Windows host can't receive UDP packets from WSL-2 guest-CodePudding

I'm trying to have a simple UDP echo client/server communicate with each other. The client program (which runs in the host Windows) sends packets to the server (which runs in WSL-2), and the server receives them, but the server's reply is never reaches the client.

import sys
from socket import *
from select import select

client = sys.argv[1].startswith("c")
host = sys.argv[2] if len(sys.argv) > 2 else "127.0.0.1"
port = 8080
sd = socket(AF_INET, SOCK_DGRAM)

def poll():
    readable, writable, errorset = select([sd], [sd], [sd], 0)
    return sd in readable

if client:
    sd.connect((host, port))
    sd.setblocking(0)
    sd.send(b"Hello!")
    while not poll():
        pass
    data, addr = sd.recvfrom(65535)
    print(f"RECV {addr} => {data}")

else:
    sd.bind((host, port))
    print(f"Listening on {host}:{port}")
    sd.setblocking(0)
    sd.setsockopt(SOL_SOCKET, SO_REUSEADDR, 1)

    while True:
        while poll():
            data, addr = sd.recvfrom(65535)
            print(f"RECV {addr} => {data}")
            sd.sendto(data.decode("utf-8").upper().encode("utf-8"), addr)

The output on Windows:

udpecho.py client 172.25.154.133

The output on Linux:

$ python3 udpecho.py server 172.25.154.133
Listening on 172.25.154.133:8080
RECV ('172.25.144.1', 57661) => b'Hello!'

And now I'm stumped. TCP connections work ok so it must only be a UDP thing, but I don't know what to try next.

Running Windows 10 Home edition and hosting Ubuntu-20.04 in WSL 2.

CodePudding user response：

This sounds like this Github issue, where UDP packets smaller than 12 bytes don't make it from WSL2 to the host Windows interface.

If so, then a reported workaround is to turn off transmission checksumming in WSL2 via:

ethtool -K eth0 tx off

It sounds like this may be a Hyper-V networking issue that can be reproduced outside of WSL2, and the Microsoft team says it is under investigation.