Specifying port number on client side - c

Question

How can I make sure that my client uses a particular port to connect to server in a typical server client program. Like in server side we are using a port number in bind system call and then listen on that particular port, is there any way we can specify the port number on the client side and uses the same port to connect to server. Server = 3456 ,
Client = 7834 ( I want to specify this port number so that client uses 7834 for connection). I am trying in C and Unix platform.

Answer 1

You do this on the client side exactly as you do it on the server side - using bind(). It's just that the client calls connect() after bind() instead of calling listen().
Note that you will only be able to run one instance of the client on each machine if you do this, and your server might see a different client port anyway if there are network middleboxes using address translation between the client and server.

Answer 2

The client program can set sockaddr...sin_port and then call bind() before connecting and it will get, subject to availability, the requested outgoing port.
The server can verify this port is set manually by calling getpeername() and checking it.
As others have said though, you dont really need to set the outgoing port usually, unless you have something weird going on, or specific requirements.

Answer 3

It's similar with server side, usually we do
local_addr.sin_port = htons(INADDR_ANY);
because port number is not important. Instead, you can use
local_addr.sin_port = htons(src_port); /Use the src_port as you like
And then call bind

Answer 4

Here is a working solution that runs in debian linux on the ts-7500 sbc. it should port pretty easy. This solution also looks up the ip address of the client machine automatically.
/*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***********************/