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Basic DNS: PTR Records And Why You Care

A.P. Lawrence
Expert Author
Published: 2005-04-12

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A PTR record is what lets someone do a "reverse" DNS lookup - that is, they have your IP address and want to know what your host/domain is. At any Unix/Linux command line, you can use "dig -x" to do a reverse lookup:

bash-2.05a$ dig -x

; <<>> DiG 9.2.1 <<>> -x
;; global options: printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 38101
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0


;; ANSWER SECTION: 1762 IN PTR pcunix.com.

;; Query time: 49 msec
;; WHEN: Fri Jul 2 11:03:29 2004
;; MSG SIZE rcvd: 67

Not every IP address has a corresponding PTR record. In fact, if you took a random sampling of addresses your firewall blocked because they were up to no good, you'd probably find most have no PTR record - a dig -x gets you no information. That's also apt to be true for mail spammers, or their PTR doesn't match up: if you do a dig -x on their IP you get a result, but if you look up that result you might not get the same IP you started with.

That's why PTR records have become important. Originally, PTR records were just intended as a convenience, and perhaps as a way to be neat and complete. There still are no requirements that you have a PTR record or that it be accurate, but because of the abuse of the internet by spammers, certain conventions have grown up. For example, you may not be able to send email to some sites if you don't have a valid PTR record, or if your pointer is "generic":

dig -x

; <<>> DiG 9.2.1 <<>> -x
;; global options: printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 55565
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0


;; ANSWER SECTION: 3570 IN PTR h00c0f06bacf1.ne.client2.attbi.com.

;; Query time: 2 msec
;; WHEN: Fri Jul 2 11:12:45 2004
;; MSG SIZE rcvd: 90

Typically, the reason you get refused is because the "generic" pointer doesn't have an MX (mail exchange record):

dig h00c0f06bacf1.ne.client2.attbi.com MX

; <<>> DiG 9.2.1 <<>> h00c0f06bacf1.ne.client2.attbi.com MX
;; global options: printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 32826
;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 0

;h00c0f06bacf1.ne.client2.attbi.com. IN MX

;; Query time: 292 msec
;; WHEN: Fri Jul 2 11:14:01 2004
;; MSG SIZE rcvd: 52

How do you get a PTR record? You might think that this is done by your domain registrar - after all, they point your domain to an IP address. Or you might think whoever handles your DNS would do this. But the PTR record isn't up to them, it's up to the ISP that "owns" the IP block it came from. They are the ones who need to create the PTR record. In some cases, that may be the same folks who handle your DNS, but the point is that it is not necessarily so: you may have no control over this whatsoever. See RFC 2317 and Avoid RFC 2317 style delegation of 'in-addr.arpa.' also.

If you don't have a PTR record, and can't get one (for example because you have a dynamic address), you will want to send outgoing email through a server that does. Your INCOMING server desn't need a PTR record - you can use a dynamically assigned IP address for a mail server as long as you have some way of updating your MX record when your IP changes. And it isn't that you can't send SMTP mail out from such a server (although some ISP's do block outgoing SMTP to any but their own server); it's just that some recipients may block you because you don't have that PTR record or because it doesn't match up wth an MX record for you.

Usually, you'll use your ISP's SMTP server as your "delegate" server - that can be done at your internal mail server or at the client machines ("Outgoing SMTP Server"). However, assuming that you aren't blocked for outgoing SMTP, you can use any server that "likes" you - that is, any server that will allow your IP address to relay mail through it. That's probably going to be another server under your control, or someone who knows you - there are few servers left that will let just anyone use them as a mail relay.

See What are reverse (PTR) records? Why do I need them? also.

*Originally published at APLawrence.com

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About the Author:
A.P. Lawrence provides SCO Unix and Linux consulting services http://www.pcunix.com

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