# Generic design of a syslogd

Written 2007-04-10 by Rainer Gerhards

The text below describes a generic approach on how a syslogd can be implemented. I created this description for some other project, where it was not used. Instead of throwing it away, I thought it would be a good addition to the rsyslog documentation. While rsyslog differs in details from the description below, it is sufficiently close to it. Further development of rsyslog will probably match it even closer to the description.

If you intend to read the rsyslog source code, I recommend reading this document here first. You will not find the same names and not all of the concepts inside rsyslog. However, I think your understanding will benefit from knowing the generic architecture.

    +-----------------+
    | "remote" PLOrig |
    +-----------------+
        |
        I  +--------+-----+-----+          +-----+-------+------+-----+
        P  | PLOrig | GWI | ... |          | GWO | Store | Disc | ... |
        C  +--------+-----+-----+          +-----+-------+------+-----+
        |        |                                          ^
        v        v                                          |
       +--------------+        +------------+          +--------------+
       | PLGenerator  |        |  RelayEng  |          | CollectorEng |
       +--------------+        +------------+          +--------------+
              |                      ^                       ^
              |                      |                       |
              v                      v                       |
       +-------------+         +------------+          +--------------+
       |   PLG Ext   |         | RelEng Ext |          | CollcEng Ext |
       +-------------+         +------------+          +--------------+
              |                      ^                       ^
              |                      |                       |
              v                      v                       |
       +--------------------------------------------------------------+
       |                      Message Router                          |
       +--------------------------------------------------------------+
                          |                            ^
                          v                            |
       +--------------------------------------------------------------+
       |           Message CoDec (e.g. RFC 3164, RFCYYYY)             |
       +--------------------------------------------------------------+
                          |                            ^
                          v                            |
       +---------------------+-----------------------+----------------+
       |    transport UDP    |    transport TLS      |      ...       |
       +---------------------+-----------------------+----------------+

                 Generic Syslog Application Architecture

- A “syslog application” is an application whose purpose is the processing of syslog messages. It may be part of a larger application with a broader purpose. An example: a database application might come with its own syslog send subsystem and not go through a central syslog application. In the sense of this document, that application is called a “syslog application” even though a casual observer might correctly call it a database application and may not even know that it supports sending of syslog messages.
- Payload is the information that is to be conveyed. Payload by itself may have any format and is totally independent from to format specified in this document. The “Message CoDec” of the syslog application will bring it into the required format.
- Payload Originators (“PLOrig”) are the original creators of payload. Typically, these are application programs.
- A “Remote PLOrig” is a payload originator residing in a different application than the syslog application itself. That application may reside on a different machine and may talk to the syslog application via RPC.
- A “PLOrig” is a payload originator residing within the syslog application itself. Typically, this PLOrig emits syslog application startup, shutdown, error and status log messages.
- A “GWI” is a inbound gateway. For example, a SNMP-to-syslog gateway may receive SNMP messages and translate them into syslog.
- The ellipsis after “GWI” indicates that there are potentially a variety of different other ways to originally generate payload.
- A “PLGenerator” is a payload generator. It takes the information from the payload-generating source and integrates it into the syslog subsystem of the application. This is a highly theoretical concept. In practice, there may not actually be any such component. Instead, the payload generators (or other parts like the GWI) may talk directly to the syslog subsystem. Conceptually, the “PLGenerator” is the first component where the information is actually syslog content.
- A “PLG Ext” is a payload generator extension. It is used to modify the syslog information. An example of a “PLG Ext” might be the addition of cryptographic signatures to the syslog information.
- A “Message Router” is a component that accepts in- and outbound syslog information and routes it to the proper next destination inside the syslog application. The routing information itself is expected to be learnt by operator configuration.
- A “Message CoDec” is the message encoder/decoder. The encoder takes syslog information and encodes them into the required format for a syslog message. The decoder takes a syslog message and decodes it into syslog information. Codecs for multiple syslog formats may be present inside a single syslog application.
- A transport (UDP, TLS, yet-to-be-defined ones) sends and receives syslog messages. Multiple transports may be used by a single syslog application at the same time. A single transport instance may be used for both sending and receiving. Alternatively, a single instance might be used for sending and receiving exclusively. Multiple instances may be used for different listener ports and receivers.
- A “RelayEng” is the relaying engine. It provides functionality necessary for receiving syslog information and sending it to another syslog application.
- A “RelEng Ext” is an extension that processes syslog information as it enters or exits a RelayEng. An example of such a component might be a relay cryptographically signing received syslog messages. Such a function might be useful to guarantee authenticity starting from a given point inside a relay chain.
- A “CollectorEng” is a collector engine. At this component, syslog information leaves the syslog system and is translated into some other form. After the CollectorEng, the information is no longer defined to be of native syslog type.
- A “CollcEng Ext” is a collector engine extension. It modifies syslog information before it is passed on to the CollectorEng. An example for this might be the verification of cryptographically signed syslog message information. Please note that another implementation approach would be to do the verification outside of the syslog application or in a stage after “CollectorEng”.
- A “GWO” is an outbound gateway. An example of this might be the forwarding of syslog information via SNMP or SMTP. Please note that when a GWO directly connects to a GWI on a different syslog application, no native exchange of syslog information takes place. Instead, the native protocol of these gateways (e.g. SNMP) is used. The syslog information is embedded inside that protocol. Depending on protocol and gateway implementation, some of the native syslog information might be lost.
- A “Store” is any way to persistently store the extracted syslog information, e.g. to the file system or to a data base.
- “Disc” means the discarding of messages. Operators often find it useful to discard noise messages and so most syslog applications contain a way to do that.
- The ellipsis after “Disc” indicates that there are potentially a variety of different other ways to consume syslog information.
- There may be multiple instances of each of the described components in a single syslog application.
- A syslog application is made up of all or some of the above mentioned components.