Execute two local socket servers simultaneously in C - c

Question

I'm trying to simulate a DHCP server as a university task. It's required to execute at least 2 servers (or more up to 255) since several clients will be sending messages to these servers using BROADCAST (that's where the limit of 'only' 255 servers comes from).
But when I'm trying to execute 2 servers in 2 different terminals in Ubuntu, the second server executed is rejected. I guess it's due to IP at the same localhost or whatever (I'm sorry but I don't know too much about communication protocols)
How can I run several servers on the same machine? Our teacher told us about using VMWare to run another Linux OS, and try to run both, but for me this solution is far away from my thoughts... I will post the server and client connection statements, although I don't know if there's a problem with the C statements, or the OS configuration, etc.
Server:
int sockfd, newsockfd, clilen, n;
struct sockaddr_in cli_addr, serv_addr ;
char host_name[200];
int size = sizeof(struct DHCP);
char buffer[size];
struct MACIP *macip;
if ((sockfd = socket(AF_INET, SOCK_DGRAM,0)) < 0) {
printf("server: can't open datagram socket\n");
return -1;
}
serv_addr.sin_family = AF_INET;
serv_addr.sin_addr.s_addr = htonl(0);
serv_addr.sin_port = htons(SERV_UDP_PORT);
if (bind(sockfd, (struct sockaddr *) &serv_addr, sizeof(serv_addr)) <0) {
printf("server: can't bind local address\n");
return -1;
}
Client:
int sockfd, n, clilen;
struct sockaddr_in serv_addr, cli_addr;
char serv_host_addr[30];
int size = sizeof(struct DHCP);
char buffer[size];
printf("Dirección IP del servidor (a.b.c.d) => ");
gets(serv_host_addr);
if ((sockfd = socket(AF_INET, SOCK_DGRAM, 0)) < 0) {
perror("client: can't open datagram socket");
return -1;
}
int opt=1;
if (setsockopt(sockfd, SOL_SOCKET, SO_BROADCAST,&opt, sizeof(opt))<0) {
perror("Error opcion socket");
close(sockfd);
return 1;
}
bzero((char *)&cli_addr,sizeof(cli_addr));
cli_addr.sin_family = AF_INET;
if (bind(sockfd, (struct sockaddr *) &cli_addr,sizeof(cli_addr)) < 0) {
perror("client: can't bind a port");
return -1;
}
serv_addr.sin_family = AF_INET;
inet_pton(AF_INET, serv_host_addr,&serv_addr.sin_addr);
//serv_addr.sin_addr.s_addr = inet_addr(serv_host_addr);
serv_addr.sin_port = htons(SERV_UDP_PORT);
Thank you very much in advance.

Answer 1

Every instance of your UDP server must listen on another port. Key line in your code is this:
serv_addr.sin_port = htons(SERV_UDP_PORT);
Looks like you have the port number as a constant (SERV_UDP_PORT). To make multiple servers run at the same time you need another port for each one.
Probably best solution in your case is to have a command line argument that would define the port on which you start your server.
Each client needs to know which port (and address of course) to connect to as well. So make your client to accept similar command line argument.

Answer 2

To run multiple servers, they should listen on different ports on same network interface. You may get away by using two network interfaces (say wifi and ethernet) on same network with different ips.

Answer 3

You might try to set up alias interfaces on an existing interface. Such alias interfaces then get their own ip address each.
Using the loopback interface (lo), which typical has 127.0.0.1 assigned, such aliases to this interface could then be called lo:0, lo:1 ... and would carry the ip addresses 127.0.0.2, 127.0.0.3, ....
For details on how to achieve this see man ifconfig.