Configure Static Routing¶

Overview¶

This guide covers static routing configuration, route management, policy-based routing, and advanced routing concepts in Linux.

Routing Concepts¶

Key Terms¶

Routing: Process of forwarding packets between networks
Static Route: Manually configured route (doesn’t change automatically)
Dynamic Route: Automatically learned route (via routing protocols)
Default Gateway: Route used when no specific route matches
Metric: Cost or priority of a route (lower is preferred)
Next Hop: Next router/gateway to forward packets
Administrative Distance: Route preference (lower is preferred)
Policy Routing: Routing based on source, not just destination

Routing Table Components¶

Destination: Target network
Gateway: Next hop IP address
Genmask/Prefix: Network mask
Interface: Outgoing network interface
Metric: Route priority

Viewing Routing Tables¶

Using `ip route` Command¶

# Show routing table
ip route show
ip route list
ip r

# Show with details
ip route show table all

# Show specific table
ip route show table main
ip route show table local

# Show IPv6 routes
ip -6 route show

# Show route to specific destination
ip route get 8.8.8.8
ip route get 192.168.1.100

# Show cached routes (deprecated in newer kernels)
ip route show cache

Using `route` Command (Legacy)¶

# Show routing table
route -n

# Show with hostname resolution
route

# Show IPv6 routes
route -A inet6 -n

Using `netstat` Command¶

# Show routing table
netstat -r
netstat -rn

# Show IPv6 routes
netstat -rn -A inet6

Routing Table Files¶

# View kernel routing table
cat /proc/net/route

# View IPv6 routing table
cat /proc/net/ipv6_route

# Format: destination, gateway, netmask, flags, metric, ref, use, interface

Managing Static Routes¶

Using `ip route` Command¶

Add Routes¶

# Add route to network via gateway
ip route add 10.0.0.0/8 via 192.168.1.1

# Add route via specific interface
ip route add 10.0.0.0/8 dev eth1

# Add route with both gateway and interface
ip route add 10.0.0.0/8 via 192.168.1.1 dev eth0

# Add default gateway
ip route add default via 192.168.1.1
ip route add 0.0.0.0/0 via 192.168.1.1

# Add route with metric
ip route add 10.0.0.0/8 via 192.168.1.1 metric 100

# Add host route (single IP)
ip route add 10.0.0.5/32 via 192.168.1.1

# Add IPv6 route
ip -6 route add 2001:db8::/32 via 2001:db8::1

# Add IPv6 default gateway
ip -6 route add default via 2001:db8::1

Delete Routes¶

# Delete specific route
ip route del 10.0.0.0/8 via 192.168.1.1

# Delete default gateway
ip route del default

# Delete IPv6 route
ip -6 route del 2001:db8::/32

Replace Routes¶

# Replace existing route
ip route replace 10.0.0.0/8 via 192.168.1.2

# Replace default gateway
ip route replace default via 192.168.1.254

Flush Routes¶

# Flush all routes
ip route flush table main

# Flush routes to specific network
ip route flush 10.0.0.0/8

# Flush cached routes
ip route flush cache

Using `route` Command (Legacy)¶

# Add route
route add -net 10.0.0.0/8 gw 192.168.1.1

# Add default gateway
route add default gw 192.168.1.1

# Add host route
route add -host 10.0.0.5 gw 192.168.1.1

# Delete route
route del -net 10.0.0.0/8

# Delete default gateway
route del default

# Add route via interface
route add -net 10.0.0.0/8 dev eth1

Persistent Static Routes¶

RHEL/CentOS/Fedora¶

Method 1: Network Scripts¶

Create route files: /etc/sysconfig/network-scripts/route-<interface>

# /etc/sysconfig/network-scripts/route-eth0
10.0.0.0/8 via 192.168.1.1
172.16.0.0/12 via 192.168.1.1 dev eth0
default via 192.168.1.254

# Format:
# network/prefix via gateway [dev interface] [metric N]

Alternative format:

# /etc/sysconfig/network-scripts/route-eth0
ADDRESS0=10.0.0.0
NETMASK0=255.0.0.0
GATEWAY0=192.168.1.1

ADDRESS1=172.16.0.0
NETMASK1=255.240.0.0
GATEWAY1=192.168.1.1

Method 2: NetworkManager¶

Using nmcli:

