I had this same problem with 17.10, but I'm betting the same applies to other recent versions as well.
Those MOTD messages are generated by scripts in
/etc/update-motd.d. The exact script that generates that specific line is
/etc/update-motd.d/91-release-upgrade, which also runs
/usr/lib/ubuntu-release-upgrader/release-upgrade-motd. This last script will check for new releases and write output into
The script will only update the file every 24 hours. So if you wait long enough, the file will go stale and the script will update the file anyway. But, if you want to update it immediately, just remove
/var/lib/ubuntu-release-upgrader/release-upgrade-available and run
/etc/update-motd.d/91-release-upgrade and it should start working correctly.
I created this problem because when I first setup my 17.10 server, I had a typo in my DNS server address.