TCP sockets over wlan - tcp

Question

I have a project that uses TCP sockets to communicate between a server and one client. As of now I have been doing this on one computer so I have just used local address of "127.0.0.1" for the address to bind and connect to on both sides and its worked fine. Now I have a second computer to act as a client, but I don't know how to change the addresses accordingly. They are connected through a network that is not connected to the Internet. Before the code looked like this -
Server -
struct addrinfo hints;
struct addrinfo *servinfo; //will point to the results
//store the connecting address and size
struct sockaddr_storage their_addr;
socklen_t their_addr_size;
memset(&hints, 0, sizeof hints); //make sure the struct is empty
hints.ai_family = AF_INET; //local address
hints.ai_socktype = SOCK_STREAM; //tcp
hints.ai_flags = AI_PASSIVE; //use local-host address
//get server info, put into servinfo
if ((status = getaddrinfo("127.0.0.1", port, &hints, &servinfo)) != 0) {
fprintf(stderr, "getaddrinfo error: %s\n", gai_strerror(status));
return false;
}
//make socket
fd = socket(servinfo->ai_family, servinfo->ai_socktype, servinfo->ai_protocol);
if (fd < 0) {
printf("\nserver socket failure %m", errno);
return false;
}
//allow reuse of port
int yes=1;
if (setsockopt(fd,SOL_SOCKET,SO_REUSEADDR,(char*) &yes,sizeof(int)) == -1) {
perror("setsockopt");
return false;
}
//unlink and bind
unlink("127.0.0.1");
if(bind (fd, servinfo->ai_addr, servinfo->ai_addrlen) < 0) {
printf("\nBind error %m", errno);
return false;
}
Client -
struct addrinfo hints;
struct addrinfo *servinfo; //will point to the results
memset(&hints, 0, sizeof hints); //make sure the struct is empty
hints.ai_family = AF_INET; //local address
hints.ai_socktype = SOCK_STREAM; //tcp
hints.ai_flags = AI_PASSIVE; //use local-host address
//get server info, put into servinfo
if ((status = getaddrinfo("127.0.0.1", port, &hints, &servinfo)) != 0) {
fprintf(stderr, "getaddrinfo error: %s\n", gai_strerror(status));
return false;
}
//make socket
fd = socket(servinfo->ai_family, servinfo->ai_socktype, servinfo->ai_protocol);
if (fd < 0) {
printf("\nserver socket failure %m", errno);
return false;
}
//connect
if(connect(fd, servinfo->ai_addr, servinfo->ai_addrlen) < 0) {
printf("\nclient connection failure %m", errno);
return false;
}
I know it should be simple, but I can't figure out how to change the IPs to get them to work. I tried setting the server computer's IP address in the quotes in these lines -
if ((status = getaddrinfo("127.0.0.1", port, &hints, &servinfo)) != 0)
and
unlink("127.0.0.1");
and then change the address in the client code to the client computer's IP address in this line -
if ((status = getaddrinfo("127.0.0.1", port, &hints, &servinfo)) != 0)
Whenever I do that, it tells me connection refused. I have also tried doing the opposite way of putting the server's address in the client's line and client's address in the server's lines along with a few other attempts. At this point I feel like I am just guessing though. So can someone please help me understand how to change this from using the local address with one computer to connecting two computers? Any help is appreciated.

Answer 1

First, unlink("127.0.0.1"); is totally wrong here, don't do that.
Then, you have two computers connected by some network. Both should have IP addresses. Replace 127.0.0.1 with the server's IP address in both client and the server. The server does not to have to know client's address beforehand - it'll get that information from the accept(2) call. The client needs server's address to know where to connect. The server needs its own address for the bind(2) call.

Answer 2

The main problem is that your putting AI_PASSIVE in your client code. AI_PASSIVE is meant for servers only (that's what it signals).
Also on the server side you should first of all not call unlink. That's for AF_UNIX sockets only, not AF_INET. Secondly you don't need to put "127.0.0.1" in the getaddrinfo line on the server side. It's better to use NULL to bind to all available addresses.
If you change those things, I believe your code should work. However you're actually supposed to loop on the getaddrinfo result using the ai_next pointer and try to connect to each result, using the first that succeeds.

Answer 3

Connection Refused usually means your client received a RST to his SYN. This is most often caused by the lack of a listening socket on the server, on the port you're trying to connect to.
Run your server
On the CLI, type netstat -ant. Do you see an entry that's in LISTEN state on your port?
Something like:
tcp4 0 0 *.3689 *.* LISTEN
I bet you do not, and therefore have a problem with your server listening socket. I also bet the changes you made this this line:
if ((status = getaddrinfo("127.0.0.1", port, &hints, &servinfo)) != 0) {
Weren't quite right. Try changing that IP to 0.0.0.0 on the server to tell it to to bind to any IP on the system. On the client, that line should have the IP address of the server. You should also remove the unlink() call in the server; unnecessary.
If you do have a listening socket, then there's probably a firewall or something in between your boxes that's blocking the SYN. Try typing service iptables stop on the CLI of both systems.