WSL Port Forwarding Example for UDP

udp-port-forwarding.ps1

#--------------------------------------------------------
# Background: WSL UDP port forwarding
#--------------------------------------------------------

# Microsoft recommends "netsh interface portproxy" for port forwarding, but it does not support UDP for now.
# See https://learn.microsoft.com/en-us/windows/wsl/networking#accessing-a-wsl-2-distribution-from-your-local-area-network-lan.

# For UDP port forwarding, you can use Windows NAT mechanism.
# See https://gist.github.com/the-moog/e7a1b5150ce9017309afbcf91848e622.

#--------------------------------------------------------
# Configure Windows NAT for port forwarding
#--------------------------------------------------------

# It's hard to configure the default WSL NAT network, which is blackbox, to do port forwarding.
# Instead, set up a new NAT network for port forwarding.
# The new NAT network must not conflict with the default WSL NAT network.

# Assign new IP address to WSL switch
# Two IP addresses must be configured, one for the WSL switch and one for the guest, as follows:
#   WSL switch: 192.168.100.1
#   WSL guest: 192.168.100.2
New-NetIPAddress -InterfaceAlias "vEthernet (WSL)" -IPAddress 192.168.100.1 -PrefixLength 24

# Add NAT network
New-NetNat -Name "WSL-NAT" -InternalIPInterfaceAddressPrefix 192.168.100.0/24

# Add UDP port forwarding from WSL host to guest on port 2222
Add-NetNatStaticMapping -NatName "WSL-NAT" -Protocol UDP -ExternalIPAddress 0.0.0.0/0 -ExternalPort 2222 -InternalIPAddress 192.168.100.2 -InternalPort 2222

#--------------------------------------------------------
# Configure WSL guest to assign IP address
#--------------------------------------------------------

# Configure new IP address on WSL
# The new IP address needs to be the same as the one specified in port forwarding
wsl -u root ip address add 192.168.100.2/24 dev eth0

I needed to change default route in WSL.

I followed those steps and got udp packets to the wsl, however no responses.
Before this nat, my wsl already the ip: 172.22.142.240/20.
and output of ip route is:

default via 192.168.100.1 dev eth0
172.22.128.0/20 dev eth0 scope link  src 172.22.142.240
192.168.100.0/24 dev eth0 scope link  src 192.168.100.2

In wireshark I saw

• on the external Ethernet the incoming packets from: external_linux_pc - to: ip_of_windows_pc_with_wsl
• on the vEthernet the incoming packets from: external_linux_pc - to: 192.168.100.2
• on the vEthernet the outgoing packets from: 172.22.142.240 - to: external_linux_pc
• no outgoing packets on the external Ethernet
  my wsl somehow responds from the ip it had before setting up this NAT, which makes windows think this is part of another "connection" and therefore not sent out the external Ethernet Interface.

To fix this, I had to change the default route to use the new address:

sudo ip route del default eh0
sudo ip route add default via 192.168.100.1 dev eth0