DNS resolution functions for PostgreSQL

nospam@example.com (Christian J. Dietrich) — Fri, 14 May 2010 21:39:00 +0200

You are working with lots of IP addresses in a PostgreSQL database? Ever wanted to quickly find out the hostname for a given IP address? Issue something like

SELECT hostbyname('www.google.com');

in order to resolve that hostname? Then pg-dns-resolve is the right thing for you. pg-dns-resolve contains PL/Python functions for DNS resolution at the SQL prompt. Check out these examples:

For all of the functions there is a variant ending in "_n" which means that on error, NULL is to be returned instead of an error string describing the cause of the error. Some functions have a "_s"-version which means they return the result as a set, i.e. multiple rows.

Here we go. Resolve the hostname for a given IP address:



db=# select dst, hostbyaddr(dst) from dns_per_ip limit 2;

      dst      |        hostbyaddr

---------------+-----------------------------

 192.168.1.1   | (1, 'Unbekannter Rechner')

 193.232.128.6 | ns5.msk-ix.net

(2 rows)

Forward resolve www.google.de to (one of) its IP address:



db=# select hostbyname('www.google.de');

 hostbyname

---------------

 74.125.43.105

(1 row)

Note that on error, NULL is returned by hostbyname_n, BUT hostbyname returns an error string instead. So if you want to know why the resolution failed, use hostbyname, otherwise use hostbyname_n.



db=# select hostbyname_n('nonexisting'), hostbyname('nonexisting');

  hostbyname_n |                  hostbyname

---------------+----------------------------------------------------

               | (-2, 'Der Name oder der Dienst ist nicht bekannt')

(1 row)





db=# select hostbyname_n('nonexistinghost') is NULL;

 ?column?

----------

 true

(1 row)

If you need all IP addresses of a hostname, use addrsbyname. DNS usually returns a different order of multiple IP addresses due to round-robin. Note, that the list of IP addresses of addrsbyname is sorted, thus two executions with the same argument return the same list. This is very useful for comparisons.



db=# select addrsbyname('www.google.de');

 addrsbyname

-------------------

 74.125.43.103

 74.125.43.104

 74.125.43.105

 74.125.43.106

 74.125.43.147

 74.125.43.99

(1 row)

If you want e.g. a comma-separated list instead of newline-separated list, use your own separator string as the second argument to addrsbyname:



db=# select addrsbyname('www.google.de', ', ');

                                    addrsbyname

-----------------------------------------------------------------------------------------

 74.125.43.103, 74.125.43.104, 74.125.43.105, 74.125.43.106, 74.125.43.147, 74.125.43.99

(1 row)

hostsbyname works similar to addrsbyname. hostsbyname returns a list of all hostnames associated with a given hostname, including aliases. As with addrsbyname there are 2 variants, one using the default newline delimiter to separate elements and one where you can specify the delimiter yourself. The list of resulting hostnames is sorted.



db=# select hostsbyname('www.google.de', ', ');

                  hostsbyname

-------------------------------------------------

 www.google.com, www.google.de, www.l.google.com

(1 row)

When working with sets, there are 4 interesting functions: addrsbyname_s and addrsbyname_ns as well as hostsbyname_s and hostsbyname_ns. Those return a set, i.e. multiple rows, instead of an aggregated string and they are useful when working with statements such as



  SELECT ... 

  FROM ...

  WHERE xxx IN ( SELECT addrsbyname_ns('www.google.com') )





db=# SELECT addrsbyname_s('www.google.com');

 addrsbyname_s

---------------

 74.125.43.103

 74.125.43.104

 74.125.43.105

 74.125.43.106

 74.125.43.147

 74.125.43.99

(6 rows)

Note the subtle difference: 6 rows instead of 1 row when comparing the output of addrsbyname_s to that of addrsbyname.



db=# SELECT '74.125.43.103'::ip4 IN ( SELECT addrsbyname_s('www.google.com') );

 ?column?

----------

 t

(1 row)



db=# SELECT hostsbyname_ns('www.google.com');

  hostsbyname_ns

------------------

 www.google.com

 www.l.google.com

(2 rows)

Query a non existing hostname with the "_ns"-variant and the result will be an empty set (0 rows):



db=# SELECT hostsbyname_ns('nonexistinghost');

 hostsbyname_ns

----------------

(0 rows)

A special case is forward-confirmed reverse DNS resolution (http://en.wikipedia.org/wiki/Forward-confirmed_reverse_DNS):



db=# SELECT fcrdns('192.203.230.10');

 fcrdns

--------

 f

(1 row)



db=# SELECT fcrdns('74.125.43.104');

 fcrdns

--------

 t

(1 row)

Like it? It's free, download it here: http://pgfoundry.org/projects/pgdnsres/. Alternatively, grab a version from http://www.cj2s.de/plpython_dns-functions.sql.

Installation is easy:

Make sure, you have ip4r installed. Get it from: http://pgfoundry.org/projects/ip4r/

Make sure, you have PL/Python installed and are allowed to add new functions

psql [YOUR OPTIONS] < plpython_dns-functions.sql

blueblog - by Christian J. Dietrich - PostgreSQL

DNS resolution functions for PostgreSQL