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Wednesday, 1 August 2018

How to Configure Linux Machine as a Router

setup-linux-machine-as-a-router

A router is a networking device that forwards data packets between computer networks. Routers perform the traffic directing functions on the Internet. We can transform a Linux machine into a Virtual Router, if we have two interfaces on the Linux machine, and each interface is connected to a different network.

The interface connected to our local network is called Private interface. Whereas, the interface connected to the outer world is called Public interface.

In this post, we will setup a CentOS 7 machine as a Virtual Router.

 

Table of Contents:

setup-linux-machine-as-a-router

System Specification:

We have configured a CentOS 7 virtual machine with following specification.

  • Operating System - CentOS 7
  • Hostname - ipaserver.example.com
  • Private Interface - eno16777728
  • Public Interface - eno33554968

 

Configure Private Interface:

Connect to the ipaserver.example.com and configure network interfaces.

Check status of network devices.

# nmcli device status
DEVICE       TYPE      STATE         CONNECTION
eno16777728  ethernet  disconnected  --
eno33554968  ethernet  disconnected  --
lo           loopback  unmanaged     --

Configure Private Interface with necessary settings for the Router setup.

# nmcli connection add con-name prv0 ifname eno16777728 type ethernet autoconnect yes ip4 192.168.113.10/24 gw4 192.168.113.10
Connection 'prv0' (0f5bebd6-e737-48ba-a34e-0c272a365982) successfully added.
# nmcli connection modify prv0 ipv4.method manual ipv4.dns 192.168.113.10 ipv6.method ignore
# nmcli connection modify prv0 ipv4.never-default yes
# nmcli connection modify prv0 connection.zone internal
# nmcli connection down prv0 ; nmcli connection up prv0
Connection successfully activated (D-Bus active path: /org/freedesktop/NetworkManager/ActiveConnection/3)

 

Configure Public Interface:

Check status of network devices.

# nmcli device status
DEVICE       TYPE      STATE         CONNECTION
eno16777728  ethernet  connected     prv0
eno33554968  ethernet  disconnected  --
lo           loopback  unmanaged     --

Configure Public Interface with necessary settings for the Router setup.

# nmcli connection add con-name pub0 ifname eno33554968 type ethernet autoconnect yes ip4 192.168.116.50/24 gw4 192.168.116.2
Connection 'pub0' (0f4bebd6-e717-49ca-a33e-0c272a336982) successfully added.
# nmcli connection modify pub0 ipv4.method manual ipv4.dns 192.168.116.2 ipv6.method ignore
# nmcli connection modify pub0 connection.zone external
# nmcli connection down pub0 ; nmcli connection up pub0
Connection successfully activated (D-Bus active path: /org/freedesktop/NetworkManager/ActiveConnection/2)

 

Configure Firewall:

Set internal zone as the default zone of the firewall.

# firewall-cmd --set-default-zone=internal
success

Check status of Firewall.

# firewall-cmd --list-all
internal (default, active)
  interfaces: eno16777728
  sources:
  services: dhcpv6-client ipp-client mdns samba-client ssh
  ports:
  masquerade: no
  forward-ports:
  icmp-blocks:
  rich rules:

# firewall-cmd --list-all --zone=external
external (active)
  interfaces: eno33554968
  sources:
  services: ssh
  ports:
  masquerade: yes
  forward-ports:
  icmp-blocks:
  rich rules:

Both interfaces are in their relevant zones.

Make sure that the IP Forwarding is enabled in Kernel settings.

# sysctl -a | grep ip_forward
net.ipv4.ip_forward = 1

 

Test Router Configuration:

Connect to a client machine client2.example.com in your private network and set the default gateway as follows.

# nmcli c a con-name eno16777728 ifname eno16777728 autoconnect yes type ethernet ip4 192.168.113.11/24 gw4 192.168.113.10

Use the tracepath command to check the network path in used now.

# tracepath 8.8.8.8
 1:  192.168.113.11                                        0.075ms pmtu 1500
 1:  192.168.113.10                                        0.403ms
 1:  192.168.113.10                                        0.178ms
 2:  192.168.116.2                                         0.328ms
 3:  no reply
 4:  no reply

It shows that our Red Hat Enterprise Linux (RHEL) 7 machine has been successfully configured as a Virtual Router.

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