I have an update on the same problem as seen on another computer - ethel, connected to an A.D.S.L. terminal adaptor (router). More details of the computer follow the commands and log excerpts. I think that this update suggests that avahi-daemon is the messenger, telling us that the NetworkManager has failed. /var/log/syslog:Aug 24 09:49:13 ethel avahi-daemon[1042]: Withdrawing address record for 192.168.41.113 on enp2s0. /var/log/syslog:Aug 24 09:49:13 ethel avahi-daemon[1042]: Leaving mDNS multicast group on interface enp2s0.IPv4 with address 192.168.41.113. /var/log/syslog:Aug 24 09:49:13 ethel avahi-daemon[1042]: Interface enp2s0.IPv4 no longer relevant for mDNS. ... some hours later .... [Mint] graham@ethel $ ip a s ... 2: enp2s0: mtu 1500 qdisc pfifo_fast state UP group default qlen 1000 link/ether d4:3d:7e:9b:55:7a brd ff:ff:ff:ff:ff:ff inet6 fe80::af3c:f15c:e5aa:34ca/64 scope link valid_lft forever preferred_lft forever Thu Aug 24 18:53:38 ~ [Mint] graham@ethel $ /etc/init.d/network-manager status * NetworkManager.service - Network Manager Loaded: loaded (/lib/systemd/system/NetworkManager.service; enabled; vendor preset: enabled) Active: active (running) since Sat 2017-07-15 19:27:36 BST; 1 months 9 days ago Docs: man:NetworkManager(8) Main PID: 1034 (NetworkManager) CGroup: /system.slice/NetworkManager.service ├─1034 /usr/sbin/NetworkManager --no-daemon ├─1402 /usr/sbin/dnsmasq --no-resolv --keep-in-foreground --no-hos... └─4973 /sbin/dhclient -d -q -sf /usr/lib/NetworkManager/nm-dhcp-he... Aug 24 18:53:47 ethel dhclient[4973]: send_packet: please consult README fi...s. Aug 24 18:53:47 ethel dhclient[4973]: dhclient.c:2386: Failed to send 300 b...e. Aug 24 18:53:58 ethel dhclient[4973]: DHCPREQUEST of 192.168.41.113 on enp2...c) Aug 24 18:53:58 ethel dhclient[4973]: send_packet: Network is unreachable Aug 24 18:53:58 ethel dhclient[4973]: send_packet: please consult README fi...s. Aug 24 18:53:58 ethel dhclient[4973]: dhclient.c:2386: Failed to send 300 b...e. Aug 24 18:54:07 ethel dhclient[4973]: DHCPREQUEST of 192.168.41.113 on enp2...c) Aug 24 18:54:07 ethel dhclient[4973]: send_packet: Network is unreachable Aug 24 18:54:07 ethel dhclient[4973]: send_packet: please consult README fi...s. Aug 24 18:54:07 ethel dhclient[4973]: dhclient.c:2386: Failed to send 300 b...e. Hint: Some lines were ellipsized, use -l to show in full. Thu Aug 24 18:54:08 ~ [Mint] graham@ethel $ /etc/init.d/avahi-daemon status * avahi-daemon.service - Avahi mDNS/DNS-SD Stack Loaded: loaded (/lib/systemd/system/avahi-daemon.service; enabled; vendor preset: enabled) Active: active (running) since Sat 2017-07-15 19:27:34 BST; 1 months 9 days ago Main PID: 1042 (avahi-daemon) Status: "avahi-daemon 0.6.32-rc starting up." CGroup: /system.slice/avahi-daemon.service ├─1042 avahi-daemon: running [ethel.local] └─1070 avahi-daemon: chroot helper Aug 19 14:15:25 ethel avahi-daemon[1042]: Interface enp2s0.IPv6 no longer re.... Aug 19 14:15:29 ethel avahi-daemon[1042]: Joining mDNS multicast group on in.... Aug 19 14:15:29 ethel avahi-daemon[1042]: New relevant interface enp2s0.IPv4.... Aug 19 14:15:29 ethel avahi-daemon[1042]: Registering new address record for.... Aug 19 14:15:31 ethel avahi-daemon[1042]: Joining mDNS multicast group on in.... Aug 19 14:15:31 ethel avahi-daemon[1042]: New relevant interface enp2s0.IPv6.... Aug 19 14:15:31 ethel avahi-daemon[1042]: Registering new address record for.... Aug 24 09:49:13 ethel avahi-daemon[1042]: Withdrawing address record for 192.... Aug 24 09:49:13 ethel avahi-daemon[1042]: Leaving mDNS multicast group on in.... Aug 24 09:49:13 ethel avahi-daemon[1042]: Interface enp2s0.IPv4 no longer re.... Hint: Some lines were ellipsized, use -l to show in full. Thu Aug 24 18:55:25 ~ [Mint] graham@ethel $ sudo /etc/init.d/avahi-daemon restart [ ok ] Restarting avahi-daemon (via systemctl): avahi-daemon.service. Thu Aug 24 18:55:51 ~ ... note that the reports above cover five days ... [Mint] liz@ethel $ ip a s ... 