# gethostbyname 和 gethostbyaddr 用来在 IPv4 地址和 hostname 之间转换. getservbyport 和 getservbyname 则是与服务相关。gethostbyname出错时不设errno而是设h_errno，并有hstrerror()函数。
# FQDN的全称是: Fully Qualified domain name. 技术上说必须以点号(period)终止.
# AAAA 被称为 "quad A" rcord, 给出了从hostname到Ipv6地址的映射。 PTR用来把IP地址到hostname.
# Entries in the DNS are known as Resource Records(RRs).
# 一个点分十进制(dotted-decimal)IPv4的地址前加 0::ffff:就是 IPv6的字符串形式。
# 与getpeername对应的函数不是gethostname而是getsockname.
# getaddrinfo函数的host参数指定为dotted-decimal IPv4或 IPv6 hex string,会使得只有IPv4或IPv6的addrinfo返回。
# 不给UDP套接字设置SO_REUSEADDR选项。We do not set the SO_REUSEADDR socket option for the UDP socket because this socket option can allow multiple sockets to bind the same UDP port on hosts that support multicasting. Since there is nothing like TCP's TIME_WAIT state for a UDP socket, there is no need to set this socket option when the server is started.
# 一般情况下，同端口的不同协议对应同样的服务。但也有例外。对于端口514，which is the rsh service with TCP, but the syslog service with UDP.
# gethostbyaddr的第一个参数是char* addr，而其实它并非指向一个char* 事实上指向in_addr结构体。
# getaddrinfo好复杂呀！hint的ai_flags设置了AI_CONONNAME成员得到host的canonical name.
# port 53 是domain service的端口号.
# 如果设置了IPV6_V6ONLY.那么一个来自ipv4 client的连接会被拒绝。
# POSIX says that specifying AF_UNSPEC will return addresses that can be used with any protocol family that can be used with the hostname and service name.
# POSIX specification also implies that if the AI_PASSIVE flag is specified without a hostname, then the IPv6 wildcard address(IN6ADDR_ANY_INIT or 0::0) should be returned as a sockaddr_in6 structure, along with the IPv4 wildcard address(INADDR_ANY or 0.0.0.0), which is returned as a sockaddr_in structure.
# An ipv6 server socket can handle both ipv4 and ipv6 on a dual-stack host. Refer to page319 in UNP for details.