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[Launchpad Bug Tracker <1817903@xxxxxxxxxxxxxxxxxx>]

You have been subscribed to a public bug by Eric Desrochers (slashd):

[impact]

systems upgraded from pre-Bionic releases to Bionic or later will
continue to use ifupdown/resolvconf for network conf and management, but
resolvconf has a new systemd service in Bionic and later that pulls
systemd-resolved stub-resolv.conf into its local configuration.  With
the recent addition of edns0 option to the stub resolver conf in systemd
to fix bug 1811471, this means resolvconf now sets up the
/etc/resolv.conf file to include upstream servers but also use edns.
For any systems where the upstream resolver(s) don't support edns, dns
lookups will break.

[test case]

create a xenial system with ifupdown/resolvconf, then upgrade to bionic
(alternately it should be possible to install bionic, then remove
netplan and install/configure ifupdown and resolvconf).  The system
ifupdown config should include an upstream name server.

After upgrade, the /etc/resolv.conf will contain both the upstream name
server as well as options edns0.

[regression potential]

this changes how resolvconf handles system dns on bionic and later:

1) networking is managed by ifupdown

resolvconf is currently adding the local stub resolver to
/etc/resolv.conf, even though in this case it doesn't know about any
upstream name servers.  This change will remove the local stub resolver
from /etc/resolv.conf; it should not be there.

2) networking is managed by systemd-networkd

resolvconf is currently setting up /etc/resolv.conf to direct all local
dns queries to the local stub resolver, similar to how systemd-resolved
itself configures /etc/resolv.conf.  This change will instead set up
/etc/resolv.conf to bypass the local stub resolver, and send all dns
queries to the upstream name server(s).

In case #1, this change has little chance for regression; in case #2
however, this change will bypass the local stub resolver and thus create
more network dns traffic (since dns queries will not be cached locally).
However, this is how pre-Bionic releases worked, and simply removing
resolvconf will restore systemd-resolved control of /etc/resolv.conf,
causing the system to again use the local stub resolver.

Additional regressions due to this change would likely be seen in dns
query failures with other system configurations.

[other info]

This affects only Bionic and later; in Xenial and earlier, resolvconf
does not include the 'resolvconf-pull-resolved' service to pull in the
systemd-resolved stub config, which is what causes this problem.

This also does not affect Debian, as it does not include the
'resolvconf-pull-resolved' service either.

original description:

--

Mint 19 (Ubuntu 18.04)

Following latest mint update done on 24/02/2019, DNS is broken....

nslookup and dig of certain domain names work as expected, ping does not
(ip works but not domain name)

After a day of trial and error, testing I found that the problem lies
with the presence of

"options edns0"

in /run/resolvconf/resolv.conf (link to by /etc/resolv.conf)

With option present many dns lookups fail with both FF and chrome browswers and thunderbird...
This is on a home network, with router set as dns proxy for external wan, not using NetworkManager

Deleting the option on live system results in the issue immediately
disappearing, but on reboot it is added back in (by systemd-resolve ?)

I cannot find any option to prevent this being added, so presumably it
is hard-coded in systemd following the update?

systemd:
  Installed: 237-3ubuntu10.13

** Affects: systemd (Ubuntu)
     Importance: Critical
     Assignee: Dan Streetman (ddstreet)
         Status: In Progress

** Affects: systemd (Ubuntu Trusty)
     Importance: Undecided
         Status: Invalid

** Affects: systemd (Ubuntu Xenial)
     Importance: Undecided
         Status: Invalid

** Affects: systemd (Ubuntu Bionic)
     Importance: Critical
     Assignee: Dan Streetman (ddstreet)
         Status: In Progress

** Affects: systemd (Ubuntu Cosmic)
     Importance: Critical
     Assignee: Dan Streetman (ddstreet)
         Status: In Progress

** Affects: systemd (Ubuntu Disco)
     Importance: Critical
     Assignee: Dan Streetman (ddstreet)
         Status: In Progress


** Tags: regression-update sts sts-sponsors
-- 
systemd-resolve appends "options edns0" to resolv.conf
https://bugs.launchpad.net/bugs/1817903
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