Script to set WSL eth0 MTU to the same as a (lower) Pulse Secure Juniper Networks adapter MTU

update_mtu.sh

#!/bin/bash

# Sets the WSL eth0 adapter to the same MTU as a Juniper Networks Virtual Adapter, from the Windows side.
# This can help make certain connections (SSH/HTTPS specifically) more stable/reliable, as they can use
# larger packets when initiating connections or sending large chunks of data.

# Recommended to run in ~/.profile to persist across reboots. Remember to make this script executable.
# Add to sudoers file if running in ~/.profile
# %sudo   ALL=(ALL) NOPASSWD: /usr/sbin/ip link set mtu * dev *


win_adapter_description="Juniper Networks Virtual Adapter"
wsl_adapter_name="eth0"

wsl_adapter_mtu="$(ip link show dev "$wsl_adapter_name" | grep -oP 'mtu \d+' | cut -d' ' -f2)"

win_adapter_name="$(ipconfig.exe /all | grep -B3 "$win_adapter_description" | head -n1 | cut -d' ' -f3- | cut -d':' -f1)"
if [ -z "$win_adapter_name" ] ; then
    exit
fi
win_adapter_mtu="$(netsh.exe interface ipv4 show interfaces | grep "$win_adapter_name" | tr -s ' ' | cut -d' ' -f4)"

if [ $UID -eq 0 ] ; then
    use_sudo=
else
    use_sudo=sudo
fi

$use_sudo /usr/sbin/ip link set mtu "$win_adapter_mtu" dev "$wsl_adapter_name"