I've just seen this issue the first time on a Ubuntu 20.04 (updated several months ago from 16.04) AWS EC2 machine. Looking closer into this, the machine lost its IP address multiple times in the last hours. The following avahi-daemon log messages made me find this bug report:
ck@focal:~$ grep avahi-daemon /var/log/syslog | grep -i withdrawing Jan 18 20:17:42 inf-mon04-p avahi-daemon[941]: Withdrawing address record for 10.10.1.41 on eth0. Jan 18 21:44:52 inf-mon04-p avahi-daemon[941]: Withdrawing address record for 10.10.1.41 on eth0. It's very interesting, that both of these events happened EXACTLY one hour after the system boot: ck@focal:~$ last | head ck pts/0 xx.xx.xx.xxx Tue Jan 18 22:11 still logged in root ttyS0 Tue Jan 18 21:50 still logged in reboot system boot 5.4.0-91-generic Tue Jan 18 20:44 still running reboot system boot 5.4.0-91-generic Tue Jan 18 19:17 - 20:39 (01:22) Running "dhclient" on the EC2 console, as suggested in comment #46, re- assigns the IP address. Relevant versions installed: ck@focal:~$ dpkg -l|egrep "(dhcp|network-manager)" ii isc-dhcp-client 4.4.1-2.1ubuntu5.20.04.2 amd64 DHCP client for automatically obtaining an IP address ii isc-dhcp-common 4.4.1-2.1ubuntu5.20.04.2 amd64 common manpages relevant to all of the isc-dhcp packages ii network-manager 1.22.10-1ubuntu2.2 amd64 network management framework (daemon and userspace tools) ii network-manager-gnome 1.8.24-1ubuntu3 amd64 network management framework (GNOME frontend) ii network-manager-pptp 1.2.8-2 amd64 network management framework (PPTP plugin core) -- You received this bug notification because you are a member of Ubuntu Touch seeded packages, which is subscribed to network-manager in Ubuntu. https://bugs.launchpad.net/bugs/1586528 Title: Avahi-daemon withdraws address record Status in avahi package in Ubuntu: Invalid Status in network-manager package in Ubuntu: Confirmed Bug description: For some reason, if I leave my Ubuntu VM up for a prolonged period of time the machine will lose connection to the network. ip addr shows that the nic port no longer has an address and an examination of the syslog shows this: May 27 14:19:38 matt-VirtualBox avahi-daemon[590]: Withdrawing address record for 10.0.2.15 on enp0s3. May 27 14:19:38 matt-VirtualBox avahi-daemon[590]: Leaving mDNS multicast group on interface enp0s3.IPv4 with address 10.0.2.15. May 27 14:19:38 matt-VirtualBox avahi-daemon[590]: Interface enp0s3.IPv4 no longer relevant for mDNS. for no known reason. The only reliable way to get the network to come back (that I have found) is a full reboot. ProblemType: Bug DistroRelease: Ubuntu 16.04 Package: avahi-daemon 0.6.32~rc+dfsg-1ubuntu2 ProcVersionSignature: Ubuntu 4.4.0-22.40-generic 4.4.8 Uname: Linux 4.4.0-22-generic x86_64 ApportVersion: 2.20.1-0ubuntu2 Architecture: amd64 CurrentDesktop: Unity Date: Fri May 27 15:11:34 2016 InstallationDate: Installed on 2015-10-22 (218 days ago) InstallationMedia: Ubuntu 15.10 "Wily Werewolf" - Release amd64 (20151021) SourcePackage: avahi UpgradeStatus: Upgraded to xenial on 2016-03-30 (58 days ago) To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/avahi/+bug/1586528/+subscriptions -- Mailing list: https://launchpad.net/~touch-packages Post to : touch-packages@lists.launchpad.net Unsubscribe : https://launchpad.net/~touch-packages More help : https://help.launchpad.net/ListHelp
