How to Use the Command `ip route get` (with examples)

The ip route get command is a powerful tool that allows users to inspect the kernel routing table to determine the exact route that packets would follow to reach a specific destination. This command is particularly useful for network administrators and IT professionals seeking to diagnose network routing problems, analyze network paths, or configure complex network setups. The command’s versatility makes it a fundamental utility in network management and troubleshooting, revealing the path a packet will take based on various criteria such as source address, incoming interface, or specific routing tables.

Use case 1: Print route to a destination

Code:

ip route get 1.1.1.1

Motivation: The primary motivation for using this command is to quickly ascertain which path data packets will take to reach a given IP address. This is critical for network diagnostics, enabling administrators to ensure that routing pathways are correctly configured and operational without packet loss or misrouting.

Explanation:

• ip route get: This command is used to inquire about the routing path from the routing table.
• 1.1.1.1: This is the destination IP address for which the route is being queried. It’s a common practice to use known IP addresses like 1.1.1.1 (Cloudflare’s DNS) for such checks due to their reliability.

Example output:

1.1.1.1 via 192.168.1.1 dev eth0 src 192.168.1.2
    cache

Use case 2: Print route to a destination from a specific source address

Code:

ip route get 1.1.1.1 from 192.168.1.2

Motivation: This use case is crucial when multiple interfaces or network paths are available. It allows the administrator to determine how a packet originating from a specific IP address will traverse the network. This is beneficial for troubleshooting or configuring source-based routing where different paths are determined by the source address.

Explanation:

• from 192.168.1.2: Specifies the source address of the packet. This is particularly useful in environments with complex routing rules that depend on the source of the packet.

Example output:

1.1.1.1 via 192.168.1.1 dev eth0 src 192.168.1.2
    cache

Use case 3: Print route to a destination for packets arriving on a specific interface

Code:

ip route get 1.1.1.1 iif eth0

Motivation: Determining the route for packets arriving on a specific interface is essential for network setups with multiple entry points or paths. This is particularly useful in diagnosing routing issues in multi-homed devices or when packets arrive from different networks/interfaces.

Explanation:

• iif eth0: Specifies the incoming interface the packet arrived on. This helps determine how the routing is handled when packets enter the network through a specific interface.

Example output:

1.1.1.1 via 192.168.1.3 dev eth0 src 192.168.1.2
    cache

Use case 4: Print route to a destination, forcing output through a specific interface

Code:

ip route get 1.1.1.1 oif eth1

Motivation: In environments where policy-based routing is being used or where there are multiple outbound interfaces, it’s critical to control outbound traffic’s exit point. This use case allows you to test and configure such scenarios, ensuring that packets are exiting through the correct interface as expected, especially when managing bandwidth or fulfilling security policies.

Explanation:

• oif eth1: Specifies the outgoing interface through which the packet should be routed. This enforces traffic to be directed through a particular interface.

Example output:

1.1.1.1 via 192.168.2.1 dev eth1 src 192.168.2.2
    cache

Use case 5: Print route to a destination with a specified Type of Service (ToS)

Code:

ip route get 1.1.1.1 tos 0x10

Motivation: Using ToS (Type of Service) allows administrators to define the priority of the packets. Checking the route with specific ToS settings is important for implementing QoS (Quality of Service) policies that prioritize traffic. This ensures compliance with organizational policies or SLAs (Service Level Agreements) concerning network performance and reliability.

Explanation:

• tos 0x10: Specifies the Type of Service field used in the packet, determining its priority and handling across the network.

Example output:

1.1.1.1 via 192.168.1.1 dev eth0 src 192.168.1.2 tos 0x10
    cache

Use case 6: Print route to a destination using a specific VRF (Virtual Routing and Forwarding) instance

Code:

ip route get 1.1.1.1 vrf myvrf

Motivation: Using VRFs allows network segmentation within the same physical infrastructure, useful in large-scale environments or multi-tenant configurations. Checking routes through a specific VRF can help validate configuration and ensure traffic isolation and proper routing between virtual networks.

Explanation:

• vrf myvrf: Specifies the VRF instance used for the routing lookup, separating routing rules for different virtualized environments.

Example output:

1.1.1.1 via 10.0.0.1 dev vrf0 src 10.0.0.2
    cache

Conclusion:

The ip route get command is an indispensable tool in network administration for diagnosing and configuring network paths in various environments. Whether you are checking a simple route to a destination or a more complex scenario involving multiple interfaces or VRFs, it offers clear insights into how traffic is handled and facilitates effective network management. Understanding and using its different options allow for a more robust network configuration and aid in troubleshooting issues quickly and efficiently.