Re: Basic routing

[Dennis Jacobfeuerborn <dennisml@conversis.de>]

On 04.10.2014 04:52, John Smithee wrote:
> Neal Murphy wrote, On 10/04/2014 03:34 AM:
>> On Friday, October 03, 2014 09:10:58 PM John Smithee wrote:
>>> Hi,
>>>
>>> I've 2 NICs on a machine, both attached to seperate networks
>>> (192.168.68.0/24 and 192.168.69.0/24). I'm trying to set up a
>>> basic gateway (or routing?) between the two networks.
>>>
>>> Is this a classical routing issue or has this to be done via iptables?
>>
>> Standard networking, standard routing. Netfilter doesn't enter the
>> equation.
>>
>> The prime directive: every router must have explicit routes to all
>> networks it
>> can reach, except that the default route can eliminate many explicit
>> routes.
>> In other words, "These routes specify how to reach these LANs; packets
>> for all
>> other networks will be sent via the default route if it exists.
>> Packets for
>> which there is no route will be dribbled into the bit bucket." Note
>> the phrase
>> "every router"; it include all of your internal routers as well as your
>> perimeter (default) gateway.
>>
>> For a router to transmit a packet it must know *where* to send it. Your
>> machine must have explicit routes to networks reachable via
>> 192.168.68.X and
>> explicit routes to networks reachable via 192.168.69.Y, where the X and Y
>> addresses are the addresses of the routers that are gateways to those
>> other
>> LANs.
>> ----
>>    ip route add 10.20.30.0/24 via 192.168.69.34
>>    ip route add 192.168.128.0/17 via 192.168.68.200
>>    etc.
>> ----
>>
>> If your 'internetwork' includes the universe (the Internet), you need a
>> default route (send all packets I don't have a route for to this
>> address).
>> ----
>>    ip route add default via 192.168.68.254
>> ----
> 
> Thx, yes this is indeed standard IP networking stuff, but unfortunately
> it still isn't working; there must be something more to it.
> 
> I think the ping error text is perhaps misleading: I guess the ping request
> does reach the destination, but the answer packet from the ping reply
> gets not forwarded to the originating second interface eth1 (192.168.69.*).
> 
> Ie. the the request from eth1 correctly goes out thru the eth0 interface
> and the reply comes as well over the same eth0 interface,
> but there is on the return path something missing to forward it from
> eth0 to eth1. Isn't it?

Have you tried doing a tcpdump on eth0 while the ping is running to see
if packets are actually leaving the system and/or returning?

Regards,
  Dennis