Linux中/etc/services文件中为什么一个端口会对应tcp和udp两种协议?

Question

比如：
http 80/tcp www # WorldWideWeb HTTP
http 80/udp # HyperText Transfer Protocol

Answer 1

请看这个文件刚开头的注释部分:

# Network services, Internet style
#
# Note that it is presently the policy of IANA to assign a single well-known
# port number for both TCP and UDP; hence, officially ports have two entries
# even if the protocol doesn't support UDP operations.