It works because linux (and XP too) maintain a cache of all routes learned.
Try: ip route show cache.
You can clean this cache: ip route flush cache.
From: Andrew McGill <andrewm@xxxxxxxxxxxxx>
To: lartc@xxxxxxxxxxxxxxx
Subject: netmask 255.255.255.255 vs ip route add via ... (bug?)
Date: Tue, 14 Nov 2006 15:48:41 +0200 (SAST)
>Greetings routing folks,
>
>I want to use the netmask 255.255.255.255 to insulate (not quite
>isolate) machines on a shared subnet from each other. This works
>just fine on win XP, but Linux iproute will not acccept the gateway
>address in one step -- neither on the command line nor via DHCP:
>
>Here's the interface, set up with a netmask of /32:
>
> # ip addr
> ...
> 2: eth0: <BROADCAST,MULTICAST,UP> mtu 1500 qdisc pfifo_fast
>qlen 1000
> link/ether 00:08:74:48:1f:0c brd ff:ff:ff:ff:ff:ff
> inet 192.168.1.6/32 brd 192.168.1.255 scope global eth0
> inet6 fe80::208:74ff:fe48:1f0c/64 scope link
> valid_lft forever preferred_lft forever
> ...
>
>And here's me trying to add the route:
>
> # ip route add default via 192.168.1.17
> RTNETLINK answers: Network is unreachable
>
>Hmm ... erk ... workaround ... add a host route first, then add it
>as a default route ...
>
> # sudo ip route add 192.168.1.17 dev eth0
> # sudo ip route add default via 192.168.1.17
>
>And this is what we get ... (yep, it works)
>
> # ip route ls
> 192.168.1.17 dev eth0 scope link
> default via 192.168.1.17 dev eth0
>
>But wait! We can delete the host route! And it works just fine (you
>*can* try this at home folks).
>
> # sudo ip route del 192.168.1.17
> # ip route ls
> default via 192.168.1.17 dev eth0
>
>So why did we need that host route?
>
>It should be possible to add the gateway directly, or it should be
>impossible to delete it once something "depends" on it. The current
>behaviour seems a little unbalanced (and, for my strange purposes,
>inconvenient :)
>
> Tested on Ubuntu 6.06 Dapper (Kernel: 2.6.15, iproute2 20041019)
> Looks the same on Fedora Core 3, (Kernel 2.6.11.8, iproute2
>2.6.9)
>
>&:-)
>
>
>--
>Disclaimer: this disclaimer and your base are us
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