C# Specify Client Port

Question

I have the following code regarding asynchronous sockets programming:

    public void ServerBeginAceept(ushort ServerPort)
    {
        try
        {
            ServerSocket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);

            IPEndPoint ipEndPoint = new IPEndPoint(IPAddress.Any, ServerPort);
            ServerSocket.Bind(ipEndPoint);
            ServerSocket.Listen(MAX_CONNECTIONS);
            IAsyncResult result = ServerSocket.BeginAccept(new AsyncCallback(ServerEndAccept), ServerSocket);
        }
    }


    public void ServerEndAccept(IAsyncResult iar)
    {
        try
        {
            ServerSocket = (Socket)iar.AsyncState;
            CommSocket = ServerSocket.EndAccept(iar);
        }
    }

    public void ClientBeginConnect(string ServerIP, ushort ServerPort)
    {
        try
        {
            CommSocket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
            IPEndPoint ipEndPoint = new IPEndPoint(IPAddress.Parse(ServerIP), ServerPort);
            IAsyncResult result = CommSocket.BeginConnect(ipEndPoint, new AsyncCallback(ClientEndConnect), CommSocket);

        }
    }


    public void ClientEndConnect(IAsyncResult iar)
    {
        try
        {
            CommSocket = (Socket)iar.AsyncState;
            CommSocket.EndConnect(iar);

            OnNetworkEvents eventArgs = new OnNetworkEvents(true, "Connected: " + CommSocket.RemoteEndPoint.ToString(), string.Empty);
            OnUpdateNetworkStatusMessage(this, eventArgs);
        }
    }

Well there is nothing wrong with the code (I have simplified it and it is presented here as proof that I'm working on it an not looking for homework help!)

My Question is: how do I communicate with the server from a specified client port? The code here is all Server based port and this works well. But I would like to implement code that talks to a specific port on the client and not just one generated randomly by the server.

Thank you for your time.

Answer 1

Why do you want to use a specific port?

The client port is chosen randomly by design, because only one socket can have a port open at one time when it's not listening on that socket.

For example, if you always used port 80 on the client end of an HTTP connection, only one socket from that machine could connect to port 80 at a time. That would mean HTTP connections could not be parallel.

Answer 2

not sure about C#, but here is a working solution. in C:

     /*dl_senderprog.c - debian linux send to server a client, datagram*/

 /***********************************************************************


 140203  lets see if we can bind to a port

 ts7500:/var/www/jon/uvir_sensor_lab/source/socket#
 ts7500:/var/www/jon/uvir_sensor_lab/source/socket# vi senderprog_bind.c
 ts7500:/var/www/jon/uvir_sensor_lab/source/socket# gcc -g senderprog_bind.c -o senderprog_bind
 ts7500:/var/www/jon/uvir_sensor_lab/source/socket# ./senderprog_bind
 Sender:Client-Usage: ./senderprog_bind <hostname> <message>
 ts7500:/var/www/jon/uvir_sensor_lab/source/socket#
 ts7500:/var/www/jon/uvir_sensor_lab/source/socket#
 ts7500:/var/www/jon/uvir_sensor_lab/source/socket# ./senderprog_bind 10.0.1.26 "dot,33,22"
 MY IP address:10.0.1.242: on port: 1043
 Sender: Client-gethostname() is OK...
 Sender: Client-socket() sockfd is OK...
 Sender: Using port: 14950
 Sender: Client-sendto() is OK...
 Sender: sent 9 bytes to 10.0.1.26
 Sender: Client-sockfd successfully closed!
 ts7500:/var/www/jon/uvir_sensor_lab/source/socket#
 ts7500:/var/www/jon/uvir_sensor_lab/source/socket#
 ts7500:/var/www/jon/uvir_sensor_lab/source/socket# # it worked!!!!!
 ts7500:/var/www/jon/uvir_sensor_lab/source/socket#



