Evaggelos Balaskas - System Engineer

WireGuard: fast, modern, secure VPN tunnel. WireGuard securely encapsulates IP packets over UDP.

Goal

What I would like to achieve, in this article, is to provide a comprehensive guide for a redirect-gateway vpn using wireguard with a twist. The client machine should reach internet through the wireguard vpn server. No other communications should be allowed from the client and that means if we drop the VPN connection, client can not go to the internet.

Intro - Lab Details

Here are my lab details. This blog post will help you understand all the necessary steps and provide you with a guide to replicate the setup. You should be able to create a wireguard VPN server-client between two points. I will be using ubuntu 20.04 as base images for both virtual machines. I am also using LibVirt and Qemu/KVM running on my archlinux host.

Wireguard Generate Keys

and the importance of them!

Before we begin, give me a moment to try explaining how the encryption between these two machines, in a high level design works.

Each linux machines creates a pair of keys.

• Private Key
• Public Key

These keys have a unique relationship. You can use your public key to encrypt something but only your private key can decrypt it. That mean, you can share your public keys via an cleartext channel (internet, email, slack). The other parties can use your public key to encrypt traffic towards you, but none other decrypt it. If a malicious actor replace the public keys, you can not decrypt any incoming traffic thus make it impossible to connect to VPN server!

Public - Private Keys

Now each party has the other’s public keys and they can encrypt traffic for each other, BUT only you can decrypt your traffic with your private key. Your private key never leaves your computer.

Hopefully you get the idea.

Wireguard Server - Ubuntu 20.04 LTS

In order to make the guide easy to follow, I will start with the server part.

Server Key Generation

Generate a random private and public key for the server as-root

wg genkey | tee /etc/wireguard/privatekey | wg pubkey > /etc/wireguard/publickey

Make the keys read-only

chmod 0400 /etc/wireguard/*key

List keys

ls -l /etc/wireguard/

total 8
-r-------- 1 root root 45 Jul 12 20:29 privatekey
-r-------- 1 root root 45 Jul 12 20:29 publickey

UDP Port & firewall

In order to run a VPN service, you need to choose a random port that wireguard VPN server can listen to it.

eg.

61194

open firewall

ufw allow 61194/udp

Rule added
Rule added (v6)

view more info

ufw status verbose

Status: active
Logging: on (low)
Default: deny (incoming), allow (outgoing), disabled (routed)
New profiles: skip

To             Action      From
--             ------      ----
22/tcp         ALLOW IN    Anywhere
61194          ALLOW IN    Anywhere
22/tcp (v6)    ALLOW IN    Anywhere (v6)
61194 (v6)     ALLOW IN    Anywhere (v6)

So you see, only SSH and our VPN ports are open.
Default Incoming Policy is: DENY.

Wireguard Server Configuration File

Now it is time to create a basic configuration wireguard-server file.

Naming the wireguard interface

Clarification the name of our interface does not matter, you can choose any name. Gets it’s name from the configuration filename! But in order to follow the majority of guides try to use something from wg+. For me it is easier to name wireguard interface:

• wg0

but you may seen it also as

• wg

without a number. All good!

Making a shell script for wireguard server configuration file

Running the below script will create a new configuration file /etc/wireguard/wg0.conf

PRIVATE_KEY=$(cat /etc/wireguard/privatekey)
NETWORK=10.0.8

cat > /etc/wireguard/wg0.conf <<EOF
[Interface]
Address = ${NETWORK}.1/24
ListenPort = 61194
PrivateKey = ${PRIVATE_KEY}

EOF

ls -l /etc/wireguard/wg0.conf
cat   /etc/wireguard/wg0.conf

Note I have chosen the network 10.0.8.0/24 for my VPN setup, you can choose any Private Network

10.0.0.0/8     - Class A
172.16.0.0/12  - Class B
192.168.0.0/16 - Class C

I chose a Class C (256 IPs) from a /8 (Class A) private network, do not be confused about this. It’s just a Class-C /24 private network.

Let’s make our first test with the server

wg-quick up wg0

[#] ip link add wg0 type wireguard
[#] wg setconf wg0 /dev/fd/63
[#] ip -4 address add 10.0.8.1/24 dev wg0
[#] ip link set mtu 1420 up dev wg0

It’s Alive!

