ident-lookup.py

gistfile1.py

#!/usr/bin/env python
# Ident lookup for incoming connections
# RFC 1413 - Identification Protocol
import socket

def lookup_ident(host, server_port, client_port):
    s = socket.socket(socket.AF_INET6, socket.SOCK_STREAM)
    s.settimeout(1)
    try:
        s.connect((host, 113))
        query = "%d,%d" % (server_port, client_port)
        s.send(query + "\r\n")
        data = ""
        while not "\r\n" in data:
            data_new = s.recv(1024)
            if not len(data_new) > 0:
                break
            data += data_new
        print data
        rows = data.split("\r\n")
        parts = rows[0].split(":")
        parts = map(lambda x: x.strip(), parts)
        if len(parts) < 4 or parts[0].replace(' ', '') != query:
            return None # Invalid reply format
        if not parts[1].upper() == 'USERID':
            return None # Error response
        opsys = parts[2].split(",")
        if len(opsys) > 1:
            charset = opsys[1]
        else:
            charset = 'US-ASCII'
        user_id = parts[3].decode(charset)
        return user_id
    except socket.error:
        return None

def receive_connections(host, port):
    s = socket.socket(socket.AF_INET6, socket.SOCK_STREAM)
    s.bind((host, port))
    s.listen(1)
    try:
        while True:
            (conn, addr) = s.accept()
            print "Received connection from", addr
            yield (conn, addr)
    finally:
        s.close()

def main():
    PORT = 1235
    for (conn,addr) in receive_connections("", PORT):
        user = lookup_ident(addr[0], addr[1], PORT)
        conn.send("%s\r\n" % user)
        conn.close()

if __name__ == '__main__':
    main()

query = "%d,%d" % (server_port, client_port) in line 11 (https://gist.github.com/joneskoo/812505#file-gistfile1-py-L11) has the arguments reversed. A valid identd query needs to be "remote_port, local_port\r\n" which would translate to query = "%d,%d" % (client_port, server_port)