/etc/network/interfaces does not overwrite /etc/resolv.conf

Question

I'm a Linux novice experimenting with setting up a DNS cache server on a Pine64 on Ubuntu 16.04. I'm trying to set up a static IP-address with a a local Bind9 DNS-server. I've followed this great tutorial, and looked at several others.

However, I'm having trouble with getting the changes in /etc/resolv.conf to stick. /: When I reboot, the settings in /etc/network/interfaces are not written to /etc/resolv.conf.

This is my /etc/network/interfaces:

# interfaces(5) file used by ifup(8) and ifdown(8)
# Include files from /etc/network/interfaces.d:
source-directory /etc/network/interfaces.d

# Disable wlan1 by default (8723bs has two intefaces)
iface wlan1 inet manual

# Loopback interface
auto lo
iface lo inet loopback

# The primary network interface
auto eth0
iface eth0 inet static
        address 192.168.0.18
        netmask 255.255.255.0
        gateway 192.168.0.1
        dns-nameservers 192.168.0.18
        dns-search green.intra

This is my /etc/default/bind9:

# run resolvconf?
RESOLVCONF=yes

# startup options for the server
OPTIONS="-4 -u bind"

From those two files, I was under the impression that the /etc/resolv.conf should become this (automatically, upon booting):

# Dynamic resolv.conf(5) file for glibc resolver(3) generated by resolvconf(8)
#     DO NOT EDIT THIS FILE BY HAND -- YOUR CHANGES WILL BE OVERWRITTEN
nameserver 192.168.0.18
search green.intra

...but instead it becomes this:

# Dynamic resolv.conf(5) file for glibc resolver(3) generated by resolvconf(8)
#     DO NOT EDIT THIS FILE BY HAND -- YOUR CHANGES WILL BE OVERWRITTEN
nameserver 83.255.255.1
nameserver 83.255.255.2
search home

If I add my desired settings to /etc/resolv.conf, everything works as I expect, until I reboot.

What am I doing wrong?

Edit: I have RTFM in trying to figure this out:

https://help.ubuntu.com/14.04/serverguide/dns-troubleshooting.html
https://help.ubuntu.com/12.04/serverguide/network-configuration.html
https://help.ubuntu.com/14.04/serverguide/dns-configuration.html

Edit: ifup and ifdown fail: As suggested in the answer to the proposed duplicate question, using service networking restart might not be reliable in this situation. Instead, I tried the following:

ubuntu@pinex:~$ sudo ifdown -v eth0
Reading directory /etc/network/interfaces.d
Parsing file /etc/network/interfaces.d/eth0
ifdown: interface eth0 not configured

and:

ubuntu@pinex:~$ sudo ifdown -v eth0
Reading directory /etc/network/interfaces.d
Parsing file /etc/network/interfaces.d/eth0
ifdown: interface eth0 not configured
ubuntu@pinex:~$ sudo ifup -v eth0
Reading directory /etc/network/interfaces.d
Parsing file /etc/network/interfaces.d/eth0
Configuring interface eth0=eth0 (inet)
/bin/run-parts --exit-on-error --verbose /etc/network/if-pre-up.d
run-parts: executing /etc/network/if-pre-up.d/wpasupplicant

/sbin/dhclient -1 -v -pf /run/dhclient.eth0.pid -lf /var/lib/dhcp/dhclient.eth0.leases -I -df /var/lib/dhcp/dhclient6.eth0.leases eth0
Internet Systems Consortium DHCP Client 4.3.3
Copyright 2004-2015 Internet Systems Consortium.
All rights reserved.
For info, please visit https://www.isc.org/software/dhcp/

