Ubuntu – Two routes for localnet, how to delete the one without GW

gatewayroutingUbuntu

I have a system were I get sporadic routing issues. When looking into the routing table I get the following:

localnet        static.xx.xx.x  255.255.255.192 UG    0      0        0 eth0
localnet        *               255.255.255.192 U     0      0        0 eth0

The first one is correct since "static.xx.xx.x" is the gateway for the local net.
For security reasons the local net is only reachable via this gateway.

What is the correct syntax for "route del …" to delete the second route? It expects me to specify a GW in the command.

Where is this route set? In /etc/network/interfaces I set the correct route on startup.

Thanks for your input on this!

Edit:
Output of "route -n" as requestet:

root@lb01:~# route -n
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
0.0.0.0         aa.b.10.214     0.0.0.0         UG    100    0        0 eth0
aa.b.10.192     aa.b.10.193     255.255.255.192 UG    0      0        0 eth0
aa.b.10.192     0.0.0.0         255.255.255.192 U     0      0        0 eth0
192.168.1.0     0.0.0.0         255.255.255.0   U     0      0        0 eth1

Best Answer

If you have an IP address on a subnet, which is what I assume you mean by local network, you will also have a route for it, intrinsically, because you have an interface on it. You can't direct all traffic to that network through a gateway because the gateway would be on that subnet too, and you need to have a route to the gateway.

Although this is absolutely terrible network architecture, you should be able to set the interface with an IP address and a subnet mast of /32. Then, add a static /32 route for the gateway on that interface, and a static /26 route for your network:

ip addr add 192.0.2.40/32 dev eth0
ip route add 192.0.2.1/32 dev eth0
ip route add 192.0.2.0/26 via 192.0.2.1 dev eth0

Of course you would have to do that without any IP addresses or routes configured on the interface. I don't recall the syntax for the debian /etc/network/interfaces file in any detail just now, but the way to configure that there should be obvious.

The problem with doing that, of course, is that other hosts on that network will not know to send traffic to your host through that gateway, because they will have a route for 192.0.2.0/26 on the link as well. This is why we use different subnets for hosts that should communicate through a router. Since you have a router anyway, set up a /30 subnet for this special host and use that instead.

For reference's sake, don't use net-tools stuff like route and ifconfig; they are ancient and there are lots of little weird things that confuse them. Use iproute2 instead. The iproute2 command to delete a route is simply to delete it, specifying the route as clearly as possible:

ip route del 192.0.2.0/26 dev eth0

It may let you do that, or it may not. Certainly, after doing it you won't have any network access on that host, if it succeeds.

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