CODEWITHSUNDEEP
csockets
mojo
mojo

Reputation: 61

different ports in netstat and program

I have this simple TCP echo server, and whenever a client connects to it, it displays the client's IP and port. But when I run netstat -a, a different port for the client is shown. I'm running the server and client on the same computer.

In my program, it shows client connected: 127.0.0.1:34997, but the result of netstat -a|grep 6969 is:

tcp        0      0 0.0.0.0:6969            0.0.0.0:*               LISTEN     
tcp        0      0 localhost:46472         localhost:6969          ESTABLISHED
tcp        0      0 localhost:6969          localhost:46472         ESTABLISHED

The code of echo server is:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>

#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
#include <arpa/inet.h>

int main(int argc,char **argv){
    int listenfd,confd,n;
    struct sockaddr_in server,client;
    pid_t pid;
    char buffer[100];

    memset(&server,0,sizeof server);
    server.sin_family=AF_INET;
    server.sin_addr.s_addr=htonl(INADDR_ANY);
    server.sin_port=htons(6969);

    listenfd=socket(AF_INET,SOCK_STREAM,0);
    
    if(bind(listenfd,(struct sockaddr *)&server,sizeof server)==-1){
        perror("bind error");return-1;
    }

    if(listen(listenfd,20)==-1){
        perror("listen error");return -1;
    }
    printf("listening for connection..\n");

    for(;;){
        socklen_t cllen = sizeof(client);
        confd=accept(listenfd,(struct sockaddr *)&client,&cllen);
        
        
        if((pid=fork())==0){    
            printf("client connected: %s:%d\n",inet_ntoa(client.sin_addr),client.sin_port);
            close(listenfd);
            for(;;){
                n=read(confd,&buffer,sizeof buffer);
                if(n==0) break;
                write(confd,&buffer,n);
            }
            printf("client disconnected: %s:%d",inet_ntoa(client.sin_addr),client.sin_port);
            exit(0);
        }
        close(confd);

    }
}

Upvotes: 0

Views: 333

Answers (1)

yano
yano

Reputation: 5305

You need to byte flip the client.sin_port.

printf("client connected: %s:%d\n",inet_ntoa(client.sin_addr), ntohs(client.sin_port));

34997 is 0x88B5. Flip the endianess of those bytes to 0xB588, and you have 46472, as shown in your netstat output. More information on the ntohs man page.

Note that client.sin_addr is most likely an unsigned short (see struct sockaddr_in definitions here and here, check your own system), but this will be promoted to an int, so the %d format specifier is fine.

Upvotes: 5

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