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How to get the TCP Client Port number by Socket Descripter?
► getsockname() function (man_page) return the socket information on second parameter by socket descripter in frisrt parameter
1. Write test.cpp
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <arpa/inet.h>
int main()
{
char strIP[16] = {0,};
unsigned short usPort = 0;;
struct sockaddr_in server_addr, my_addr;
int sockfd = -1; // default Error
// Create TCP Client Socket
if((sockfd = socket(AF_INET, SOCK_STREAM, 0)) < 0)
{
perror("Can't open stream socket.");
return -1;
}
// Set server_addr
bzero(&server_addr, sizeof(server_addr));
server_addr.sin_family = AF_INET;
server_addr.sin_addr.s_addr = inet_addr("172.16.3.204"); // Server IP
server_addr.sin_port = htons(80); // Server Port
// Connect to server
if (connect(sockfd, (struct sockaddr *) &server_addr, sizeof(server_addr)) < 0)
{
perror("Connect server error");
close(sockfd);
return -1;
}
// Get Client IP and Port
bzero(&my_addr, sizeof(my_addr));
socklen_t len = sizeof(my_addr);
getsockname(sockfd, (struct sockaddr *) &my_addr, &len);
printf("Local ip address: %s\n", inet_ntoa(my_addr.sin_addr));
printf("Local port : %u\n", ntohs(my_addr.sin_port)); // Network is Big endian
return 0;
}
2. Check the result
Compile and execute the program
[root@localhost test]# g++ -o test test.cpp && ./test
Local ip address: 10.211.55.4
Local port : 38804
Check the packet by tcpdump
[root@localhost test]# tcpdump -i eth0 -s 0 -n tcp port 80
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
19:38:57.622896 IP 10.211.55.4.38804 > 172.16.3.204.http: Flags [S], seq 2413665406, win 14600, options [mss 1460,sackOK,TS val 724875807 ecr 0,nop,wscale 7], length 0
19:38:57.626132 IP 172.16.3.204.http > 10.211.55.4.38804: Flags [S.], seq 3993806964, ack 2413665407, win 32768, options [mss 1460,wscale 1,nop], length 0
19:38:57.626189 IP 10.211.55.4.38804 > 172.16.3.204.http: Flags [.], ack 1, win 115, length 0
19:38:57.626901 IP 10.211.55.4.38804 > 172.16.3.204.http: Flags [F.], seq 1, ack 1, win 115, length 0
19:38:57.627027 IP 172.16.3.204.http > 10.211.55.4.38804: Flags [.], ack 2, win 16384, length 0
19:38:57.633470 IP 172.16.3.204.http > 10.211.55.4.38804: Flags [F.], seq 1, ack 2, win 16384, length 0
19:38:57.633518 IP 10.211.55.4.38804 > 172.16.3.204.http: Flags [.], ack 2, win 115, length 0
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