why can't bind to 0.0.0.0:80 and 192.168.1.1:80 simultaneously?

Question

My python test code:

import socket

s1 = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s1.bind(('192.168.1.1', 80)) 
s1.listen(5)

s2 = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s2.bind(('0.0.0.0', 80)) 
s2.listen(5)

I got this error:

fpemud-workstation test # ./test.py
Traceback (most recent call last):
  File "./test.py", line 11, in <module>
    s2.bind(('0.0.0.0', 80)) 
  File "/usr/lib64/python2.7/socket.py", line 224, in meth
    return getattr(self._sock,name)(*args)
socket.error: [Errno 98] Address already in use

192.168.1.1 is the ip address of my eth0 interface.
I think 0.0.0.0:80 and 192.168.1.1:80 should be able to co-exist.
Packets with dst-addr 192.168.1.1 goes to socket s1, packets with other dst-addr goes to socket s2.

Answer 1

You cannot bind to both 0.0.0.0:80 and any other IP on port 80, because 0.0.0.0 covers every IP that exists on the machine, including your 192.168.1.1 address. It doesn't mean 'any other destination address', it means 'all interfaces on this box'.

Answer 2

Because it's a contradiction in terms. 0.0.0.0 means 'accept connections from any local IP address'. 192.168.1.1 means 'accept connections only that are addressed to 192.168.1.1'. What exactly do you expect to happen if someone connects to 192.168.1.1?

Answer 3

Despite what other answers have said, this should be possible - it's just that the way bind works is implementation dependent.

On Windows, for example, your code will probably work fine as is. On some *nix operating systems I believe you can get it to work by setting the SO_REUSEADDR socket option. On Linux, I've been able to get it to work using the SO_REUSEPORT socket option, but only on kernel version 3.9 or later.

Unfortunately the SO_REUSEPORT property isn't directly supported in current versions of python, so we have to define it manually.

Basically your code should look like this:

# This adds support for the SO_REUSEPORT constant if not already defined.
if not hasattr(socket, 'SO_REUSEPORT'):
  socket.SO_REUSEPORT = 15

s1 = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s1.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEPORT, 1)
s1.bind(('192.168.1.1', 80)) 
s1.listen(5)

s2 = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s2.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEPORT, 1)
s2.bind(('0.0.0.0', 80)) 
s2.listen(5)