Home-made PC Router and NAS Server [Page 3] - Router

Creating a router

Create network

Before starting, keep your server with its WAN socket connected to your existing LAN (as per previous page). Only plug the Internet into your router server once it is fully configured from below.

Previous versions of Ubuntu and other distros used /etc/network/interfaces for configuring the network

This has been replaced with a new method - netplan - and the following setup has been changed to use netplan instead.

First, let's remind ourselves about the two network card interfaces:

Our WAN interface (for me known as enp4s0) connects to the Internet and will be set to receive an IP address and gateway automatically (DHCP), whereas our LAN interface (for me known as enp2s0) will have an IP address specified (static, no DHCP).

To avoid any conflicts with your existing LAN setup during configuration, we'll configure our new setup to use 10.0.1.x addresses. This is within the local reserved IPV4 range, but I've not seen any home routers use them (usually they are 192.168.x.x or 10.0.0.x range).

During the setup, the WAN port should be plugged into our existing LAN, so the network card which will be the new WAN port, will receive an IP of say 192.168.0.100 (or similar) from our old router.

Netplan is setup during the install and is modified by using a YAML file.

YAML (YAML Ain't Markup Language) is a data serialisation language often used for configuration files, and netplan uses it. The important thing to note is that indentation (whitespace) matters, and tabs cannot be used to do that indentation.

I'm editing the default one that Ubuntu server comes configured with - /etc/netplan/50-cloud-init.yaml

sudo nano /etc/netplan/50-cloud-init.yaml

The following configuration is what I use for my setup. If you configured the network during install, it should look very similar. Substitute in your correct network interfaces and preferred static IP address for the LAN. I added the additional 'optional' attribute set to true, so that I can still boot if either connection is down. The WAN adaptor also has DNS name servers set to the Google DNS IPs 8.8.8.8 and 8.8.4.4 (rather than accepting the DNS my Internet provider ISP gives me).

# This file is generated from information provided by
# the datasource.  Changes to it will not persist across an instance.
# To disable cloud-init's network configuration capabilities, write a file
# /etc/cloud/cloud.cfg.d/99-disable-network-config.cfg with the following:
# network: {config: disabled}
network:
    ethernets:
        enp2s0:
            addresses:
            - 10.0.1.2/8
            dhcp4: false
            nameservers:
                addresses: []
                search: []
            optional: true
        enp4s0:
            addresses: []
            dhcp4: true
            nameservers:
                addresses: [8.8.8.8,8.8.4.4]
            optional: true
    version: 2

Let's try it:

sudo netplan --debug generate
sudo netplan apply
ip address

The output of ip address should show an IP assigned by our old router on enp4s0, and the static address 10.0.1.2 on enp2s0

NAT - enable forwarding

For port forwarding, I will continue to use the kernel and iptables to do this. In theory netplan can also do it, but I could not get it to work.

To enable port forwarding (permanently), edit /etc/sysctl.conf

sudo nano /etc/sysctl.conf

Uncomment the line (remove hash) #net.ipv4.ip_forward=1

# Uncomment the next line to enable packet forwarding for IPv4
net.ipv4.ip_forward=1

# Uncomment the next line to enable packet forwarding for IPv6
#  Enabling this option disables Stateless Address Autoconfiguration
#  based on Router Advertisements for this host
#net.ipv6.conf.all.forwarding=1

Save and Exit (Ctrl+O, Ctrl+X)

Reload the parameters by running:

sudo sysctl -p

NAT - IP tables

Since moving to netplan, the previous setup using if-pre-up.d scripts no longer works. To restore the iptable rules during boot and network disconnect/reconnects, iptables-persistent works fine.

It does need to be installed though, and these commands will do that:

sudo apt-add-repository universe
sudo apt install iptables-persistent

Now create an iptables script:

sudo nano /etc/iptables/rules.v4

Here is an example of a 'vanilla' script that will do the router tasks like a dedicated router would when brought from the store. That is, traffic from inside your LAN if it is DNS and DHCP can reach both the server, LAN traffic is forwarded to the WAN (Internet), but no traffic on the WAN can reach your server or any other machines on your LAN.

