/lib/dhcpcd/dhcpcd-hooks/10-mtu is in dhcpcd5 5.2.12-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 | # Configure the MTU for the interface
mtu_dir="$state_dir/mtu"
if [ "$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 ifconfig "$interface" mtu "$new_interface_mtu"; then
syslog info "$interface: 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 "$interface: MTU restored to $mtu"
ifconfig "$interface" mtu "$mtu"
rm "$mtu_dir/$interface"
fi
fi
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