linux socket通讯获取本地的源端口号的实现方法

吾爱主题阅读：121 2024-04-05 13:51:50 评论：0

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

1 2	`#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

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 #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

1 2 3 4 $ 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

1 2 3 4 5 6 7 $ 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)