This script adds “primary” IP addresses (both IPv4 and IPv6) of the machine to /etc/issue (text for login screen). Add it to /etc/network/if-post-up.d and /etc/network/if-post-down.d.

update-issue

#!/bin/sh

msg_file='/etc/issue'
test_ips='208.67.222.222 2620:0:ccc::2'

my_ips=$(printf '%s\n' $test_ips \
	| xargs -rn 1 ip route get 2>/dev/null \
	| grep -v unreachable \
	| sed -En 's/.* src ([0-9a-f.:]+) .*/\1/p' \
	| xargs)

if [ -z "$my_ips" ]; then
	sed -i '/^IP /d' $msg_file

elif grep -q '^IP ' $msg_file; then
	sed -i "s/IP .*/IP $my_ips/" $msg_file

else
	sed -i '${/^$/d;}' $msg_file
	printf '%s\n\n' "IP $my_ips" >> $msg_file
fi

exit 0