Re: IP_alias under Red Hat

[Brian Candler <B.Candler at pobox.com>]

On Wed, Jan 15, 2003 at 12:56:48PM -0000, Moussa Bagayoko wrote:
> Thank for your quick response. I tried with eth0:0 and eth0:eth0.
> Actually the alias is ok with . All two address is reachable trough
> internet. So I'm using this machine as modem server and some client get
> connected with ip adress in the server ip alias bloc and they can do nothing
> outside the server. But they can ping all two server ip address. If they get
> connected with ip in the server primary ip bloc, all is right.

I'm not sure exactly what you mean here.

You can't give the same IP address to both the dial-in server and the
client:

                                         10.0.0.1 and
                    modem link           10.0.0.2 
        client - - - - - - - - - dial-up -----------------R---> Internet
        10.0.0.2                 server

This simply won't work. When an inbound packet arrives from the Internet to
10.0.0.2, it will be delivered to the dial-up server (which says "10.0.0.2
is one of my own IP addresses, this packet is for me") and it won't be
forwarded down the modem link.

So the next thing you might try is to remove 10.0.0.2 as an alias:

                    modem link           10.0.0.1
        client - - - - - - - - - dial-up -----------------R---> Internet
        10.0.0.2                 server

The problem here is that when a machine on the Internet tries to send a
packet to 10.0.0.2, it will get as far as the router R. R will try to send
an ARP packet for 10.0.0.2, but nobody will respond. So the packet won't be
delivered.

You have two solutions:

(1) Put a static route on router R,

          ip route 10.0.0.2 255.255.255.255 10.0.0.1

    (note: if your dial-up server has more than one phone line then you
     should allocate a block of IPs, which is in a different subnet, and
     use that as your dialup pool)

(2) Enable 'proxy-ARP' on the dial-up server for 10.0.0.2. Then, when
    router R ARPs for 10.0.0.2, the dial-up server will respond saying
    "yes that's me". R will send the packet over the ethernet to that
    MAC address. When R receives the packet, it will forward it using its
    own forwarding table.

With Linux pppd, you could configure than easily: something like this

[/etc/ppp/options.ttyS0]
   :10.0.0.2 proxyarp

With multiple phone-lines you write a separate options file with a different
IP address for each line.

If I have misunderstood your problem - then a diagram like one of the above
would be very helpful in trying to understand what you're doing.

Regards,

Brian.

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