I have been affected by this one for a time now on a 18.04 LTS install
on a laptop with both wired and wireless ethernet. Not a fresh install,
but one that had been upgraded from earlier releases.

I have both Wireless and wired ethernet configured. NetworkManager is
taking care of network IP address assignment, which is done dynamically
on both cases through DHCP from the AP/router.

Every now and then, I get both network interfaces with no IP address assigned 
whatsoever. It may happen after resuming from suspend (laptop being powered 
down from hours to days) or while actively using the laptop. In all cases it 
seems NetworkManager service is still running and "dhclient" processes are up 
and running for the interfaces. For the latest time this has occured to me:
"""
# service NetworkManager status
● NetworkManager.service - Network Manager
   Loaded: loaded (/lib/systemd/system/NetworkManager.service; enabled; vendor 
preset: enabled)
   Active: active (running) since Fri 2018-11-16 21:31:31 CET; 4 days ago
     Docs: man:NetworkManager(8)
 Main PID: 845 (NetworkManager)
    Tasks: 5 (limit: 4915)
   CGroup: /system.slice/NetworkManager.service
           ├─  845 /usr/sbin/NetworkManager --no-daemon
           ├─12154 /sbin/dhclient -d -q -sf 
/usr/lib/NetworkManager/nm-dhcp-helper -pf /run/dhclient-wlp3s0.pid -lf 
/var/lib/NetworkManager/dhclient-cef3df46-08a4-4e76-b0d8-3a2c21cd504e-wlp3s0.lease
 -cf /var/lib/NetworkManager/dhclient-wl
           └─23839 /sbin/dhclient -d -q -sf 
/usr/lib/NetworkManager/nm-dhcp-helper -pf /run/dhclient-enp0s25.pid -lf 
/var/lib/NetworkManager/dhclient-7d46e9cc-ee9c-366d-a025-73d355f9d046-enp0s25.lease
 -cf /var/lib/NetworkManager/dhclient-

# ps -efa | grep Netwo
root       845     1  0 nov16 ?        00:00:20 /usr/sbin/NetworkManager 
--no-daemon
root      1207  2521  0 14:13 pts/1    00:00:00 grep --color=auto Netwo
root     12154   845  0 nov20 ?        00:00:00 /sbin/dhclient -d -q -sf 
/usr/lib/NetworkManager/nm-dhcp-helper -pf /run/dhclient-wlp3s0.pid -lf 
/var/lib/NetworkManager/dhclient-cef3df46-08a4-4e76-b0d8-3a2c21cd504e-wlp3s0.lease
 -cf /var/lib/NetworkManager/dhclient-wlp3s0.conf wlp3s0
root     23839   845  0 nov20 ?        00:00:00 /sbin/dhclient -d -q -sf 
/usr/lib/NetworkManager/nm-dhcp-helper -pf /run/dhclient-enp0s25.pid -lf 
/var/lib/NetworkManager/dhclient-7d46e9cc-ee9c-366d-a025-73d355f9d046-enp0s25.lease
 -cf /var/lib/NetworkManager/dhclient-enp0s25.conf enp0s25
"""

For me, getting IP addresses assigned again to interfaces is as easy as
running "service NetworkManager restart".

Last significant logs in /var/log/syslog prior to losing connectivity (IP 
address) on both network interfaces (wired "enp0s25" and wireless "wlp3s0") are 
as below:
"""
#### Wireless interface goes "timed out"
Nov 21 12:13:40 laptop NetworkManager[845]: <info>  [1542798820.8236] 
connectivity: (wlp3s0) timed out
Nov 21 12:13:40 laptop avahi-daemon[841]: Withdrawing address record for 
192.168.1.17 on wlp3s0.
Nov 21 12:13:40 laptop avahi-daemon[841]: Leaving mDNS multicast group on 
interface wlp3s0.IPv4 with address 192.168.1.17.
Nov 21 12:13:40 laptop avahi-daemon[841]: Interface wlp3s0.IPv4 no longer 
relevant for mDNS.