 ***********************************************************************/




 #include <stdio.h>
 #include <stdlib.h>
 #include <unistd.h>
 #include <errno.h>
 #include <string.h>
 #include <sys/types.h>
 #include <sys/socket.h>
 #include <netinet/in.h>
 #include <arpa/inet.h>
 #include <netdb.h>

 #include <sys/ioctl.h>
 #include <net/if.h>





 /* the port users will be connecting to 14950 is the port on the windows machine
    that I have the server running on */
 #define TOPORT 14950
 #define MYPORT 1043

 void my_ip( char *myniccard, char *myipaddr) {
      int fd;
      struct ifreq ifr;

      myipaddr[0]=0;

      fd = socket(AF_INET, SOCK_DGRAM, 0);

      /* I want to get an IPv4 IP address */
      ifr.ifr_addr.sa_family = AF_INET;

      /* I want IP address attached to "eth0" */
      //strncpy(ifr.ifr_name, "eth0", IFNAMSIZ-1);
      strncpy(ifr.ifr_name, myniccard, IFNAMSIZ-1);

      ioctl(fd, SIOCGIFADDR, &ifr);

      close(fd);

      /* display result */
      sprintf(myipaddr,"%s"
        , inet_ntoa(((struct sockaddr_in *)&ifr.ifr_addr)->sin_addr));
      printf("MY IP address:%s: on port: %d\n", myipaddr, MYPORT);

      }   // my_ip


 int main(int argc, char *argv[ ])
 {
 int sockfd;
 /* connectors address information */
 struct sockaddr_in their_addr;
 struct sockaddr_in localaddr;
 char myipaddressm[22];   //buffer for ip address
 char *myniccardm ="eth0";   // check with ipconfig for correct ethernet port

 struct hostent *he;
 int numbytes;

 if (argc != 3) {
      fprintf(stderr, "Sender:Client-Usage: %s <hostname> <message>\n", argv[0]);
      exit(1);
      }

 my_ip(myniccardm, myipaddressm);


 /* get the host info */
 if ((he = gethostbyname(argv[1])) == NULL) {
      perror("Sender: Client-gethostbyname() error lol!");
      exit(1);
      }
  else
      printf("Sender: Client-gethostname() is OK...\n");

 if((sockfd = socket(AF_INET, SOCK_DGRAM, 0)) == -1) {
       perror("Sender: Client-socket() error lol!");
       exit(1);
       }
   else
       printf("Sender: Client-socket() sockfd is OK...\n");


 // Bind to a specific network interface
 // (this is unusual, as you normally do not want a specific
 //  port for the client, but we have a specific server in
 //  this case that will not accept connects unless its on
 //  a specific port )
 localaddr.sin_family = AF_INET;
 localaddr.sin_addr.s_addr = inet_addr(myipaddressm);
 localaddr.sin_port = htons(MYPORT);  // Any local port will do
 bind(sockfd, (struct sockaddr *)&localaddr, sizeof(localaddr));



 /* host byte order */
 their_addr.sin_family = AF_INET;
 /* short, network byte order */
 printf("Sender: Using port: %d\n",TOPORT);
 their_addr.sin_port = htons(TOPORT);
 their_addr.sin_addr = *((struct in_addr *)he->h_addr);
 /* zero the rest of the struct */
 memset(&(their_addr.sin_zero), '\0', 8);

 if((numbytes = sendto(sockfd, argv[2],
                       strlen(argv[2]),
                       0,
                       (struct sockaddr *)&their_addr,
                       sizeof(struct sockaddr))) == -1) {
       perror("Sender: Client-sendto() error lol!");
       exit(1);
       }
   else
       printf("Sender: Client-sendto() is OK...\n");

 printf("Sender: sent %d bytes to %s\n", numbytes, inet_ntoa(their_addr.sin_addr));

 if (close(sockfd) != 0)
       printf("Sender: Client-sockfd closing is failed!\n");
   else
       printf("Sender: Client-sockfd successfully closed!\n");
 return 0;

 }//main


 /*******************************************EOF***********************/