Книга: Linux Network Administrator Guide, Second Edition

The Inner Workings of uucico

The Inner Workings of uucico

To understand why uucico needs to know particular information, a quick description of how it actually connects to a remote system is helpful.

When you execute uucico -s system from the command line, uucico first has to connect physically. The actions taken depend on the type of connection to open. Thus, when using a telephone line, it has to find a modem and dial out. Over TCP, it has to call gethostbyname to convert the name to a network address, find out which port to open, and bind the address to the corresponding socket.

A successful connection is followed by authorization. This procedure generally consists of the remote system asking for a login name and possibly a password. This exchange is commonly called the login chat. The authorization procedure is performed either by the usual getty/login suite, or on TCP sockets by uucico itself. If authorization succeeds, the remote end fires up uucico. The local copy of uucico that initiated the connection is referred to as master, and the remote copy as slave.

Next follows the handshake phase: the master sends its hostname plus several flags. The slave checks this hostname for permission to log in, send, and receive files, etc. The flags describe (among other things) the maximum grade of spool files to transfer. If enabled, a conversation count or call sequence number check takes place here. With this feature, both sites maintain a count of successful connections, which are compared. If they do not match, the handshake fails. This is useful to protect yourself against impostors.

Finally, the two uucico s try to agree on a common transfer protocol. This protocol governs the way data is transferred, checked for consistency, and retransmitted in case of an error. There is a need for different protocols because of the differing types of connections supported. For example, telephone lines require a "safe" protocol, which is pessimistic about errors, while TCP transmission is inherently reliable and can use a more efficient protocol that foregoes most extra error checking.

After the handshake is complete, the actual transmission phase begins. Both ends turn on the selected protocol driver. At this point, the drivers possibly perform a protocol-specific initialization sequence.

The master then sends all files queued for the remote system whose spool grade is high enough. When it has finished, it informs the slave that it is done and that the slave may now hang up. The slave now can either agree to hang up or take over the conversation. This is a change of roles: now the remote system becomes master, and the local one becomes slave. The new master now sends its files. When done, both uucico s exchange termination messages and close the connection.

If you need additional information on UUCP, please refer to the source code. There is also a really antique article floating around the Net, written by David A. Novitz, which gives a detailed description of the UUCP protocol.[90] The Taylor UUCP FAQ also discusses some details UUCP's implementation. It is posted to comp.mail.uucp regularly.

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