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resolv.conf mismatch netplan configuration

Ask Ubuntu Asked by il_mix on November 26, 2021

I’m setting up an Ubuntu Server 20.04 system as a gateway between my LAN and the WAN. I’m not able to configure the DNS properly.

I have 2 netplan config files for my 2 interfaces.

Internal eth interface (LAN)

[email protected]:~# cat /etc/netplan/01-eth_int.yaml
# This is the network config written by 'subiquity'
network:
  ethernets:
        enp4s0:
            addresses: ['10.0.10.1/24']
            match:
                macaddress: 00:60:e0:76:9e:e7
            set-name: eth_int
  version: 2

External eth interface (WAN)

[email protected]:~# cat /etc/netplan/02-eth_ext.yaml
# This is the network config written by 'subiquity'
network:
  ethernets:
        enp5s0:
            addresses: ['10.200.56.254/24']
            gateway4: 10.200.56.1
            nameservers:
                addresses: [195.78.215.228, 195.78.223.228]
                addresses: [8.8.8.8, 8.8.4.4]
            match:
                macaddress: 00:60:e0:76:9e:e8
            set-name: eth_ext
  version: 2

On reboot the interfaces are configured according to netplan config files. But /etc/resolv.conf is misconfigured

[email protected]:~# 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 "resolvectl 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
options edns0

Test/check performed:

  • /etc/resolv.conf is a symlink of ../run/systemd/resolve/stub-resolv.conf
  • systemd-resolve –status returns the correct DNS for eth_ext interface, but no global DNS (can be an issue?)
    [email protected]:~# systemd-resolve --status
    Global
           LLMNR setting: no
    MulticastDNS setting: no
      DNSOverTLS setting: no
          DNSSEC setting: no
        DNSSEC supported: no
              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 5 (eth_ext)
          Current Scopes: DNS
    DefaultRoute setting: yes
           LLMNR setting: yes
    MulticastDNS setting: no
      DNSOverTLS setting: no
          DNSSEC setting: no
        DNSSEC supported: no
             DNS Servers: 195.78.215.228
                          195.78.223.228
                          8.8.8.8
                          8.8.4.4

    Link 4 (enp0s31f6)
          Current Scopes: none
    DefaultRoute setting: no
           LLMNR setting: yes
    MulticastDNS setting: no
      DNSOverTLS setting: no
          DNSSEC setting: no
        DNSSEC supported: no

    Link 3 (eth_int)
          Current Scopes: none
    DefaultRoute setting: no
           LLMNR setting: yes
    MulticastDNS setting: no
      DNSOverTLS setting: no
          DNSSEC setting: no
        DNSSEC supported: no

    Link 2 (enp3s0)
          Current Scopes: none
    DefaultRoute setting: no
           LLMNR setting: yes
    MulticastDNS setting: no
      DNSOverTLS setting: no
          DNSSEC setting: no
        DNSSEC supported: no
  • edited /etc/systemd/resolved.conf, setting and uncommenting DNS entry. A global DNS entry appears running systemd-resolve –status, but /etc/resolv.conf won’t change

Obviously, this mismatch will prevent DNS to work properly

[email protected]:~# ping google.com
ping: google.com: Temporary failure in name resolution

[email protected]:~#  nslookup heise.de 127.0.0.53
;; connection timed out; no servers could be reached

How can I solve this?

2 Answers

You've mentioned, your /etc/resolv.conf is symlink of /run/systemd/resolve/stub-resolv.conf

To resolve this issue:

  1. Unlink it: $ sudo unlink /etc/resolv.conf
  2. $ sudo ln -s /run/systemd/resolve/resolv.conf /etc/resolv.conf
  3. sudo systemctl restart systemd-resolved.service

The following information from man systemd-resolved:

/ETC/RESOLV.CONF
       Four modes of handling /etc/resolv.conf (see resolv.conf(5)) are supported:

       •   systemd-resolved maintains the /run/systemd/resolve/stub-resolv.conf file for compatibility with
           traditional Linux programs. This file may be symlinked from /etc/resolv.conf. This file lists the
           127.0.0.53 DNS stub (see above) as the only DNS server. It also contains a list of search domains that are
           in use by systemd-resolved. The list of search domains is always kept up-to-date. Note that
           /run/systemd/resolve/stub-resolv.conf should not be used directly by applications, but only through a
           symlink from /etc/resolv.conf. This file may be symlinked from /etc/resolv.conf in order to connect all
           local clients that bypass local DNS APIs to systemd-resolved with correct search domains settings. This
           mode of operation is recommended.

       •   A static file /usr/lib/systemd/resolv.conf is provided that lists the 127.0.0.53 DNS stub (see above) as
           only DNS server. This file may be symlinked from /etc/resolv.conf in order to connect all local clients
           that bypass local DNS APIs to systemd-resolved. This file does not contain any search domains.

       •   systemd-resolved maintains the /run/systemd/resolve/resolv.conf file for compatibility with traditional
           Linux programs. This file may be symlinked from /etc/resolv.conf and is always kept up-to-date, containing
           information about all known DNS servers. Note the file format's limitations: it does not know a concept of
           per-interface DNS servers and hence only contains system-wide DNS server definitions. Note that
           /run/systemd/resolve/resolv.conf should not be used directly by applications, but only through a symlink
           from /etc/resolv.conf. If this mode of operation is used local clients that bypass any local DNS API will
           also bypass systemd-resolved and will talk directly to the known DNS servers.

       •   Alternatively, /etc/resolv.conf may be managed by other packages, in which case systemd-resolved will read
           it for DNS configuration data. In this mode of operation systemd-resolved is consumer rather than provider
           of this configuration file.

       Note that the selected mode of operation for this file is detected fully automatically, depending on whether
       /etc/resolv.conf is a symlink to /run/systemd/resolve/resolv.conf or lists 127.0.0.53 as DNS server.

Answered by Gryu on November 26, 2021

I notice you didn't specify dhcp4: false on your lan and didn't assign a nameserver - that may be your problem?

I just recently setup my ubuntu machine as a router and after a lot of struggle I gave up on naming the interfaces and let them as eno1 / enp1s0 in the yaml file (BTW I combined their description in a single file that I named 01-router-all.yaml so that I only have one file to edit when making changes). I think that I faced a race condition with the dhcp server service starting before the network service had named the interfaces as per my .yaml.

Here is my working /etc/netplan/01-router-all.yaml if it helps (I am somewhat cheating and assigning public DNS, as I still need to setup a DNS server on my machine):

network:
  version: 2
  renderer: networkd

  ethernets:
    # eno1 = wan1
    eno1:
      dhcp4: false
      addresses: [192.168.1.8/24]
      nameservers:
        addresses: [9.9.9.9, 1.1.1.1]
        search: []

    # enp1s0 = lan
    enp1s0:
      dhcp4: false
      addresses: [192.168.9.1/24]
      nameservers:
        addresses: [9.9.9.9, 1.1.1.1]
        search: []

Answered by bobmorane on November 26, 2021

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