18.04 Unable to connect to server due to "Temporary failure in name resolution"

Question

I am at a bit of a loss. I am able to perform an nslookup and a dig on the address "imap.comcast.net" but if I try to ping it or connect to it from my application I get a "Temporary failure in name resolution". Something must be amiss in my DNS configuration but I am unsure what it is.

> ping imap.comcast.net
ping: imap.comcast.net: Temporary failure in name resolution

> systemd-resolve --status --no-pager
Global
          DNSSEC NTA: 10.in-addr.arpa
                      16.172.in-addr.arpa
                      168.192.in-addr.arpa
                      17.172.in-addr.arpa
                      18.172.in-addr.arpa
                      19.172.in-addr.arpa
                      20.172.in-addr.arpa
                      21.172.in-addr.arpa
                      22.172.in-addr.arpa
                      23.172.in-addr.arpa
                      24.172.in-addr.arpa
                      25.172.in-addr.arpa
                      26.172.in-addr.arpa
                      27.172.in-addr.arpa
                      28.172.in-addr.arpa
                      29.172.in-addr.arpa
                      30.172.in-addr.arpa
                      31.172.in-addr.arpa
                      corp
                      d.f.ip6.arpa
                      home
                      internal
                      intranet
                      lan
                      local
                      private
                      test

Link 2 (ens3)
      Current Scopes: DNS
       LLMNR setting: yes
MulticastDNS setting: no
      DNSSEC setting: no
    DNSSEC supported: no
         DNS Servers: 10.0.0.2
          DNS Domain: ec2.internal

> cat /etc/resolv.conf
# This file is managed by man:systemd-resolved(8). Do not edit.
#
# This is a dynamic resolv.conf file for connecting local clients to the
# internal DNS stub resolver of systemd-resolved. This file lists all
# configured search domains.
#
# Run "systemd-resolve --status" to see details about the uplink DNS servers
# currently in use.
#
# Third party programs must not access this file directly, but only through the
# symlink at /etc/resolv.conf. To manage man:resolv.conf(5) in a different way,
# replace this symlink by a static file or a different symlink.
#
# See man:systemd-resolved.service(8) for details about the supported modes of
# operation for /etc/resolv.conf.

nameserver 127.0.0.53
search ec2.internal

Some more forensic information /etc/resolv.conf is a symlink as seen here

> readlink -f /etc/resolv.conf
/run/systemd/resolve/stub-resolv.conf

If I change the symlink and restart systemd.resolve I can ping the address but it appears to disable dns caching

>sudo service systemd-resolved stop
>sudo rm -f /etc/resolv.conf
>sudo ln -s /run/systemd/resolve/resolv.conf /etc/resolv.conf
>sudo service systemd-resolved start
>ping imap.comcast.net
>sudo systemd-resolve --statistics
DNSSEC supported by current servers: no

Transactions
Current Transactions: 0
  Total Transactions: 0

Cache
  Current Cache Size: 0
          Cache Hits: 0
        Cache Misses: 0

DNSSEC Verdicts
              Secure: 0
            Insecure: 0
               Bogus: 0
       Indeterminate: 0

Solution 1

I think I had this issue before. I used the first answer in this question to resolve it: Wrong nameserver set by resolvconf and NetworkManager

The answer is to change the resolv.conf symlink:

sudo rm -f /etc/resolv.conf
sudo ln -s /run/systemd/resolve/resolv.conf /etc/resolv.conf
reboot

Solution 2

For me the answer was that the DNS entries were missing in my Netpan setup. I edited them manually (in my case /etc/netplan/50-cloud-init.yaml but I realize that this might be specific to my environment, and might not "stick").

I changed

network:
ethernets:
    ens160:
        addresses:
        - 10.8.25.116/28
        dhcp4: false
        gateway4: 10.8.25.113
        nameservers:
            addresses: []
            search: []
version: 2

to

network:
ethernets:
    ens160:
        addresses:
        - 10.8.25.116/28
        dhcp4: false
        gateway4: 10.8.25.113
        nameservers:
            addresses: [8.8.8.8,4.2.2.2]
            search: []
version: 2

and then applied it using

netplan apply

Comments

[yeary]

I attempted to change the symlink like so sudo service systemd-resolved stop sudo rm -f /etc/resolv.conf sudo ln -s /run/systemd/resolve/resolv.conf /etc/resolv.conf sudo service systemd-resolved start But, although it does solve my name resolution error it also seems to disable systemd.resolve's dns caching completely

[Andrew Lamarra]

What are you using to determine if systemd-resolved's dns caching is enabled or not?