Source: modDB
Source: Frontline Message Board
Related: ammahls
This is from the HL SDK mailers list:

Here is my first pass at writing a spec for the RCON format, feel free to ask questions.

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Source Server RCON format

The protocol is based around command/response packets encapsulated in a TCP/IP stream. The stream can have multiple outstanding commands and can be extended to allow for multiple sub-channels of data.

The command packet format consists of:

packet size (int)
request id (int)
command (int)
string1 (null delimited string)
string2 (null delimited string)