# Add static route to connection
nmcli connection modify eth0 +ipv4.routes "10.0.0.0/8 192.168.1.1"

# Add multiple routes
nmcli connection modify eth0 +ipv4.routes "10.0.0.0/8 192.168.1.1, 172.16.0.0/12 192.168.1.1"

# Add route with metric
nmcli connection modify eth0 +ipv4.routes "10.0.0.0/8 192.168.1.1 100"

# Remove route
nmcli connection modify eth0 -ipv4.routes "10.0.0.0/8 192.168.1.1"

# View routes
nmcli connection show eth0 | grep ipv4.routes

# Apply changes
nmcli connection up eth0

# IPv6 routes
nmcli connection modify eth0 +ipv6.routes "2001:db8::/32 2001:db8::1"

Method 3: NetworkManager Configuration Files¶

Edit: /etc/NetworkManager/system-connections/<connection>.nmconnection

[ipv4]
method=manual
address1=192.168.1.100/24,192.168.1.1
route1=10.0.0.0/8,192.168.1.1
route2=172.16.0.0/12,192.168.1.1,100
dns=8.8.8.8;8.8.4.4;

[ipv6]
method=manual
address1=2001:db8::100/64,2001:db8::1
route1=2001:db8:1::/48,2001:db8::1

Reload NetworkManager:

nmcli connection reload
nmcli connection up eth0

Ubuntu/Debian (Netplan)¶

Edit: /etc/netplan/01-netcfg.yaml

network:
  version: 2
  renderer: networkd
  ethernets:
    eth0:
      addresses:
        - 192.168.1.100/24
      gateway4: 192.168.1.1
      routes:
        - to: 10.0.0.0/8
          via: 192.168.1.1
          metric: 100
        - to: 172.16.0.0/12
          via: 192.168.1.1
        - to: 0.0.0.0/0
          via: 192.168.1.254
          metric: 200
      nameservers:
        addresses:
          - 8.8.8.8
          - 8.8.4.4

IPv6 example:

network:
  version: 2
  ethernets:
    eth0:
      addresses:
        - 2001:db8::100/64
      gateway6: 2001:db8::1
      routes:
        - to: 2001:db8:1::/48
          via: 2001:db8::1

Apply configuration:

netplan try      # Test configuration
netplan apply    # Apply configuration

systemd-networkd¶

Configuration: /etc/systemd/network/

# /etc/systemd/network/20-wired.network
[Match]
Name=eth0

[Network]
Address=192.168.1.100/24
Gateway=192.168.1.1

[Route]
Destination=10.0.0.0/8
Gateway=192.168.1.1
Metric=100

[Route]
Destination=172.16.0.0/12
Gateway=192.168.1.1

[Route]
Destination=0.0.0.0/0
Gateway=192.168.1.254
Metric=200

Restart service:

systemctl restart systemd-networkd

Multiple Default Gateways¶

Metric-Based Selection¶

# Primary default gateway (lower metric)
ip route add default via 192.168.1.1 metric 100

# Backup default gateway (higher metric)
ip route add default via 192.168.2.1 metric 200

# View
ip route show

Multiple Gateways (Load Balancing)¶

# Equal-cost multi-path routing
ip route add default \
    nexthop via 192.168.1.1 dev eth0 weight 1 \
    nexthop via 192.168.2.1 dev eth1 weight 1

Policy-Based Routing¶

Routing Tables¶

View Routing Tables¶

# List all tables
cat /etc/iproute2/rt_tables

# Default tables:
# 0     unspec
# 253   default
# 254   main
# 255   local

# Add custom table
echo "100 custom" >> /etc/iproute2/rt_tables

Manage Routes in Custom Tables¶

# Add route to custom table
ip route add 10.0.0.0/8 via 192.168.1.1 table custom

# Add default gateway to custom table
ip route add default via 192.168.1.1 table custom

# Show routes in custom table
ip route show table custom

# Delete route from table
ip route del 10.0.0.0/8 table custom

Routing Rules (Policy Routing)¶

View Rules¶

# Show routing rules
ip rule show
ip rule list

# Default rules:
# 0: from all lookup local
# 32766: from all lookup main
# 32767: from all lookup default

Add Rules¶

Source-based routing:

# Route traffic from specific source via custom table
ip rule add from 192.168.1.100 table custom priority 100

# Route traffic from network
ip rule add from 192.168.1.0/24 table custom priority 100

Destination-based routing:

