DNS resolved to a backwards IP?!

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aw

296 posts

Ultimate Geek
+1 received by user: 30

#76115 28-Jan-2011 23:10

I just tried to telnet into a client's SMTP server to check it was up (with the cyclone and all) and something weird happened.

The relevant IP is 58.28.ABC.DEF (letters obviously masking the actual address) but when I telnetted it by name, telnet said "Trying DEF.ABC.28.58..." and failed.

The bit values of the IP were reversed. Has anyone seen behaviour like that before? A couple of minutes later it was resolving correctly.

My home office DNS server is BIND9, and the domain is hosted by Openhost and managed by me.

So weird.

This was using telnet on an openSUSE 11.1 box (old I know) .

oldmaknz

536 posts

Ultimate Geek

#432481 28-Jan-2011 23:12

Telnet D:

aw

296 posts

Ultimate Geek
+1 received by user: 30

#432910 30-Jan-2011 19:25

By telnet I meant telnet port 25. Anyway, that's not the bit that went funny, it was the DNS resolution that did - just thought the reversed IP that came back was strange and wondered if anyone had seen that before.

gzt

18678 posts

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Lifetime subscriber

#433331 31-Jan-2011 18:05

aw: The relevant IP is 58.28.ABC.DEF (letters obviously masking the actual address) but when I telnetted it by name, telnet said "Trying DEF.ABC.28.58..." and failed.

Forward-confirmed reverse DNS failure?

heydonms

27 posts

Geek

#435049 3-Feb-2011 21:15

What telnet client were you using?

It may simply be a reverse DNS entry. it's not uncommon for ISPs to have an rDNS of the form 4.3.2.1.cpe.isp.com for the address 1.2.3.4

kyhwana2

2572 posts

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#435050 3-Feb-2011 21:24

Yes, that's how it works. Im sure there's a RFC for that, can't find it atm.