When I ping the server sip3.voip-centrex.net, I get this output:

PING sip3.voip-centrex.net ( 56(84) bytes of data.
64 bytes from mpfrrelay.michaelpage.fr ( icmp_req=1 ttl=50 time=35.7 ms
64 bytes from mpfrrelay.michaelpage.fr ( icmp_req=2 ttl=50 time=38.1 ms
64 bytes from mpfrrelay.michaelpage.fr ( icmp_req=3 ttl=50 time=34.6 ms

Domain name does not match. What does it mean ?


The DNS system has many kinds of records. "A" records map names to IP addresses. "PTR" records map IP addresses to names. The mapping is not always transitive, and there can be multiple A records that map the the same IP address.

Strictly speaking, PTR records map special names to other names. The IP address first must be translated to, then a PTR request can be made. You can see this in the detailed output of the dig command:

$ dig -x

; <<>> DiG 9.8.1-P1 <<>> -x
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 21897
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0

;    IN  PTR

;; ANSWER SECTION: 86400 IN    PTR mpfrrelay.michaelpage.fr.

;; Query time: 148 msec
;; WHEN: Sat Jan 26 08:47:32 2013
;; MSG SIZE  rcvd: 82
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If you run dig sip3.voip-centrex.net, you will get the IP (normal lookup request).

If you run dig -x, you will get the name mpfrrelay.michaelpage.fr (reverse lookup request).

That's it.

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mpfrrelay.michaelpage.fr is simply the reverse DNS name for It only means, that the domain name your pinging is not the reverse DNS for the IP. Nothing more.

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