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Message-ID: <20240418174437.26662-1-kuniyu@amazon.com>
Date: Thu, 18 Apr 2024 10:44:37 -0700
From: Kuniyuki Iwashima <kuniyu@...zon.com>
To: <bob.mcmahon@...adcom.com>
CC: <netdev@...r.kernel.org>, <kuniyu@...zon.com>
Subject: Re: UDP recvfrom/recv question on connected/unconnected sockets
From: Bob McMahon <bob.mcmahon@...adcom.com>
Date: Thu, 18 Apr 2024 10:07:51 -0700
> Hi All,
>
> I have a question about the OS routing UDP packets to threads and connect
> vs unconnected sockets. Same src/dst IP and same dst port, different src
> port.
>
> If there are two UDP sockets listening on the same port, each serviced by
> its own thread and they both hang a recvfrom() or recv() (for the connected
> socket,) will the OS route packets only to the thread with a connected
> socket vs the thread with th unconnected socket? If not, what will happen?
Connected sockets receive packets matching 4-tuple, and unconnected sockets
receive packets that no connected socket matches.
$ python3
>>> from socket import *
>>>
>>> s1 = socket(AF_INET, SOCK_DGRAM)
>>> s1.setsockopt(SOL_SOCKET, SO_REUSEADDR, 1)
>>> s1.bind(('0', 0))
>>>
>>> s2 = socket(AF_INET, SOCK_DGRAM)
>>> s2.setsockopt(SOL_SOCKET, SO_REUSEADDR, 1)
>>> s2.bind(s1.getsockname())
>>>
>>> s1.connect(('10.0.0.53', 8000))
>>> s1
<socket.socket fd=3, family=AddressFamily.AF_INET, type=SocketKind.SOCK_DGRAM, proto=0, laddr=('10.0.0.53', 28947), raddr=('10.0.0.53', 8000)>
>>> s2
<socket.socket fd=4, family=AddressFamily.AF_INET, type=SocketKind.SOCK_DGRAM, proto=0, laddr=('0.0.0.0', 28947)>
>>>
>>> s3 = socket(AF_INET, SOCK_DGRAM)
>>> s3.bind(('10.0.0.53', 8000))
>>>
>>> s4 = socket(AF_INET, SOCK_DGRAM)
>>> s4.bind(('10.0.0.53', 8080))
>>>
>>> s3.sendto(b'hello', s1.getsockname())
5
>>> s4.sendto(b'world', s1.getsockname())
5
>>>
>>> s1.recv(10)
b'hello'
>>> s2.recv(10)
b'world'
Then, if you create a new unconnected socket, the old unconnected socket will
no longer receive packets until the new one is close()d.
>>> s5 = socket(AF_INET, SOCK_DGRAM)
>>> s5.setsockopt(SOL_SOCKET, SO_REUSEADDR, 1)
>>> s5.bind(s1.getsockname())
>>>
>>> s4.sendto(b'test', s1.getsockname())
4
>>> s2.recv(10)
^CTraceback (most recent call last):
File "<stdin>", line 1, in <module>
KeyboardInterrupt
>>> s5.recv(10)
b'test'
SO_REUSEADDR allows the newer socket to take over the port from the old
ones.
If you want to blance the loads across unconnected sockets, use SO_REUSPORT
instead.
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