Re: arp-proxy

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Date: Mon, 21 Nov 2005 11:28:56 +0000
To: Jon Otterholm <jon.otterholm@ide.resurscentrum.se>

> > On Thu, Nov 17, 2005 at 04:52:03PM +0100, Jon Otterholm wrote:
> > > Scenario#1:
> > > -I have a range of ip's, for example 215.10.10.0 - 215.10.10.255.
> > > -I want to distrubute theese ip's to my customers via DHCP.
> > > -They are all atached to me via a VLAN-trunk on a unique VID
> > > -I have 200+ customers.

Let me see if I can summarise your requirements:

        vlan1
 cust1 --------,
                \ service network
        vlan2 \ trunked single subnet x.y.z.0/24
 cust2 -----------===============*------+------------+-------
                 / | |
        vlan3 / DHCP srv router ---> Internet
 cust3 --------'

The constraints are:

- cust1, cust2, cust3 are all on the same subnet x.y.z.0/24 and get an IP
  address allocated by DHCP

- However, the MAC address for cust1 must never appear as a source MAC
  address on vlan2 or vlan3, as this would confuse the provider's
  trunked switching infrastructure

So you can't just bridge all the VLANs together. Rather:

1. a broadcast from custX must appear on the service network, but not
   on any of the other customer VLANs. More strongly, no packet from
   custX may appear on any other VLAN apart from the service network. [*]

2. a broadcast from the service subnet should appear on all customer VLANs
   (e.g. ARP from DHCP server or router)

3. an ARP request for custY from custX must be proxy-ARP responded to by
   a device on the service network, otherwise customers wouldn't be able to
   communicate with each other.

4. a unicast packet from the service network to a customer must be forwarded
   to the correct customer VLAN

Is that a reasonable summary? Taking that as the base:

Point (4) implies that a bridge forwarding table must still be built,
because the device performing this function (labelled '*' in the above
diagram) needs to associate a MAC address with a VLAN, and do so by
learning rather than static configuration.

Point (1) says that you can't just bridge all the VLANs together, because
otherwise a packet to a broadcast address or to an unknown MAC address would
be forwarded to all the other VLANs. We want this to happen for packets
originating from the service network (point (2)), but not for packets which
originate from the customer networks. So, some sort of L2 forwarding filter
should do the trick: configure it so that packets may only be forwarded to
customer VLANs if they originate from the service network. If this filter is
applied, you guarantee that a customer's MAC address will never appear as a
source on any other VLAN.

Point (3), proxy ARP, is easy enough. You know all the possible customer IPs
- they are exactly the range assigned to the DHCP server to allocate - and
therefore you can proxy-ARP respond for any IP address within the DHCP
range, as long as the request originated from a customer VLAN (which can
also be determined by the ARP source IP). The router itself could perform
this proxy ARP function, or else any server on the service network running
something like choparp (which can give out the router's MAC address in the
ARP responses). IP datagrams from custX to custY then go
custX->router->custY.

So actually, when thought about like this, the L2 masquerading requirement
vanishes, and what you really need is bridging plus some L2 filtering based
on ingress and egress interfaces.

Unfortunately, I don't know if FreeBSD has this level of L2 filtering (I
note that the bridge(4) documentation says that ipf/ipfw filtering only
works for IP datagrams). However a frob on the bridging code should be
possible; call the first interface 'master' and the rest 'slaves', and have
a rule so that packets to a 'slave' interface are only forwarded if they
originated from the 'master' interface.

Aside: the network as designed above has an obvious flaw that any customer
can DHCP for as many different machines as they wish, and therefore exhaust
your DHCP pool. You could have a separate mini DHCP server listening on each
VLAN and only handing out a single IP, but that doesn't stop customers
stealing other customer's IPs through static configuration. So actually, I
think you need anti-spoofing filters on each VLAN too.

Doing that, you end up statically routing a separate IP down each VLAN, in
which case what you *really* want is to be able to configure each VLAN
subinterface as if it were a point-to-point interface. But I don't think
FreeBSD supports that on broadcast media.

Regards,

Brian.

[*] Or if it did appear on the other customer VLANs, it would have to be
    masqueraded to appear as if it came from a MAC address on the service
    network; however I believe this isn't actually necessary, as the only
    broadcasts we really care about here are ARP requests. All others
    can be dropped, and indeed probably should be dropped so that all
    your customers don't get drowned in each other's broadcast traffic.
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