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I'm running into a DNS issue in our office's network. We had a power outage and when the router rebooted a few of the servers on our network started to have problems connecting to other devices on the network.

All our servers are using Ubuntu 20.04 and are set to DHCP. We don't alter any network settings on these either. This is the second time this has happened, the first it fixed itself after a few hours. I'm trying to determine the root cause to prevent this from happening again. A majority of our servers are fine, this is just on 7 of them. All of our servers are on the same subnet too.

These servers cannot ping anything in the LAN including the router. It will not ping google.com, but it can reach 8.8.8.8. If I change their netplan yaml to use a public DNS like google the issue resolves itself, but I'd like to avoid doing that to all of our devices as a permanent solution. Any ideas on what could be causing this?

lsb_release -a

Distributor ID: Ubuntu
Description:    Ubuntu 20.04.6 LTS
Release:    20.04
Codename:   focal

cat /etc/netplan/*.yaml

    network:
  ethernets:
    eno1:
      dhcp4: true
    enp3s0:
      dhcp4: true
    enp5s0:
      dhcp4: true
  version: 2

ifconfig -a

docker0: flags=4099<UP,BROADCAST,MULTICAST>  mtu 1500
        inet 172.17.0.1  netmask 255.255.0.0  broadcast 172.17.255.255
        ether 02:42:c7:32:42:55  txqueuelen 0  (Ethernet)
        RX packets 0  bytes 0 (0.0 B)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 0  bytes 0 (0.0 B)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

eno1: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 192.168.0.40  netmask 255.255.255.0  broadcast 192.168.0.255
        inet6 fe80::2be:43ff:fe9f:69d5  prefixlen 64  scopeid 0x20<link>
        ether 00:be:43:9f:69:d5  txqueuelen 1000  (Ethernet)
        RX packets 55032  bytes 12198105 (12.1 MB)
        RX errors 0  dropped 62  overruns 0  frame 0
        TX packets 14856  bytes 2663136 (2.6 MB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0
        device interrupt 16  memory 0xa4100000-a4120000

enp3s0: flags=4099<UP,BROADCAST,MULTICAST>  mtu 1500
        ether 00:be:43:9f:69:d6  txqueuelen 1000  (Ethernet)
        RX packets 0  bytes 0 (0.0 B)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 0  bytes 0 (0.0 B)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

enp5s0: flags=4099<UP,BROADCAST,MULTICAST>  mtu 1500
        ether 50:7c:6f:5d:05:f9  txqueuelen 1000  (Ethernet)
        RX packets 0  bytes 0 (0.0 B)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 0  bytes 0 (0.0 B)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0
        device memory 0xa4b00000-a4bfffff

lo: flags=73<UP,LOOPBACK,RUNNING>  mtu 65536
        inet 127.0.0.1  netmask 255.0.0.0
        inet6 ::1  prefixlen 128  scopeid 0x10<host>
        loop  txqueuelen 1000  (Local Loopback)
        RX packets 7931  bytes 816614 (816.6 KB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 7931  bytes 816614 (816.6 KB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

tailscale0: flags=4305<UP,POINTOPOINT,RUNNING,NOARP,MULTICAST>  mtu 1280
        inet 100.96.215.19  netmask 255.255.255.255  destination 100.96.215.19
        inet6 fe80::2af2:5a40:4399:cbd  prefixlen 64  scopeid 0x20<link>
        inet6 fd7a:115c:a1e0::6b01:d713  prefixlen 128  scopeid 0x0<global>
        unspec 00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00  txqueuelen 500  (UNSPEC)
        RX packets 2724  bytes 222118 (222.1 KB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 6047  bytes 845973 (845.9 KB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

ping -c 5 google.com

ping: google.com: Temporary failure in name resolution

ping -c 5 8.8.8.8

PING 8.8.8.8 (8.8.8.8) 56(84) bytes of data.
64 bytes from 8.8.8.8: icmp_seq=1 ttl=53 time=2.39 ms
64 bytes from 8.8.8.8: icmp_seq=2 ttl=53 time=2.64 ms
64 bytes from 8.8.8.8: icmp_seq=3 ttl=53 time=2.53 ms
64 bytes from 8.8.8.8: icmp_seq=4 ttl=53 time=2.61 ms
64 bytes from 8.8.8.8: icmp_seq=5 ttl=53 time=2.61 ms

--- 8.8.8.8 ping statistics ---
5 packets transmitted, 5 received, 0% packet loss, time 4008ms
rtt min/avg/max/mdev = 2.393/2.557/2.644/0.090 ms

nslookup google.com

Server:     127.0.0.53
Address:    127.0.0.53#53

** server can't find google.com: SERVFAIL
Improve this question
7
  • Sometimes the simplest answer will give results, have you tried stopping and starting network service on problem servers?
    – amar
    Commented Jun 27 at 19:00
  • I have restarted network manager and closed and reopened the eth interface with ip link set.
    – bae
    Commented Jun 27 at 19:09
  • Only eno1 has an IP address. Are the other two supposed to? Because of the power outage, it seems to me like some of your network devices are not starting back up properly. I would do a hard shutdown and reboot of everything.
    – mpboden
    Commented Jun 27 at 19:52
  • IP addresses, and pointers to DNS servers are provided by the DHCP server. Check its configuration and logs. On the failing systems: Look at the network logs with the terminal command: sudo journalctl -b 0 -u NetworkManager. Also do: service NetworkManager status and service --status-all.
    – waltinator
    Commented Jun 27 at 22:58
  • Have you rebooted the router? Is the router maybe confused or damaged?
    – David
    Commented Jun 28 at 7:46

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