multihome network

Steve Bertrand iaccounts at
Fri Nov 16 05:54:33 PST 2007

Girish Venkatachalam wrote:
> On 00:18:42 Nov 16, alexus wrote:
>> Hello,
>> I have two NICs on my box, one (primary) connected to switch and have
>> private IP. that IP also have a static route on Cisco PIX for
>> accessing this box from outside. the other interface has public IP
>> that is connected to another switch, i configure both IPs through
>> /etc/rc.conf, but I can not for some reason access my box through that
>> public IP, no firewall rules would prevent me from doing so. here is
>> my output for netstat -rn

-- snip

> Your default route is and not

Yes, but if he changes that, then he won't be able to access the box via
the PIX (private) connection.

I will make these assumptions, then elaborate:

The box in question is at your office. You are at home trying to access
it. The connection works by connecting to the public IP of the PIX (that
gets port-forwarded back), but does not work when accessing the direct
Internet facing port.

I'm willing to bet that if you run a tcpdump on your machine at home you
are attempting the connection to the 216.112.241.x IP, you will actually
find that the machine is getting back to you just fine. However, many
OS's will drop a 'spoofed' packet. Essentially what is likely happening
is this:

- you send from home a packet to 216.112.241.x.
- the office router/box accepts it
- the office router looks up in it's routing table a path back to your
home IP
- it has no particular route, so it sends it out the default gateway
- your pc at home notices that the packet was sent to a destination IP,
but it came back from a different one (the outside IP of the PIX)
- the packet is dropped as the source address is spoofed

There are a couple ways to fix this. The first and easiest is if you are
only trying to connect to this box's public IP from one location, add a
static route on the office box to that network that routes to it's
public upstream

The other way is to utilize policy-based routing. IPFW can do this, and
(from what I understand) so can PF. (In Cisco-land, you would use a


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