Static routing

A route is a rule set in the kernel that is used to determine which physical network interface or gateway is needed in order to reach a particular network (or single host). There are many types of routed protocols; this article covers routing of the IP protocol in the Linux kernel.

Although IP routes are stored in the kernel, they are modifiable by userspace tools as described in the following examples.

Showing routes

Show the routing table with iproute2:

user $ip route

default via 192.168.1.1 dev wlan1 metric 1
192.168.50.0/24 dev lan proto kernel scope link src 192.168.50.1
127.0.0.0/8 via 127.0.0.1 dev lo 
192.168.1.0/24 dev wlan1 proto kernel scope link src 192.168.1.1

Or show the routing table using sys-apps/net-tools:

user $netstat -rn

Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
192.168.50.0    0.0.0.0         255.255.255.0   U     0      0        0 lan
192.168.1.0     0.0.0.0         255.255.255.0   U     2000   0        0 wlan1
127.0.0.0       0.0.0.0         255.0.0.0       U     0      0        0 lo
0.0.0.0         192.168.1.1     0.0.0.0         UG    2000   0        0 wlan1

Adding a static route

The IP address, subnet mask (CIDR), and gateway are necessary prerequisite information before adding a static route.

In this example the 10.10.10.0 network with a 255.255.255.0 subnet mask will be routed to the 192.168.1.50 gateway. CIDR style netmasks are required when adding routes using commands from the sys-apps/iproute2 package (ip). The following example will add the 10.10.10.0/24 route:

root #ip route add 10.10.10.0/24 via 192.168.1.50

Show the routing table using the ip route command:

user $ip route

default via 192.168.1.1 dev wlan1 metric 1
10.10.10.0/24 dev wlan1 via 192.168.1.50 src 10.10.10.1
192.168.50.0/24 dev lan proto kernel scope link src 192.168.50.1
127.0.0.0/8 via 127.0.0.1 dev lo 
192.168.1.0/24 dev wlan1 proto kernel scope link src 192.168.1.1

Older systems may possibly only have the netstat or route commands (via sys-apps/net-tools) instead of ip used in the above example.

Adding the same static route as described above using the route command:

root #route add -net 10.10.10.0 netmask 255.255.255.0 gw 192.168.1.50

Show routing table using netstat (sys-apps/net-tools):

user $netstat -rn

Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
10.10.10.0      192.168.1.50    255.255.255.0   UG    0      0        0 wlan1
192.168.50.0    0.0.0.0         255.255.255.0   U     0      0        0 lan
192.168.1.0     0.0.0.0         255.255.255.0   U     2000   0        0 wlan1
127.0.0.0       0.0.0.0         255.0.0.0       U     0      0        0 lo
0.0.0.0         192.168.1.1     0.0.0.0         UG    2000   0        0 wlan1

The routing table is sorted from most specific routes to most general. This is how it is read by the routing process. Longest prefix match - means the the smallest network, or the network with the largest netmask, or the most specific route f.e. 255.255.255.255 is at first position in the routing table.

Adding a permanent static route

For users of the netifrc scripts (OpenRC's standard network tools), permanent static routes can be added by opening a preferred text editor to /etc/conf.d/net and adjusting the file accordingly.

Reference the current routing table for help.

FILE /etc/conf.d/net

routes_wlan1="10.10.10.0/24 via 192.168.1.50
    default via 192.168.1.1"

With dhcpcd as network manager the static route goes into /etc/dhcpcd.conf instead.

Both statements above mean:

1. IP packets destined to the 10.10.10.0/24 network are send to 192.168.1.50.
2. IP packets destined to all 0.0.0.0/0 other networks are send to 192.168.1.1.

Note
0.0.0.0/0 means all other networks without a prefix (Subnet mask), the default route.

The default route 0.0.0.0/0 is used if:

• The host has no physical or logical IP interface in the target network segment.
• The host has to send IP packets outside of its own IP network segment, and there is no specific route found in the routing table for target IP network.

See also

• iproute2 — a tool developed to unify network interface configuration, routing, and tunneling for Linux systems.
• Network management — describes possibilities for managing the network stack.

External resources

• Longest prefix match (on Wikipedia)
• Gentoo Bug 5409326