rick@ofa123.FIDONET.ORG (Rick Ellis) (12/01/89)
I've sometimes seen references to MX host or records. What are they?
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Rick Ellis
...!{dhw68k,conexch}!ofa123!rick rick@ofa123.FIDONET.ORG
714 544-0934 2400/1200/300eps@toaster.SFSU.EDU (Eric P. Scott) (12/03/89)
In article <95.25762542@ofa123.FIDONET.ORG> rick@ofa123.FIDONET.ORG (Rick Ellis) writes: >I've sometimes seen references to MX host or records. What are they? Generally this refers to hosts known to the Internet Domain Name Service that don't have IP addresses ("A records"). They really could be called MX-only hosts, since all hosts should have at least one MX record. MX stands for Mail eXchanger, and has two parts - a preference value (if multiple MX records appear, the mail system tries lower numeric values first). - the name of a host which will accept mail for this host, which must resolve to an IP address (i.e. MXes cannot be cascaded). Here's an excerpt from the nslookup output for sun.com, which is an Internet site: > sun.com sun.com inet address = 192.9.9.1 sun.com CPU=SUN-3/180 OS=UNIX sun.com inet address = 192.9.9.1, protocol = tcp ftp telnet smtp domain finger sun.com inet address = 192.9.9.1, protocol = udp echo time tftp sun.com preference = 10, mail exchanger = Sun.COM Sun.COM inet address = 192.9.9.1 The first line is an A record, that gives sun.com's IP address as 192.9.9.1; you don't need to know this number, that's just for the machines to be able to find each other--every Internet site behaves as though it is directly connected to every other Internet machine, subject only to administrative prohibitions. The second is an HINFO record, identifying the type of machine and its operating system. The next two lines are WKS (Well Known Service) records, identifying which network services are available. smtp is Simple Mail Transfer Protocol; this host is capable of handling mail. The next line is an MX record that says that sun.com handles its own mail. Case is not significant in domain names, but we got another A record just to make sure. The DNS allows non-Internet sites to be registered, so you can't tell by looking at a domain name whether it's "on the Internet." Here's a .com site that isn't: > claris.com claris.com preference = 20, mail exchanger = ames.arc.nasa.gov ames.arc.nasa.gov inet address = 128.102.18.3 What this means is that any mail addressed to something@claris.com is accepted by ames.arc.nasa.gov for delivery. ames has a non- Internet link to claris, so claris is really two "hops" away from any site on the Internet. If claris wanted to avoid delays in case ames goes down, they could list additional MXs. In your case, there is no MX record specifically for ofa123.FIDONET.ORG, so the "catch-all" wildcard for *.FIDONET.ORG points it at handies.ucar.edu in Boulder, Colorado. This is probably suboptimal. If I received mail on a local (unnamed) FIDONET host, the MX for Net 125 would point at a San Francisco Bay Area Internet host, from which the telephone charges would be reasonable. >>> Non-Internet sites can access nslookup by mail; messages sent to nslookup@sh.cs.net are processed by a program that runs nslookup and mails the results back. >>> Any site, however small, can register in the US domain. Send mail to service@nic.ddn.mil with a Subject: line of NETINFO US-DOMAIN.TXT for details. -=EPS=-