HOWTO: Configuring a router on a Mini-PC with CentOS

Over half an year later, I realized, that I never published my article about operating system and software setup of my Mini-PC router. This is a follow-up post about the Qotom-hardware I wrote earlier. So, its probably about time to do that!

To get the ball rolling on new CentOS installation, a good start is to download it, Rufus it into an USB-stick and install the minimal setup into router-PC. The CentOS installation is so well documented and trivial process, I won't go into any details of it. Read something like Installing Red Hat Enterprise Linux 7.4 on all architectures for details of that.

Goal

Every project needs a goal. In any kind of engineering there is a specification and criteria, that the goal has been met.

The goal of this project is to create a Linux-server capable of securing a local network from the Internet and allow traffic to pass from the LAN to the wild-wild-net.

Spec:

• There is a working CentOS Linux running on the MiniPC
• ISP's cable modem is configured as bridge, no double NATting done
  • MiniPC gets a public IP-address from ISP
  • MiniPC can be accessed from the Net via the IP-address
• Configurations persist a reboot on the MiniPC
• MiniPC issues dynamic IP-addresses to LAN-clients
• MiniPC acts as a caching nameserver to LAN-clients
  • Any requests from the Net are not served
• Wireless access point is configured not do do any routing, aka. it is in access point mode
• The setup is secure with attack surface minimized
• LAN IP-address range is 192.168.1.0/24

Definition of done:

• Internet works!
  • MiniPC can connect to net
  • MiniPC can be connected from net and LAN via SSH
  • Wired clients can connect to net via Ethernet cable without any manual configuration
  • Wireless clients can connec to the net via Wi-Fi without any manual configuration

Step 1: Packages

After minimal installation, the set of tools and packages required includes:
net-tools bind-utils screen tcpdump policycoreutils-python setools

• net-tools: mostly for netstat, using route or ifconfig is deprecated
• bind-utils: for dig and nslookup
• screen: for full-screen window manager
• tcpdump: taking a look into Ethernet and TCP/IP-packages, when something goes wrong, getting detailed view is very important
• policycoreutils-python setools: for managing SELinux

Step 2: Remove NetworkManager

Packages to install: -none needed-

Why a server would have GNOME NetworkManager installed on a server is beyond me. I simply cannot comprehend what CentOS-people are thinking, when they as default treat my server as a laptop. But the main thing is, that this piece of shit needs to go! The quicker, the better!

DANGER!

When you actually run the yum-command to remove NetworkManager, your system will lose all network connectivity. So, please, do run this at a console, not via SSH-connection.

DANGER!

Run command as root on console:

yum erase NetworkManager

Now your system's networking is royally messed up.

Step 3: Setup NICs

Packages to install: -none needed-

Check that NetworkManager created and left ifcfg-files into /etc/sysconfig/network-scripts/. If the appropriate ifcfg-files (one for each interface) are gone, you need to start learning how to write one fast. A good starting point on that would be RedHat Enterprise Linux 7 product documentation, Networking Guide, section 2.2 Editing Network Configuration Files.

LAN interface

Out of the two Ethernet-interfaces, 50/50 coin-flip ended as enp3s0 LAN and enp1s0 WAN. For any practical purposes, it really doesn't matter which one is which, but I'm describing here my setup. If you're using some other hardware, your interface names won't match those.

For any sensible use of your LAN-side, this interface should be connected to a network switch, so that your local network can be shared by your PC, Playstation, TV, Wi-Fi access point or whatever you have there running. Of course you can run it with only one host connected directly to the router.

This is critical: Your LAN-interface MUST have a static IP-address for it. It really cannot act as LAN-side of a router without one.

I chose my LAN to be private IP-range 192.168.1.0/24, so I edited /etc/sysconfig/network-scripts/ifcfg-enp3s0 to contain:

TYPE=Ethernet
BOOTPROTO=none
DEFROUTE=yes
IPV6INIT=yes
NAME=enp3s0
UUID=-don't-touch-this-
DEVICE=enp3s0
ONBOOT=yes

NETWORK=192.168.1.0
BROADCAST=193.168.1.255
USERCTL=no

IPADDR=192.168.1.1
PREFIX=24
IPV4_FAILURE_FATAL=no
IPV6_AUTOCONF=yes
IPV6_DEFROUTE=yes
IPV6_PEERDNS=yes
IPV6_PEERROUTES=yes
IPV6_FAILURE_FATAL=no

WAN interface

For WAN, there is no need to touch enp1s0 setup (much). When your WAN-interface (enp1s0) starts, it will obtain an IP-address from your ISP. With that, it will also get your ISP's DNS-address an overwrite your precious manual changes in /etc/resolv.conf. You don't want that to happen. So, prevent that and edit /etc/sysconfig/network-scripts/ifcfg-enp1s0 and add:

PEERDNS="no"

Well, that was easy!

IP-forwarding

For routing to work, it requires Linux kernel to have IP-forwarding enabled. It will allow network packets to travel between interfaces.

