Implementation: Server side very easy recvfrom(2) decode request gettimeofday(2) build reply sendmsg(2) oups, not that easy... we might reply with the wrong src address many implementations will refuse our answer listen on each individual IP, so we know which IP the request was sent to and can use that as src address when replying use getifaddrs(3) to get the IPs not available on Solaris, so there people have to specify the addresses to listen on manually until Sun gets a clue at least