# Route traffic to specific destination via custom table
ip rule add to 10.0.0.0/8 table custom priority 100

Interface-based routing:

# Route traffic arriving on interface
ip rule add iif eth0 table custom priority 100

# Route traffic leaving on interface
ip rule add oif eth1 table custom priority 100

TOS-based routing:

# Route based on Type of Service
ip rule add tos 0x10 table custom priority 100

Fwmark-based routing:

# Route based on firewall mark
ip rule add fwmark 1 table custom priority 100

Combined rules:

# Complex rule
ip rule add from 192.168.1.0/24 to 10.0.0.0/8 table custom priority 100

Delete Rules¶

# Delete by specification
ip rule del from 192.168.1.100 table custom

# Delete by priority
ip rule del priority 100

# Flush all rules (dangerous!)
ip rule flush

Complete Policy Routing Example¶

Scenario: Route traffic from different networks through different gateways

# Create custom routing tables
echo "100 isp1" >> /etc/iproute2/rt_tables
echo "200 isp2" >> /etc/iproute2/rt_tables

# Add routes to tables
ip route add default via 10.0.1.1 table isp1
ip route add default via 10.0.2.1 table isp2

# Add routing rules
ip rule add from 192.168.1.0/24 table isp1 priority 100
ip rule add from 192.168.2.0/24 table isp2 priority 200

# Add routes for local networks in both tables
ip route add 192.168.1.0/24 dev eth1 table isp1
ip route add 192.168.2.0/24 dev eth2 table isp2

# Flush routing cache
ip route flush cache

Source-Based Routing Example¶

Route different users through different gateways:

# Setup
echo "100 admin_table" >> /etc/iproute2/rt_tables

# Add default gateway for admin table
ip route add default via 192.168.1.1 table admin_table

# Add local network routes
ip route add 192.168.1.0/24 dev eth0 table admin_table

# Route admin user (192.168.1.100) through specific gateway
ip rule add from 192.168.1.100 table admin_table priority 100

# Verify
ip rule show
ip route show table admin_table

Equal-Cost Multi-Path (ECMP) Routing¶

Load Balancing Between Gateways¶

# Add multi-path default route
ip route add default \
    nexthop via 192.168.1.1 dev eth0 weight 1 \
    nexthop via 192.168.2.1 dev eth1 weight 1

# Unequal weight distribution (2:1 ratio)
ip route add default \
    nexthop via 192.168.1.1 dev eth0 weight 2 \
    nexthop via 192.168.2.1 dev eth1 weight 1

# View
ip route show

Reverse Path Filtering¶

Configure rp_filter¶

# Check current settings
cat /proc/sys/net/ipv4/conf/all/rp_filter
cat /proc/sys/net/ipv4/conf/eth0/rp_filter

# Values:
# 0 = No source validation
# 1 = Strict mode (recommended)
# 2 = Loose mode

# Set temporarily
echo 1 > /proc/sys/net/ipv4/conf/all/rp_filter

# Set permanently
echo "net.ipv4.conf.all.rp_filter = 1" >> /etc/sysctl.conf
echo "net.ipv4.conf.default.rp_filter = 1" >> /etc/sysctl.conf
sysctl -p

Advanced Routing Features¶

Nexthop Objects (Newer Kernels)¶

# Create nexthop object
ip nexthop add id 1 via 192.168.1.1 dev eth0
ip nexthop add id 2 via 192.168.2.1 dev eth1

# Create nexthop group
ip nexthop add id 10 group 1/2

# Use nexthop in route
ip route add 10.0.0.0/8 nhid 10

# View nexthops
ip nexthop show

Route Metrics and Preferences¶

# Lower metric is preferred
ip route add 10.0.0.0/8 via 192.168.1.1 metric 10
ip route add 10.0.0.0/8 via 192.168.2.1 metric 20

# With both routes present, 192.168.1.1 is preferred

Administrative Distance¶

Not directly configurable in Linux, but protocols have default preferences: - Connected: 0 - Static: 1 - OSPF: 110 - RIP: 120

Route Attributes¶

# Add route with specific attributes
ip route add 10.0.0.0/8 via 192.168.1.1 \
    metric 100 \
    mtu 1400 \
    advmss 1360

# Show route with all attributes
ip route show 10.0.0.0/8

Troubleshooting Routing¶

Verify Routes¶

# Check routing table
ip route show

# Test route to destination
ip route get 8.8.8.8
ip route get 10.0.0.5

# Check specific table
ip route show table custom

# Check rules
ip rule show

Trace Route Path¶

# Traceroute
traceroute 8.8.8.8
traceroute -n 8.8.8.8  # No DNS resolution

# MTR (better)
mtr 8.8.8.8
mtr -n 8.8.8.8

Check IP Forwarding¶

# Check if enabled
cat /proc/sys/net/ipv4/ip_forward

# Enable temporarily
echo 1 > /proc/sys/net/ipv4/ip_forward

# Enable permanently
echo "net.ipv4.ip_forward = 1" >> /etc/sysctl.conf
sysctl -p

Debug Routing Issues¶

# Check interface status
ip link show
ip addr show

# Check if gateway is reachable
ping -c 4 192.168.1.1

# Check ARP table
ip neigh show
arp -n

# Monitor routing changes
ip monitor route

# Check routing cache (older kernels)
ip route show cache

Common Issues¶

Issue: No route to host

# Add missing route
ip route add 10.0.0.0/8 via 192.168.1.1

# Or add default gateway
ip route add default via 192.168.1.1

Issue: Asymmetric routing

# May need to disable rp_filter
echo 0 > /proc/sys/net/ipv4/conf/all/rp_filter

# Or use policy routing

Issue: Routes not persisting

# Add to configuration files
# RHEL: /etc/sysconfig/network-scripts/route-*
# Ubuntu: /etc/netplan/*.yaml
# Or use NetworkManager

Routing with Multiple Interfaces¶

Setup Routing Between Interfaces¶

# Enable IP forwarding
sysctl -w net.ipv4.ip_forward=1

# Add routes
ip route add 10.0.0.0/8 via 192.168.1.1 dev eth0
ip route add 172.16.0.0/12 via 192.168.2.1 dev eth1

# Default route
ip route add default via 192.168.1.1 dev eth0

Interface-Specific Routing¶

# Force traffic out specific interface
ip route add 10.0.0.0/8 dev eth1
ip route add 172.16.0.0/12 dev eth0

# Source-based interface selection
ip rule add from 192.168.1.0/24 oif eth0
ip rule add from 192.168.2.0/24 oif eth1

Monitoring Routing¶

Real-Time Monitoring¶

# Monitor route changes
ip monitor route

# Monitor all IP events
ip monitor

# Monitor specific table
ip monitor route table custom

# Watch routing table
watch -n 1 'ip route show'

Routing Statistics¶

# View route cache statistics
ip -s route show cache

# Interface statistics
ip -s link show eth0

# Routing protocol statistics (if running dynamic routing)
vtysh -c "show ip route"

Quick Reference Commands¶

View Routes¶

ip route show                          # Show routing table
ip route show table all                # All tables
ip route get 8.8.8.8                  # Route to destination
ip rule show                          # Show routing rules

Add Routes¶

ip route add 10.0.0.0/8 via 192.168.1.1                    # Basic route
ip route add default via 192.168.1.1                       # Default gateway
ip route add 10.0.0.0/8 via 192.168.1.1 metric 100        # With metric
ip route add 10.0.0.0/8 via 192.168.1.1 table custom      # Custom table

Policy Routing¶

echo "100 custom" >> /etc/iproute2/rt_tables              # Add table
ip route add default via 192.168.1.1 table custom         # Add route to table
ip rule add from 192.168.1.0/24 table custom priority 100 # Add rule

Delete Routes¶

ip route del 10.0.0.0/8               # Delete route
ip route del default                   # Delete default
ip rule del priority 100               # Delete rule

Persistent Configuration¶

# RHEL/CentOS
vim /etc/sysconfig/network-scripts/route-eth0
nmcli connection modify eth0 +ipv4.routes "10.0.0.0/8 192.168.1.1"

# Ubuntu
vim /etc/netplan/01-netcfg.yaml
netplan apply

Practical Examples¶

Example 1: Multi-Homed Host¶

# Host with two network connections
# eth0: 192.168.1.100/24 (ISP1 - gateway 192.168.1.1)
# eth1: 192.168.2.100/24 (ISP2 - gateway 192.168.2.1)

# Setup tables
echo "100 isp1" >> /etc/iproute2/rt_tables
echo "200 isp2" >> /etc/iproute2/rt_tables

# Add routes
ip route add default via 192.168.1.1 table isp1
ip route add default via 192.168.2.1 table isp2
ip route add 192.168.1.0/24 dev eth0 table isp1
ip route add 192.168.2.0/24 dev eth1 table isp2

# Add rules
ip rule add from 192.168.1.100 table isp1
ip rule add from 192.168.2.100 table isp2

# Main table default (for locally generated traffic)
ip route add default via 192.168.1.1 metric 100
ip route add default via 192.168.2.1 metric 200

Example 2: VPN Routing¶

# Route specific traffic through VPN
# VPN interface: tun0, VPN gateway: 10.8.0.1

# Add route for specific network through VPN
ip route add 10.0.0.0/8 via 10.8.0.1 dev tun0

# Or use policy routing
echo "100 vpn" >> /etc/iproute2/rt_tables
ip route add default via 10.8.0.1 dev tun0 table vpn
ip rule add from 192.168.1.100 table vpn

Example 3: DMZ Routing¶

# Router with three interfaces
# eth0: WAN (Internet) - 203.0.113.5
# eth1: LAN - 192.168.1.1/24
# eth2: DMZ - 10.0.0.1/24

# Enable forwarding
echo 1 > /proc/sys/net/ipv4/ip_forward

# Routes
ip route add default via 203.0.113.1 dev eth0
ip route add 192.168.1.0/24 dev eth1
ip route add 10.0.0.0/24 dev eth2

# Allow LAN to DMZ
# (firewall rules needed too)

Exam Tips¶

Know how to add/delete/modify routes with ip route
Understand routing tables and policy-based routing
Be familiar with persistent route configuration
Know how to troubleshoot with ip route get
Understand metrics and route selection
Practice multi-path routing scenarios
Know the difference between runtime and persistent routes
Be comfortable with both RHEL and Debian-based configurations
Understand source-based routing concepts
Know how to verify routing with traceroute/mtr
Remember to enable IP forwarding for routing between interfaces
Practice reading and understanding routing table output

Configure Static Routing¶

Overview¶

Routing Concepts¶

Key Terms¶

Routing Table Components¶

Viewing Routing Tables¶

Using ip route Command¶

Using route Command (Legacy)¶

Using netstat Command¶

Routing Table Files¶

Managing Static Routes¶

Using ip route Command¶

Add Routes¶

Delete Routes¶

Replace Routes¶

Flush Routes¶

Using route Command (Legacy)¶

Persistent Static Routes¶

RHEL/CentOS/Fedora¶

Method 1: Network Scripts¶

Method 2: NetworkManager¶

Method 3: NetworkManager Configuration Files¶

Ubuntu/Debian (Netplan)¶

systemd-networkd¶

Multiple Default Gateways¶

Metric-Based Selection¶

Multiple Gateways (Load Balancing)¶

Policy-Based Routing¶

Routing Tables¶

View Routing Tables¶

Manage Routes in Custom Tables¶

Routing Rules (Policy Routing)¶

View Rules¶

Add Rules¶

Delete Rules¶

Complete Policy Routing Example¶

Source-Based Routing Example¶

Equal-Cost Multi-Path (ECMP) Routing¶

Load Balancing Between Gateways¶

Reverse Path Filtering¶

Configure rp_filter¶

Advanced Routing Features¶

Nexthop Objects (Newer Kernels)¶

Route Metrics and Preferences¶

Administrative Distance¶

Route Attributes¶

Troubleshooting Routing¶

Verify Routes¶

Trace Route Path¶

Check IP Forwarding¶

Debug Routing Issues¶

Common Issues¶

Routing with Multiple Interfaces¶

Setup Routing Between Interfaces¶

Interface-Specific Routing¶

Monitoring Routing¶

Real-Time Monitoring¶

Routing Statistics¶

Quick Reference Commands¶

View Routes¶

Add Routes¶

Policy Routing¶

Delete Routes¶

Persistent Configuration¶

Practical Examples¶

Example 1: Multi-Homed Host¶

Example 2: VPN Routing¶

Example 3: DMZ Routing¶

Exam Tips¶

Using `ip route` Command¶

Using `route` Command (Legacy)¶

Using `netstat` Command¶

Using `ip route` Command¶

Using `route` Command (Legacy)¶