This won't work on Solaris / MacOS / OpenBSD (I think anyway), as they don't support the SO_PEERCRED getsockopt() sub-call.

Solaris is of particular interest to me; I did some digging and found that it supports getting the credentials of the remote peer (to include PF_INET sockets, so long as the remote peer is on the same system e.g. a loopback connection, a process running in another zone, or running at a different level of trust). It does it through a completely different API though; to get started, see the manpages for ucred_get() (describes the whole family) and in particular getpeerucred().

Sun supplies Sun::Solaris::Ucred.pm with the Sun-provided Perl packages, which exposes this API to Perl.

Here's my slapdash implementation for getting peer credentials on both Linux and Solaris (disclaimer: this is a proof of concept, I /know/ there are several glaring problems):

use IO::Socket::UNIX qw( SOCK_STREAM SOMAXCONN SOL_SOCKET 
                                      SO_PEERCRED );

# Assume that $socket represents a connected UNIX-domain socket, 
#   and use it like so:
my ($pid, $uid, $gid) = socket_peercreds($socket);

sub socket_peercreds {
        my $socket = shift;

        my($gid, $os, $packed, $pid, $ucred, $uid);

        chomp($os = `uname -s`);

        if($os eq 'Linux'){
                $packed = getsockopt($socket, SOL_SOCKET, SO_PEERCRED)
+;

                 # these are lowercase Ls
                ($pid, $uid, $gid) = unpack('lll', $packed);

                return ($pid, $uid, $gid);
        }
        elsif($os eq 'SunOS'){
                eval "use Sun::Solaris::Ucred qw(getpeerucred ucred_ge
+t ucred_geteuid ucred_getegid ucred_getpid);
";

                $ucred = getpeerucred(fileno($socket));
                if(! defined($ucred)){
                        print "ERROR: getpeerucred() failed: $!\n";
                        print "\$ucred: $ucred\n";
                        return undef;
                }

                $uid = ucred_geteuid($ucred);
                $gid = ucred_getegid($ucred);
                $pid = ucred_getpid($ucred);
                return ($pid, $uid, $gid);
        }
        else {
                print "ERROR: Unknown os, can't get peer's creds\n";
                return undef;
        }
}
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