I'm reading a book on network programming with Go. One of the chapters deals with the /etc/services file. Something I noticed while exploring this file is that certain popular entries like HTTP and SSH, both of which use TCP at the transport layer, have a second entry for UDP. For example on Ubuntu 14.04:
ubuntu@vm1:~$ grep ssh /etc/services ssh 22/tcp # SSH Remote Login Protocol ssh 22/udp ubuntu@vm1:~$ grep http /etc/services http 80/tcp www # WorldWideWeb HTTP http 80/udp # HyperText Transfer Protocol
Anyone know why these have two entries? I don't believe SSH or HTTP ever use UDP (confirmed by this question for SSH).