Verifique se há vazamentos de DNS no servidor Ubuntu

Atualmente, executo o servidor Ubuntu 18.04.1 LTS. Configurei a VPN de túnel dividido e parece funcionar bem, mas quero verificar se há vazamentos de DNS. Existe uma maneira de fazer isso com a linha de comando? Realmente não é possível abrir um navegador no servidor Ubuntu ...

Atualizado com informações para heynnema:

ls -al /etc/openvpn

total 52
drwxrwxrwx  4 root root  4096 Nov  2 21:32 .
drwxr-xr-x 99 root root  4096 Nov  3 12:40 ..
-rwxrwxrwx  1 root root  1403 Nov  2 20:35 ca.crt
drwxrwxrwx  2 root root  4096 Sep  5 14:43 client
-rwxrwxrwx  1 root root  1597 Nov  2 20:39 iptables.sh
-rwxrwxrwx  1 root root    18 Nov  2 20:36 login.txt
-rwxrwxrwx  1 root root   670 Nov  2 21:29 openvpn.conf
-rwxrwxrwx  1 root root   623 Nov  2 20:40 routing.sh
drwxrwxrwx  2 root root  4096 Sep  5 14:43 server
-rwxrwxrwx  1 root root   636 Nov  2 20:35 tls.key
-rwxrwxrwx  1 root root 11773 Nov 12  2017 update-systemd-resolved

#

grep -i hosts /etc/nsswitch.conf
hosts:          files resolve [!UNAVAIL=return] dns

# A seção "evitar vazamento de DNS" que obtive do guia on-line, no entanto, a VPN não funcionará para mim se eu habilitar esse comando.

up/down/down-pre
#up and down scripts to be executed when VPN starts or stops
up /etc/openvpn/iptables.sh
down /etc/openvpn/update-systemd-resolved
down-pre

# prevent DNS leakage
#dhcp-option DOMAIN-ROUTE .

Mais informações:

#! /bin/bash
# Niftiest Software – www.niftiestsoftware.com
# Modified version by HTPC Guides – www.htpcguides.com

export INTERFACE="tun0"
export VPNUSER="vpn"
export LOCALIP="192.168.1.10"
export NETIF="enp1s0"

# flushes all the iptables rules, if you have other rules to use then add them into the script
iptables -F -t nat
iptables -F -t mangle
iptables -F -t filter

# mark packets from $VPNUSER
iptables -t mangle -A OUTPUT -j CONNMARK --restore-mark
iptables -t mangle -A OUTPUT ! --dest $LOCALIP -m owner --uid-owner $VPNUSER -j MARK --set-mark 0x1
iptables -t mangle -A OUTPUT --dest $LOCALIP -p udp --dport 53 -m owner --uid-owner $VPNUSER -j MARK --set-mark 0x1
iptables -t mangle -A OUTPUT --dest $LOCALIP -p tcp --dport 53 -m owner --uid-owner $VPNUSER -j MARK --set-mark 0x1
iptables -t mangle -A OUTPUT ! --src $LOCALIP -j MARK --set-mark 0x1
iptables -t mangle -A OUTPUT -j CONNMARK --save-mark

# allow responses
iptables -A INPUT -i $INTERFACE -m conntrack --ctstate ESTABLISHED -j ACCEPT

# block everything incoming on $INTERFACE to prevent accidental exposing of ports
iptables -A INPUT -i $INTERFACE -j REJECT

# let $VPNUSER access lo and $INTERFACE
iptables -A OUTPUT -o lo -m owner --uid-owner $VPNUSER -j ACCEPT
iptables -A OUTPUT -o $INTERFACE -m owner --uid-owner $VPNUSER -j ACCEPT

# all packets on $INTERFACE needs to be masqueraded
iptables -t nat -A POSTROUTING -o $INTERFACE -j MASQUERADE

# reject connections from predator IP going over $NETIF
iptables -A OUTPUT ! --src $LOCALIP -o $NETIF -j REJECT

# Start routing script
/etc/openvpn/routing.sh

exit 0