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Routing

Routing is the process of determining the path that data packets take from source to destination across a network. On Linux, the kernel maintains a routing table that directs packets to the appropriate network interface or next hop.

Routing Tables

The kernel routing table maps destination networks to next hops and interfaces.

View the routing table with ip:

ip route show

A typical routing table contains:

default via 192.168.1.1 dev eth0
192.168.1.0/24 dev eth0 proto kernel scope link src 192.168.1.10
172.16.0.0/16 via 10.0.0.1 dev eth1
10.0.0.0/8 via 10.0.0.1 dev eth1 metric 100
  • default: The default route (gateway of last resort).
  • 192.168.1.0/24: Directly connected network (no gateway needed).
  • 172.16.0.0/16: Route via next hop 10.0.0.1.

Route Types

  • unicast: Standard point-to-point route.
  • broadcast: Deliver to all hosts on a network.
  • unreachable: Destination is unreachable (ICMP unreachable returned).
  • prohibit: Destination is forbidden (ICMP prohibited returned).
  • local: Destination is assigned to a local interface.
  • blackhole: Packets are silently dropped (used for filtering).
  • throw: Route lookup continues without finding a match (policy routing).

Static Routes

Add a static route:

ip route add 10.0.0.0/8 via 192.168.1.254

Remove a static route:

ip route del 10.0.0.0/8

Dynamic Routing

For complex networks with redundant paths, dynamic routing protocols automatically compute routes:

  • RIP (Routing Information Protocol): Distance-vector protocol, simple but limited to 15 hops.
  • OSPF (Open Shortest Path First): Link-state protocol, fast convergence, suitable for large networks.
  • BGP (Border Gateway Protocol): Path-vector protocol, used for internet routing between autonomous systems.

Linux implementations include quagga (FRR), bird, and bgpd.

Policy Routing

Linux supports policy-based routing, where different routing tables are selected based on source address, firewall mark, or incoming interface.

Create an additional routing table in /etc/iproute2/rt_tables:

200     custom

Add rules:

ip rule add from 192.168.1.10 table custom
ip route add default via 10.0.0.1 table custom

Metrics and Multipath

Routes can have metrics (administrative distance) to prioritise paths:

ip route add default via 192.168.1.1 metric 100
ip route add default via 10.0.0.1 metric 200

ECMP (Equal-Cost Multi-Path) allows multiple next hops for the same destination:

ip route add 192.168.0.0/16 nexthop via 192.168.1.1 weight 1 nexthop via 10.0.0.1 weight 1