[Bug 1624317] Re: systemd-resolved breaks VPN with split-horizon DNS

[ChristianEhrhardt]

I dup'ed another bug onto this, as I think they are essentially the
same.

While Martin worked on the domain-restricted cases the reporter and
others outlined that this is not what this bug is about.

TL;DR:
- anything (like vpn) provides new DNS servers on an extra link
  - lets call them "internal" and "external" sources of DNS info
- systemd-resolved asks ALL local interfaces concurrently and takes the fastest answer
- now we have two breaking use cases:
  - if "internal" and "external" are intentionally configured to report differently (override a DNS name to internal host) you can no more rely on getting the internal resolution
  - by asking "external" even if "internal" holds an answer is a dns leak as in [1]

At least for the VPN cases - and most here seem to be that - what we
would need is a way to "disable" the DNS resolving on the original
interface.


>From man systemd-resolved:
Multi-label names are routed to all local interfaces that have a DNS sever configured, plus the
globally configured DNS server if there is one. [...]

If lookups are routed to multiple interfaces, the first successful response is returned (thus
effectively merging the lookup zones on all matching interfaces). If the lookup failed on all
interfaces, the last failing response is returned.


>From the bug I dupped we have the output of
$ systemd-resolve --status
(There is also tcpdump data there)
Both confirms that is lists all links and their DNS
Link 4 (tun0)
      Current Scopes: DNS
[...]
         DNS Servers: 209.222.18.222
                      209.222.18.218
Link 3 (wlp3s0)
      Current Scopes: DNS LLMNR/IPv4 LLMNR/IPv6
[...]
         DNS Servers: 192.168.0.1

Combined with the concurrent-quere-prefer-first behavior above that
might be the root cause for all or most of the people chiming in here.

[1]: https://www.dnsleaktest.com/

** Changed in: systemd (Ubuntu)
   Importance: Medium => High

** Tags added: zesty

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https://bugs.launchpad.net/bugs/1624317

Title:
  systemd-resolved breaks VPN with split-horizon DNS

Status in systemd:
  New
Status in systemd package in Ubuntu:
  Confirmed

Bug description:
  I use a VPN configured with network-manager-openconnect-gnome in which
  a split-horizon DNS setup assigns different addresses to some names
  inside the remote network than the addresses seen for those names from
  outside the remote network.  However, systemd-resolved often decides
  to ignore the VPN’s DNS servers and use the local network’s DNS
  servers to resolve names (whether in the remote domain or not),
  breaking the split-horizon DNS.

  This related bug, reported by Lennart Poettering himself, was closed with the current Fedora release at the time reaching EOL:
  https://bugzilla.redhat.com/show_bug.cgi?id=1151544

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