Question

I have a socket server that listens and accepts connections from client, which works as follow:

... do some pre-processing (socket, binds, etc)
//listen to client
if (listen(sockfd, BACKLOG) == -1) {
        perror("listen");
        exit(1);
}

printf("server: waiting for connections...\n");

while(1) {  // main accept() loop
    sin_size = sizeof client_addr;
    new_fd = accept(sockfd, (struct sockaddr *)&their_addr, &sin_size);
    if (new_fd == -1) {
        perror("accept");
        continue;
    }
     //do something .....
    .....
}

How can I restrict the server so it only accepts connection from specific IP addresses? For instance, I can create a text file containing a white list of IP addresses to accept, in the following format:
202.168.2.5 – 202.168.2.127
92.104.3.1 – 92.104.4.254
//and so on

So basically I want to reject connection from all the IP addresses not included in the whitelist. If the socket library API does not support this, I am okay with the idea of accepting the connections first, then just immediately close the socketfd if the peeraddress is not in the whitelist. But how to perform this, how can I check that a specific IP address is within the range specified in my whitelist? Any examples would be appreciated.

Answer 1

You want to call getpeername to get the address information from the client. Then check if their IP address is found in the whitelist. If not, disconnect them.

In order to check that their ip address lies within a given range, you want to convert the address bytes into one number. You can do that with the following:

unsigned int n = bytes[0] << 24 | bytes[1] << 16 | bytes[2] << 8 | bytes[3];

If the lower bound of the address range is A, and the upper bound is B, and the client’s ip address is X, then they are white listed if (A <= X && X <= B).

If each range of ip addresses tests false, then they aren’t on the white list and you should disconnect them.