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If I add a route with

ip route add 172.10.1.0/24 via 10.0.0.100 dev eth0

and then execute the same command a second time, it fails because the route is already present.

I don't see that behavior documented anywhere. Is it safe to depend on this behavior, or might different / future versions of Linux allow duplicate routes?

use-case

I'm writing a bash script that configures the routing table. I want to make it safe to run the script multiple times.

I see two options:

  • Put the ip route add command within an if statement that runs ip route list and uses regex to check if the route is already in place.

  • Just run ip route add and ignore if it fails because the route already exists.

The first doesn't seem very robust, as I have to depend on the output format of ip route list. The second seems to depend on undocumented behavior.

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  • 1
    No. You can add multiple routes just now if they have different metrics: for m in 1 2 3; do ip route add 172.10.1.0/24 via 10.0.0.100 dev eth0 metric $m; done. A route with a metric != 0 is still a route. You don't have to use ip list | grep regex (yuck), you can check for the existence of a route explicitly: just replace the add with list in your command. Commented Dec 4, 2019 at 21:50

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You can use ip route replace instead of add.

This takes the same parameters as add but does not fail, when the route currently exists.

As man ip-route tells us

ip route replace
change or add new one

it will silently add the route when it is not already set and "change"(replace) it with whatever you specify.

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  • This is very nice! Thanks a lot. Too bad the same does not exist ip rule. Commented Dec 4, 2019 at 22:59

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