Foundations of Python Network Programming 978-1-4302-3004-5

CHAPTER 2 ■ UDP
• SO_DONTROUTE: Only be willing to send packets that are addressed to hosts on
subnets to which this computer is connected directly. My laptop, for example, at
this moment would be willing to send packets to the networks 127.0.0.0/8 and
192.168.5.0/24 if this socket option were set, but would not be willing to send
them anywhere else.
• SO_TYPE: When passed to getsockopt(), this returns to you regardless of whether a
socket is of type SOCK_DGRAM and can be used for UDP, or it is of type SOCK_STREAM
and instead supports the semantics of TCP (see Chapter 3).
The next chapter will introduce some further socket options that apply specifically to TCP sockets.
Broadcast
If UDP has a superpower, it is its ability to support broadcast: instead of sending a packet to some
specific other host, you can point it at an entire subnet to which your machine is attached and have the
physical network card broadcast the packet so that all attached hosts see it without its having to be
copied separately to each one of them.
Now, it should be immediately mentioned that broadcast is considered passé these days, because a
more sophisticated technique called “multicast” has been developed, which lets modern operating
systems take better advantage of the intelligence built into many networks and network interface
devices. Also, multicast can work with hosts that are not on the local subnet, which is what makes
broadcast unusable for many applications! But if you want an easy way to keep something like gaming
clients or automated scoreboards up-to-date on the local network, and each client can survive the
occasional dropped packet, then UDP broadcast is an easy choice.
Listing 2–4 shows an example of a server that can receive broadcast packets and a client that can
send them. And if you look closely, you will see that there is pretty much just one difference between this
listing and the techniques we were using in previous listings: before using this socket object, we are
using its setsockopt() method to turn on broadcast. Aside from that, the socket is used quite normally
by both server and client.
Listing 2–4. UDP Broadcast
#!/usr/bin/env python
# Foundations of Python Network Programming - Chapter 2 - udp_broadcast.py
# UDP client and server for broadcast messages on a local LAN
s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
s.setsockopt(socket.SOL_SOCKET, socket.SO_BROADCAST, 1)
MAX = 65535
PORT = 1060
if 2