[ale] Does Linux discover routes by magic? Does Traceroute fib?

[Alan Hightower]

Is the Sonicwall router NAT'ing?  If so you wont see an echo by design.

-Alan

Sent from my iPhone 4S

On Jan 17, 2013, at 12:41 PM, Neal Rhodes <neal at mnopltd.com> wrote:

> Maybe this is basic linux routing that I've been oblivious to all these years. 
> 
> Picture a local lan with a primary Sonicwall router at 192.168.220.1, 
> and a little Cisco router to handle one side of a T1 circuit sitting at 192.168.220.254, and the network on the other side of the T1 is xxx.yyy.47.0. 
> 
> The servers have nothing but the most minimal routing: 
> Kernel IP routing table on EV4:
> Destination     Gateway         Genmask         Flags   MSS Window  irtt Iface
> 192.168.220.0   0.0.0.0         255.255.255.0   U         0 0          0 eth0
> 169.254.0.0     0.0.0.0         255.255.0.0     U         0 0          0 eth0
> 0.0.0.0         192.168.220.1   0.0.0.0         UG        0 0          0 eth0
> 
> When we traceroute to a host on the other side of the T1...
> traceroute to xxx.yyy.47.9 (xxx.yyy.47.9), 30 hops max, 60 byte packets
> 1  192.168.220.254 (192.168.220.254)  2.043 ms  2.055 ms  2.128 ms
> 2  xxx.yyy.42.129 (xxx.yyy.42.129)  20.148 ms  20.974 ms  22.040 ms
> 3  xxx.yyy.41.41 (xxx.yyy.41.41)  23.397 ms  24.409 ms  25.533 ms
> 4  * * *
> 5  xxx.yyy.47.9 (xxx.yyy.47.9)  30.735 ms  31.734 ms  33.216 ms
> 
> one gets the impression that it never hit its default gateway at 192.168.220.1, but somehow figured out that the .254 router could get it there. 
> 
> Is that accurate? Or was there an access to the default gateway that isn't shown by the traceroute?    Looking at several other hosts, including our own,  it looks like traceroute or mtr to "google.com" never shows me hitting our local gateway.    If I had two different routes, two local routers, how would I know which one it took? 
> 
> 
> What I'm trying to do is configure the routes such that access through this T1 be independent of failure of the primary Sonicwall router.    But adding a specific route for the xxx.yyy LAN appears to have no effect, as least from what I'm seeing. 
> 
> Is there a layer deeper that I should be looking at?  Does the gateway just not count as a hop?  The route cache shows no reference to xxx.yyy. 
> 
> BTW, this is a Centos 6.x (mumble) server.    (2.6.32-279.1.1.el6.x86_64 #1 SMP)
> 
> Regards, 
> Neal
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