Resolving the MX records

This basic example shows how to create a resolver which asks for MX records which contain the information about mail servers.

#!/usr/bin/python
#
# MX is a small program that prints out the mx records for a particular domain
#
import ldns

resolver = ldns.ldns_resolver.new_frm_file("/etc/resolv.conf")

dname = ldns.ldns_dname("nic.cz")

pkt = resolver.query(dname, ldns.LDNS_RR_TYPE_MX, ldns.LDNS_RR_CLASS_IN, ldns.LDNS_RD)
if (pkt):
        mx = pkt.rr_list_by_type(ldns.LDNS_RR_TYPE_MX, ldns.LDNS_SECTION_ANSWER)
        if (mx):
                mx.sort()
                print mx

Resolving step by step

First of all we import ldns extension module which make LDNS functions and classes accessible:

import ldns

If importing fails, it means that Python cannot find the module or ldns library.

Then we create the resolver by ldns.ldns_resolver.new_frm_file() constructor

resolver = ldns.ldns_resolver.new_frm_file("/etc/resolv.conf")

and domain name variable dname:

dname = ldns.ldns_dname("nic.cz")

To create a resolver you may also use:

resolver = ldns.ldns_resolver.new_frm_file(None)

which behaves in the same manner as the command above.

In the third step we tell the resolver to query for our domain, type MX, of class IN:

pkt = resolver.query(dname, ldns.LDNS_RR_TYPE_MX, ldns.LDNS_RR_CLASS_IN, ldns.LDNS_RD)

The function should return a packet if everything goes well and this packet will contain resource records we asked for. Note that there exists a simplier way. Instead of using a dname variable, we can use a string which will be automatically converted.

pkt = resolver.query("fit.vutbr.cz", ldns.LDNS_RR_TYPE_MX, ldns.LDNS_RR_CLASS_IN, ldns.LDNS_RD)

Now, we test whether the resolver returns a packet and then get all RRs of type MX from the answer packet and store them in list mx:

if (pkt):
        mx = pkt.rr_list_by_type(ldns.LDNS_RR_TYPE_MX, ldns.LDNS_SECTION_ANSWER)

If this list is not empty, we sort and print the content to stdout:

if (mx):
        mx.sort()
        print mx