Foundations of Python Network Programming 978-1-4302-3004-5

CHAPTER 4 ■ SOCKET NAMES AND DNS
often requires two round-trips, this record type is unpopular these days, but you still might run across it
occasionally.
That MX record is crucial, by the way, and is something that newcomers to network programming
often get confused! Sending e-mail to a domain is a completely different act from trying to make an
HTTP or SSH connection to a domain; if you want to send e-mail to someone @python.org, then do not
try making an SMTP connection to the host named python.org! Always rely on MX records to point you to
your destination, if they exist; try making an SMTP connection to an A record for the domain named in
an e-mail address only if there are no MX records returned for that domain name.
Resolving Mail Domains
I mentioned previously that resolving an e-mail domain is a very legitimate use of raw DNS in most
Python programs. The rules for doing this resolution are specified most recently in RFC 5321. They are,
briefly, that if MX records exist, then you must try to contact those SMTP servers, and return an error to
the user (or put the message on a re-try queue) if none of them will accept the message. If instead no MX
records exist, but an A or AAAA record is provided for the domain, then you are allowed to try an SMTP
connection to that address. If neither record exists, but a CNAME is specified, then the domain name it
provides should be searched for MX or A records using the same rules.
Listing 4–4 shows how you might implement this algorithm. By doing a series of DNS queries, it
works its way through the possible destinations, printing out its decisions as it goes. By adjusting a
routine like this to return addresses rather than just printing them out, you could power a Python mail
dispatcher that needed to deliver e-mail to remote hosts.
Listing 4–4. Resolving an E-mail Domain Name
#!/usr/bin/env python
# Foundations of Python Network Programming - Chapter 4 - dns_mx.py
# Looking up a mail domain - the part of an email address after the `@`
import sys, DNS
if len(sys.argv) != 2:
» print >>sys.stderr, 'usage: dns_basic.py '
» sys.exit(2)
def resolve_hostname(hostname, indent=0):
» """Print an A or AAAA record for `hostname`; follow CNAMEs if necessary."""
» indent = indent + 4
» istr = ' ' * indent
» request = DNS.Request()
» reply = request.req(name=sys.argv[1], qtype=DNS.Type.A)
» if reply.answers:
» » for answer in reply.answers:
» » » print istr, 'Hostname', hostname, '= A', answer['data']
» » return
» reply = request.req(name=sys.argv[1], qtype=DNS.Type.AAAA)
» if reply.answers:
» » for answer in reply.answers:
» » » print istr, 'Hostname', hostname, '= AAAA', answer['data']
» » return
» reply = request.req(name=sys.argv[1], qtype=DNS.Type.CNAME)
» if reply.answers:
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