Kernel Module

Verify that wireguard is loaded as a kernel module on your system.

lsmod | grep wireguard

wireguard             212992  0
ip6_udp_tunnel         16384  1 wireguard
udp_tunnel             16384  1 wireguard

Wireguard is in the Linux kernel since March 2020.

Show IP address

ip address show dev wg0

3: wg0: <POINTOPOINT,NOARP,UP,LOWER_UP> mtu 1420
qdisc noqueue state UNKNOWN group default qlen 1000
    link/none
    inet 10.0.8.1/24 scope global wg0
       valid_lft forever preferred_lft forever

Listening Connections

Verify that wireguard server listens to our specific UDP port

ss -nulp '( sport = :61194 )' | column -t

State   Recv-Q  Send-Q  Local Address:Port  Peer Address:Port  Process
UNCONN  0       0           0.0.0.0:61194       0.0.0.0:*
UNCONN  0       0              [::]:61194          [::]:*

Show wg0

wg show wg0

interface: wg0
  public key: <public key>
  private key: (hidden)
  listening port: 61194

Show Config

wg showconf wg0

[Interface]
ListenPort = 61194
PrivateKey = <private key>

Close wireguard

wg-quick down wg0

What is IP forwarding

In order for our VPN server to forward our client’s network traffic from the internal interface wg0 to it’s external interface and on top of that, to separate it’s own traffic from client’s traffic, the vpn server needs to masquerade all client’s traffic in a way that knows who to forward and send back traffic to the client. In a nuthshell this is IP forwarding and perhaps the below diagram can explain it a little better.

To do that, we need to enable the IP forward feature on our linux kernel.

sysctl -w net.ipv4.ip_forward=1

The above command does not persist the change across reboots. To persist this setting we need to add it, to sysctl configuration. And although many set this option global, we do not need to have to do that!

Wireguard provides four (4) stages, to run our own scripts.

Wireguard stages

• PreUp
• PostUp
• PreDown
• PostDown

So we can choose to enable IP forwarding at wireguard up and disable it on wireguard down.

IP forwarding in wireguard configuration file

PRIVATE_KEY=$(cat /etc/wireguard/privatekey)
NETWORK=10.0.8

cat > /etc/wireguard/wg0.conf <<EOF

[Interface]
Address    = ${NETWORK}.1/24
ListenPort = 61194
PrivateKey = ${PRIVATE_KEY}

PreUp    = sysctl -w net.ipv4.ip_forward=1
PostDown = sysctl -w net.ipv4.ip_forward=0
EOF

verify above configuration

wg-quick up wg0

[#] sysctl -w net.ipv4.ip_forward=1
net.ipv4.ip_forward = 1
[#] ip link add wg0 type wireguard
[#] wg setconf wg0 /dev/fd/63
[#] ip -4 address add 10.0.8.1/24 dev wg0
[#] ip link set mtu 1420 up dev wg0

Verify system control setting for IP forward:

sysctl net.ipv4.ip_forward

net.ipv4.ip_forward = 1

Close wireguard

wg-quick down wg0

[#] wg showconf wg0
[#] ip link delete dev wg0
[#] sysctl -w net.ipv4.ip_forward=0
net.ipv4.ip_forward = 0

Verify that now IP Forward is not enabled.

sysctl net.ipv4.ip_forward

net.ipv4.ip_forward = 0

Routing via firewall rules

Next on our agenda, is to add some additional rules on our firewall. We already enabled IP Forward feature with the above and now it is time to update our firewall so it can masquerade our client’s network traffic.

Default Route

We need to identify the default external network interface on the ubuntu server.

DEF_ROUTE=$(ip -4 -json route list default | jq  -r .[0].dev)

usually is eth0 or ensp3 or something similar.

firewall rules

1. We need to forward traffic from the internal interface to the external interface
2. We need to masquerade the traffic of the client.

iptables -A FORWARD -i %i -o ${DEF_ROUTE} -s ${NETWORK}.0/24 -j ACCEPT

iptables -t nat -A POSTROUTING -s ${NETWORK}.0/24 -o ${DEF_ROUTE} -j MASQUERADE

and also we need to drop these rules when we stop wireguard service

iptables -D FORWARD -i %i -o ${DEF_ROUTE} -s ${NETWORK}.0/24 -j ACCEPT

iptables -t nat -D POSTROUTING -s ${NETWORK}.0/24 -o ${DEF_ROUTE} -j MASQUERADE

See the -D after iptables.

iptables is a CLI (command line interface) for netfilters. So think the above rules as filters on our network traffic.

