How to create a simple Proxy to access web servers in C

Question

I’m trying to create an small Web Proxy in C. First, I’m trying to get a webpage, sending a GET frame to the server. I don’t know what I have missed, but I am not receiving any response. I would really appreciate if you can help me to find what is missing in this code.

int main (int argc, char** argv) {
   int cache_size,     //size of the cache in KiB
       port,
       port_google = 80,
       dir,
       mySocket,
       socket_google;

   char google[] = "www.google.es", ip[16];
   struct sockaddr_in socketAddr;
   char buffer[10000000];

   if (GetParameters(argc,argv,&cache_size,&port) != 0)
       return -1;

   GetIP (google, ip);
   printf("ip2 = %s\n",ip);

   dir = inet_addr (ip);
   printf("ip3 = %i\n",dir);

   /* Creation of a socket with Google */
   socket_google = conectClient (port_google, dir, &socketAddr);
   if (socket_google < 0) return -1;
   else printf("Socket created\n");

   sprintf(buffer,"GET /index.html HTTP/1.1\r\n\r\n");
   if (write(socket_google, (void*)buffer, MESSAGE_LENGTH+1) < 0 )
       return 1;
   else printf("GET frame sent\n");

   strcpy(buffer,"\n");
   read(socket_google, buffer, sizeof(buffer));

   // strcpy(message,buffer);
   printf("%s\n", buffer);

   return 0;
}

And this is the code I use to create the socket. I think this part is OK, but I copy it just in case.

int conectClient (int puerto, int direccion, struct sockaddr_in *socketAddr) {
   int mySocket;
   char error[1000];

   if ( (mySocket = socket(AF_INET, SOCK_STREAM, 0)) == -1) {
       printf("Error when creating the socket\n");
       return -2;
   }

   socketAddr->sin_family = AF_INET;
   socketAddr->sin_addr.s_addr = direccion;
   socketAddr->sin_port = htons(puerto);

   if (connect (mySocket, (struct sockaddr *)socketAddr,sizeof (*socketAddr)) == -1) {
       snprintf(error, sizeof(error), "Error in %s:%d\n", __FILE__, __LINE__);
       perror(error);
       printf("%s\n",error);
       printf ("-- Error when stablishing a connection\n");
       return -1;
   }
   return mySocket;
}

Thanks!

Answer 1

First, you're not checking how many bytes the write(2) call actually wrote to the socket. The return value of the call tells you that. Same for the read(2). TCP socket is a bi-directional stream, so as a rule always do both in a loop until expected number of bytes is transferred, EOF is read (zero return from read(2)), or an error occurred (which you are not checking for when reading either).

Then HTTP is rather complex protocol. Make yourself familiar with RFC 2616, especially application level connection management and transfer encodings.

Edit 0:

Hmm, there's no such thing as "simple" proxy. You need to manage multiple connections (at least client-to-proxy and proxy-to-server), so it's probably best to look into select(2)/poll(2)/epoll(4)/kqueue(2) family of system call, which allow you to multiplex I/O. This is usually combined with non-blocking sockets. Look into helper libraries like libevent. Look at how this is done in good web-servers/proxies like nginx. Sound like it's a lot for you to discover, but don't worry, it's fun :)