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Mixing UUCP and RFC-822

The best cure against the problems of mail routing in UUCP networks so far is the adoption of the domain name system in UUCP networks. Of course, you can't query a name server over UUCP. Nevertheless, many UUCP sites have formed small domains that coordinate their routing internally. In the Maps, these domains announce one or two host as their mail gateways, so that there doesn't have to be a map entry for each host in the domain. The gateways handle all mail that flows into and out of the domain. The routing scheme inside the domain is completely invisible to the outside world.

This works very well with the smart-host routing scheme described above. Global routing information is maintained by the gateways only; minor hosts within a domain will get along with only a small hand-written paths file that lists the routes inside their domain, and the route to the mail hub. Even the mail gateways do not have to have routing information for every single UUCP host in the world anymore. Beside the complete routing information for the domain they serve, they only need to have routes to entire domains in their databases now. For instance, the pathalias entry shown below will route all mail for sites in the domain to smurf:

Any mail addressed to will be sent to swim with an envelope address of smurf!jones!claire.

The hierarchical organization of the domain name space allows mail servers to mix more specific routes with less specific ones. For instance, a system in France may have specific routes for subdomains of fr, but route any mail for hosts in the us domain toward some system in the U.S. In this way, domain-based routing (as this technique is called) greatly reduces the size of routing databases as well as the administrative overhead needed.

The main benefit of using domain names in a UUCP environment, however, is that compliance with RFC-822 permits easy gatewaying between UUCP networks and the Internet. Many UUCP domains nowadays have a link with an Internet gateway that acts as their smart-host. Sending messages across the Internet is faster, and routing information is much more reliable because Internet hosts can use DNS instead of the Usenet Maps.

In order to be reachable from the Internet, UUCP-based domains usually have their Internet gateway announce an MX record for them (MX records were described above). For instance, assume that moria belongs to the domain. acts as their Internet gateway. moria would therefore use gcc2 as its smart-host, so that all mail for foreign domains is delivered across the Internet. On the other hand, gcc2 would announce an MX record for, and deliver all incoming mail for orcnet sites to moria.

The only remaining problem is that the UUCP transport programs can't deal with fully qualified domain names. Most UUCP suites were designed to cope with site names of up to eight characters, some even less, and using non-alphanumeric characters such as dots is completely out of the question for most.

Therefore, some mapping between RFC-822 names and UUCP hostnames is needed. The way this mapping is done is completely implementation-dependent. One common way of mapping FQDNs to UUCP names is to use the pathalias file for this:

This will produce a pure UUCP-style bang path from an address that specifies a fully qualified domain name. Some mailers provide a special files for this; sendmail, for instance, uses the uucpxtable for this.

The reverse transformation (colloquially called domainizing) is sometimes required when sending mail from a UUCP network to the Internet. As long as the mail sender uses the fully qualified domain name in the destination address, this problem can be avoided by not removing the domain name from the envelope address when forwarding the message to the smart-host. However, there are still some UUCP sites that are not part of any domain. They are usually domainized by appending the pseudo-domain uucp.


Andrew Anderson
Thu Mar 7 23:22:06 EST 1996