# -----------------------------------------
#  Enabling NAT
# -----------------------------------------
*nat
:POSTROUTING ACCEPT [0:0]
# enp4s0 is WAN interface, enp2s0 is LAN interface
-A POSTROUTING -o enp4s0 -j MASQUERADE

COMMIT

*filter
:INPUT ACCEPT [0:0]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]

# -----------------------------------------
#  Allowing DNS and DHCP client access
# -----------------------------------------
# basic global accept rules - ICMP, loopback, traceroute, established all accepted
-A INPUT -s 127.0.0.0/8 -d 127.0.0.0/8 -i lo -j ACCEPT
-A INPUT -p icmp -j ACCEPT
-A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT

# enable traceroute rejections to get sent out
-A INPUT -p udp -m udp --dport 33434:33523 -j REJECT --reject-with icmp-port-unreachable

# DNS - accept from LAN
-A INPUT -i enp2s0 -p tcp --dport 53 -j ACCEPT
-A INPUT -i enp2s0 -p udp --dport 53 -j ACCEPT

# DHCP client requests - accept from LAN
-A INPUT -i enp2s0 -p udp --dport 67:68 -j ACCEPT

# SSH - accept from LAN
-A INPUT -i enp2s0 -p tcp --dport 22 -j ACCEPT

# drop all other inbound traffic
-A INPUT -j DROP

# -----------------------------------------
#  Allowing traffic out to the Internet
# -----------------------------------------
# Forwarding rules
# forward packets along established/related connections
-A FORWARD -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT

# forward from LAN (enp2s0) to WAN (enp4s0)
-A FORWARD -i enp2s0 -o enp4s0 -j ACCEPT

# drop all other forwarded traffic
-A FORWARD -j DROP

COMMIT

To expand it further, such as opening ports for specific services as well as forwarding VPN traffic, here is mine in full. It shows:

  1. How I forward Open VPN to the host machine, for VPN access
  2. How I configured VOIP to forward to my VOIP ATA, only from the IP of my VOIP provider (the -s parameter has an example of the IP address of my VOIP provider, 141.0.0.0)
# -----------------------------------------
#  Enabling NAT
# -----------------------------------------
*nat
:POSTROUTING ACCEPT [0:0]
# enp4s0 is WAN interface, enp2s0 is LAN interface
-A POSTROUTING -o enp4s0 -j MASQUERADE

# NAT holes: Open all VOIP ports to the voip
-A PREROUTING -p udp -m udp -i enp4s0 -s 141.0.0.0 --dport 50601 -j DNAT --to-destination 10.0.1.118
-A PREROUTING -p udp -m udp -i enp4s0 -s 141.0.0.0 --dport 5004 -j DNAT --to-destination 10.0.1.118

# Allow traffic from OpenVPN client to LAN and WAN interface
-A POSTROUTING -s 10.8.0.0/24 -o enp2s0 -j MASQUERADE
-A POSTROUTING -s 10.8.0.0/24 -o enp4s0 -j MASQUERADE

COMMIT

*filter
:INPUT ACCEPT [0:0]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]

# -----------------------------------------
#  Allowing pings and traceroutes (LAN and WAN)
# -----------------------------------------
# basic global accept rules - ICMP, loopback, traceroute, established all accepted
-A INPUT -s 127.0.0.0/8 -d 127.0.0.0/8 -i lo -j ACCEPT
-A INPUT -p icmp -j ACCEPT
-A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT

# enable traceroute rejections to get sent out
-A INPUT -p udp -m udp --dport 33434:33523 -j REJECT --reject-with icmp-port-unreachable

# -----------------------------------------
#  Ports that are open to the LAN only
# -----------------------------------------
# DNS - accept from LAN
-A INPUT -i enp2s0 -p tcp --dport 53 -j ACCEPT
-A INPUT -i enp2s0 -p udp --dport 53 -j ACCEPT

# DHCP client requests - accept from LAN
-A INPUT -i enp2s0 -p udp --dport 67:68 -j ACCEPT

# SSH - accept from LAN
-A INPUT -i enp2s0 -p tcp --dport 22 -j ACCEPT

# Samba - accept from LAN
-A INPUT -i enp2s0 -m state --state NEW -p tcp --dport 137 -j ACCEPT
-A INPUT -i enp2s0 -m state --state NEW -p tcp --dport 138 -j ACCEPT
-A INPUT -i enp2s0 -m state --state NEW -p tcp --dport 139 -j ACCEPT
-A INPUT -i enp2s0 -m state --state NEW -p tcp --dport 445 -j ACCEPT

