Interesting network problem

[Konstantin Svist]

David G. Mackay wrote:
> On Thu, 2007-08-16 at 14:46 -0700, Konstantin Svist wrote:
>   
>> David G. Mackay wrote:
>>     
>>> I have a netopia 2241N dsl modem provided by my ISP (AT&T), which I
>>> access through an ethernet port.  The unit can be configured in pass
>>> through mode so that the public internet address is bridged, i.e. my
>>> ethernet card is assigned the public address.  This has been working
>>> well for some time now.
>>>
>>> There was a network outage the other day, and when service was restored,
>>> the gateway on the remote side had changed.  The fun part is that the
>>> gateway is on a different class A ip net than my public ip.  I can
>>> resolve all that by doing:
>>> route add -net e.f.g.h netmask 255.0.0.0 dev ethx
>>> route add default gw e.f.g.h
>>> where e.f.g.h is the external gateway ip.
>>> This borks up the /etc/sysconfig/network-scripts/ifcfg-ethx scripting.
>>>
>>> Is there a better way to handle this, other than kludging things in
>>> rc.local?
>>>   
>>>       
>> Normally, DHCP takes care of these little problems.. but if you're not 
>> using it (and using static IP config instead), just use the GUI utility 
>> system-config-network - it
>> should set all these values correctly
>>     
>
> It is a static address, so DHCP is not appropriate.  And, I suppose,
> system-config-network would accept the gateway and ip addresses.  The
> problem is that the gateway is on a different subnet than the ip by most
> normal networking rules.  The modem doesn't show the netmask, so I
> suppose that AT&T might have CIDRed a couple of class A addresses
> together.  But, without that, the gateway will be unreachable until you
> add the route add -net, which doesn't normally occur in the network init
> scripts.
>
> Dave
>   

you get to write in any gateway in system-config-network, it's not 
limited to same subnet.