Unix Network Programming Episode 78

'getaddrinfo' Function

The gethostbyname and gethostbyaddr functions only support IPv4. The API for resolving IPv6 addresses went through several iterations, as will be described in Section 11.20(See 8.9.20); the final result is the getaddrinfo function.

The POSIX definition of this function comes from an earlier proposal by Keith Sklower for a function named getconninfo. This function was the result of discussions with Eric Allman, William Durst, Michael Karels, and Steven Wise, and from an early implementation written by Eric Allman. The observation that specifying a hostname and a service name would suffice for connecting to a service independent of protocol details was made by Marshall Rose in a proposal to X/Open.

复制代码
#include <netdb.h>
int getaddrinfo (const char *hostname, const char *service, const struct addrinfo *hints, struct addrinfo **result) ;

This function returns through the result pointer a pointer to a linked list of addrinfo structures, which is defined by including <netdb.h>.

复制代码
struct addrinfo {
	int ai_flags; /* AI_PASSIVE, AI_CANONNAME */
	int ai_family; /* AF_xxx */
	int ai_socktype; /* SOCK_xxx */
	int ai_protocol; /* 0 or IPPROTO_xxx for IPv4 and IPv6 */
	socklen_t ai_addrlen; /* length of ai_addr */
	char *ai_canonname; /* ptr to canonical name for host */
	struct sockaddr *ai_addr; /* ptr to socket address structure */
	struct addrinfo *ai_next; /* ptr to next structure in linked list */
};

The members of the hints structure that can be set by the caller are:

  • ai_flags (zero or more AI_XXX values OR'ed together)
  • ai_family (an AF_xxx value)
  • ai_socktype (a SOCK_xxx value)
  • ai_protocol

If the function returns success (0), the variable pointed to by the result argument is filled in with a pointer to a linked list of addrinfo structures, linked through the ai_next pointer. There are two ways that multiple structures can be returned:

1.If there are multiple addresses associated with the hostname, one structure is returned for each address that is usable with the requested address family (the ai_family hint, if specified).

2.If the service is provided for multiple socket types, one structure can be returned for each socket type, depending on the ai_socktype hint. (Note that most getaddrinfo implementations consider a port number string to be implemented only by the socket type requested in ai_socktype; if ai_socktype is not specified, an error is returned instead.)

For example, if no hints are provided and if the domain service is looked up for a host with two IP addresses, four addrinfo structures are returned:

  • One for the first IP address and a socket type of SOCK_STREAM
  • One for the first IP address and a socket type of SOCK_DGRAM
  • One for the second IP address and a socket type of SOCK_STREAM
  • One for the second IP address and a socket type of SOCK_DGRAM

If we were to enumerate all 64 possible inputs to getaddrinfo (there are six input variables), many would be invalid and some would make little sense. Instead, we will look at the common cases.

  • Specify the hostname and service. This is normal for a TCP or UDP client. On return, a TCP client loops through all returned IP addresses, calling socket and connect for each one, until the connection succeeds or until all addresses have been tried. We will show an example of this with our tcp_connect function in Figure 11.10(See 8.9.12).
  • For a UDP client, the socket address structure filled in by getaddrinfo would be used in a call to sendto or connect. If the client can tell that the first address doesn't appear to work (either by receiving an error on a connected UDP socket or by experiencing a timeout on an unconnected socket), additional addresses can be tried.
  • If the client knows it handles only one type of socket (e.g., Telnet and FTP clients handle only TCP; TFTP clients handle only UDP), then the ai_socktype member of the hints structure should be specified as either SOCK_STREAM or SOCK_DGRAM.
  • A typical server specifies the service but not the hostname, and specifies the AI_PASSIVE flag in the hints structure. The socket address structures returned should contain an IP address of INADDR_ANY (for IPv4) or IN6ADDR_ANY_INIT (for IPv6). A TCP server then calls socket, bind, and listen. If the server wants to malloc another socket address structure to obtain the client's address from accept, the returned ai_addrlen value specifies this size.
  • A UDP server would call socket, bind, and then recvfrom. If the server wants to malloc another socket address structure to obtain the client's address from recvfrom, the returned ai_addrlen value specifies this size.
  • As with the typical client code, if the server knows it only handles one type of socket, the ai_socktype member of the hints structure should be set to either SOCK_STREAM or SOCK_DGRAM. This avoids having multiple structures returned, possibly with the wrong ai_socktype value.
  • The TCP servers that we have shown so far create one listening socket, and the UDP servers create one datagram socket. That is what we assume in the previous item. An alternate server design is for the server to handle multiple sockets using select or poll. In this scenario, the server would go through the entire list of structures returned by getaddrinfo, create one socket per structure, and use select or poll.
相关推荐
sali-tec3 分钟前
C# 基于halcon的视觉工作流-章46-不匀面划痕
人工智能·算法·计算机视觉·c#
是良辰4 分钟前
Linux 服务器NFS文件共享
运维·服务器
JJJJ_iii6 分钟前
【机器学习03】学习率与特征工程、多项式回归、逻辑回归
人工智能·pytorch·笔记·学习·机器学习·回归·逻辑回归
沐浴露z13 分钟前
【深入理解计算机网络11】应用层详解:DNS,WWW,HTTP
服务器·网络协议·计算机网络·http
迎風吹頭髮16 分钟前
Linux服务器编程实践22-TCP头部选项解析:MSS、窗口扩大因子与SACK
服务器·网络·tcp/ip
Net_Walke17 分钟前
【Linux系统】系统编程
linux·运维·服务器
大明者省20 分钟前
人工智能学科主要单词
人工智能
BEOL贝尔科技20 分钟前
不稳定的冰箱如何做权限管理?冰箱锁加入远程管理功能后生物保存工作发生了哪些变化?
大数据·人工智能
AI拉呱_23 分钟前
第144期《2025年AI现状报告》解读(二):产业篇
人工智能·chatgpt
啊吧怪不啊吧27 分钟前
初识SQL
服务器·数据库·sql