[ILUG] Question about SOA & MX

[Colm Buckley]

> What or Who is SOA & MX?

They are both types of DNS record.  SOA ("Start of Authority") is the
record in the DNS which indicates where the primary name server for a
zone is, who's responsible for it, and what time-out and cacheing
paramaters should be used for it.  For example:

linux.ie.               29m32s IN SOA   lugh.tuatha.org. admin.linux.ie. (
                                        2000010701      ; serial
                                        12H             ; refresh
                                        2H              ; retry
                                        1W              ; expiry
                                        1D )            ; minimum

(This is the result of "dig linux.ie. SOA")

This indicates that the zone "linux.ie" has a primary name server on
"lugh.tuatha.org", that its administrator is "admin at linux.ie" (the dot
is there instead of the @ for historical reasons), that the serial
number of the zone file is 2000010701 (convention is that the serial
number shows the date of last update), that the secondary name servers
will check the primary for updates every 12 hours, that the secondaries
will re-check the primary every 2 hours following an incident where they
were unable to check the primary, that the secondaries will still
deliver data for up to one week if the primary is unavailable, and that
data in this zone may be cached for one day before checking with an
authoritative name server.

SOA records are required for each zone in the DNS.

MX records indicate which hosts are accepting mail for a given domain.
For example:

linux.ie.               20m32s IN MX    10 mail.tuatha.org.
linux.ie.               20m32s IN MX    20 mail.heanet.ie.
linux.ie.               20m32s IN MX    30 dagda.tuatha.cs.tcd.ie.

(This is the result of "dig linux.ie. MX")

This indicates that mail destined for "linux.ie." should first be sent
to "mail.tuatha.org." - if that host is unavailable, then
"mail.heanet.ie." is to be tried, and if that is unavailable, then
"dagda.tuatha.cs.tcd.ie." should be tried.  Failing all of those, the
mail is undeliverable.  Generally, the "mail secondaries" are set up to
"store and forward" emails if the primary mail server is down - they
just hold them until the primary is back up again.

Hope this helps,

           Colm

-- 
Colm Buckley B.A. B.F. # colm at tuatha.org colm.buckley at cs.tcd.ie
Department of Computer Science      # +353 87 2469146 # whois cb3765
Trinity College, Dublin 2, Ireland. # http://www.tuatha.org/~colm/
Early bird gets the worm, but the second mouse gets the cheese.