I have used syslog for over 30 years now but other than knowing that it uses UDP and port 514, I have never looked at the underlying protocol in any detail.
This document describes the syslog protocol, which is used to convey
event notification messages. This protocol utilizes a layered
architecture, which allows the use of any number of transport
protocols for transmission of syslog messages. It also provides a
message format that allows vendor-specific extensions to be provided
in a structured way.
This RFC does not define any transports. They are defined in other documents. One such transport is defined in RFC 5426 and is consistent with the traditional UDP transport.
Here is the ABNF (Augmented Backus–Naur Form) definition for a syslog message:
SYSLOG-MSG = HEADER SP STRUCTURED-DATA [SP MSG] HEADER = PRI VERSION SP TIMESTAMP SP HOSTNAME SP APP-NAME SP PROCID SP MSGID PRI = "< " PRIVAL ">" PRIVAL = 1*3DIGIT ; range 0 .. 191 VERSION = NONZERO-DIGIT 0*2DIGIT HOSTNAME = NILVALUE / 1*255PRINTUSASCII APP-NAME = NILVALUE / 1*48PRINTUSASCII PROCID = NILVALUE / 1*128PRINTUSASCII MSGID = NILVALUE / 1*32PRINTUSASCII TIMESTAMP = NILVALUE / FULL-DATE "T" FULL-TIME FULL-DATE = DATE-FULLYEAR "-" DATE-MONTH "-" DATE-MDAY DATE-FULLYEAR = 4DIGIT DATE-MONTH = 2DIGIT ; 01-12 DATE-MDAY = 2DIGIT ; 01-28, 01-29, 01-30, 01-31 based on ; month/year FULL-TIME = PARTIAL-TIME TIME-OFFSET PARTIAL-TIME = TIME-HOUR ":" TIME-MINUTE ":" TIME-SECOND [TIME-SECFRAC] TIME-HOUR = 2DIGIT ; 00-23 TIME-MINUTE = 2DIGIT ; 00-59 TIME-SECOND = 2DIGIT ; 00-59 TIME-SECFRAC = "." 1*6DIGIT TIME-OFFSET = "Z" / TIME-NUMOFFSET TIME-NUMOFFSET = ("+" / "-") TIME-HOUR ":" TIME-MINUTE STRUCTURED-DATA = NILVALUE / 1*SD-ELEMENT SD-ELEMENT = "[" SD-ID *(SP SD-PARAM) "]" SD-PARAM = PARAM-NAME "=" %d34 PARAM-VALUE %d34 SD-ID = SD-NAME PARAM-NAME = SD-NAME PARAM-VALUE = UTF-8-STRING ; characters '"', '' and ; ']' MUST be escaped. SD-NAME = 1*32PRINTUSASCII ; except '=', SP, ']', %d34 (") MSG = MSG-ANY / MSG-UTF8 MSG-ANY = *OCTET ; not starting with BOM MSG-UTF8 = BOM UTF-8-STRING BOM = %xEF.BB.BF UTF-8-STRING = *OCTET ; UTF-8 string as specified ; in RFC 3629 OCTET = %d00-255 SP = %d32 PRINTUSASCII = %d33-126 NONZERO-DIGIT = %d49-57 DIGIT = %d48 / NONZERO-DIGIT NILVALUE = "-"
Syslog message size limits are dictated by the syslog transport in use. There is no upper limit per se. Any transport receiver must be able to accept messages of up to and including 480 octets in length, should be able to accept messages of up to and including 2048 octets in length and may accept messages larger than 2048 octets in length. If a transport receiver receives a message with a length larger than it supports, it should truncate the message or it may discard the message.
I was not aware that facility and severity values are not normative. They are described in the RFC purely for informational purposes. Facility values must be in the range of 0 to 23 inclusive. Severity values must be in the range of 0 to 7 inclusive.
The RFC also defines an optional but useful set of structured data elements. For example the SD-ID timeQuality may be used by an originator to describe its notion of system time and should be written if the originator is not properly synchronized with a reliable external time source or if it does not know whether its time zone information is correct.
Interestingly, TCP port 512 is reserved not for syslog but for remote shells such as rsh and remsh.