How to implement socket communication to obtain the local source port number in Linux

This article mainly introduces the relevant information about linux socket communication to obtain the local source port number. Friends in need can refer to it

There is a lot of information about TCP IP network communication. TCP IP passes IP data packets mode for end-to-end communication. A typical TCP data packet is as follows

You can see that the data packet contains the source port number and the destination port number. When the client socket initiates a connection to the server, the system will give the socket Randomly assign a source port number, and we can obtain the original port information of the successfully connected socket through getsocketname.

Function prototype

#include <sys/socket.h> 
int getsockname(int sockfd, struct sockaddr *addr, socklen_t *addrlen);

Parameters:

sockfd socket connection handle

addr Network address pointer, used to store local socket address information,

addrlen addr space size

Return result, if the call is successful, return 0, and store the local network address information in addr , returns -1 on failure, and reflects error information through errno.

source_port.cpp

#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; 
 } 
}

This program will first start a socket to connect to an ordinary http server (baidu, qq, 163, csdn). When the socket When connected, the local address bound to the connection is obtained through getsocketname, and the source port number is obtained through this address.

Terminal 1: Compile and run

$ g++ source_port.cpp
$ ./a.out www.baidu.com
connect to www.baidu.com success.
local port: 39702

Terminal 2: Verify by tcpdump packet capture

$ 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)

Comparing terminal one and terminal two shows that the obtained source port address is correct.

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