Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!tut.cis.ohio-state.edu!giza.cis.ohio-state.edu!karl From: karl@giza.cis.ohio-state.edu (Karl Kleinpaste) Newsgroups: comp.mail.misc Subject: Re: What is an MX Record? Message-ID: Date: 7 Jun 89 03:18:02 GMT References: <2213@ur-cc.UUCP> Sender: news@tut.cis.ohio-state.edu Organization: OSU Lines: 42 In-reply-to: msir@uhura.cc.rochester.edu's message of 6 Jun 89 20:31:55 GMT msir@uhura.cc.rochester.edu (Mark Sirota) writes: What exactly is an MX Record? I understand the functionality, but how are they implemented? I assume it's not just a line in the sendmail.cf which aliases one name to another... MX records are resource records (RRs) in your nameserver. They identify the host to which mail for a given domain entity should be given. They are particularly useful for, e.g., non-IP sites (notably, UUCP sites) which wish to be domain-addressable. Syntax in a nameserver zone file is IN MX as in $ORIGIN rochester.edu. some-uucp-host-in-your-domain IN MX 10 your-gateway.rochester.edu. The provided is used by your mailer to determine preference order in the case of multiple MX's. There can be an arbitrarily large number of MX records for any given , though I can't recall having ever seen more than about 5. The s provided tell the mailer which to try first; and if it doesn't answer the phone, as it were, which to try second, and so on. Sendmail as of (about) 5.51 (5.54? I have long since lost track) learned to put MX records to use automatically. When the mailer chosen in S0 is P=[IPC], then MX records are requested from the nameservers, and delivery is actually attempted to the hosts so identified. If no MX records exist, then sendmail expects to find an A (address) record for the indicated destination, so that it can deliver directly. Of course, the host which is advertised as being the MX host for some given had better have some sort of schema for detecting those hosts for which it does MX'ing, and must not resolve in S0 to P=[IPC]. I do it with a set of classes identifying those domains for which I MX, and hand them to a mailer P=/bin/smail, with a paths file which knows the mapping to real UUCP hosts for those domains. --Karl