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Re: Quick question about routing

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On Sat, 27 Apr 2019 09:14:14 +0200,
  Pascal Hambourg <pascal@plouf.fr.eu.org> wrote:
>  Le 27/04/2019 ?? 04:17, szr a ??crit??:
> > 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.

Very clear explanation, thank you!

-- 
Ted Heise      <theise@panix.com>       West Lafayette, IN, USA

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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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