Understanding Internet Security - Implementing SSL/TLS Using Cryptography and PKI - page 23

( struct sockaddr * ) &client_addr,
&client_addr_len ) ) != -1 )
{
// TODO: ideally, this would spawn a new thread.
process_http_request( connect_sock );
}
if ( connect_sock == -1 )
{
perror( “Unable to accept socket” );
}
return 0;
}
This code is standard sockets fare. It issues the four required system calls
that are required for a process to act as a TCP protocol server: socket , bind ,
listen , and accept . The accept call will block — that is, not return — until a
client somewhere on the Internet calls connect with its IP and port number.
The inside of this while loop handles the request. Note that there's nothing
HTTP specifi c about this loop yet; this could just as easily be an e-mail server,
an ftp server, an IRC server, and so on. If anything goes wrong, these calls
return -1, perror prints out a description of what happened, and the process
terminates.
There are two points to note about this routine:
The call to setsockopt( listen_socket, SOL_SOCKET, SO_REUSEADDR,
&on, sizeof( on ) ) . This enables the same process to be restarted if
it terminates abnormally. Ordinarily, when a server process terminates
abnormally, the socket is left open for a period of time referred to as the
TIME_WAIT period. The socket is in TIME_WAIT state if you run netstat .
This enables any pending client FIN packets to be received and processed
correctly. Until this TIME_WAIT period has ended, no process can listen on
the same port. SO_REUSEADDR enables a process to take up ownership of a
socket that is in the TIME_WAIT state, so that on abnormal termination, the
process can be immediately restarted. This is probably what you always
want, but you have to ask for it explicitly.
Notice the arguments to bind . The bind system call tells the OS which port
you want to listen on and is, of course, required. However, bind accepts a
port as well as an interface name/IP address. By supplying an IP address
here, you can specify that you're only interested in connections coming
into a certain interface. You can take advantage of that and bind this socket
with the loopback address (127.0.0.1) to ensure that only connections from
this machine are accepted (see Listing 1-18).