Linux Routing Configuration: ip route Commands and netplan

Linux routing decides which network interface and gateway packets are sent through. The modern tool is the ip command from the iproute2 package.

Essential ip Commands

# Show all IP addresses
ip addr show
ip a  # short form

# Show the routing table
ip route show
ip r  # short form

# Show the route to a specific host
ip route get 8.8.8.8

# ARP table (IP → MAC mapping)
ip neigh show

Temporary Changes (until reboot)

# Add an IP address to an interface
sudo ip addr add 192.168.1.100/24 dev eth0

# Remove an IP address
sudo ip addr del 192.168.1.100/24 dev eth0

# Add a route through a specific gateway
sudo ip route add 10.0.0.0/8 via 192.168.1.1

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

# Remove a route
sudo ip route del 10.0.0.0/8

# Bring the interface up/down
sudo ip link set eth0 up
sudo ip link set eth0 down

Persistent Routes via Netplan (Ubuntu 22.04+)

# /etc/netplan/01-netcfg.yaml
network:
  version: 2
  ethernets:
    eth0:
      dhcp4: false
      addresses:
        - 192.168.1.10/24
      routes:
        - to: default
          via: 192.168.1.1
        - to: 10.0.0.0/8
          via: 10.0.0.1
          metric: 100
      nameservers:
        addresses: [8.8.8.8, 8.8.4.4]

sudo netplan apply

Two Interfaces with Different Gateways

# /etc/netplan/01-netcfg.yaml
network:
  version: 2
  ethernets:
    eth0:                          # Main one (internet)
      dhcp4: false
      addresses: [1.2.3.4/24]
      routes:
        - to: default
          via: 1.2.3.1
    eth1:                          # Private network
      dhcp4: false
      addresses: [10.0.0.10/24]
      routes:
        - to: 10.0.0.0/8
          via: 10.0.0.1

Diagnosing common routing errors

"RTNETLINK answers: Network is unreachable" when adding a route through a gateway means the given gateway is not directly reachable — it is not in the same subnet as the interface address.

sudo ip route add 10.0.0.0/8 via 192.168.1.1
RTNETLINK answers: Network is unreachable

Check that the interface already has an address from the same subnet as the gateway: ip addr show dev eth0. If the gateway is 192.168.1.1 but the interface only has address 10.0.0.5/24, the route cannot be added until an address from network 192.168.1.0/24 appears.

After netplan apply, the internet disappears completely — almost always a YAML indentation mistake (YAML is strict about spaces), or a conflict with NetworkManager/systemd-networkd managing the same interface.

sudo netplan apply
Error in network definition //etc/netplan/01-netcfg.yaml: invalid indentation

Check the syntax before applying: sudo netplan try — the command shows the error and automatically rolls back the change after 120 seconds if you do not confirm it, which saves you from losing SSH access over a typo.

Route configuration methods compared

When static routes are not enough

• The network topology changes often (multiple providers, backup links) — you need dynamic routing (BGP, OSPF) through FRRouting or Bird, not a list of static entries.
• You need automatic failover between providers — a static route does not switch itself when a link goes down; that needs dedicated monitoring mechanisms and scripts or routing protocols.
• The server serves dozens of VLANs with independent routing — manual management through ip route or Netplan becomes hard to maintain, and a dedicated router or FRRouting is worth considering.

Verifying the result

1. Check the applied routes: ip route show must contain the expected entry with the correct gateway and metric.
2. Check connectivity through a specific route: ip route get 10.0.0.5 must show which interface and gateway the packet will use.
3. Confirm the settings survive a reboot: sudo netplan generate && sudo reboot, then run ip route show again.
4. Check both interfaces in a multihomed setup: ping -I eth0 8.8.8.8 and ping -I eth1 10.0.0.1 must both succeed independently.

Before configuring routes it helps to understand subnet masks — see the article on CIDR, and the general address translation mechanism is covered in the article on NAT.