No network in recovery mode (systemd-resolved does not start)

[Lastique]

When booting into recovery mode and enabling networking, networking does not get enabled. DNS lookup doesn't work because systemd-resolved does not start and /etc/resolv.conf is a broken symlink.

Steps to reproduce:

1. Reboot.
2. In the grub menu, select "Advanced options for Ubuntu" and then select boot in recovery mode.
3. When the menu appears, select "networking". In the console you may notice a few errors that /etc/resolv.conf file not found.
4. Select "drop to root" console.
5. Try pinging some hosts, like "ping ubuntu.com". You will see that name resolution doesn't work.
6. You can check that the network interface is up, "ip addr" displays valid info and the interface has an assigned local IP.
7. You can check that /etc/resolv.conf is in fact a dangling link to ../run/resolvconf/resolv.conf.
8. You can see that systemd-resolved is not running: "systemctl status systemd-resolved" shows the service is dead.
9. Invoking "systemctl start systemd-resolved" hangs indefinitely, the service not starting.

The workaround to get DNS working is to replace /etc/resolv.conf with a valid link or a file with a DNS server.

The DNS lookup and networking in general works fine if the system is booted normally.

ProblemType: Bug
DistroRelease: Ubuntu 17.10
Package: systemd 234-2ubuntu12.1
ProcVersionSignature: Ubuntu 4.13.0-21.24-generic 4.13.13
Uname: Linux 4.13.0-21-generic x86_64
NonfreeKernelModules: nvidia_uvm nvidia_drm nvidia_modeset nvidia
ApportVersion: 2.20.7-0ubuntu3.7
Architecture: amd64
CurrentDesktop: KDE
Date: Wed Jan 10 02:18:14 2018
InstallationDate: Installed on 2015-05-01 (984 days ago)
InstallationMedia: Kubuntu 15.04 "Vivid Vervet" - Release amd64 (20150422)
MachineType: System manufacturer System Product Name
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-4.13.0-21-generic root=UUID=a03f1835-52f9-4409-9da7-fe45770637ae ro quiet splash nomdmonddf nomdmonisw
SourcePackage: systemd
SystemdDelta:
 [MASKED] /etc/systemd/system/samba-ad-dc.service → /lib/systemd/system/samba-ad-dc.service
 [EXTENDED] /lib/systemd/system/rc-local.service → /lib/systemd/system/rc-local.service.d/debian.conf
 [EXTENDED] /lib/systemd/system/user@.service → /lib/systemd/system/user@.service.d/timeout.conf

 3 overridden configuration files found.
UpgradeStatus: Upgraded to artful on 2017-11-03 (67 days ago)
dmi.bios.date: 11/09/2012
dmi.bios.vendor: American Megatrends Inc.
dmi.bios.version: 3603
dmi.board.asset.tag: To be filled by O.E.M.
dmi.board.name: P8Z68-V PRO
dmi.board.vendor: ASUSTeK Computer INC.
dmi.board.version: Rev 1.xx
dmi.chassis.asset.tag: Asset-1234567890
dmi.chassis.type: 3
dmi.chassis.vendor: Chassis Manufacture
dmi.chassis.version: Chassis Version
dmi.modalias: dmi:bvnAmericanMegatrendsInc.:bvr3603:bd11/09/2012:svnSystemmanufacturer:pnSystemProductName:pvrSystemVersion:rvnASUSTeKComputerINC.:rnP8Z68-VPRO:rvrRev1.xx:cvnChassisManufacture:ct3:cvrChassisVersion:
dmi.product.family: To be filled by O.E.M.
dmi.product.name: System Product Name
dmi.product.version: System Version
dmi.sys.vendor: System manufacturer