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Re: Quick question about routing

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On 4/27/2019 0:14 AM, Pascal Hambourg wrote:
> Le 27/04/2019 à 04:17, szr a écrit :
>> Ok, I have a LAN with the following notes:
>>
>> 192.168.1.254 - Main router / default gateway for LAN
>> 192.168.1.4   - Linux server (eth0)
>> 192.168.1.8   - Windows laptop
>>
>> Linux server (192.168.1.4) has ip(v4) forwarding enabled.
>
> Note : IP forwarding is not involved in your scenario.
>
>> As an experiment I added an ip, 10.0.0.4, as eth0:0.
>
> Note : use of eth0:0 alias is deprecated. You can just add multiple
> addresses to eth0.
>
>> Then I added a static route for 10.0.0.0/8 using 192.168.1.4 as the
>> gateway on the main router.
>>
>> After that, I ran `ping 10.0.0.4` from the Windows laptop and the ping
>> was successful.
>>
>> While I do have plenty of networking experience and I'm not surprised
>> that this worked, one thing sort of bugs me.
>>
>> I'm able to ping an ip in the 10.0.0.0/8 subnet from a host that only
>> has an ip in the 192.168.1.0/24 subnet. I know that the laptop is
>> using the route from the default gateway (the main router)
>
> No. A routing table is local. The laptop just uses its own routing
> table. So does any other involved node (host or router) in the path.
>
> The laptop routing table has 2 routes :
> 192.168.1.0/24 direct
> 0.0.0.0/0 via 192.168.1.254
>
> 10.0.0.4 does not match the first route so use the default route and
> send to the router.
>
> The router routing table has 3 routes :
> 192.168.1.0/24 direct
> 10.0.0.0/8 via 192.168.1.4
> 0.0.0.0/0 via some external ISP router
>
> 10.0.0.4 matches the second route so forward to 192.168.1.4 (server).
>
> [Note : the router may also send an ICMP "redirect" message to the
> laptop to inform it that 10.0.0.4 is directly reachable on the network
> segment, and the laptop may update its routing table to reflect this so
> that it sends subsequent packets to 10.0.0.4 directly to the server.]
>
> The server routing table has 3 routes :
> 192.168.1.0/24 direct
> 10.0.0.0/8 direct
> 0.0.0.0/0 via 192.168.1.254
>
> 10.0.0.4 is assigned to a local interface so accept the packet and reply
> to the source 192.168.1.8 (laptop).
> 192.168.1.8 matches the first route so send directly to the laptop.
>
>> but how exactly is the ping getting back when the source of the ping
>> isn't coming from another 10.0.0.0/8 address? Is this more due to ip
>> forwarding, or
>
> No. As I wrote, no IP forwarding is involved on the server because the
> destination is the host itself. It is just acting as a host with
> multiple addresses.
>
>> because it's on the same physical network segment?
>
> Not really. It would work even if the source was in a distinct network
> segment. The server would just send the reply back to the router and the
> router would forward it to the other network segment.


Thanks, this all makes complete sense now. I must have been a bit tired 
as my mind was somehow crossing up this with trying to reach hosts 
behind a masqueraded ip, vlan, or some similar scenario. Just one of 
those cases where after a good nights rest it all becomes clear. Thanks 
again.

-- 
szr

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Thread

Quick question about routing szr <szr@sREzMOrVEoman.com> - 2019-04-26 19:17 -0700
  Re: Quick question about routing William Unruh <unruh@invalid.ca> - 2019-04-27 03:02 +0000
  Re: Quick question about routing Pascal Hambourg <pascal@plouf.fr.eu.org> - 2019-04-27 09:14 +0200
    Re: Quick question about routing Theodore Heise <theise@panix.com> - 2019-04-27 14:28 +0000
    Re: Quick question about routing szr <szr@sREzMOrVEoman.com> - 2019-04-27 08:03 -0700

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