WG Server Conf

To put everything all together and also use wireguard PostUp and PreDown to

wg genkey | tee /etc/wireguard/privatekey | wg pubkey > /etc/wireguard/publickey

chmod 0400 /etc/wireguard/*key

PRIVATE_KEY=$(cat /etc/wireguard/privatekey)
NETWORK=10.0.8
DEF_ROUTE=$(ip -4 -json route list default | jq  -r .[0].dev)
WG_PORT=61194

cat > /etc/wireguard/wg0.conf <<EOF
[Interface]
Address    = ${NETWORK}.1/24
ListenPort = ${WG_PORT}
PrivateKey = ${PRIVATE_KEY}

PreUp    = sysctl -w net.ipv4.ip_forward=1
PostDown = sysctl -w net.ipv4.ip_forward=0

PostUp  = iptables -A FORWARD -i %i -o ${DEF_ROUTE} -s ${NETWORK}.0/24 -j ACCEPT ; iptables -t nat -A POSTROUTING -s ${NETWORK}.0/24 -o ${DEF_ROUTE} -j MASQUERADE
PreDown = iptables -D FORWARD -i %i -o ${DEF_ROUTE} -s ${NETWORK}.0/24 -j ACCEPT ; iptables -t nat -D POSTROUTING -s ${NETWORK}.0/24 -o ${DEF_ROUTE} -j MASQUERADE

EOF

testing up

wg-quick up wg0

[#] sysctl -w net.ipv4.ip_forward=1
net.ipv4.ip_forward = 1
[#] ip link add wg0 type wireguard
[#] wg setconf wg0 /dev/fd/63
[#] ip -4 address add 10.0.8.1/24 dev wg0
[#] ip link set mtu 1420 up dev wg0
[#] iptables -A FORWARD -i wg0 -o ens3 -s 10.0.8.0/24 -j ACCEPT ; iptables -t nat -A POSTROUTING -s 10.0.8.0/24 -o ens3 -j MASQUERADE

testing down

wg-quick down wg0

[#] iptables -D FORWARD -i wg0 -o ens3 -s 10.0.8.0/24 -j ACCEPT ; iptables -t nat -D POSTROUTING -s 10.0.8.0/24 -o ens3 -j MASQUERADE
[#] ip link delete dev wg0
[#] sysctl -w net.ipv4.ip_forward=0
net.ipv4.ip_forward = 0

Start at boot time

systemctl enable wg-quick@wg0

and status

systemctl status wg-quick@wg0

Get the wireguard server public key

We have NOT finished yet with the server !

Also we need the output of:

cat /etc/wireguard/publickey

output should be something like this:

wr3jUAs2qdQ1Oaxbs1aA0qrNjogY6uqDpLop54WtQI4=

Wireguard Client - Ubuntu 20.04 LTS

Now we need to move to our client virtual machine.
It is a lot easier but similar with above commands.

Client Key Generation

Generate a random private and public key for the client

wg genkey | tee /etc/wireguard/privatekey | wg pubkey > /etc/wireguard/publickey

Make the keys read-only

chmod 0400 /etc/wireguard/*key

Wireguard Client Configuration File

Need to replace <wireguard server public key> with the output of the above command.

Configuration is similar but simpler from the server

PRIVATE_KEY=$(cat /etc/wireguard/privatekey)
NETWORK=10.0.8
WG_PORT=61194
WGS_IP=$(ip -4 -json route list default | jq -r .[0].prefsrc)
WSG_PUBLIC_KEY="<wireguard server public key>"

cat > /etc/wireguard/wg0.conf <<EOF
[Interface]
Address    = ${NETWORK}.2/24
PrivateKey = ${PRIVATE_KEY}

[Peer]
PublicKey  = ${WSG_PUBLIC_KEY}
Endpoint   = ${WGS_IP}:${WG_PORT}
AllowedIPs = 0.0.0.0/0

EOF

verify

wg-quick up wg0

[#] ip link add wg0 type wireguard
[#] wg setconf wg0 /dev/fd/63
[#] ip -4 address add 10.0.8.2/24 dev wg0
[#] ip link set mtu 1420 up dev wg0
[#] wg set wg0 fwmark 51820
[#] ip -4 route add 0.0.0.0/0 dev wg0 table 51820
[#] ip -4 rule add not fwmark 51820 table 51820
[#] ip -4 rule add table main suppress_prefixlength 0
[#] sysctl -q net.ipv4.conf.all.src_valid_mark=1
[#] iptables-restore -n

start client at boot

systemctl enable wg-quick@wg0

Get the wireguard client public key

We have finished with the client, but we need the output of:

cat /etc/wireguard/publickey

Wireguard Server - Peers

As we mentioned above, we need to exchange public keys of server & client to the other machines in order to encrypt network traffic.

