary | off | no | none

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

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
array | none | no | none

Note: this parameter is mandatory depending on the value of TLS.AuthMode but the code does currently not check this.

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

This contains either remote system names or fingerprints, depending on the value of parameter TLS.AuthMode. One or more values may be entered.

When a non-permitted peer is connected to, the refusal is logged together with the given remote peer identify. This is especially useful in fingerprint authentication mode: if the administrator knows this was a valid request, he can simply 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.

Note that in name authentication mode wildcards are supported. This can be done as follows:

    tls.permittedPeer="*.example.com"

Of course, there can also be multiple names used, some with and some without wildcards:

    tls.permittedPeer=["*.example.com", "srv1.example.net", "srv2.example.net"]

#### TLS.AuthMode

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
string | none | no | none

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.

The permitted names or fingerprints are configured via TLS.PermittedPeer.

### About Chained Certificates

Added in version 8.2008.0.

With librelp 1.7.0, you can use chained certificates. If using “openssl” as tls.tlslib, we recommend at least OpenSSL Version 1.1 or higher. Chained certificates will also work with OpenSSL Version 1.0.2, but they will be loaded into the main OpenSSL context object making them available to all librelp instances (omrelp/imrelp) within the same process.

If this is not desired, you will require to run rsyslog in multiple instances with different omrelp configurations and certificates.

#### TLS.CaCert

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
string | none | no | none

The CA certificate that can verify the machine certs.

#### TLS.MyCert

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
string | none | no | none

The machine public certificate.

#### TLS.MyPrivKey

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
string | none | no | none

The machine private key.

#### TLS.PriorityString

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
string | none | no | none

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.

#### tls.tlscfgcmd

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
string | none | no | none

Added in version 8.2001.0.

The setting can be used if tls.tlslib is set to “openssl” to pass configuration commands to the openssl library. OpenSSL Version 1.0.2 or higher is required for this feature. A list of possible commands and their valid values can be found in the documentation: https://docs.openssl.org/1.0.2/man3/SSL_CONF_cmd/

The setting can be single or multiline, each configuration command is separated by linefeed (n). Command and value are separated by equal sign (=). Here are a few samples:

### Example 1

This will allow all protocols except for SSLv2 and SSLv3:

    tls.tlscfgcmd="Protocol=ALL,-SSLv2,-SSLv3"

### Example 2

This will allow all protocols except for SSLv2, SSLv3 and TLSv1. It will also set the minimum protocol to TLSv1.2

    tls.tlscfgcmd="Protocol=ALL,-SSLv2,-SSLv3,-TLSv1
    MinProtocol=TLSv1.2"

#### LocalClientIp

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
word | none | no | none

Omrelp uses ip_address as local client address while connecting to remote logserver.

## Examples

### Sending msgs with omrelp

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

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

### Sending msgs with omrelp via TLS

This is the same as the previous example but uses TLS (via OpenSSL) for operations.

Certificate files must exist at configured locations. Note that authmode “certvalid” is not very strong - you may want to use a different one for actual deployments. For details, see parameter descriptions.

    module(load="omrelp" tls.tlslib="openssl")
    action(type="omrelp"
                 target="centralserv" port="2514" tls="on"
                 tls.cacert="tls-certs/ca.pem"
                 tls.mycert="tls-certs/cert.pem"
                 tls.myprivkey="tls-certs/key.pem"
                 tls.authmode="certvalid"
                 tls.permittedpeer="rsyslog")

## `obsolete legacy` 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>

Support: rsyslog Assistant | GitHub Discussions | GitHub Issues: rsyslog source project

Contributing: Source & docs: rsyslog source project

© 2008–2025 Rainer Gerhards and others. Licensed under the Apache License 2.0.