# -----------------------------------------
#  Ports that are open to the WAN (Internet)
# -----------------------------------------
# openvpn - accept from WAN
-A INPUT -i enp4s0 -m state --state NEW -m udp -p udp --dport 1194 -j ACCEPT
# openvpn - accept all incoming traffic from the vpn tun0 interface
-A INPUT -i tun0 -j ACCEPT

# drop all other inbound traffic
-A INPUT -j DROP

# -----------------------------------------
#  Allowing traffic out to the Internet
# -----------------------------------------
# Forwarding rules

# forward packets along established/related connections
-A FORWARD -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT

# forward from LAN (enp2s0) to WAN (enp4s0)
-A FORWARD -i enp2s0 -o enp4s0 -j ACCEPT

# -----------------------------------------
#  Allowing traffic to reach another system on the LAN (e.g. VOIP)
# -----------------------------------------
# VOIP traffic allow
-A FORWARD -p udp -d 10.0.1.118 --dport 50601 -j ACCEPT
-A FORWARD -p udp -d 10.0.1.118 --dport 5004 -j ACCEPT

# -----------------------------------------
#  Allowing traffic from Open VPN to reach LAN and WAN
# -----------------------------------------
# Allow forwarding from OpenVPN LAN
-A FORWARD -i tun0 -j ACCEPT

# drop all other forwarded traffic
-A FORWARD -j DROP

COMMIT

To apply the rules immediately, run iptables-restore:

sudo iptables-restore /etc/iptables/rules.v4

That completes the IP Tables setup.

You now have a router!

DHCP server

At this stage, any client that needs to use the router, and plugged into the LAN network card, would have to have a static IP defined, and also manually entering the gateway and DNS name servers.

We can use the server to run a DHCP server though. To install the DHCP server, use:

sudo apt-get install isc-dhcp-server
sudo nano /etc/default/isc-dhcp-server

Set your interface here, this will be the LAN interface, in my case:

INTERFACESv4="enp2s0"
INTERFACESv6=""
sudo nano /etc/dhcp/dhcpd.conf

Change the options near the top (I'm using Google's free DNS nameservers, which are fast):

option domain-name "youname.home";

option domain-name-servers 8.8.8.8, 8.8.4.4;

authoritative;      (uncomment this)

Add this next section to the bottom. Note IPs:

  • 10.0.1.2 is the server and gateway host LAN IP (static)
  • 10.0.1.100 to 10.0.1.199 allows 99 clients (laptops, phones, TVs, consoles, all the rest ...!)
  • I've used 10.0.1.118 as an IP reserved for my VOIP ATA, using its specific MAC address. This is to ensure the port forwarding is always going to the right place (IP address). If you don't need this, remove the whole block host voip.
subnet 10.0.1.0 netmask 255.255.255.0 {
    range 10.0.1.100 10.0.1.199;
    option routers 10.0.1.2;
    option domain-name-servers 8.8.8.8, 8.8.4.4;
    option broadcast-address 10.0.1.255;

    host voip {
        hardware ethernet 00:0b:82:88:24:7d;
        fixed-address 10.0.1.118;
        ddns-hostname "voip";
    }
}

To apply:

sudo service isc-dhcp-server start

To check if it started OK:

service isc-dhcp-server status

If it fails for any reason, more detailed messages may be seen by checking 'journalctl -xe'

At this stage, you can try plugging in a client into the LAN interface (i.e. with a straight cable to a laptop). It should automatically assign itself an IP address.

Here is ipconfig from a Windows client (my laptop):

Ethernet adapter Ethernet:

   Connection-specific DNS Suffix  . : portal.home
   IPv4 Address. . . . . . . . . . . : 10.0.1.100
   Subnet Mask . . . . . . . . . . . : 255.255.255.0
   Default Gateway . . . . . . . . . : 10.0.1.2

Finally, I found with the netplan setup and Ubuntu 18.04, after a machine restart or cold boot, the DHCP service was not working.