Listening on LPF/eth0/36:c9:e3:f1:b8:05
Sending on   LPF/eth0/36:c9:e3:f1:b8:05
Sending on   Socket/fallback
DHCPREQUEST of 192.168.0.18 on eth0 to 255.255.255.255 port 67 (xid=0x79b015c0)
DHCPACK of 192.168.0.18 from 192.168.0.1
RTNETLINK answers: File exists
bound to 192.168.0.18 -- renewal in 5280 seconds.
/bin/run-parts --exit-on-error --verbose /etc/network/if-up.d
run-parts: executing /etc/network/if-up.d/000resolvconf
run-parts: executing /etc/network/if-up.d/bind9
run-parts: executing /etc/network/if-up.d/openssh-server
run-parts: executing /etc/network/if-up.d/upstart
run-parts: executing /etc/network/if-up.d/wpasupplicant
Configuring interface eth0=eth0 (inet)
/bin/run-parts --exit-on-error --verbose /etc/network/if-pre-up.d
run-parts: executing /etc/network/if-pre-up.d/wpasupplicant
/bin/ip addr add 192.168.0.18/255.255.255.0 broadcast 192.168.0.255       dev eth0 label eth0
RTNETLINK answers: File exists
Failed to bring up eth0.

... which led me to the answer (see below).

Solution

As stated in my question, the configuration is accurate according to the Ubuntu documentation. However, ifdown and ifup did not work. After half a day of googling around, I gathered enough clues to find a solution.

TL;DR;

Comment out (or remove) the lines from /etc/network/interfaces.d/eth0 that originally was:

auto eth0
iface eth0 inet dhcp

The long answer:

I'm sorry I cannot provide an explanation as to why I had this problem. I know too little of Linux. /: The problem seems to be that resolvconf, which is triggered when booting (I think), was unable to correctly update /etc/resolv.conf because it found the same interface twice: once in /etc/network/interfaces.d/eth0 and once in my custom configuration /etc/network/interfaces.

Restarting the network service gave me this:

ubuntu@pinex:~$ sudo service networking restart
Job for networking.service failed because the control process exited with error code. See "systemctl status networking.service" and "journalctl -xe" for details.
ubuntu@pinex:~$ systemctl status networking.service
● networking.service - Raise network interfaces
   Loaded: loaded (/lib/systemd/system/networking.service; enabled; vendor preset: enabled)
  Drop-In: /run/systemd/generator/networking.service.d
           └─50-insserv.conf-$network.conf
   Active: failed (Result: exit-code) since Mon 2016-12-19 13:59:48 CET; 10s ago
     Docs: man:interfaces(5)
  Process: 2433 ExecStart=/sbin/ifup -a --read-environment (code=exited, status=1/FAILURE)
  Process: 2428 ExecStartPre=/bin/sh -c [ "$CONFIGURE_INTERFACES" != "no" ] && [ -n "$(ifquery --read-environm
 Main PID: 2433 (code=exited, status=1/FAILURE)

Dec 19 13:59:47 pinex ifup[2433]: DHCPACK of 192.168.0.18 from 192.168.0.1
Dec 19 13:59:47 pinex ifup[2433]: RTNETLINK answers: File exists
Dec 19 13:59:47 pinex dhclient[2446]: bound to 192.168.0.18 -- renewal in 4214 seconds.
Dec 19 13:59:47 pinex ifup[2433]: bound to 192.168.0.18 -- renewal in 4214 seconds.
Dec 19 13:59:48 pinex ifup[2433]: RTNETLINK answers: File exists
Dec 19 13:59:48 pinex ifup[2433]: Failed to bring up eth0.
Dec 19 13:59:48 pinex systemd[1]: networking.service: Main process exited, code=exited, status=1/FAILURE
Dec 19 13:59:48 pinex systemd[1]: Failed to start Raise network interfaces.
Dec 19 13:59:48 pinex systemd[1]: networking.service: Unit entered failed state.
Dec 19 13:59:48 pinex systemd[1]: networking.service: Failed with result 'exit-code'.

I remember seeing something about the /etc/network/interfaces.d/eth0 when googling around. When commenting out those lines, restarting the network service worked fine. However, the /etc/resolv.conf was still the same as before...

I had to do the following:

ubuntu@pinex:~$ sudo resolvconf -u
/etc/resolvconf/update.d/libc: Warning: /etc/resolv.conf is not a symbolic link to /run/resolvconf/resolv.conf
ubuntu@pinex:~$ sudo rm /etc/resolv.conf
ubuntu@pinex:~$ sudo ln -s /run/resolvconf/resolv.conf /etc/resolv.conf

That updated the /etc/resolv.conf to my expected values! After a reboot of the device, the file was automatically properly populated!

I hope this is of help to someone else as well! And, if anyone can explain the problem, that would be appreciated. (: