Networking, Part 3: Building a simple TCP Client

socket

int socket(int domain, int type, int protocol);

Socket creates a socket with domain (usually AF_INET for IPv4), type is whether to use UDP or TCP, protocol is any additional options. This creates a socket object in the kernel with which one can communicate with the outside world/network. This returns a fd so you can use it like a normal file descriptor! Remember you want to do your reads or writes from the socketfd because that represents the socket object only as a client, otherwise you want to respect the convention of the server.

getaddressinfo

We saw this in the last section! You're experts at this.

connect

int connect(int sockfd, const struct sockaddr *addr, socklen_t addrlen);

Pass it the sockfd, then the address you want to go to and its length and you will be off connecting (as long as you check the error). Remember, network calls are ultra perceptible to failing.

read/write

Once we have a successful connection we can read or write like any old file descriptor. Keep in mind if you are connected to a website, you want to conform to the HTTP protocol specification in order to get any sort of meaningful results back. There are libraries to do this, usually you don't connect at the socket level because there are other libraries or packages around it

Complete Simple TCP Client Example

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
#include <unistd.h>

int main(int argc, char **argv)
{
	int s;
	int sock_fd = socket(AF_INET, SOCK_STREAM, 0);

	struct addrinfo hints, *result;
	memset(&hints, 0, sizeof(struct addrinfo));
	hints.ai_family = AF_INET; /* IPv4 only */
	hints.ai_socktype = SOCK_STREAM; /* TCP */

	s = getaddrinfo("www.illinois.edu", "80", &hints, &result);
	if (s != 0) {
	        fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(s));
        	exit(1);
	}

	if(connect(sock_fd, result->ai_addr, result->ai_addrlen) == -1){
                perror("connect");
                exit(2);
        }

	char *buffer = "GET / HTTP/1.0\r\n\r\n";
	printf("SENDING: %s", buffer);
	printf("===\n");
	write(sock_fd, buffer, strlen(buffer));


	char resp[1000];
	int len = read(sock_fd, resp, 999);
	resp[len] = '\0';
	printf("%s\n", resp);

    return 0;
}

Example output:

SENDING: GET / HTTP/1.0

===
HTTP/1.1 200 OK
Date: Mon, 27 Oct 2014 19:19:05 GMT
Server: Apache/2.2.15 (Red Hat) mod_ssl/2.2.15 OpenSSL/1.0.1e-fips mod_jk/1.2.32
Last-Modified: Fri, 03 Feb 2012 16:51:10 GMT
ETag: "401b0-49-4b8121ea69b80"
Accept-Ranges: bytes
Content-Length: 73
Connection: close
Content-Type: text/html

Provided by Web Services at Public Affairs at the University of Illinois

Comment on HTTP request and response

The example above demonstrates a request to the server using Hypertext Transfer Protocol. A web page (or other resources) are requested using the following request:

GET / HTTP/1.0

There are four parts (the method e.g. GET,POST,...); the resource (e.g. / /index.html /image.png); the proctocol "HTTP/1.0" and two new lines (\r\n\r\n)

The server's first response line describes the HTTP version used and whether the request is successful using a 3 digit response code:

HTTP/1.1 200 OK

If the client had requested a non existing file, e.g. GET /nosuchfile.html HTTP/1.0 Then the first line includes the response code is the well-known 404 response code:

HTTP/1.1 404 Not Found