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Routing — How Linux Sends Network Traffic

Routing is the process of determining the best path for network packets to travel from a source to a destination. Every Linux server, cloud instance, Kubernetes node, and network device relies on routing tables to decide where packets should be sent. Understanding routing is essential for Linux administrators, DevOps engineers, Cloud Architects, Network Engineers, and Site Reliability Engineers (SREs).

Learning Path

Linux Mastery → Module 8: Networking → Lesson 4

Difficulty: Beginner → Intermediate

Reading Time: 70 Minutes

Course Progress

Course: Linux Mastery

Module: Networking

Lesson: 4 of 13

What You'll Learn

After completing this lesson, you'll be able to:

  • Understand routing
  • Learn how routing tables work
  • Understand default gateways
  • Configure static routes
  • View routing information
  • Troubleshoot routing issues
  • Apply routing concepts in production

Prerequisites

Complete:

  • Module 1 – Linux Fundamentals
  • Module 2 – Linux Command Line Essentials
  • Module 3 – Text Processing
  • Module 4 – File Management
  • Module 5 – Users and Groups
  • Module 6 – Process Management
  • Module 7 – Package Management
  • Module 8 Lessons 1–3

Why Learn Routing?

Imagine:

  • A Linux server cannot reach another network.
  • A Kubernetes node cannot communicate with another node.
  • SSH works locally but fails across subnets.
  • A cloud VM has Internet access but cannot reach a private network.

In many cases, the cause is an incorrect routing configuration.

What is Routing?

Routing is the process of forwarding network packets from one network to another.

A router or a Linux system examines the destination IP address and determines the next hop based on its routing table.

How Routing Works

Application
      │
      ▼
Destination IP
      │
      ▼
Routing Table Lookup
      │
      ▼
Next Hop Selected
      │
      ▼
Packet Sent

What is a Routing Table?

A routing table is a list of routes that tells the operating system how to reach different networks.

Each route typically contains:

  • Destination network
  • Gateway (next hop)
  • Network interface
  • Metric (priority)

View the Routing Table

Display all routes.

ip route

Example:

default via 192.168.1.1 dev ens160

192.168.1.0/24 dev ens160 proto kernel

Understanding a Route

Example:

192.168.10.0/24 via 192.168.1.1 dev ens160

Meaning:

  • Destination network:
192.168.10.0/24
  • Next hop:
192.168.1.1
  • Interface:
ens160

Default Route

The default route is used when no more specific route matches the destination.

Example:

default via 192.168.1.1

Without a default route, a system typically cannot communicate with external networks such as the Internet.

Routing Decision Process

Linux searches the routing table using the longest prefix match.

Example:

Destination

↓

192.168.1.50

↓

Matches

192.168.1.0/24

↓

Forward Packet

The most specific matching route is selected.

Add a Temporary Route

Add a route to another network.

sudo ip route add 10.10.0.0/16 via 192.168.1.1

Verify:

ip route

This route is temporary and will not survive a reboot.

Delete a Route

sudo ip route del 10.10.0.0/16

Replace the Default Route

sudo ip route replace default via 192.168.1.254

Persistent Routes

Routes added with the ip command are temporary.

Persistent routes are configured using the operating system's networking tools.

Examples:

  • Ubuntu: Netplan
  • RHEL / Rocky / AlmaLinux: NetworkManager (nmcli) or network configuration files

Route Lookup

Check which route Linux will use.

ip route get 8.8.8.8

Example:

8.8.8.8 via 192.168.1.1 dev ens160

This command is extremely useful when troubleshooting routing decisions.

Enable IP Forwarding

Linux can act as a router by forwarding packets.

Temporarily enable IP forwarding.

sudo sysctl -w net.ipv4.ip_forward=1

Verify:

sysctl net.ipv4.ip_forward

For permanent configuration, update the appropriate sysctl configuration file.

Common Commands

Display routing table.

ip route

View a specific route.

ip route get 8.8.8.8

Add route.

sudo ip route add 10.10.0.0/16 via 192.168.1.1

Delete route.

sudo ip route del 10.10.0.0/16

Enable IP forwarding.

sudo sysctl -w net.ipv4.ip_forward=1

Real Production Examples

Verify default gateway.

ip route

Check Internet route.

ip route get 8.8.8.8

Add a route to a private subnet.

sudo ip route add 172.16.0.0/16 via 192.168.1.1

Enable packet forwarding on a gateway server.

sudo sysctl -w net.ipv4.ip_forward=1

Production Perspective

Routing is essential for:

  • Cloud virtual machines
  • Kubernetes clusters
  • VPN gateways
  • Firewalls
  • Load balancers
  • Multi-subnet environments
  • Hybrid cloud networks
  • Enterprise data centers

Incorrect routing is one of the most common causes of network connectivity problems.

