Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!crdgw1!uunet!stan!imp
From: imp@solbourne.com (Warner Losh)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: Confirming DNS name matches local host name
Message-ID: <1991May30.214701.9756@solbourne.com>
Date: 30 May 91 21:47:01 GMT
References: <891@bcstec.boeing.com>
Distribution: na
Organization: Solbourne, User Interface Group
Lines: 44

In article <891@bcstec.boeing.com> ced@bcstec.uucp (Charles Derykus) writes:
>Given an internet IP, is there a way to retrieve what the host at that IP
>actually calls itself. In other words, I want to confirm that what DNS
>says actually matches the local host name.

gethostbyaddr(3) should do the trick (don't know if it will or not if
you are running NIS):

# include <sys/types.h>
# include <sys/socket.h>
# include <netdb.h>

int main (argc, argv)
        int argc;
        char **argv;
{
        struct hostent *h;
	u_long	ip

/* assume ip gets set somehow */

        h = gethostbyname (&ip, sizeof (u_long), AF_INET);
        if (h == NULL) {
                fprintf (stderr, "Can't get host info for this host\n");
                exit (1);
        }

        printf ("official name: %s\n", h -> h_name);
}

alternatively, there is nslookup:

Given IP address 1.2.3.4

nslookup
> set q=ptr
> 4.3.2.1.in-addr.arpa.

should also do the trick. 

Warner
-- 
Warner Losh		imp@Solbourne.COM
Free to a good home: 10,000 Miller Moths.  Must promise not to breed them.