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Finding IP Address without knowing the Hostname

Posted on 2000-05-17
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Last Modified: 2010-04-02
If I cannot get the hostname from gethostname, is there another way to get the IP address that is independent of knowing the hostname?
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Question by:paulca
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jkr earned 120 total points
ID: 2818674
If you cannot determine the hostname, IP isn't configured correctly (->'gethostname()'). But as every host has a loopback IF (i.e. IP 127.0.0.1), you can use this one in a call to 'gethostbyaddr()' and extract the other addresses form the 'h_addr_list' member of the 'HOSTENT' struct returned by this call.
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by:paulca
ID: 2823521
The code I'm using is which gives me an error:

pHost = gethostbyaddr("127.0.0.1", 4, PF_INET);

for(j=0;j<pHost->h_length;j++) {

                        if(j>0) {
                              strcat(addrStrings, ".");
                        }
                        sprintf(buf, "%u", (unsigned int)((unsigned char*)pHost->h_addr_list[i])[j]);
                        strcat(addrStrings, buf);
                  }
                        printf (addrStrings);
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LVL 86

Expert Comment

by:jkr
ID: 2827429
Ooops, it seems that the comment I posted some hours ago is gone...

So, again:

The addresses are stored as DWORDs in network byte order - use sth like

char    *BinIP2StrIP    (   long    lnIP)
{
    static char     s_acStrIP   [   16];

    PUCHAR          puc =   ( unsigned char *) &lnIP;
    unsigned int    aun [   4];

    for (   int i = 0;  i < 4;  aun [   i++]    =   *puc++);

    sprintf (   s_acStrIP,
                "%d.%d.%d.%d",
                aun [   0],
                aun [   1],
                aun [   2],
                aun [   3]
            );

    return  (   s_acStrIP);
}


DWORD dwIP;
for(j=0;dwIP = pHost->h_addr_list [j];j++)
{
 printf ( "%s", BinIP2StrIP ( dwIP));
}

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