2: enp2s0: mtu 1500 qdisc pfifo_fast state UP group default qlen 1000 link/ether d4:3d:7e:9b:55:7a brd ff:ff:ff:ff:ff:ff inet6 fe80::af3c:f15c:e5aa:34ca/64 scope link valid_lft forever preferred_lft forever Thu Aug 24 18:56:07 ~ ... note that restarting avahi-daemon has not provided a connection ... [Mint] graham@ethel $ sudo /etc/init.d/network-manager restart [ ok ] Restarting network-manager (via systemctl): network-manager.service. Thu Aug 24 18:56:45 ~ [Mint] liz@ethel $ ip a s ... 2: enp2s0: mtu 1500 qdisc pfifo_fast state UP group default qlen 1000 link/ether d4:3d:7e:9b:55:7a brd ff:ff:ff:ff:ff:ff inet 192.168.41.113/24 brd 192.168.41.255 scope global dynamic enp2s0 valid_lft 86397sec preferred_lft 86397sec inet6 fe80::af3c:f15c:e5aa:34ca/64 scope link valid_lft forever preferred_lft forever Thu Aug 24 18:56:50 ~ ... it is the restarting of network-manager that restores the connection ... [Mint] graham@ethel $ /etc/init.d/network-manager status * NetworkManager.service - Network Manager Loaded: loaded (/lib/systemd/system/NetworkManager.service; enabled; vendor preset: enabled) Active: active (running) since Thu 2017-08-24 18:56:45 BST; 31s ago Docs: man:NetworkManager(8) Main PID: 376 (NetworkManager) CGroup: /system.slice/NetworkManager.service ├─376 /usr/sbin/NetworkManager --no-daemon ├─416 /sbin/dhclient -d -q -sf /usr/lib/NetworkManager/nm-dhcp-hel... └─433 /usr/sbin/dnsmasq --no-resolv --keep-in-foreground --no-host... Aug 24 18:56:47 ethel dnsmasq[433]: started, version 2.75 cache disabled Aug 24 18:56:47 ethel dnsmasq[433]: compile time options: IPv6 GNU-getopt D...fy Aug 24 18:56:47 ethel dnsmasq[433]: DBus support enabled: connected to syst...us Aug 24 18:56:47 ethel dnsmasq[433]: warning: no upstream servers configured Aug 24 18:56:47 ethel NetworkManager[376]: [1503597407.3243] dnsmasq...0 Aug 24 18:56:47 ethel dnsmasq[433]: setting upstream servers from DBus Aug 24 18:56:47 ethel dnsmasq[433]: using nameserver 192.168.41.1#53(via enp2s0) Aug 24 18:56:51 ethel NetworkManager[376]: [1503597411.0778] manager...e Aug 24 18:56:56 ethel NetworkManager[376]: [1503597416.0785] manager...d Aug 24 18:56:56 ethel NetworkManager[376]: [1503597416.0785] manager...d Hint: Some lines were ellipsized, use -l to show in full. Thu Aug 24 18:57:16 ~ Here are some more details of the system. [Mint] liz@ethel $ uname -a Linux ethel 4.4.0-53-generic #74-Ubuntu SMP Fri Dec 2 15:59:10 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux Thu Aug 24 19:38:23 ~ [Mint] liz@ethel $ cat /etc/network/interfaces # interfaces(5) file used by ifup(8) and ifdown(8) auto lo iface lo inet loopback Thu Aug 24 19:39:00 ~ [Mint] liz@ethel $ lspci | grep -i "net" 02:00.0 Ethernet controller: Realtek Semiconductor Co., Ltd. RTL8101/2/6E PCI Express Fast/Gigabit Ethernet controller (rev 05) Thu Aug 24 19:39:47 ~ [Mint] liz@ethel $ cat /etc/os-release NAME="Linux Mint" VERSION="18.1 (Serena)" ID=linuxmint ID_LIKE=ubuntu PRETTY_NAME="Linux Mint 18.1" VERSION_ID="18.1" HOME_URL="http://www.linuxmint.com/" SUPPORT_URL="http://forums.linuxmint.com/" BUG_REPORT_URL="http://bugs.launchpad.net/linuxmint/" VERSION_CODENAME=serena UBUNTU_CODENAME=xenial Thu Aug 24 19:40:27 ~