Fix "DNS request from ... to None" messages.

Some methods are unable to determine the destination address of DNS
packets that we capture. When this happens, change the message so it
just shows where the DNS requests are from.

sshuttle/client.py

@@ -419,7 +419,12 @@ def ondns(listener, method, mux, handlers):
     if t is None:
         return
     srcip, dstip, data = t
-    debug1('DNS request from %r to %r: %d bytes\n' % (srcip, dstip, len(data)))
+    # dstip is None if we are using a method where we can't determine
+    # the destination IP of the DNS request that we captured from the client.
+    if dstip is None:
+        debug1('DNS request from %r: %d bytes\n' % (srcip, len(data)))
+    else:
+        debug1('DNS request from %r to %r: %d bytes\n' % (srcip, dstip, len(data)))
     chan = mux.next_channel()
     dnsreqs[chan] = now + 30
     mux.send(chan, ssnet.CMD_DNS_REQ, data)