pinging and routing tables

pinging and routing tables

[sinkyh]

Hello all,

I have, I hope, a simple question. From my understanding, by default  
ping looks up up the IP address in the Linux kernel routing tables.  
What happens if the IP is not in the routing table? Is the ping packet  
simply dropped?

The reason I ask this is that I have a few wireless devices for  
testing purposes and I am using iptables to mimic a particular  
topology. Mainly forcing the presence of only two possible routes from  
a source to a destination over a series of hops. These devices have  
their own routing protocol and basically routes are added/ deleted  
from the Linux kernel routing tables based on metric measurements bla  
bla bla. That is just fine and normal.

However, since the devices are all in range of each other (due to  
space restriction), if lets say the destination node IP is temporarily  
lost from the source's routing table, for some reason the source is  
still able to ping the destination directly. I assumed that if an IP  
is not in the source's routing table that the outgoing ping would be  
dropped/ ignored until the IP reappears. Not continue on ignoring  
routing tables....

Can anyone shed some light on this?

Thanks for your help and suggestions.

Re: pinging and routing tables

[Mart Frauenlob]

sinkyh@onid.orst.edu wrote:
> Hello all,
> 
> I have, I hope, a simple question. From my understanding, by default 
> ping looks up up the IP address in the Linux kernel routing tables. What 
> happens if the IP is not in the routing table? Is the ping packet simply 
> dropped?
> 
> The reason I ask this is that I have a few wireless devices for testing 
> purposes and I am using iptables to mimic a particular topology. Mainly 
> forcing the presence of only two possible routes from a source to a 
> destination over a series of hops. These devices have their own routing 
> protocol and basically routes are added/ deleted from the Linux kernel 
> routing tables based on metric measurements bla bla bla. That is just 
> fine and normal.
> 
> However, since the devices are all in range of each other (due to space 
> restriction), if lets say the destination node IP is temporarily lost 
> from the source's routing table, for some reason the source is still 
> able to ping the destination directly. I assumed that if an IP is not in 
> the source's routing table that the outgoing ping would be dropped/ 
> ignored until the IP reappears. Not continue on ignoring routing tables....
> 
> Can anyone shed some light on this?
> 
> Thanks for your help and suggestions.

Hello,

without being a network/routing guru, imho if there's not route to host 
X, you should see a message like 'no route to host'...
Did you flush the cache?
`ip route flush cache` - should clear it, so old records are removed.

btw: bit offtopic here.... (not netfilter, tc, related)

Hope it helps.

regards

Mart