关于TCP IP网络通讯的资料非常多，TCP IP通过IP数据包模式进行端对端通讯。典型的TCP数据包如下
可以看到数据包包含了源端口号和目的端口号，客户端socket向服务端发起连接时，系统会给socket随机分配一个源端口号，我们可以通过getsocketname来获取连接成功的socket的原端口信息。
函数原型
[cpp] view plain copy
- #include <sys/socket.h>
- int getsockname(int sockfd, struct sockaddr *addr, socklen_t *addrlen);
参数:
sockfd socket连接的句柄
addr 网络地址指针，用来存储本地端socket地址信息，
addrlen addr的空间大小
返回结果，如果调用成功，返回0，并将本地网络地址信息存放在addr里面，失败返回-1，并通过errno反应错误信息。
source_port.cpp
[cpp] view plain copy
- #include <cstring>
- #include <cstdio>
- #include <cstdlib>
- #include <sys/socket.h>
- #include <sys/types.h>
- #include <netinet/in.h>
- #include <netinet/ip.h>
- #include <netdb.h>
- #include <errno.h>
- #include <unistd.h>
- #include <arpa/inet.h>
- void safe_close(int &sock);
- int main(int argc, char *argv[]) {
- int sockfd = 0, n = 0;
- socklen_t len = 0;
- char host[512] = {0};
- char buf[1024] = {0};
- struct hostent *server;
- struct sockaddr_in serv_addr, loc_addr;
- if (argc < 2) {
- printf("Please input host name\n");
- exit(-1);
- }
- strncpy(host, argv[1], sizeof(host));
- server = gethostbyname(host);// 判断输入的域名是否正确
- if (NULL == server) {
- printf("find host: %s failed.\n", host);
- exit(-1);
- }
- if (-1 == (sockfd = socket(AF_INET, SOCK_STREAM, 0))) {// 创建socket
- memset(buf, 0, sizeof(buf));
- snprintf(buf, sizeof(buf), "new socket failed. errno: %d, error: %s", errno, strerror(errno));
- perror(buf);
- exit(-1);
- }
- memset(&serv_addr, 0, sizeof(serv_addr));
- serv_addr.sin_family = AF_INET;
- serv_addr.sin_port = htons(80);// http标准端口号
- memcpy(&serv_addr.sin_addr.s_addr, server->h_addr, server->h_length);
- if (-1 == inet_pton(AF_INET, host, &serv_addr.sin_addr)) {
- memset(buf, 0, sizeof(buf));
- snprintf(buf, sizeof(buf), "inet_pton failed. errno: %d, error: %s", errno, strerror(errno));
- perror(buf);
- exit(-1);
- }
- if (-1 == connect(sockfd, (struct sockaddr *)&serv_addr, sizeof(serv_addr))) {// 连接socket
- memset(buf, 0, sizeof(buf));
- snprintf(buf, sizeof(buf), "connect socket failed. errno: %d, error: %s", errno, strerror(errno));
- perror(buf);
- exit(-1);
- }
- printf("connect to %s success.\n", host);
- len = sizeof(sizeof(loc_addr));
- memset(&loc_addr, 0, len);
- if (-1 == getsockname(sockfd, (struct sockaddr *)&loc_addr, &len)) {// 获取socket绑定的本地address信息
- memset(buf, 0, sizeof(buf));
- snprintf(buf, sizeof(buf), "get socket name failed. errno: %d, error: %s", errno, strerror(errno));
- perror(buf);
- safe_close(sockfd);
- exit(-1);
- }
- if (loc_addr.sin_family == AF_INET) {// 打印信息
- printf("local port: %u\n", ntohs(loc_addr.sin_port));
- }
- safe_close(sockfd);
- return 0;
- }
- void safe_close(int &sock) {
- if (-1 != sock) {
- shutdown(sock, SHUT_RDWR);
- sock = -1;
- }
- }
本程序首先会启动一个socket连接一个普通的http服务器（baidu，qq，163，csdn），当socket连通时就通过getsocketname获取连接绑定的本地地址，并通过该地址获取源端口号。
终端1： 编译及运行
$ g++ source_port.cpp
$ ./a.out www.baidu.com
connect to www.baidu.com success.
local port: 39702
终端2： 通过tcpdump抓包验证
$ sudo tcpdump host www.baidu.com -v
tcpdump: listening on eth0, link-type EN10MB (Ethernet), capture size 65535 bytes
18:38:32.381448 IP (tos 0x0, ttl 64, id 35033, offset 0, flags [DF], proto TCP (6), length 60)
icentos.39702 > 220.181.111.188.http: Flags [S], cksum 0x8cd2 (incorrect -> 0x596a), seq 2381397554, win 29200, options [mss 1460,sackOK,TS val 3513497323 ecr 0,nop,wscale 7], length 0
18:38:32.425904 IP (tos 0x0, ttl 55, id 35033, offset 0, flags [DF], proto TCP (6), length 60)
220.181.111.188.http > icentos.39702: Flags [S.], cksum 0xc315 (correct), seq 3561856904, ack 2381397555, win 8192, options [mss 1424,sackOK,nop,nop,nop,nop,nop,nop,nop,nop,nop,nop,nop,wscale 5], length 0
18:38:32.425930 IP (tos 0x0, ttl 64, id 35034, offset 0, flags [DF], proto TCP (6), length 40)
对比终端一和终端二表明获取的源端口地址是正确的。