Why is my ISP DNS still in resolv.conf after a VPN connection and how can this be fixed?

Question

Ubuntu 15.10 and dns=dnsmasq is commented out in /etc/NetworkManager/NetworkManager.conf

Before I connect to a VPN /etc/resolv.conf contains

nameserver 2xx.xx.xx.xx <-- ISP DNS 1
nameserver 2xx.xx.xx.xx <-- ISP DNS 2

after a VPN connection /etc/resolv.conf contains

nameserver 1xx.xx.xx.xx <-- VPN DNS 1
nameserver 1xx.xx.xx.xx <-- VPN DNS 2
nameserver 2xx.xx.xx.xx <-- ISP DNS 1

The regular wired connection and the VPN have DNS servers set in network manager with automatic (only addresses). The ISP server shouldn't be there at all. What else can I change? (removing dns=dnsmasq was one change to stop split DNS).

Answer 1

NetworkManager can either:

• update resolv.conf itself;

• delegate to resolvconf (for the NetworkManager interface);

• or use netconfig.

The different configurations coming from each interface are simply aggregated (see update_dns()).

If you do not use NetworkManager for the VPN, you might use openresolv exclusive mode (-x) in order to override the nameservers from NetworkManager with the ones from the VPN instead of adding them. This can be done with this (ugly) script (OpenVPN hook):

#!/bin/sh

# Dump all foreign options (coming from environment variables foreign_option_N) to stdout
foreign_options() {
    local i
    i=1
    while true; do
      local varname=foreign_option_$i
      local value="$(eval echo \$$varname)"
      if [ -z "$value" ]; then
        return
      fi
      echo $value
      i=$((i+1))
  done
}

#Create a resolv.conf file from OpenVPN environment variables
create_resolvconf() {
    foreign_options | grep "^dhcp-option DNS " | sed "s/^dhcp-option DNS /nameserver /"
}

route_up() {    
    create_resolvconf | resolvconf -x -a $dev
}

down() {
    resolvconf -d $dev
}

case "$script_type" in
    route-up) route_up "$@" ;;
    down) down "$@" ;;
esac

You should be able to adapt this to be used as a NetworkManager dispatcher script (see man 8 NetworkManager) using:

• VPN_IP4_NAMESERVERS
• VPN_IP6_NAMESERVERS

I didn't test it but something like this should do the trick:

#!/bin/sh

create_resolvconf() {
    for ip in $VPN_IP4_NAMESERVERS $VPN_IP6_NAMESERVERS; do
         echo "nameserver $ip"
    done
}

up() {
   create_resolvconf | resolvconf -x -a $VPN_IP_IFAC
}

down() {
   resolvconf -d $VPN_IP_IFAC
}

if [ -z "$VPN_IP_IFACE" ]; then
  return 0
fi

case "$2" in
   up) up ;;
   down) down ;;
esac