alpine-overlay/kubezero/zdt-base/dhcpcd-mtu.hook

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# From https://chromium.googlesource.com/chromiumos/third_party/dhcpcd/+/refs/tags/dhcpcd-6.0.0/dhcpcd-hooks
# Configure the MTU for the interface
mtu_dir="$state_dir/mtu"
set_mtu()
{
local mtu=$1
if [ -w /sys/class/net/$interface/mtu ]; then
echo "$mtu" >/sys/class/net/$interface/mtu
else
ifconfig "$interface" mtu "$mtu"
fi
}
if [ "$reason" = PREINIT -a -e "$mtu_dir/$interface" ]; then
rm "$mtu_dir/$interface"
elif [ -n "$new_interface_mtu" ] && $if_up; then
# The smalled MTU dhcpcd can work with is 576
if [ "$new_interface_mtu" -ge 576 ]; then
if set_mtu "$new_interface_mtu"; then
syslog info "MTU set to $new_interface_mtu"
# Save the MTU so we can restore it later
if [ ! -e "$mtu_dir/$interface" ]; then
mkdir -p "$mtu_dir"
echo "$ifmtu" > "$mtu_dir/$interface"
fi
fi
fi
elif [ -e "$mtu_dir/$interface" ]; then
if $if_up || $if_down; then
# No MTU in this state, so restore the prior MTU
mtu=$(cat "$mtu_dir/$interface")
syslog info "MTU restored to $mtu"
set_mtu "$mtu"
rm "$mtu_dir/$interface"
fi
fi