Hands-on Lab

Task 1

Display the routing table.

ip route

Task 2

Identify the default route.

ip route

Task 3

Determine the route to Google's DNS server.

ip route get 8.8.8.8

Task 4

Add a temporary static route.

sudo ip route add 10.10.0.0/16 via 192.168.1.1

Replace the gateway with one appropriate for your network.

Task 5

Verify the new route.

ip route

Task 6

Delete the temporary route.

sudo ip route del 10.10.0.0/16

Task 7

Check whether IP forwarding is enabled.

sysctl net.ipv4.ip_forward

Task 8

Enable IP forwarding temporarily.

sudo sysctl -w net.ipv4.ip_forward=1

Command Deep Dive

Command Purpose Production Example
ip route Display routing table Verify routes
ip route get Show route to a destination Troubleshooting
ip route add Add static route Multi-network connectivity
ip route del Delete route Cleanup
ip route replace Replace default route Gateway updates
sysctl net.ipv4.ip_forward Check IP forwarding Router verification

Static vs Dynamic Routing

Feature Static Routing Dynamic Routing
Configuration Manual Automatic
Scalability Small networks Large networks
Maintenance Higher Lower
Complexity Simple More complex
Common Use Small LANs, cloud VMs Enterprise networks

Dynamic routing commonly uses protocols such as:

  • OSPF
  • BGP
  • RIP
  • EIGRP (Cisco)

Linux servers typically use static routes unless participating in a routing infrastructure.

Production Troubleshooting Scenario

Scenario

A Linux application server cannot communicate with a database located in another subnet.

Investigation:

Check the routing table.

ip route

Verify the route.

ip route get 10.20.30.40

Output:

Network unreachable

Add the required route.

sudo ip route add 10.20.30.0/24 via 192.168.1.1

Verify:

ip route get 10.20.30.40

The route now exists, and the application can communicate with the database.

A permanent route is then configured using the operating system's network management tools.

Best Practices

  • Use the ip command instead of the deprecated route command.
  • Verify the routing table before troubleshooting applications.
  • Configure persistent routes through supported network management tools.
  • Use static routes only when necessary.
  • Document custom routes in production environments.
  • Verify routing after network configuration changes.

Common Mistakes

❌ Forgetting that routes added with ip route add are temporary.

✅ Remember to that routes added with ip route add are temporary.

❌ Configuring an incorrect default gateway.

✅ Avoid this mistake: configuring an incorrect default gateway.

❌ Adding overlapping or conflicting routes.

✅ Avoid this mistake: adding overlapping or conflicting routes.

❌ Ignoring routing when troubleshooting connectivity issues.

✅ Always review routing when troubleshooting connectivity issues.

Interview Questions

Beginner

  1. What is routing?
  2. What is a routing table?
  3. What is the purpose of a default gateway?
  4. Which command displays the routing table?

Intermediate

  1. What is the difference between a static route and a dynamic route?
  2. How do you add a temporary route?
  3. What does ip route get do?
  4. What is IP forwarding?

Architect Level

  1. How would you design routing for a multi-subnet cloud environment?
  2. How would you troubleshoot communication between Kubernetes worker nodes?
  3. Why is the longest prefix match important in routing decisions?

Summary

In this lesson, you learned:

  • Routing fundamentals
  • Routing tables
  • Default gateways
  • Static routes
  • Route lookup
  • IP forwarding
  • Route troubleshooting
  • Production networking best practices

Routing determines how Linux systems send packets across networks. A solid understanding of routing tables, gateways, and static routes enables administrators to build reliable, scalable, and well-connected infrastructure.

Key Takeaways

  • Routing determines the path packets take to reach their destination.
  • The routing table contains information about reachable networks.
  • The default gateway handles traffic for unknown networks.
  • Use ip route to view and manage routes.
  • Use ip route get to understand routing decisions.
  • Routes added with the ip command are temporary unless configured persistently.

What's Next?

ping — Testing Network Connectivity in Linux

You'll explore:

  • How ICMP works
  • Testing network connectivity
  • Measuring latency
  • Understanding packet loss
  • Common ping options
  • Troubleshooting network issues
  • Production best practices

The ping command is one of the most widely used tools for verifying network connectivity and diagnosing communication problems.