Enable IP-forwarding immediately:

sysctl -w net.ipv4.ip_forward=1

Enable IP-forwarding on boot:

sysctl net.ipv4.ip_forward > /etc/sysctl.d/1_ip_forward.conf

Finalize network setup

When your network interface configurations are ok, restart everything by running following as root:

systemctl enable network
systemctl restart network

Now your system:

• has both interfaces on-line
• is reachable from a machine on your wired-LAN using a static IP-address other than 192.168.1.1.
  Note: your router doesn'ht have DHCPd running yet, so you need to figure out how to configure a static IP-address to your device
• still gets an IP-address from your ISP from your external interface
• can reach IP-addresses via both external and internal interfaces

If these criteria are not met, there is simply no point in proceeding. Your system won't work as a router without those prerequisites.

Finally, make sure that your IPtables-rules have effect. Your box is connected to Internet and it can be accessed/bombarded from there, so run following to secure your setup:

systemctl restart firewalld

Now your system is ready to become a router.

Step 4: Firewalld

Packages to install: -none needed-

Zones

Out-of-box CentOS has firewalld enabled. It has only one zone defined for public wild-wild-net, also TCP/22 SSH is open for the world. This needs to be run as root. First split off LAN into own zone home:

# firewall-cmd --zone home --change-interface enp3s0 --permanent

Check the zones and their assigned interfaces:

# firewall-cmd --get-active-zones
home
  interfaces: enp3s0
public
  interfaces: enp1s0

Setup network address translation (NAT) and allow traffic to flow from your LAN to outside world. Any relevant traffic is allowed to flow in from Internet back to your LAN. Commands to run:

# firewall-cmd --permanent --direct --add-rule ipv4 nat POSTROUTING 0 -o enp1s0 -j MASQUERADE
# firewall-cmd --permanent --direct --add-rule ipv4 filter FWDI_home_allow 0 -o enp1s0 -j ACCEPT
# firewall-cmd --permanent --direct --add-rule ipv4 filter FWDI_public_allow 0 -o enp3s0 -m state --state RELATED,ESTABLISHED -j ACCEPT

Enable the DNS-server we'll setup later, also disable any outgoing DNS-queries from your LAN (a security measure):

# firewall-cmd --permanent --zone home --add-service dns
# firewall-cmd --permanent --direct --add-rule ipv4 filter FWDI_home_deny 0 -p udp -m udp --dport 53 -j REJECT

At this point do a reload:

# firewall-cmd --reload

... and test your firewall setup from router:

• You still must be able to access Internet from your router
• Your LAN does work at this point. A client with a static IP must be able to access Internet.

Step 5: Named

Packages to install: bind-chroot

Out-of-box the BIND 9.9.4 does not server anybody else than localhost. To fix this, find following two lines in /etc/named.conf:

listen-on port 53 { 127.0.0.1; };
allow-query     { localhost; };

Edit them to contain:

listen-on port 53 { 127.0.0.1; 192.168.1.1; };
allow-query     { localhost; 192.168.1.0/24; };

Finally, change your system's default name resolver by editing /etc/resolv.conf to contain a single line:

nameserver 127.0.0.1

Start the server and enable it to start on boot:

systemctl start named-chroot
systemctl enable named-chroot

Now you're ready to test the setup. Just host www.google.com or your favorite site. Successful reply will include IP-address(es) for your query.

Step 6: DHCP

Packages to install: dhcp

ddns-update-style interim;
ignore client-updates;
authoritative;
default-lease-time 14400;
max-lease-time 86400;
option subnet-mask 255.255.255.0;
option broadcast-address 192.168.1.255;
option routers 192.168.1.1;
option domain-name "my.own.lan";
option domain-name-servers 192.168.1.1;
subnet 192.168.1.0 netmask 255.255.255.0 {
        range 192.168.1.50 192.168.1.99;
}

That piece of configuration will use your router as DNS for the clients and issue them addresses from range .50 - .99.

Start the server and enable it to start on boot:

systemctl start dhcpd
systemctl enable dhcpd

At this point, configure your client to use DHCP for IP-addressing. You must get an IP from the above range, also DNS-resolve and NAT should work, but that's the next step to do. Test it all.

Step 7: Testing it

Make sure:

1. A client in your LAN gets an IP-address from DHCP
2. A client in your LAN can ping your router at 192.168.1.1
3. A client in your LAN can ping something in the Internet, like Google's name server at 8.8.8.8
4. A client in your LAN resolves names, for example: nslookup www.google.com returns IP-addresses
5. A client in your LAN can access https://www.google.com/ via a web-browser

That's it! What else would you need?

Done!

Congratulations on your new router!

What I did next was set up my own DNS-zone so that my hosts had FQDNs. But that's beyond this blog post. Read something like How To Configure BIND as a Private Network DNS Server on CentOS 7 by DigitalOcean for that.