Routes onlink - missing doc

Installing and Using OpenWrt
1

hello
is it possible to say whats the meaning of the on-link flag here?

image
image1396×184 15.3 KB

thanks

2

On-Link route means one that the addresses that belong to it can be resolved locally and gateway is not needed.
For example a route to 127.0.0.0/8 is on-link via interface lo0.
In your example the use of on-link is wrong.

1 Like
3

duh dont understand :frowning:
do i have to set it to No?

4

That's right.

5

but for a what purpose do you set it Yes/ON?

6

The onlink option is used in very rare cases. When you create a static route, the next hop address must be directly reachable by the specified device, meaning it must be from the same IP subnet. Otherwise you'll get an error response.

Example:

root@OpenWrt:~# ip address show dev br-lan | grep 'inet '
    inet 192.168.92.15/24 brd 192.168.92.255 scope global br-lan
root@OpenWrt:~#
root@OpenWrt:~# ip route add 10.0.0.0/8 via 192.168.100.1 dev br-lan
Error: Nexthop has invalid gateway.

192.168.100.1 is not in the same IP subnet (192.168.92.0/24), so the route was rejected.

Now let's use the onlink option:

root@OpenWrt:~# ip route add 10.0.0.0/8 via 192.168.100.1 dev br-lan onlink
root@OpenWrt:~#
root@OpenWrt:~# ip route show 10.0.0.0/8
10.0.0.0/8 via 192.168.100.1 dev br-lan onlink
root@OpenWrt:~#

Using this option, we tell the kernel that the next hop is located in the same Layer 2 segment.
If a packet must be sent to network 10.0.0.0/8, an ARP request will be generated on the br-lan interface (asking "who has 192.168.1.100"), and the packet will be forwarded to the returned MAC address.

3 Likes
7

thanks for the explanation!