How might I perform DNS lookups using C/C++ on Linux?

Question

How do I get similar functionality to the host command using a c api (or any other language for that matter)? I need more information than just an IP address given by gethostbyname(); specifically, the SMTP-related data.

Answer 1

I don't think there is a function in the C standard library for this, but many scripting languages do have this functionality 'built in'. For example, Perl has the Net::DNS package:

use Net::DNS;
my @mx = mx("example.com");
foreach $host (@mx) {
  print $host;
}

If you need to do this in C, a quick google shows up a few C libraries out there which you can use:

• adns
• udns
• dns.c
• FireDNS (as mentioned by ko-dos)

Answer 2

I'd suggest FireDNS. It's a very fast C library for all kinds of dns queries.

Answer 3

If a blocking (synchronous) query is ok, just use res_query(), and link your program with -lresolv.

 len = res_query(host, C_IN, T_MX, &answer, sizeof(answer));

Answer 4

And I would add, unless you're writing a mail relay you almost certainly shouldn't be looking up MX records - you should be passing the mail on to a user-configured mail relay instead.