This was found to be because the DHCP service was starting before the network interfaces were fully up. Upon checking service dependencies, they seemed fine so a quick solution to it is to add a delay to the start of the DHCP service. 15 seconds is enough and works reliably. Feel free to test restarts without this fix though in case the bug has been fixed since.

sudo nano /lib/systemd/system/isc-dhcp-server.service

Find the line starting 'exec dhcpd ...' and add 'sleep 15;' in front:

[Unit]
Description=ISC DHCP IPv4 server
Documentation=man:dhcpd(8)
Wants=network-online.target cloud-config.service
After=network-online.target cloud-config.service
After=time-sync.target
ConditionPathExists=/etc/default/isc-dhcp-server
ConditionPathExists=|/etc/ltsp/dhcpd.conf
ConditionPathExists=|/etc/dhcp/dhcpd.conf

[Service]
EnvironmentFile=/etc/default/isc-dhcp-server
RuntimeDirectory=dhcp-server
# The leases files need to be root:dhcpd even when dropping privileges
ExecStart=/bin/sh -ec '\
    CONFIG_FILE=/etc/dhcp/dhcpd.conf; \
    if [ -f /etc/ltsp/dhcpd.conf ]; then CONFIG_FILE=/etc/ltsp/dhcpd.conf; fi; \
    [ -e /var/lib/dhcp/dhcpd.leases ] || touch /var/lib/dhcp/dhcpd.leases; \
    chown root:dhcpd /var/lib/dhcp /var/lib/dhcp/dhcpd.leases; \
    chmod 775 /var/lib/dhcp ; chmod 664 /var/lib/dhcp/dhcpd.leases; \
    sleep 15; exec dhcpd -user dhcpd -group dhcpd -f -4 -pf /run/dhcp-server/dhcpd.pid -cf $CONFIG_FILE $INTERFACES'

[Install]
WantedBy=multi-user.target

Test by issuing:

sudo systemctl daemon-reload
sudo service isc-dhcp-server start
sudo service isc-dhcp-server status

Then do a full reboot and confirm that the service is starting fine and assigning IP addresses to clients connected to the LAN port.

Local DNS

Optionally you can install local DNS caching, which may help with slower Internet connections. This can be done by installing bind9:

sudo apt-get install bind9

You can then edit the DHCP config above and replace Google's IP addresses with the servers own IP (10.0.1.2). And it's that simple.

SSH security

Our SSH setup allows connections from anywhere, password authenticated.

Though we cannot connect to SSH from the Internet unless we open an iptables rule, as added safety it is still good to tell SSH to only listen on the LAN adaptor.

To get SSH from the outside, we can use Open VPN to access the server first, and then SSH to the relevant LAN IP instead.

To tell SSH to only listen on certain IPs, you can edit /etc/ssh/sshd_config

sudo nano /etc/ssh/sshd_config
ListenAddress 10.0.1.2

Uncomment the ListenAddress line and put the static IP for the LAN address (10.0.1.2 in my case). SSH will need a restart before it takes effect, and since you may already be connected via SSH anyway, it is easiest to restart the machine (otherwise the SSH stop will drop the connection and you cannot reconnect to start again or restart the machine).

You can check whether it works by issuing 'netstat -tunlp':

Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address           Foreign Address         State       PID/Program name
tcp        0      0 10.0.1.2:53             0.0.0.0:*               LISTEN      -
tcp        0      0 188.172.155.73:53       0.0.0.0:*               LISTEN      -
tcp        0      0 127.0.0.1:53            0.0.0.0:*               LISTEN      -
tcp        0      0 10.0.1.2:22             0.0.0.0:*               LISTEN      -
tcp        0      0 127.0.0.1:953           0.0.0.0:*               LISTEN      -
tcp6       0      0 :::53                   :::*                    LISTEN      -
tcp6       0      0 ::1:953                 :::*                    LISTEN      -
udp        0      0 10.0.1.2:53             0.0.0.0:*                           -
udp        0      0 188.172.155.73:53       0.0.0.0:*                           -
udp        0      0 127.0.0.1:53            0.0.0.0:*                           -
udp        0      0 0.0.0.0:68              0.0.0.0:*                           -
udp6       0      0 :::53                   :::*                                -

We can see that in the Local Address column, port 22 (ending with :22) only shows for 10.0.1.2.

0.0.0.0 or ::: indicates a port/service listening on all IP addresses. 127.0.0.1 indicates it is listening on the localhost only.

SSH is now only accessible on the LAN. Take care if assigning a new static IP via SSH though - if you change it and do not update the SSH configuration, you will lose access.

You can go further with SSH security and prevent password authentication. This involves generating a public-private key pair and authenticating with that instead of the password.

The main advantage of this is it prevents brute force attacks as your SSH server would continue to listen for the right password. I would say though just keep it off the Internet unless you really need it.

References and more reading:
Ars Technica - The Ars guide to building a Linux router from scratch
Netplan.io - Examples
Linux Config - How to configure static IP address on Ubuntu 18.04
Server Fault - Ubuntu - dhcp server 'not configured to listen on any interfaces