So as we get the client public key and run the below script to the server

WSC_PUBLIC_KEY="<wireguard client public key>"
NETWORK=10.0.8

wg set wg0 peer ${WSC_PUBLIC_KEY} allowed-ips ${NETWORK}.2

after that we can verify that our wireguard server and connect to the wireguard client

wireguard server ping to client

$ ping -c 5 10.0.8.2

PING 10.0.8.2 (10.0.8.2) 56(84) bytes of data.
64 bytes from 10.0.8.2: icmp_seq=1 ttl=64 time=0.714 ms
64 bytes from 10.0.8.2: icmp_seq=2 ttl=64 time=0.456 ms
64 bytes from 10.0.8.2: icmp_seq=3 ttl=64 time=0.557 ms
64 bytes from 10.0.8.2: icmp_seq=4 ttl=64 time=0.620 ms
64 bytes from 10.0.8.2: icmp_seq=5 ttl=64 time=0.563 ms

--- 10.0.8.2 ping statistics ---
5 packets transmitted, 5 received, 0% packet loss, time 4103ms
rtt min/avg/max/mdev = 0.456/0.582/0.714/0.084 ms

wireguard client ping to server

$ ping -c 5 10.0.8.1

PING 10.0.8.1 (10.0.8.1) 56(84) bytes of data.
64 bytes from 10.0.8.1: icmp_seq=1 ttl=64 time=0.752 ms
64 bytes from 10.0.8.1: icmp_seq=2 ttl=64 time=0.639 ms
64 bytes from 10.0.8.1: icmp_seq=3 ttl=64 time=0.622 ms
64 bytes from 10.0.8.1: icmp_seq=4 ttl=64 time=0.625 ms
64 bytes from 10.0.8.1: icmp_seq=5 ttl=64 time=0.597 ms

--- 10.0.8.1 ping statistics ---
5 packets transmitted, 5 received, 0% packet loss, time 4093ms
rtt min/avg/max/mdev = 0.597/0.647/0.752/0.054 ms

wireguard server - Peers at configuration file

Now the final things is to update our wireguard server with the client Peer (or peers).

wg showconf wg0

the output should be something like that:

[Interface]
ListenPort = 61194
PrivateKey = <server: private key>

[Peer]
PublicKey = <client: public key>
AllowedIPs = 10.0.8.2/32
Endpoint = 192.168.122.68:52587

We need to append the peer section to our configuration file.

so /etc/wireguard/wg0.conf should look like this:

[Interface]
Address    = 10.0.8.1/24
ListenPort = 61194
PrivateKey = <server: private key>

PreUp    = sysctl -w net.ipv4.ip_forward=1
PostDown = sysctl -w net.ipv4.ip_forward=0

PostUp  = iptables -A FORWARD -i %i -o ens3 -s 10.0.8.0/24 -j ACCEPT ; iptables -t nat -A POSTROUTING -s 10.0.8.0/24 -o ens3 -j MASQUERADE
PreDown = iptables -D FORWARD -i %i -o ens3 -s 10.0.8.0/24 -j ACCEPT ; iptables -t nat -D POSTROUTING -s 10.0.8.0/24 -o ens3 -j MASQUERADE

[Peer]
PublicKey = <client: public key>
AllowedIPs = 10.0.8.2/32
Endpoint = 192.168.122.68:52587

SaveConfig

In None of server or client wireguard configuration file, we didn’t declare this option. If set, then the configuration is saved on the current state of wg0 interface. Very useful !

SaveConfig = true

client firewall

It is time to introduce our twist!

If you mtr or traceroute our traffic from our client, we will notice that in need our network traffic goes through the wireguard vpn server !

               My traceroute  [v0.93]
wgc (10.0.8.2)                        2021-07-21T22:41:44+0300

                          Packets               Pings
 Host                Loss%   Snt   Last   Avg  Best  Wrst StDev
 1. 10.0.8.1         0.0%    88    0.6   0.6   0.4   0.8   0.1
 2. myhomepc         0.0%    88    0.8   0.8   0.5   1.0   0.1
 3. _gateway         0.0%    88    3.8   4.0   3.3   9.6   0.8
 ...

