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Im running a mixed network of Ubuntu (all 20.04) MacOS and Windows 10.

I run Ubuntu server which runs dnsmasq and acts as the DHCP server for the network (the machine is called 'serv2' and has IP address 192.168.2.4). Serv2 also provides a cached DNS for the network (and allows the resolving of local host names) (again using dnsmasq).

My system uses IPv4 and I have never tried to configure IPv6

A new problem kicked off that sometimes (and randomly on startup) a Ubuntu desktop client couldn't resolve the name of the other hosts no my network. This could be solved by a reboot.

I've done a lot of poking about, and I think that the following factors are significant:

resolve.conf contains:

nameserver 192.168.2.4
nameserver fd52:a81c:df85:1:ded9:16ff:fed5:9a73
search njj.chickenkiller.com connect

looking at systemd-resolve --status the inability to resolve local host names only happens when fd52... is the first listed nameserver.

My ifconfig output:

enp6s0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
inet 192.168.2.11  netmask 255.255.255.0  broadcast 192.168.2.255
inet6 fe80::a37a:ffa8:2056:1d1e  prefixlen 64  scopeid 0x20<link>
inet6 fd52:a81c:df85:1:389:3ab9:d587:3bb7  prefixlen 64  scopeid 0x0<global>
inet6 fd52:a81c:df85:1:cc9f:3c2f:ad73:fd29  prefixlen 64  scopeid 0x0<global>
ether b4:2e:99:41:5d:bc  txqueuelen 1000  (Ethernet)

and can see that I've been allocated IPv6 addresses using the prefix fd52:a81c:df85. Similarly, my server has been allocated IPv6 address fd52:a81c:df85:1:1a31:bfff:fe6a:35bc

In my systemd log (journalctl -b|grep dhcp), I have the following entries:

