C code for DNS lookups

Cedric Puddy cedric at itactics.itactics.com
Tue Nov 9 16:26:43 UTC 1999

On Mon, 8 Nov 1999, Rui Prior wrote:

> As I had no response to my previous message and I really need to know how to
> implement DNS access in C (making queries, parsing results), I'm going to
> repost it. I'd be very grateful if someone could help me.
> Sorry for any inconvenience.
> Rui Prior

Hi Rui, 

Please find the code for a program I call "fastlook" below.
It does not do all of the things that you desire -- indeed
it merely calls the resolvers and hopes that _someone_
set the resolv.conf up and so on.  Writing this wasn't
as easy as one might have thought it would be (the
dig/nslookup code, linux libraries, and bind documentation
combined to make a nice morass), but here it be.  :)



------------- fastlook.c start ---------------
*  fastlook - a bare bones tool for resolving IP addresses/names into
*  canical DNS names.
*  (c)1999 james Partanen, Cedric Puddy
*  $Header: /usr/src/cp/fastlook/fastlook.c,v 1.2 1999/04/23 20:56:37 cedric Exp $
*  The tool can act as a filter, or take a list of addresses on the
*  command line.  It will output the addresses in the form of
*	<queryed addr><tab><resolved address>
*  if running as filter, there will be no superfluous newlines, etc
*  output at any time.
*  The reason for the creation of the program is simple.  Our Linux
*  firewalls log raw IP addresses.  A quick skim of docs and such
*  suggested that they could not be easily made to do lookups as they
*  logged, and some brief thought indicated that this would not be
*  a good idea anyway (perhaps there's a reason the option doesn't
*  exist).  A client wanted to pull the logs into a database and
*  run reports based on this data and other data using some
*  reporting tool.  The solution used was to pull the raw data into
*  one table, and pull the IP<->Name mappings (that we generate with
*  fastlook) into a second table.  When he wants reports with
*  regular names, he uses the IP<->Name mapping table as if it where
*  an associative array, and all is well.  (Crufty, yes, but it suits
*  the application and need:).
*  The program was written on a stock RedHat 5.2 linux machine,
*  with a 2.0.35 kernel and headers.  It does nothing particularly
*  complicated or strange, and would probably compile without
*  difficulty on other machines.  As time goes by, I will probably
*  build and make avaiable binaries for Linux 2.0.x, BSD/OS (BSDI),
*  UnixWare, and Interix 2.x (the platforms I have ready access to)
*  Naturally, this code comes completely without warrentee of
*  any kind.  It is offered and made available with the express
*  understanding that you, the user, undertake any and all risk
*  in the implementation and use hereof.  This code is public
*  domain, or if you are the social type, postcard-ware.  :)
*	Cedric Puddy, cedric at thinkers.org
*	ClearLine Services
*	118 Louisa Street, Kitchener, Ontario, N2H 5M3, Canada

#include <errno.h>
#include <string.h>
#include <limits.h>
#include <stdio.h>
#include <unistd.h>
#include <netdb.h>
#include <sys/socket.h>

int main(int argc, char *argv[])
    struct hostent *host;
    char **args, **names, ch, *s, buf[LINE_MAX];
    unsigned int filter;

    if ((char *)0 == argv[1])
	filter = 1;
	filter = 0;

    if (isatty(1))

    if (!filter)
	args = &argv[1];
    while ((char *)0 != (s=(filter)?gets(buf):*(args++))) {

	if (   (struct hostent *)0 == (host=gethostbyname(s))
	    || (struct hostent *)0 == (host=gethostbyaddr(host->h_addr_list[0],
							  AF_INET))) {

	    /* Verbose error reporting may not be desirable.  If it is
	       required/desired, then use this printf instead. */
	    /* printf("%s\tFailed: %s\n", s, hstrerror(h_errno)); */
	    printf("%s\t%s",s ,s);

	} else {

	    printf("%s\t%s", s, host->h_name);
	    for (names=&host->h_aliases[0]; (char *)0 != *names; names++)
		printf(" %s", *names);


--------------------- fastlook.c end --------------

|  CCj/ClearLine - Unix/NT Administration and TCP/IP Network Services
|  118 Louisa Street, Kitchener, Ontario, N2H 5M3, 519-741-2157
   Cedric Puddy, IS Director		cedric at thinkers.org
     PGP Key Available at: 		http://www.thinkers.org/cedric

More information about the bind-users mailing list