DNS Reverse Lookup (newbie)

Posted on 2003-11-26
Last Modified: 2007-11-27
I did an nslookup (plus reverse lookup) of my domain name on a variety of nameservers.

The results can be seen on

I am a little new to this.

5 of the 11 nameservers queried showed a FAILURE in reverse lookup.

Is that a potential problem and what might be causing this problem?

Are reverse lookups still used to block spam?

Question by:eamonroche
LVL 18

Accepted Solution

chicagoan earned 125 total points
ID: 9827524
Let's see why:
Start at the root:
Default Server:
>set type=ptr
> root
Default Server:  A.ROOT-SERVERS.NET

Address:        nameserver = chia.ARIN.NET        nameserver = dill.ARIN.NET        nameserver = henna.ARIN.NET        nameserver = indigo.ARIN.NET        nameserver = epazote.ARIN.NET        nameserver = figwort.ARIN.NET        nameserver = ginseng.ARIN.NET

ok- arin has control of that netblock - let's ask them what they know about it:

> server
Default Server:

Address: nameserver = nameserver =

ok - that netblock is delegated to webcontrolcenter

let's ask them:

> server
Default Server:


*** can't find Non-existent domain

oops - hmmm

let's try the other one:

> server
Default Server:

Address:     name =     name =     name =     name =     name =     name =     name =     name =     name =     name =

ah! so - I'd say ns1.webcontrolcenter is hosed

in-arpa lookups that hit that will cache the negative answer for a while, lookups that hit NS2 will cache the good answer

SMTP servers can and very often are configured to reject mail from an ip the doesn't have a reverse lookup, one that doesn't match, doesn;t have mx records, etc. THis produces a lot of false positives as far as spam goes, but it's built right into sendmail so you gotta live with it. Your best bet is to use your ISP's mail relay as a smart relay for your mail server.


Expert Comment

ID: 9827763
He... That's some good info, and excellent material on uses...
Thanks! 8-)
LVL 18

Expert Comment

ID: 9827843
no problem, give your dns provider a buzz
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Expert Comment

ID: 9827968

Effectively you we can see that you do not have the reverse entry setup properly in this quick report:

FAIL Reverse DNS entries for MX records ERROR: One or more of your mail server(s) have no reverse DNS (PTR) entries (if you see "Timeout" below, it may mean that your DNS servers did not respond fast enough). RFC1912 2.1 says you should have a reverse DNS for all your mail servers. It is strongly urged that you have them, as many mailservers will not accept mail from mailservers with no reverse DNS entry.

You need to setup a revrese zone in your DNS (supposing you are hosting your DNS)  an arpa like:

After you set this up and if your ISP owns the public internet addresses you are using you need to call them and ask them to give your DNS server the delegation Right on the reverse addresses.

And to answer your question more and more company set their mail server to block emails from servers that do not reversely resolve ip's.  

I have had the same problem but since the reverse zone was delegated to my own DNS server and I entered the ip addresses of our smtp server in the zones it's working perfectly.

Also you can setup as much reverse zone in your DNS as you have different range of addreses.

Bruno Gagnon

Expert Comment

ID: 10283707
The good news is: You're not blackholed:

The bad news:
; <<>> DiG 9.2.2 <<>>
;; global options:  printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id:
;; flags: qr aa rd ra; QUERY: 1, ANSWER: 0, AUTHORITY:

;                 IN      A

.                       86400   IN      SOA    
GN-GRS.COM. 2004020500 1800 900 604800 86400

;; Query time: 200 msec
;; WHEN: Thu Feb 05 14:10:00 2004
;; MSG SIZE  rcvd: 106

(translation: No reverse DNS)

Sendmail blocks mail that has an unresolvable domain
name in the MAIL FROM: part of the SMTP transaction.
It doesn't block mail from or to servers that have
unresolvable IP numbers. Mail servers that have IP
numbers with no reverse lookups can still send mail
with this turned on. However, some servers DO block
mail if the connecting node has no reverse configured.

DNS names and numbers are resolved in two seperate
systems. The NAMES are owned by individuals and
administered by name registrars at the root servers
and thereafter byt the operator of the DNS servers
specified there.

IP addresses are owned by ISP's, organizations and
(rarely) individuals and are administered by ARIN in
north america, and thereafter by the the netblock
owner. Your address lies in a block registered to
Network Availability

and that's who has to enter the reverse entry. It
would be nice if the reverse entry matched the A
record, but any ole' thing will do (often they like to
use the '' name which is a representation of
the IP address and something meaningful to the ISP.

Note that there is no uber-spam-filter. overzealous or
uninformed (or just plain dumb) administrators can and
do setup crazy filters and some big outfits like AOL
and Hotmail have proprietary systems and their own
ideas. Sometimes it's easier to relay through your ISP
or at least use them as a smart host for problematic

You SMTP logs should have a reason the mail was
rejected, get this fixed and keep an eye on the logs
to fine tune things.


--- Laxman Subramanian
<> wrote:
> Frank,

> I was browsing through answers for why mails from my
> mailserver are rejected by *few isp's* and the
> reason was that they tried to do the reverse dns
> lookup and the ip resolves to domain not found. I am
> kind of lost here as to who actually is the person I
> need to square this with is it the ISP or the DNS
> registeration provider EASYDNS. This just ahppened
> recently after we moved to a new ip block and ever
> since we are not able to get reverse dns lookups .

> When I asked my DNS registrar he replied back saying
> its the ISP who needs to fix this.

> "The easyDNS interface does not support reverse
> lookup, because we cannot provide our clients with
> reverse DNS in the vast majority of cases and so
> there is no way through the interface to manage that
> zone. This is because the nameservers have to have a
> delegation for the entire netblock that IP address
> resides in for us to provide a reverse lookup. In
> most cases, connectivity providers will not delegate
> blocks of their IP addresses to third party
> nameservers (unless they are themselves a customer)
> So you will need to contact your upstream
> connectivity provider to set up the reverse lookup.
> "

> My mailserver is and it
> resolves to where as the reverse fails
> . can you help me here and educate with me with what
> is happening and how i can square this.

> Laxman Subramanian, CISSP

LVL 18

Expert Comment

ID: 10283762
so... what did your ISP say?
LVL 18

Expert Comment

ID: 10283769
I think you want to open a new question...

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