In C: Proxy HTTP requests to another server - c

Question

I'm trying to proxy HTTP requests to another HTTP server. The hostname and port number of the upstream HTTP server, respectively, are server_proxy_hostname and server_proxy_port.
The first step is to do a DNS lookup of the server_proxy_hostname.
Secondly, I create a network socket and connet it to the IP address I got from DNS.
Last step: Wait for new data on both sockets. When data arrives, I immediately read it to a buffer and then write it to the other
socket. This maintains a 2-way communication between the HTTP client and
the upstream HTTP server.
If any of the socket is closed, I close the other one.
The problem right now is that it is not working. (It times out)
I believe that the way I'm getting my IP addresses is correct, so the problem has to be in either my while loop (it never terminates?) or the part where I called connect(). I tried adding error termination for read() and select() but that didn't work either.
void handle_proxy_request(int fd) {
char * targetHostName = server_proxy_hostname;
int targetPort = server_proxy_port;
struct hostent *info;
info = gethostbyname(targetHostName);
struct in_addr ** ipAddresslist;
ipAddresslist = (struct in_addr **) (info -> h_addr_list);
struct in_addr * ipAddress = ipAddresslist[0];
printf("ip address is %s\n", inet_ntoa(*ipAddress));
/*ip for in_addr struct*/
unsigned long ip = inet_addr(inet_ntoa(*ipAddress));
struct in_addr addressIp = {ip};
struct sockaddr_in address = {PF_INET, htons(targetPort), addressIp};
int socket_num = socket(PF_INET, SOCK_STREAM, 0);
connect(socket_num, (struct sockaddr *)&address, sizeof(address));
/*portion for select()*/
char buf[10000];
int nfds = (fd > socket_num)?fd:socket_num;
fd_set readSet;
fd_set writeSet;
while (1) {
FD_ZERO(&readSet);
FD_ZERO(&writeSet);
FD_SET(fd, &readSet);
FD_SET(socket_num, &readSet);
FD_SET(fd, &writeSet);
FD_SET(socket_num, &writeSet);
int selectReturn = select(nfds, &readSet, &writeSet, NULL, NULL);
if (selectReturn == -1){
break;
}
if(FD_ISSET(fd, &readSet)){
int readStat = read(fd, buf, sizeof(buf));
int status = write(socket_num, buf, sizeof(buf));
if (status == -1 || readStat == -1){
close(socket_num);
close(fd);
break;
}
/*memset(buf, 0, sizeof(buf));*/
}
if(FD_ISSET(socket_num, &readSet)){
int readStat2 = read(socket_num, buf, sizeof(buf));
int status2 = write(fd, buf, sizeof(buf));
if (status2 == -1 || readStat2 == -1){
close(socket_num);
close(fd);
break;
}
}
}
}

Answer 1

int socket_num = socket(PF_INET, SOCK_STREAM, 0);
Unchecked. Check this for errors.
connect(socket_num, (struct sockaddr *)&address, sizeof(address));
Ditto.
FD_SET(fd, &writeSet);
FD_SET(socket_num, &writeSet);
Remove. This is poor practice. Sockets are almost always ready to write, so you shouldn't use the writeSet unless you have previously encountered a case where a socket wasn't ready to write, i.e. write() returned -1 with errno == EAGAIN/EWOULDBLOCK.
int readStat = read(fd, buf, sizeof(buf));
int status = write(socket_num, buf, sizeof(buf));
That should be
int status = write(socket_num, buf, readStat);
in both socket cases.
and it should be preceded by tests for readStat == 0, indicating end of stream, and readStat == -1, indicating an error, which you should trace.
You can't get a timeout in this code, as you haven't set any.
There's a wrinkle. If you get end of stream reading a socket you should shutdown the other socket for output. If you get end of stream on a socket you've already shutdown for output, close them both. This correctly propagates FINs in both directions at the correct times.
When you get any error from any system call, you must immediately call perror() or log it with the result strerror(), before you call any other system calls.