Richard Freeman on 30 Aug 2009 13:37:30 -0700

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Re: [PLUG] network fixer hat

Eric wrote:
> Paul DiSciascio wrote:
>> You're correct, it's being passed to the default gateway.  You won't  
>> be able to talk to anything in the subnet without a NIC  
>> on that subnet or a router that knows about it.   Even if you were to  
>> add a route for that subnet to your routing table, the return traffic  
>> won't know how to get back to the subnet.
> This makes sense.  Oddly, I can get return traffic from the ftp daemon
> Unfortunately, I cannot log in so maybe it's not working as it appears.

Ok, return traffic back to the single linux host that you've multi-homed 
in this manner will work just fine.  However, that linux box will not 
automatically forward packets on to the other subnet unless you set up 
some netfilter rules.  With appropriate settings you could actually set 
up a router that forwards traffic as appropriate even though it is all 
on the same physical network.

As far as ftp/etc goes - what interface is your server listening on?  If 
the FTP server is bound to then it should get the traffic from 
both subnets (not sure if it matters whether you start the server after 
creating the interface).  However, if your FTP server is bound to 
10.10.10.### then it will not receive traffic from 192.168.x.y since it 
isn't listening on that interface.  There is a chance you might need to 
adjust your FTP configuration.

Note that if this same box is also connected to another interface to the 
internet (the box is effectively a router/firewall) be careful about 
binding to since now your FTP server will be open to the 
internet as well (unless you have a netfilter rule set to block traffic 
to that port from outside).

Also - if you do have netfilter set up you need to make sure that it 
isn't blocking incoming traffic on the new interface.

I've done this trick myself to configure new appliances/etc to get them 
on my normal network without having to reconfigure one of my own 
comptuers to talk to them.
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