On 2013-10-18 10:46 , Doug wrote: > Hello, > > $ idig plus.google.com mx > plus.google.com. 1200 IN CNAME plus-china.l.google.com. > plus-china.l.google.com. 600 IN MX 40 alt3.aspmx.l.google.com. > plus-china.l.google.com. 600 IN MX 50 alt4.aspmx.l.google.com. > plus-china.l.google.com. 600 IN MX 10 aspmx.l.google.com. > plus-china.l.google.com. 600 IN MX 20 alt1.aspmx.l.google.com. > plus-china.l.google.com. 600 IN MX 30 alt2.aspmx.l.google.com. > > I never saw this type of MX. Are they valid records?
The records are valid, though against the RFC1912 section 2.4. The example is MX -> CNAME, but the rule is: "Don't use CNAMEs in combination with RRs which point to other names" And thus CNAME -> MX -> A falls under that too. The problem with the above specifically is that Sendmail will cause some issues, as it will lookup the CNAME, and replace all headers with the destination, thus if you have: From: exam...@plus.google.com To: someth...@example.net Sendmail will rewrite this to: From: exam...@plus-china.l.google.com To: someth...@example.net Sendmail is one of the few and maybe only SMTP server that does though and hence you will just get very inconsistent results depending if the remote site (which you do not control) still uses that. Hence, why one should never do the above. It is btw rather odd that you get the CNAME, as from a variety of queries around the world they all seem to return a A/AAAA/MX set directly instead. You might thus want to check *who* is actually returning the above as the Google folks definitely know about the problems caused by CNAMEs and thus likely would never use it in the above case; they just return a different A/AAAA/MX set when queried instead from their DNS server. Greets, Jeroen -- $ host plus.google.com plus.google.com has address 173.194.116.39 plus.google.com has address 173.194.116.41 plus.google.com has address 173.194.116.33 plus.google.com has address 173.194.116.32 plus.google.com has address 173.194.116.40 plus.google.com has address 173.194.116.46 plus.google.com has address 173.194.116.37 plus.google.com has address 173.194.116.36 plus.google.com has address 173.194.116.38 plus.google.com has address 173.194.116.34 plus.google.com has address 173.194.116.35 plus.google.com has IPv6 address 2a00:1450:400a:806::1003 plus.google.com mail is handled by 10 aspmx.l.google.com. plus.google.com mail is handled by 50 alt4.aspmx.l.google.com. plus.google.com mail is handled by 30 alt2.aspmx.l.google.com. plus.google.com mail is handled by 20 alt1.aspmx.l.google.com. plus.google.com mail is handled by 40 alt3.aspmx.l.google.com. _______________________________________________ dns-operations mailing list dns-operations@lists.dns-oarc.net https://lists.dns-oarc.net/mailman/listinfo/dns-operations dns-jobs mailing list https://lists.dns-oarc.net/mailman/listinfo/dns-jobs
