Proxy-ARP-Subnet(1).pdf

ProxyARP Subnetting HOWTO

Bob Edwards

Robert.Edwards@anu.edu.au

v2.0, 27 August 2000

This HOWTO discusses using Proxy Address Resolution Protocol (ARP) with subnetting in order to make a

small network of machines visible on another Internet Protocol (IP) subnet (I call it sub−subnetting). This

makes all the machines on the local network (network 0 from now on) appear as if they are connected to the

main network (network 1).

This is only relevent if all machines are connected by Ethernet or ether devices (ie. it won't work for

SLIP/PPP/CSLIP etc.)

ProxyARP Subnetting HOWTO

Table of Contents

1. Acknowledgements .........................................................................................................................................1

2. Why use Proxy ARP with subnetting?. .........................................................................................................2

3. How Proxy ARP with subnetting works. ......................................................................................................3

4. Setting up Proxy ARP with subnetting. ........................................................................................................5

5. Other alternatives to Proxy ARP with subnetting. ......................................................................................7

6. Other Applications of Proxy ARP with subnetting .....................................................................................8

7. Copying conditions. .........................................................................................................................................9

1. Acknowledgements

This document, and my Proxy ARP implementation could not have been made possible without the help of:

Andrew Tridgell, who implemented the subnetting options for arp in Linux, and who personally

assisted me in getting it working

the Proxy−ARP mini−HOWTO, by Al Longyear

the Multiple−Ethernet mini−HOWTO, by Don Becker

· the arp (8) source code and man page by Fred N. van Kempen and Bernd Eckenfels

1. Acknowledgements

2. Why use Proxy ARP with subnetting?

The applications for using Proxy ARP with subnetting are fairly specific.

In my case, I had a wireless Ethernet card that plugs into an 8−bit ISA slot. I wanted to use this card to

provide connectivity for a number of machines at once. Being an ISA card, I could use it on a Linux machine,

after I had written an appropriate device driver for it − this is the subject of another document. From here, it

was only necessary to add a second Ethernet interface to the Linux machine and then use some mechanism to

join the two networks together.

For the purposes of discussion, let network 0 be the local Ethernet connected to the Linux box via an

NE−2000 clone Ethernet interface on eth0. Network 1 is the main network connected via the wireless

Ethernet card on eth1. Machine A is the Linux box with both interfaces. Machine B is any TCP/IP machine

on network 0 and machine C is likewise on network 1.

Normally, to provide the connectivity, I would have done one of the following:

Used the IP−Bridge software (see the Bridge mini−HOWTO) to bridge the traffic between the two

network interfaces. Unfortunately, the wireless Ethernet interface cannot be put into "Promiscuous"

mode (ie. it can't see all packets on network 1). This is mainly due to the lower bandwidth of the

wireless Ethernet (2MBit/sec) meaning that we don't want to carry any traffic not specifically

destined to another wireless Ethernet machine − in our case machine A − or broadcasts. Also,

bridging is rather CPU intensive!

· Alternatively, use subnets and an IP−router to pass packets between the two networks (see the

IP−Subnetworking mini−HOWTO). This is a protocol specific solution, where the Linux kernel can

handle the Internet Protocol (IP) packets, but other protocols (such as AppleTalk) need extra software

to route. This also requires the allocation of a new IP subnet (network) number, which is not always

an option.

In my case, getting a new subnet (network) number was not an option, so I wanted a solution that allowed all

the machines on network 0 to appear as if they were on network 1. This is where Proxy ARP comes in. Other

solutions are used to connect other (non−IP) protocols, such as netatalk to provide AppleTalk routing.

2. Why use Proxy ARP with subnetting?

3. How Proxy ARP with subnetting works

The Proxy ARP is actually only used to get packets from network 1 to network 0. To get packets back the

other way, the normal IP routing functionality is employed.

In my case, network 1 has an 8−bit subnet mask (255.255.255.0). I have chosen the subnet mask for network

0 to be 4−bit (255.255.255.240), allowing 14 IP nodes on network 0 (2 ^ 4 = 16, less two for the all zeros and

all ones cases). Note that any size of subnet mask up to, but not including, the size of the mask of the other

network is allowable here (eg. 2, 3, 4, 5, 6 or 7 bits in this case − for one bit, just use normal Proxy ARP!)

All the IP numbers for network 0 (16 in total) appear in network 1 as a subset. Note that it is very important,

in this case, not to allow any machine connected directly to network 1 to have an IP number in this range! In

my case, I have "reserved" the IP numbers of network 1 ending in 64 .. 79 for network 0. In this case, the IP

numbers ending in 64 and 79 can't actually be used by nodes − 79 is the broadcast address for network 0.

Machine A is allocated two IP numbers, one within the network 0 range for it's real Ethernet interface (eth0)

and the other within the network 1 range, but outside of the network 0 range, for the wireless Ethernet

interface (eth1).

Say machine C (on network 1) wants to send a packet to machine B (on network 0). Because the IP number

of machine B makes it look to machine C as though it is on the same physical network, machine C will use

the Address Resolution Protocol (ARP) to send a broadcast message on network 1 requesting the machine

with the IP number of machine B to respond with it's hardware (Ethernet or MAC layer) address. Machine B

won't see this request, as it isn't actually on network 1, but machine A, on both networks, will see it.

The first bit of magic now happens as the Linux kernel arp code on machine A, with a properly configured

Proxy ARP with subnetting entry, determines that the ARP request has come in on the network 1 interface

(eth1) and that the IP number being ARP'd for is in the subnet range for network 0. Machine A then sends it's

own hardware (Ethernet) address back to machine C as an ARP response packet.

Machine C then updates it's ARP cache with an entry for machine B, but with the hardware (Ethernet)

address of machine A (in this case, the wireless Ethernet interface). Machine C can now send the packet for

machine B to this hardware (Ethernet) address, and machine A receives it.

Machine A notices that the destination IP number in the packet is that of machine B, not itself. Machine A's

Linux kernel IP routing code attempts to forward the packet to machine B by looking at it's routing tables to

determine which interface contains the network number for machine B. However, the IP number for machine

B is valid for both the network 0 interface (eth0), and for the network 1 interface (eth1).

At this point, something else clever happens. Because the subnet mask for the network 0 interface has more 1

bits (it is more specific) than the subnet mask for the network 1 interface, the Linux kernel routing code will

match the IP number for machine B to the network 0 interface, and not keep looking for the potential match

with the network 1 interface (the one the packet came in on).

Now machine A needs to find out the "real" hardware (Ethernet) address for machine B (assuming that it

doesn't already have it in the ARP cache). Machine A uses an ARP request, but this time the Linux kernel arp

code notes that the request isn't coming from the network 1 interface (eth1), and so doesn't respond with the

Proxy address of eth1. Instead, it sends the ARP request on the network 0 interface (eth0), where machine B

will see it and respond with it's own (real) hardware (Ethernet) address. Now machine A can send the packet

(from machine C) onto machine B.

3. How Proxy ARP with subnetting works

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