route Command Linux: Configure Network Routing Tables (Deprecated Guide)

The route command in Linux has been a fundamental tool for network administrators to view and manipulate the kernel’s IP routing table. While this command has been deprecated in favor of the more modern ip command, understanding its functionality remains crucial for system administrators working with legacy systems or preparing for certification exams.

What is the route Command?

The route command is part of the net-tools package that allows users to display and modify the kernel’s IP routing table. This table determines how network packets are forwarded between different network interfaces and destinations. The routing table contains information about network destinations, gateways, and network interfaces.

Basic Syntax and Options

The basic syntax of the route command follows this pattern:

route [options] [add|del] [target] [netmask Nm] [gw GW] [metric N] [dev If]

Common Options

• -n: Show numerical addresses instead of resolving hosts
• -v: Verbose output
• -e: Use netstat format for displaying routing table
• -A family: Use specified address family (inet, inet6)
• -C: Display kernel routing cache

Command Actions

• add: Add a new route
• del: Delete an existing route
• -host: Target is a host
• -net: Target is a network

Viewing the Routing Table

The most common use of the route command is to display the current routing table:

$ route
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
default         192.168.1.1     0.0.0.0         UG    100    0        0 eth0
192.168.1.0     0.0.0.0         255.255.255.0   U     100    0        0 eth0
192.168.1.1     0.0.0.0         255.255.255.255 UH    100    0        0 eth0

For numerical output without hostname resolution:

$ route -n
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
0.0.0.0         192.168.1.1     0.0.0.0         UG    100    0        0 eth0
192.168.1.0     0.0.0.0         255.255.255.0   U     100    0        0 eth0
192.168.1.1     0.0.0.0         255.255.255.255 UH    100    0        0 eth0

Understanding Route Table Fields

• Destination: The target network or host
• Gateway: The gateway IP address (0.0.0.0 means no gateway needed)
• Genmask: The network mask for the destination
• Flags: Route flags (U=Up, G=Gateway, H=Host, etc.)
• Metric: Distance to target (lower is preferred)
• Ref: Number of references to this route
• Use: Count of lookups for this route
• Iface: Network interface used for this route

Adding Routes

Adding a Default Gateway

To add a default gateway (route for all traffic):

$ sudo route add default gw 192.168.1.1

This command adds a default route through the gateway 192.168.1.1. All traffic not matching other specific routes will use this gateway.

Adding a Network Route

To add a route to a specific network:

$ sudo route add -net 192.168.2.0 netmask 255.255.255.0 gw 192.168.1.10

This adds a route to the 192.168.2.0/24 network through gateway 192.168.1.10.

Adding a Host Route

To add a route to a specific host:

$ sudo route add -host 10.0.0.5 gw 192.168.1.10

This creates a specific route for the host 10.0.0.5 through gateway 192.168.1.10.

Adding Route Through Interface

To add a route directly through a network interface:

$ sudo route add -net 172.16.0.0 netmask 255.255.0.0 dev eth1

Deleting Routes

Deleting Default Gateway

$ sudo route del default

To delete a specific default gateway:

$ sudo route del default gw 192.168.1.1

Deleting Network Routes

$ sudo route del -net 192.168.2.0 netmask 255.255.255.0

Deleting Host Routes

$ sudo route del -host 10.0.0.5

Advanced Examples

Multiple Gateway Configuration

Setting up multiple gateways for different networks:

# Add route for internal network through first gateway
$ sudo route add -net 10.0.0.0 netmask 255.0.0.0 gw 192.168.1.1

# Add route for DMZ network through second gateway  
$ sudo route add -net 172.16.0.0 netmask 255.240.0.0 gw 192.168.1.2

# Verify routes
$ route -n

Setting Route Metrics

Adding routes with specific metrics for load balancing:

$ sudo route add default gw 192.168.1.1 metric 100
$ sudo route add default gw 192.168.1.2 metric 200

Route Flags Explanation

Understanding the flags in the routing table:

• U: Route is up
• G: Route uses a gateway
• H: Route is to a host (not a network)
• R: Route was set up by the routing daemon
• D: Route was set up dynamically by a redirect
• M: Route was modified by a redirect
• A: Route was installed by addrconf
• C: Route was cached
• !: Route is rejected

Troubleshooting with route Command

Common Network Issues

Check if default gateway is configured:

$ route -n | grep '^0.0.0.0'
0.0.0.0         192.168.1.1     0.0.0.0         UG    100    0        0 eth0

Verify specific network routes:

$ route -n | grep '192.168.2'

Route Cache Information

Display kernel routing cache:

$ route -C

Modern Alternative: ip Command

Since the route command is deprecated, here are equivalent ip command alternatives:

Viewing Routes

# route -n equivalent
$ ip route show

# More detailed information
$ ip route show table all

Adding Routes

# Add default gateway
$ sudo ip route add default via 192.168.1.1

# Add network route
$ sudo ip route add 192.168.2.0/24 via 192.168.1.10

# Add host route
$ sudo ip route add 10.0.0.5/32 via 192.168.1.10

Deleting Routes

# Delete default gateway
$ sudo ip route del default

# Delete network route
$ sudo ip route del 192.168.2.0/24

Best Practices and Security Considerations

Route Configuration Best Practices

• Always backup current routing configuration before making changes
• Use specific network masks to avoid routing conflicts
• Document route changes for future reference
• Test connectivity after route modifications
• Use metrics appropriately for load balancing scenarios

Security Implications

• Incorrect routing can lead to traffic interception
• Always verify route destinations and gateways
• Monitor for unauthorized route changes
• Implement proper access controls for route modification

Common Errors and Solutions

Permission Denied

Error: SIOCADDRT: Operation not permitted

Solution: Use sudo for route modifications

Network Unreachable

Error: SIOCADDRT: Network is unreachable

Solution: Ensure the gateway is reachable and interfaces are up

File Exists Error

Error: SIOCADDRT: File exists

Solution: Route already exists, delete existing route first

Migration to ip Command

As the route command is deprecated, system administrators should migrate to using the ip command. The ip command provides more features, better performance, and is actively maintained. Here’s a quick migration reference:

Conclusion

While the route command has been deprecated, understanding its functionality remains valuable for Linux system administrators. The command provides essential network routing capabilities that form the foundation of network configuration in Linux systems. However, modern systems should utilize the ip command from the iproute2 package for better performance, enhanced features, and continued support.

For production environments and new deployments, always prefer the ip command over the deprecated route command. The transition ensures compatibility with modern Linux distributions and access to advanced networking features not available in the legacy net-tools package.