Programming Perl, 4th Edition

gethostbyaddr

gethostbyaddr ADDR, ADDRTYPE

This function translates addresses into names (and alternate addresses). ADDR should be a packed binary network address, and ADDRTYPE should in practice usually be AF_INET (from the Socket module). The return value in list context is:

#  0      1         2          3       4 ...
($name, $aliases, $addrtype, $length, @addrs) =
    gethostbyaddr($packed_binary_address, $addrtype);

where @addrs is a list of packed binary addresses. In the Internet domain, each address is (historically) four bytes long and can be unpacked by saying something like:

($a, $b, $c, $d) = unpack("C4", $addrs[0]);

Alternatively, you can convert directly to dot-vector notation with the v modifier to sprintf:

$dots = sprintf "%vd", $addrs[0];

The inet_ntoa function from the Socket module is useful for producing a printable version. This approach will become important if and when we all ever manage to switch over to IPv6.

use Socket;
$printable_address = inet_ntoa($addrs[0]);

In scalar context, gethostbyaddr returns only the hostname.

To produce an ADDR from a dot vector, say this:

use Socket;
$ipaddr = inet_aton("127.0.0.1");       # localhost
$claimed_hostname = gethostbyaddr($ipaddr, AF_INET);

See the section Sockets in Chapter 15 for more examples. The Net::hostent module supports a by-name interface to this function. ...