The dispatch script fails for me when bringing up an interface with DHCP:

Looking at the journal logs for NetworkManager-dispatcher.service:

Jan 06 18:10:03 bellman nm-dispatcher[1981]: + STATUS=up
Jan 06 18:10:03 bellman nm-dispatcher[1981]: + RT_TABLE=/etc/iproute2/rt_tables
Jan 06 18:10:03 bellman nm-dispatcher[1981]: + '[' enp5s0 = lo ']'
Jan 06 18:10:03 bellman nm-dispatcher[1981]: + '[' -z enp5s0 ']'
Jan 06 18:10:03 bellman nm-dispatcher[1981]: ++ grep -c enp5s0 /etc/iproute2/rt_tables
Jan 06 18:10:03 bellman nm-dispatcher[1981]: + '[' 0 -eq 0 ']'
Jan 06 18:10:03 bellman nm-dispatcher[1981]: + echo 'Adding new routing table enp5s0'
Jan 06 18:10:03 bellman nm-dispatcher[1981]: Adding new routing table enp5s0
Jan 06 18:10:03 bellman nm-dispatcher[1981]: + logger -p user.info -t mptcp 'Adding new routing table enp5s0'
Jan 06 18:10:03 bellman nm-dispatcher[1981]: ++ wc -l
Jan 06 18:10:03 bellman nm-dispatcher[1981]: + NUM=13
Jan 06 18:10:03 bellman nm-dispatcher[1981]: + echo -e '\n13  enp5s0'
Jan 06 18:10:03 bellman nm-dispatcher[1981]: + '[' -n 192.168.0.104 ']'
Jan 06 18:10:03 bellman nm-dispatcher[1981]: ++ cut -d / -f 2
Jan 06 18:10:03 bellman nm-dispatcher[1981]: ++ echo 192.168.0.104/24 192.168.0.1
Jan 06 18:10:03 bellman nm-dispatcher[1981]: ++ cut -d ' ' -f 1
Jan 06 18:10:03 bellman nm-dispatcher[1981]: + SUBNET=24
Jan 06 18:10:03 bellman nm-dispatcher[1981]: + ip route add to /24 dev enp5s0 scope link table enp5s0
Jan 06 18:10:03 bellman nm-dispatcher[1981]: Error: any valid prefix is expected rather than "/24".
Jan 06 18:10:03 bellman nm-dispatcher[1981]: req:4 'up' [enp5s0], "/etc/NetworkManager/dispatcher.d/03userscript0001": complete: failed with Script '/etc/NetworkManager/dispatcher.d/03userscript0001' exited with error status 1.

It appears that $DHCP4_NETWORK_NUMBER is not set. Moreover this script is being called on any change to the interface. It should probably be conditional on $NM_DISPATCHER_ACTION = "up". Finally, there isn't any mechanism to clean up the routing table / rules when an interface is brought down.

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