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#include <sys/socket.h>

int getsockname(int sockfd, struct sockaddr *addr, socklen_t *addrlen);

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#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;

}

}

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$ g++ source_port.cpp

$ ./a.out www.baidu.com

connect to www.baidu.com success.

local port: 39702

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$ 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)