The first entry is:

1. 10.0.8.1  

if we drop our vpn connection.

wg-quick down wg0

We still go to the internet.

                     My traceroute  [v0.93]
wgc (192.168.122.68)                     2021-07-21T22:45:04+0300

                            Packets               Pings
 Host                   Loss%   Snt   Last   Avg  Best  Wrst StDev
 1. myhomepc             0.0%     1    0.3   0.3   0.3   0.3   0.0
 2. _gateway             0.0%     1    3.3   3.3   3.3   3.3   0.0

This is important because in some use cases, we do not want our client to directly or unsupervised talk to the internet.

UFW alternative for the client

So to avoid this issue, we will re-rewrite our firewall rules.

A simple script to do that is the below. Declares the default policy to DENY for everything and only accepts ssh incoming traffic and outgoing traffic through the vpn.

DEF_ROUTE=$(ip -4 -json route list default | jq -r .[0].dev)
WGS_IP=192.168.122.69
WG_PORT=61194

## reset
ufw --force reset

## deny everything!

ufw default deny incoming
ufw default deny outgoing
ufw default deny forward

## allow ssh
ufw allow 22/tcp

## enable
ufw --force enable

## allow traffic out to the vpn server
ufw allow out on ${DEF_ROUTE} to ${WGS_IP} port ${WG_PORT}

## allow tunnel traffic out
ufw allow out on wg+

# ufw status verbose

Status: active
Logging: on (low)
Default: deny (incoming), deny (outgoing), disabled (routed)
New profiles: skip

To                         Action      From
--                         ------      ----
22/tcp                     ALLOW IN    Anywhere
22/tcp (v6)                ALLOW IN    Anywhere (v6)             

192.168.122.69 61194       ALLOW OUT   Anywhere on ens3
Anywhere                   ALLOW OUT   Anywhere on wg+
Anywhere (v6)              ALLOW OUT   Anywhere (v6) on wg+

DNS

There is caveat. Please bare with me.

Usually and especially in virtual machines they get DNS setting through a local LAN. We can either allow traffic on the local vlan or update our local DNS setting so it can go only through our VPN

cat > /etc/systemd/resolved.conf <<EOF
[Resolve]
DNS=88.198.92.222

EOF

systemctl restart systemd-resolved

let’s try this

# ping google.com
ping: google.com: Temporary failure in name resolution

Correct, fire up wireguard vpn client

wg-quick up wg0

ping -c4 google.com

PING google.com (142.250.185.238) 56(84) bytes of data.
64 bytes from fra16s53-in-f14.1e100.net (142.250.185.238): icmp_seq=1 ttl=115 time=43.5 ms
64 bytes from fra16s53-in-f14.1e100.net (142.250.185.238): icmp_seq=2 ttl=115 time=44.9 ms
64 bytes from fra16s53-in-f14.1e100.net (142.250.185.238): icmp_seq=3 ttl=115 time=43.8 ms
64 bytes from fra16s53-in-f14.1e100.net (142.250.185.238): icmp_seq=4 ttl=115 time=43.0 ms

--- google.com ping statistics ---
4 packets transmitted, 4 received, 0% packet loss, time 3004ms
rtt min/avg/max/mdev = 42.990/43.795/44.923/0.707 ms

mtr shows:

mtr google.com

                My traceroute  [v0.93]
wgc (10.0.8.2)                          2021-07-21T23:01:16+0300

                                        Packets               Pings
 Host                                 Loss%   Snt   Last   Avg  Best  Wrst StDev
 1. 10.0.8.1                           0.0%     2    0.6   0.7   0.6   0.7   0.1
 2. _gateway                           0.0%     1    0.8   0.8   0.8   0.8   0.0
 3. 192.168.1.1                        0.0%     1    3.8   3.8   3.8   3.8   0.0

drop vpn connection

# wg-quick down wg0 

[#] ip -4 rule delete table 51820
[#] ip -4 rule delete table main suppress_prefixlength 0
[#] ip link delete dev wg0
[#] iptables-restore -n

# ping -c4 google.com
ping: google.com: Temporary failure in name resolution

and that is perfect !

No internet access for our client. Only when our vpn connection is up!

WireGuard: fast, modern, secure VPN tunnel

That’s it!