Wrong resolv.conf used on Ubuntu 19 (systemd-resolved enabled)

[NickStemerdink]

Description
I'm running Ubuntu 19.04 which has systemd-resolved enabled by default in mode one. The systemd-resolved manual states that traditional linux programs should use /etc/resolv.conf. Currently because of commit e353e7e however it will use /run/systemd/resolve/resolv.conf when systemd-resolved is running.

/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.

Describe the results you received:
/run/systemd/resolve/stub-resolv.conf is used.

Describe the results you expected:
I would expect that /etc/resolv.conf is used.

Output of docker version:

Docker version 19.03.2, build 6a30dfca03

Output of docker info:

Client:
 Debug Mode: false

Server:
 Containers: 19
  Running: 0
  Paused: 0
  Stopped: 19
 Images: 75
 Server Version: 19.03.2
 Storage Driver: overlay2
  Backing Filesystem: extfs
  Supports d_type: true
  Native Overlay Diff: true
 Logging Driver: json-file
 Cgroup Driver: cgroupfs
 Plugins:
  Volume: local
  Network: bridge host ipvlan macvlan null overlay
  Log: awslogs fluentd gcplogs gelf journald json-file local logentries splunk syslog
 Swarm: inactive
 Runtimes: runc
 Default Runtime: runc
 Init Binary: docker-init
 containerd version: 894b81a4b802e4eb2a91d1ce216b8817763c29fb
 runc version: 425e105d5a03fabd737a126ad93d62a9eeede87f
 init version: fec3683
 Security Options:
  apparmor
  seccomp
   Profile: default
 Kernel Version: 5.0.0-29-generic
 Operating System: Ubuntu 19.04
 OSType: linux
 Architecture: x86_64
 CPUs: 4
 Total Memory: 15.56GiB
 Name: nick-desktop
 ID: 7QKQ:QK7I:U7A5:YLHI:BO2G:KTQV:QHSN:I5HT:HRQR:CKXN:BAL3:OFZM
 Docker Root Dir: /var/lib/docker
 Debug Mode: false
 Registry: https://index.docker.io/v1/
 Labels:
 Experimental: false
 Insecure Registries:
  127.0.0.0/8
 Live Restore Enabled: false

[thaJeztah]

Thanks for reporting; this is indeed something that needs a solution (but there's some complications); there is an existing issue about that #38243 (also see the discussion on moby/libnetwork#2385 (comment))

let me close this one as duplicate of #38243 to prevent the discussion from diverging