The concept of jobs is vital to understanding UUCP.
Every transfer that a user initiates with uucp or
uux is called a job. It is made up of
a command to be executed on a remote system, a collection of files to
be transferred between sites, or both.
As an example, the following command makes UUCP copy the file
netguide.ps to a remote host named
pablo and execute the
lpr command on
to print the file:
$ uux -r pablo!lpr !netguide.ps
UUCP does not generally call the remote system immediately to execute a job
(or else you could make do with kermit). Instead, it
temporarily stores the job description away. This is called
spooling. The directory tree under which jobs are stored
is therefore called the spool directory and is generally
located in /var/spool/uucp. In our example, the job
description would contain information about the remote command to be executed
(lpr), the user who requested the execution, and a couple
of other items. In addition to the job description, UUCP has to store the
input file netguide.ps.
The exact location and naming of spool files may vary, depending on
some compile-time options. HDB-compatible UUCPs generally store spool
files in a /var/spool/uucp subdirectory with the name
of the remote site. When compiled for Taylor configuration, UUCP creates
subdirectories below the site-specific spool directory for different types
of spool files.
At regular intervals, UUCP dials up the remote system. When a connection to
the remote machine is established, UUCP transfers the files describing the
job, plus any input files. The incoming jobs will not be executed
immediately, but only after the connection terminates. Execution is handled
by uuxqt, which also takes care of forwarding any jobs that
are designated for another site.
To distinguish between more and less important jobs, UUCP associates a
grade with each job. This is a single digit ranging
from 0 through 9, A through Z, and a through z, in decreasing precedence. Mail
is customarily spooled with grade B or C, while news is spooled with grade
N. Jobs with higher grades are transferred earlier. Grades may be assigned
using the –g flag when invoking uucp
You can also prohibit the transfer of jobs below a given grade at certain
times. To do this we set the maximum spool grade
that will be prohibited during a conversation. The maximum spool grade
defaults to z, meaning all grades will be transferred every time. Note the
semantic ambiguity here: a file is transferred only if it has a grade
equal to or above the maximum spool grade threshold.
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
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 uucicos 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
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. The Taylor UUCP FAQ also
discusses some details UUCP's implementation. It is posted to
In this section, we describe the most important command-line options for
- – – system, –s system
Calls the named system unless prohibited by call-time
- –S system
Calls the named system unconditionally.
- – –master, –r1
Starts uucico in master mode. This is the default when
–s or –S is given. All by itself,
the –r1 option causes uucico to try
to call all systems in the sys file described in the
next section of this chapter, unless prohibited by call or retry time
- – –slave, –r0
Starts uucico in slave mode. This is the default
when no –s or –S is
given. In slave mode, either standard input/output are assumed to be
connected to a serial port, or the TCP port specified by the
–p option is used.
- – –ifwork, –C
This option supplements –s or
–S and tells uucico to call the
named system only if there are jobs spooled for it.
- – –debug type, –x type, –X type
Turns on debugging of the specified type. Several types can be given as a
comma-separated list. The following types are valid:
Using all turns on all options. For
compatibility with other UUCP implementations, a number may be specified
instead, which turns on debugging for the first n
items from the above list.
Debugging messages will be logged to the Debug
file below /var/spool/uucp.