Skip to main content

Chapter 7.2 - Routing and Routing Tables

Learning Objectives​

By the end of this chapter, you will be able to:

  • Understand Linux routing
  • View and modify routing tables
  • Configure the default route
  • Understand static routes
  • Troubleshoot routing

What is Routing?​

Routing determines how network packets are sent from a source to a destination.

Function:

  • Decide where to send packets
  • Based on routing tables
  • Default route for unknown destinations

Routing Tables​

Viewing Routes​

# View all routes
ip route show
ip route
route -n  # With net-tools

# View specific route
ip route get 8.8.8.8

Structure of a Route​

destination via gateway dev interface

Example:

default via 192.168.1.1 dev eth0
192.168.1.0/24 dev eth0 proto kernel scope link src 192.168.1.100

Default Route​

Viewing the Default Route​

# With ip
ip route show default
ip route | grep default

# With route
route -n | grep "^0.0.0.0"

Adding a Default Route​

# With ip
sudo ip route add default via 192.168.1.1 dev eth0

# With route
sudo route add default gw 192.168.1.1 eth0

Removing the Default Route​

# With ip
sudo ip route del default

# With route
sudo route del default

Static Routes​

Adding a Route​

# Route to specific network
sudo ip route add 10.0.0.0/24 via 192.168.1.1 dev eth0

# Route to specific host
sudo ip route add 10.0.0.50 via 192.168.1.1 dev eth0

# Route via local interface
sudo ip route add 172.16.0.0/16 dev eth1

Removing a Route​

# Remove route
sudo ip route del 10.0.0.0/24

# Or specify completely
sudo ip route del 10.0.0.0/24 via 192.168.1.1 dev eth0

Examples​

# Route to private network via VPN
sudo ip route add 192.168.100.0/24 via 10.8.0.1 dev tun0

# Route to specific server
sudo ip route add 203.0.113.50 via 192.168.1.1 dev eth0

Multiple Routing Tables​

Viewing the Tables​

# List of tables
ip route show table all

# Specific table
ip route show table main
ip route show table local

Special Tables​

  • main: Main table (default)
  • local: Local routes (127.0.0.0/8, etc.)
  • default: Default routes

Adding Route in Specific Table​

# Route in custom table
sudo ip route add 10.0.0.0/24 via 192.168.1.1 table 100

# View table
ip route show table 100

Permanent Configuration​

Ubuntu/Debian (Netplan)​

# /etc/netplan/01-netcfg.yaml
network:
  version: 2
  ethernets:
    eth0:
      addresses:
        - 192.168.1.100/24
      routes:
        - to: 10.0.0.0/24
          via: 192.168.1.1
        - to: 0.0.0.0/0
          via: 192.168.1.1
      nameservers:
        addresses:
          - 8.8.8.8

Apply:

sudo netplan apply

Red Hat/CentOS​

# File: /etc/sysconfig/network-scripts/route-eth0
10.0.0.0/24 via 192.168.1.1 dev eth0
default via 192.168.1.1 dev eth0

Troubleshooting​

Traceroute​

# Trace the path
traceroute 8.8.8.8
tracepath 8.8.8.8  # With iputils

# With mtr (install if necessary)
sudo apt install mtr
mtr 8.8.8.8

Check Connection​

# Ping
ping -c 4 8.8.8.8
ping -c 4 google.com

# See used route
ip route get 8.8.8.8

Check Default Route​

# Default route
ip route show default

# Gateway accessible?
ping -c 2 $(ip route | grep default | awk '{print $3}')

Practical Examples​

Routing Configuration Script​

#!/bin/bash
# Routing configuration

INTERFACE="eth0"
GATEWAY="192.168.1.1"

# Default route
sudo ip route add default via $GATEWAY dev $INTERFACE

# Route to private network
sudo ip route add 10.0.0.0/24 via $GATEWAY dev $INTERFACE

# Verify
ip route show

Routing Verification​

#!/bin/bash
# Verify routing configuration

echo "=== Routes ==="
ip route show

echo "=== Default Route ==="
ip route show default

echo "=== Test Connectivity ==="
ping -c 2 8.8.8.8

Summary​

In this chapter, you learned:

Routing: Decision of sending network packets
Tables: main, local, custom tables
Default Route: Gateway for unknown destinations
Static Routes: Specific routes to networks/hosts
Configuration: Netplan, route-* files
Troubleshooting: traceroute, ping, ip route get

Next Steps​

Chapter 7.3: DNS and Name Resolution
Chapter 7.4: Network Tools

Chapter created on: December 2024