May 08 12:16:03 kenickie audit[626]: AVC apparmor="STATUS" operation="profile_load" profile="unconfined" name="/usr/lib/NetworkManager/nm-dhcp-client.action" pid=626 comm="apparmor_parser"
May 08 12:16:03 kenickie audit[626]: AVC apparmor="STATUS" operation="profile_load" profile="unconfined" name="/usr/lib/NetworkManager/nm-dhcp-helper" pid=626 comm="apparmor_parser"
May 08 12:16:05 kenickie NetworkManager[687]: <info>  [1588936565.2384] dhcp-init: Using DHCP client 'internal'
May 08 12:16:08 kenickie NetworkManager[687]: <info>  [1588936568.1844] dhcp4 (enp6s0): activation: beginning transaction (timeout in 45 seconds)
May 08 12:16:08 kenickie NetworkManager[687]: <info>  [1588936568.2316] dhcp4 (enp6s0): option dhcp_lease_time      => '43200'
May 08 12:16:08 kenickie NetworkManager[687]: <info>  [1588936568.2316] dhcp4 (enp6s0): option domain_name          => 'njj.chickenkiller.com'
May 08 12:16:08 kenickie NetworkManager[687]: <info>  [1588936568.2316] dhcp4 (enp6s0): option domain_name_servers  => '192.168.2.4'
May 08 12:16:08 kenickie NetworkManager[687]: <info>  [1588936568.2316] dhcp4 (enp6s0): option expiry               => '1588979768'
May 08 12:16:08 kenickie NetworkManager[687]: <info>  [1588936568.2316] dhcp4 (enp6s0): option host_name            => 'kenickie'
May 08 12:16:08 kenickie NetworkManager[687]: <info>  [1588936568.2316] dhcp4 (enp6s0): option ip_address           => '192.168.2.11'
May 08 12:16:08 kenickie NetworkManager[687]: <info>  [1588936568.2317] dhcp4 (enp6s0): option next_server          => '192.168.2.4'
May 08 12:16:08 kenickie NetworkManager[687]: <info>  [1588936568.2317] dhcp4 (enp6s0): option ntp_servers          => '192.168.2.4'
May 08 12:16:08 kenickie NetworkManager[687]: <info>  [1588936568.2317] dhcp4 (enp6s0): option requested_broadcast_address => '1'
May 08 12:16:08 kenickie NetworkManager[687]: <info>  [1588936568.2317] dhcp4 (enp6s0): option requested_domain_name => '1'
May 08 12:16:08 kenickie NetworkManager[687]: <info>  [1588936568.2317] dhcp4 (enp6s0): option requested_domain_name_servers => '1'
May 08 12:16:08 kenickie NetworkManager[687]: <info>  [1588936568.2317] dhcp4 (enp6s0): option requested_domain_search => '1'
May 08 12:16:08 kenickie NetworkManager[687]: <info>  [1588936568.2317] dhcp4 (enp6s0): option requested_host_name  => '1'
May 08 12:16:08 kenickie NetworkManager[687]: <info>  [1588936568.2317] dhcp4 (enp6s0): option requested_interface_mtu => '1'
May 08 12:16:08 kenickie NetworkManager[687]: <info>  [1588936568.2317] dhcp4 (enp6s0): option requested_ms_classless_static_routes => '1'
May 08 12:16:08 kenickie NetworkManager[687]: <info>  [1588936568.2317] dhcp4 (enp6s0): option requested_nis_domain => '1'
May 08 12:16:08 kenickie NetworkManager[687]: <info>  [1588936568.2317] dhcp4 (enp6s0): option requested_nis_servers => '1'
May 08 12:16:08 kenickie NetworkManager[687]: <info>  [1588936568.2318] dhcp4 (enp6s0): option requested_ntp_servers => '1'
May 08 12:16:08 kenickie NetworkManager[687]: <info>  [1588936568.2318] dhcp4 (enp6s0): option requested_rfc3442_classless_static_routes => '1'
May 08 12:16:08 kenickie NetworkManager[687]: <info>  [1588936568.2318] dhcp4 (enp6s0): option requested_root_path  => '1'
May 08 12:16:08 kenickie NetworkManager[687]: <info>  [1588936568.2318] dhcp4 (enp6s0): option requested_routers    => '1'
May 08 12:16:08 kenickie NetworkManager[687]: <info>  [1588936568.2318] dhcp4 (enp6s0): option requested_static_routes => '1'
May 08 12:16:08 kenickie NetworkManager[687]: <info>  [1588936568.2318] dhcp4 (enp6s0): option requested_subnet_mask => '1'
May 08 12:16:08 kenickie NetworkManager[687]: <info>  [1588936568.2318] dhcp4 (enp6s0): option requested_time_offset => '1'
May 08 12:16:08 kenickie NetworkManager[687]: <info>  [1588936568.2318] dhcp4 (enp6s0): option requested_wpad       => '1'
May 08 12:16:08 kenickie NetworkManager[687]: <info>  [1588936568.2318] dhcp4 (enp6s0): option routers              => '192.168.2.1'
May 08 12:16:08 kenickie NetworkManager[687]: <info>  [1588936568.2318] dhcp4 (enp6s0): option subnet_mask          => '255.255.255.0'
May 08 12:16:08 kenickie NetworkManager[687]: <info>  [1588936568.2318] dhcp4 (enp6s0): state changed unknown -> bound
May 08 12:16:09 kenickie NetworkManager[687]: <info>  [1588936569.9981] dhcp6 (enp6s0): activation: beginning transaction (timeout in 45 seconds)
May 08 12:16:10 kenickie NetworkManager[687]: <info>  [1588936570.0162] dhcp6 (enp6s0): option dhcp6_domain_search  => 'connect'
May 08 12:16:10 kenickie NetworkManager[687]: <info>  [1588936570.0166] dhcp6 (enp6s0): option dhcp6_name_servers   => 'fd52:a81c:1234:1:ded9:16ff:fed5:9a73'
May 08 12:16:10 kenickie NetworkManager[687]: <info>  [1588936570.0169] dhcp6 (enp6s0): state changed unknown -> bound

Note the one from last line where the IPv6 name server gets allocated

Looking at my router, it has an IPv6 setting (my ISP does not support IPv6 in my area) offering a ULA prefix of fd52:a81c:df85:: (ie the one appearing on my network). The text along side the IPv6 settings states "This page allows to configure IP addresses used in your home network. In case DHCP is used, Vodafone Connect automatically assignes an IP address to devices connected to the network." However, DHCP is turned off on the router.

So four questions:

  1. How is the ULA prefix getting shared around my network? (is it a router bug)?
  2. How are the IPv6 addresses getting allocated
  3. Where does the dhcp6 allocation of a name server come from?
  4. Is there an elegant way to stop the the dhcp6 allocation of a name server?
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  • Increasingly playing around. fd52:a81c:1234:1:ded9:16ff:fed5:9a73 is the router. Which despite having DHCP turned off, seems happy to offer services. Disabling ULA seems to have solved the problem. – Nick Sillito May 8 '20 at 12:41
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Sorry to answer my own question, but the issue relates to VDSL router (and ultimately nothing to do with Ubuntu). It appears in the last firmware update IPv6 was turned on and the router was allocating IPv6 addresses to clients, and name resolution.

Its covered in this post on my ISP's problem page

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