Question

I have this code for server code in c, under unix:

/* A simple server in the internet domain using TCP
   The port number is passed as an argument */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h> 
#include <sys/socket.h>
#include <netinet/in.h>

void error(const char *msg)
{
    perror(msg);
    exit(1);
}

int main(int argc, char *argv[])
{
     int sockfd, newsockfd, portno;
     socklen_t clilen;
     char buffer[256];
     struct sockaddr_in serv_addr, cli_addr;
     int n;
     if (argc < 2) {
         fprintf(stderr,"ERROR, no port provided\n");
         exit(1);
     }
     sockfd = socket(AF_INET, SOCK_STREAM, 0);
     if (sockfd < 0) 
        error("ERROR opening socket");
     bzero((char *) &serv_addr, sizeof(serv_addr));
     portno = atoi(argv[1]);
     serv_addr.sin_family = AF_INET;
     serv_addr.sin_addr.s_addr = INADDR_ANY;
     serv_addr.sin_port = htons(portno);
     if (bind(sockfd, (struct sockaddr *) &serv_addr,
              sizeof(serv_addr)) < 0) 
              error("ERROR on binding");
     listen(sockfd,5);
     clilen = sizeof(cli_addr);
     newsockfd = accept(sockfd, 
                 (struct sockaddr *) &cli_addr, 
                 &clilen);
     if (newsockfd < 0) 
          error("ERROR on accept");
     bzero(buffer,256);
     n = read(newsockfd,buffer,255);
     if (n < 0) error("ERROR reading from socket");
     printf("Here is the message: %s\n",buffer);
     n = write(newsockfd,"I got your message",18);
     if (n < 0) error("ERROR writing to socket");
     close(newsockfd);
     close(sockfd);
     return 0; 
}

I took this code from here.

i run the code on port 8888
my question is why don’t I see it when I run those command in the bash:

netstat -a | grep 8888

or

ifconfig -a | grep 8888

but if I do :

sudo cat /etc/services | grep 8888

I can see it.

Is there any other relevant shell command I can use to see it?

Answer 1

If port 8888 is listed in your /etc/services file, then netstat will substitute the port number by the name given there. This is why grep doesn’t find it. You can turn this off with the -n flag:

$ netstat -a
...
tcp        0      0 *:ssh                   *:*                     LISTEN     
...
$ netstat -an
...
tcp        0      0 0.0.0.0:22              0.0.0.0:*               LISTEN     
...

I generally use netstat -lpn -t tcp to show only listening TCP sockets; then the list is usually short enough to grep using my eyes. The -p flag shows which program is listening, which is often helpful.