UUCP is quite a flexible system. With that flexibility comes a need to carefully control access to its features to prevent abuse, whether it be intentional or accidental. The primary features of concern to the UUCP administrator are remote command execution, file transfer, and forwarding. Taylor UUCP provides a means of limiting the freedom that remote UUCP hosts have in exercising each of these features. With careful selection of permissions, the UUCP administrator can ensure that the host's security is preserved.
UUCP's task is to copy files from one system to another and to request execution of certain commands on remote hosts. Of course, you as an administrator would want to control what rights you grant other systems—allowing them to execute any command they choose on your system is definitely not a good idea.
By default, the only commands Taylor UUCP allows other systems to execute on your machine are rmail and rnews, which are commonly used to exchange email and Usenet News over UUCP. To change the set of commands for a particular system, you can use the commands keyword in the sys file. Similarly, you may want to limit the search path to just those directories containing the allowed commands. You can change the search path allowed for a remote host with the command-path statement. For instance, you may want to allow system pablo to execute the bsmtp command in addition to rmail and rnews:[1]
system pablo ... commands rmail rnews bsmtp |
Taylor UUCP also allows you to fine-tune file transfers in great detail. At one extreme, you can disable transfers to and from a particular system. Just set request to no, and the remote system will not be able to either retrieve files from your system or send it any files. Similarly, you can prohibit your users from transferring files to or from a system by setting transfer to no. By default, users on both the local and the remote system are allowed to upload and download files.
In addition, you can configure the directories that files may be copied to and from. Usually you will want to restrict access from remote systems to a single directory hierarchy, but still allow your users to send files from their home directory. Commonly, remote users are allowed to receive files only from the public UUCP directory /var/spool/uucppublic. This is the traditional place to make files publicly available, very much like FTP servers on the Internet.[2]
Taylor UUCP provides four different commands to configure the directories for sending and receiving files. They are: local-send, which specifies the list of directories a user may ask UUCP to send files from; local-receive, which gives the list of directories a user may ask to receive files to; and remote-send and remote-receive, which do the analogous for requests from a foreign system. Consider the following example:
system pablo ... local-send /home ~ local-receive /home ~/receive remote-send ~ !~/incoming !~/receive remote-receive ~/incoming |
The local-send command allows users on your host to send any files below /home and from the public UUCP directory to pablo. The local-receive command allows them to receive files either to the world-writable receive directory in the uucppublic, or any world-writable directory below /home. The remote-send directive allows pablo to request files from /var/spool/uucppublic, except for files from the incoming and receive directories. This is signaled to uucico by preceding the directory names with exclamation marks. Finally, the last line allows pablo to upload files to incoming.
A major problem with file transfers using UUCP is that it receives files only to directories that are world-writable. This may tempt some users to set up traps for other users. However, there's no way to escape this problem outside of disabling UUCP file transfers altogether.
UUCP provides a mechanism to have other systems execute file transfers on your behalf. For instance, suppose your system has uucp access to a system called seci, but not to another system called uchile. This allows you to make seci retrieve a file from uchile for you and send it to your system. The following command would achieve this:
$ uucp -r seci!uchile!~/find-ls.gz ~/uchile.files.gz |
This technique of passing a job through several systems is called forwarding. On your own UUCP system, you would want to limit the forwarding service to a few hosts you trust not to run up a horrendous phone bill by making you download the latest X11R6 source release for them.
By default, Taylor UUCP prohibits forwarding altogether. To enable forwarding for a particular system, you can use the forward command. This command specifies a list of sites the system may request you to forward jobs to and from. For instance, the UUCP administrator of seci would have to add the following lines to the sys file to allow pablo to request files from uchile:
#################### # pablo system pablo ... forward uchile #################### # uchile system uchile ... forward-to pablo |
The forward-to entry for uchile is necessary so that any files returned by it are actually passed on to pablo. Otherwise UUCP would drop them. This entry uses a variation of the forward command that permits uchile to send files only to pablo through seci, not the other way round.
To permit forwarding to any system, use the special keyword ANY (capital letters required).
| [1] | bsmtp is used to deliver mail with batched SMTP. |
| [2] | You may use a tilde (~) character to refer to the UUCP public directory, but only in UUCP configuration files; outside it usually translates to the user's home directory. |