Routing table entry appears to be ignored on my Mac

Question

I have a pretty standard setup at home, which consists of a single subnet 192.168.240.0/24, and my DSL router sits on 192.168.240.1. Other devices occupy 192.168.240.64 onwards.

But I've now added a Linux server on 192.168.240.16, which has two NICs. One obviously sat on 192.168.240.0/24 and another sitting on 192.168.241.0/24 (192.168.241.16).

At the moment, there is nothing else on 192.168.241.0/24, but I intend to move my DSL modem onto this subnet in the near future.

I'm trying to get routing configured on the Linux box between the two interfaces, and I believe I have that working. But before I move the DSL modem, I thought I'd make sure that routing was working.

I checked the routing table on the Mac (using netstat -r) before I did anything, which (minus irrelevant entries) showed as:

Destination        Gateway           Flags        Refs      Use   Netif Expire
default            192.168.240.1     UGSc         4665        0     en0
192.168.240        link#4            UCS             8        0     en0

So I then configured a static route on the Mac as follows:

route add -net 192.168.241.0/24 192.168.240.16

Then the routing table on the Mac showed:

Destination        Gateway           Flags        Refs      Use   Netif Expire
default            192.168.240.1     UGSc         4665        0     en0
192.168.240        link#4            UCS             8        0     en0
192.168.241        192.168.240.16    UGSc            0        1     en0

When I then attempted to ping the second interface on my Linux box from the Mac, I received no reply. Checking the routing tables again, to my amazement, an extra line appeared.

Destination        Gateway           Flags        Refs      Use   Netif Expire
default            192.168.240.1     UGSc         4665        0     en0
192.168.240        link#4            UCS             8        0     en0
192.168.241        192.168.240.16    UGSc            0        1     en0
192.168.241.16     192.168.240.1     UGHDI           0       17     en0

Why did this last line appear, and why did it not honour the route I'd created previously? What do I need to do to force it to use the static route I've created?

Answer 1

I think the D in UGHDI means dynamic. The Linux box told the Mac that it didn't know a route to that subnet and told it that it would be better to route that data to the DSL router (because that is the Linux box's default gateway and so assumed to know how to route everywhere).

This implies the Linux box isn't routing data between it's NICs. I'd check it's configuration.

Answer 2

It looks like you need to not only add the new subnet, but also add a new default route which appears to be what your doing wrong.

Anecdotally, I frequently have trouble with basic name resolution and networking on Apple devices (ssh hostname fails with "couldn't resolve host hostname" but nslookup hostname returns the IP), so this doesn't surprise me in the least.