Twisted: source IP address for outbound connections

Question

I'm in the process of implementing a service -- written in Python with the Twisted framework, running on Debian GNU/Linux -- that checks the availability of SIP servers. For this I use the OPTIONS method (a SIP protocol feature), as this seems to be a commonplace practice. In order to construct correct and RFC compliant headers, I need to know the source IP address and the source port for the connection that is going to be established. [How] can this be done with Twisted?

This is what I tried: I subclassed protocol.DatagramProtocol and within startProtocol(self) I used self.transport.getHost().host and self.transport.getHost().port. The latter is indeed the port that's going to be used, whereas the former only yields 0.0.0.0.

I guess that at this point Twisted doesn't [yet?] know which interface and as such which source IP address will be used. Does Twisted provide a facility that could help me with this or do I need to interface with the OS (routing) in a different way? Or did I just use self.transport.getHost().host incorrectly?

Answer 1

If you are using UDP then the endpoint is determined by either:

1. calling bind() on the socket and explicitly giving it an address
2. sending a packet

If you want a few more details, check this response.

The problem is that I'm not that familiar with twisted. From what I can tell by a quick perusal of the source, it looks like you might want to use a reactor like t.i.d.SelectReactor instead. This is what t.n.d.DNSDatagramProtocol does under the hood.

If you take twisted out of the picture, then the following snippet shows what is going on:

>>> import socket
>>> s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM, 0)
<socket._socketobject object at 0x10025d670>
>>> s.getsockname()           # this is an unbound or unnamed socket
('0.0.0.0', 0)
>>> s.bind( ('0.0.0.0', 0) )  # 0.0.0.0 is INADDR_ANY, 0 means pick a port
>>> s.getsockname()           # IP is still zero, but it has picked a port
('0.0.0.0', 56814)

Get the host name is a little trickier if you need to support multiple network interfaces or IPv4 and IPv6. If you can make the interface used configurable, then pass it in as the first member of the tuple to socket.bind() and you are set.

Now the hard part is doing this within the confines of the abstractions that twisted provides. Unfortunately, I can't help a whole lot there. I would recommend looking for examples on how you can get access to the underlying socket or find a way to pass the socket information into the framework.

Good luck.

Answer 2

Did you see if that you want to do is possible with the SIP implementation that is part of Twisted?

In any case, how you set the source address and port for UDP in Twisted is quite similar to how you set them without Twisted. In Twisted, reactor.listenUDP(port, protocol, interface) binds an UDP socket to a specific port and interface and handles the received datagrams to your protocol. Inside the protocol, self.transport.write(msg, addr) sends a datagram to addr using the address that the protocol is bound to as source address.

Reading your question again, I think the only part you were missing was passing interface to reactor.listenUDP(...).

Answer 3

For the sake of completeness I answer my own question:

Make sure you use connect() on the transport before trying to determine the host's source IP address. The following excerpt shows the relevant part of a protocol implementation:

class FooBarProtocol(protocol.DatagramProtocol):
    def startProtocol(self):
        self.transport.getHost().host   # => 0.0.0.0
        self.transport.connect(self.dstHost, self.dstPort)
        self.transport.getHost().host   # => 192.168.1.102