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Protocol Requests and Replies

19.2. Protocol Requests and Replies

Options used:

  • remote_support=Remote operations supported

After a connection has been established, a request can be sent to the lpd server. The request consists of a single octet indicating the request type, followed by the printer (or print queue) name, followed by a set of options for the request, followed by a LF (line feed) character.

Table 19-1. RFC1179 Commands

NNN RFC1179 Operation program
1 yes start print lpc
2 yes transfer a printer job lpr
3 yes print short form of queue status lpr
4 yes print long form of queue status lpr
5 yes remove jobs lprm
6 LPRng do control operation lpc
7 LPRng transfer a block format print job lpr
8 LPRng secure command transfer lpc
9 LPRng verbose status information lpr

After the request has been sent, then a reply will be returned. In general the reply has the following form:

    \000\n    Success
    \NNN\n    Failure (NNN is error code)
    text\n    Text or status information

As can be seen, this protocol is extremely simple, but there are a set of problems due to the loosely written language of RFC1179.

  1. Firstly, while RFC1179 sets limits on the lengths of commands, it does not strictly set limits on the characters set used in the commands. This can result in problems when trying to print status information, headers on banners, and other details.

  2. The original RFC1179 protocol did not provide any way to do remote control of queues or lpd servers. This has been added to the protocol. As a side effect, if you try to use lpc to control a non-LPRng printer, it will not work.

  3. You can specify that a network printer is non-LPRng by using the remote_support=RQVMC option and specify the operations supported by the printer. The letters R, Q, M, and C stand for lpr, lpq, lprm, and lpc operations respectively, and indicate that these are supported. If remote_support does not allow a particular operation, then the LPRng software will not send a corresponding request to the printer. For example, remote_support=R would restrict operations to spooling jobs only, and the LPRng software would not query the printer for status.