#### And exactly 20 minutes later so does the wired interface...
Nov 21 12:33:40 laptop NetworkManager[845]: <info>  [1542800020.8236] 
connectivity: (enp0s25) timed out
Nov 21 12:33:40 laptop NetworkManager[845]: <info>  [1542800020.8237] manager: 
NetworkManager state is now CONNECTED_SITE
Nov 21 12:33:40 laptop avahi-daemon[841]: Withdrawing address record for 
192.168.1.19 on enp0s25.
Nov 21 12:33:40 laptop systemd[1]: Starting Network Manager Script Dispatcher 
Service...
Nov 21 12:33:40 laptop avahi-daemon[841]: Leaving mDNS multicast group on 
interface enp0s25.IPv4 with address 192.168.1.19.
Nov 21 12:33:40 laptop gsd-sharing[1976]: Failed to StopUnit service: 
GDBus.Error:org.freedesktop.systemd1.NoSuchUnit: Unit 
gnome-user-share-webdav.service not loaded.
Nov 21 12:33:40 laptop avahi-daemon[841]: Interface enp0s25.IPv4 no longer 
relevant for mDNS.
Nov 21 12:33:40 laptop gsd-sharing[1976]: Failed to StopUnit service: 
GDBus.Error:org.freedesktop.systemd1.NoSuchUnit: Unit rygel.service not loaded.
Nov 21 12:33:40 laptop dbus-daemon[812]: [system] Activating via systemd: 
service name='org.freedesktop.nm_dispatcher' 
unit='dbus-org.freedesktop.nm-dispatcher.service' requested by ':1.9' (uid=0 
pid=845 comm="/usr/sbin/NetworkManager --no-daemon " label="unconfined")
Nov 21 12:33:40 laptop gsd-sharing[1976]: Failed to StopUnit service: 
GDBus.Error:org.freedesktop.systemd1.NoSuchUnit: Unit 
gnome-remote-desktop.service not loaded.
Nov 21 12:33:40 laptop dbus-daemon[812]: [system] Successfully activated 
service 'org.freedesktop.nm_dispatcher'
Nov 21 12:33:40 laptop systemd[1]: Started Network Manager Script Dispatcher 
Service.
Nov 21 12:33:40 laptop nm-dispatcher: req:1 'connectivity-change': new request 
(1 scripts)
Nov 21 12:33:40 laptop nm-dispatcher: req:1 'connectivity-change': start 
running ordered scripts...
Nov 21 12:33:40 laptop nm-dispatcher: req:2 'connectivity-change': new request 
(1 scripts)
Nov 21 12:33:40 laptop nm-dispatcher: req:2 'connectivity-change': start 
running ordered scripts...
Nov 21 12:34:00 laptop whoopsie[1357]: [12:34:00] Cannot reach: 
https://daisy.ubuntu.com
Nov 21 12:34:00 laptop whoopsie[1357]: [12:34:00] offline
"""

It can't be a coincidence both network devices lose the IP addresses due
to some "timed out" event exactly 20 minutes apart. As the event comes
from NetworkManager, so must be the bug causing the observed behavior.

Please check if possible to strengthen NetworkManager in the current and 
upcoming releases. Mine being now:
"""
# cat /etc/os-release 
NAME="Ubuntu"
VERSION="18.04.1 LTS (Bionic Beaver)"
ID=ubuntu
ID_LIKE=debian
PRETTY_NAME="Ubuntu 18.04.1 LTS"
VERSION_ID="18.04"
HOME_URL="https://www.ubuntu.com/";
SUPPORT_URL="https://help.ubuntu.com/";
BUG_REPORT_URL="https://bugs.launchpad.net/ubuntu/";
PRIVACY_POLICY_URL="https://www.ubuntu.com/legal/terms-and-policies/privacy-policy";
VERSION_CODENAME=bionic
UBUNTU_CODENAME=bionic
"""

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https://bugs.launchpad.net/bugs/1771749

Title:
  Ethernet connection lost after few minutes since starting

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