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ProxyARP Subnetting HOWTO

Bob Edwards

   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.)
   Table of Contents
   1. [1]Acknowledgements
   2. [2]Why use Proxy ARP with subnetting?
   3. [3]How Proxy ARP with subnetting works
   4. [4]Setting up Proxy ARP with subnetting
   5. [5]Other alternatives to Proxy ARP with subnetting
   6. [6]Other Applications of Proxy ARP with subnetting
   7. [7]Copying conditions
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
2. Why use Proxy ARP with subnetting?

   The applications for using Proxy ARP with subnetting are fairly
   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
     * 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.
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 ( I have
   chosen the subnet mask for network 0 to be 4-bit (,
   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
   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.
   Machine B gets the packet from machine C (via machine A) and then
   wants to send back a response. This time, machine B notices that
   machine C in on a different subnet (machine B's subnet mask of excludes all machines not in the network 0 IP address
   range). Machine B is setup with a "default" route to machine A's
   network 0 IP number and sends the packet to machine A. This time,
   machine A's Linux kernel routing code determines the destination IP
   number (of machine C) as being on network 1 and sends the packet onto
   machine C via Ethernet interface eth1.
   Similar (less complicated) things occur for packets originating from
   and destined to machine A from other machines on either of the two
   Similarly, it should be obvious that if another machine (D) on network
   0 ARP's for machine B, machine A will receive the ARP request on it's
   network 0 interface (eth0) and won't respond to the request as it is
   set up to only Proxy on it's network 1 interface (eth1).
   Note also that all of machines B and C (and D) are not required to do
   anything unusual, IP-wise. In my case, there is a mixture of Suns,
   Macs and PC/Windoze 95 machines on network 0 all connecting through
   Linux machine A to the rest of the world.
   Finally, note that once the hardware (Ethernet) addresses are
   discovered by each of machines A, B, C (and D), they are placed in the
   ARP cache and subsequent packet transfers occur without the ARP
   overhead. The ARP caches normally expire entries after 5 minutes of
4. Setting up Proxy ARP with subnetting

   I set up Proxy ARP with subnetting on a Linux kernel version 2.0.30
   machine, but I am told that the code works right back to some kernel
   version in the 1.2.x era.
   The first thing to note is that the ARP code is in two parts: the part
   inside the kernel that sends and receives ARP requests and responses
   and updates the ARP cache etc.; and other part is the arp(8) command
   that allows the super user to modify the ARP cache manually and anyone
   to examine it.
   The first problem I had was that the arp(8) command that came with my
   Slackware 3.1 distribution was ancient (1994 era!!!) and didn't
   communicate with the kernel arp code correctly at all (mainly
   evidenced by the strange output that it gave for "arp -a").
   The arp(8) command in "net-tools-1.33a" available from a variety of
   places, including (from the README file that came with it)
   [8] works properly and
   includes new man pages that explain stuff a lot better than the older
   arp(8) man page.
   Armed with a decent arp(8) command, all the changes I made were in the
   /etc/rc.d/rc.inet1 script (for Slackware - probably different for
   other flavours). First of all, we need to change the broadcast
   address, network number and netmask of eth0:
NETMASK= # for a 4-bit host part
NETWORK=x.y.z.64        # our new network number (replace x.y.z with your net)
BROADCAST=x.y.z.79      # in my case

   Then a line needs to be added to configure the second Ethernet port
   (after any module loading that might be required to load the driver
/sbin/ifconfig eth1 (name on net 1) broadcast (x.y.z.255) netmask

   Then we add a route for the new interface:
/sbin/route add -net (x.y.z.0) netmask

   And you will probably need to change the default gateway to the one
   for network 1.
   At this point, it is appropriate to add the Proxy ARP entry:
/sbin/arp -i eth1 -Ds ${NETWORK} eth1 netmask ${NETMASK} pub

   This tells ARP to add a static entry (the s) to the cache for network
   ${NETWORK}. The -D tells ARP to use the same hardware address as
   interface eth1 (the second eth1), thus saving us from having to look
   up the hardware address for eth1 and hardcoding it in. The netmask
   option tells ARP that we want to use subnetting (ie. Proxy for all (IP
   number) & ${NETMASK} == ${NETWORK} & ${NETMASK}). The pub option tells
   ARP to publish this ARP entry, ie. it is a Proxy entry, so respond on
   behalf of these IP numbers. The -i eth1 option tells ARP to only
   respond to requests that come in on interface eth1.
   Hopefully, at this point, when the machine is rebooted, all the
   machines on network 0 will appear to be on network 1. You can check
   that the Proxy ARP with subnetting entry has been correctly installed
   on machine A. On my machine (names changed to protect the innocent) it
bash$ /sbin/arp -an
Address                 HWtype  HWaddress           Flags Mask            Iface
x.y.z.1                 ether   00:00:0C:13:6F:17   C     *               eth1
x.y.z.65                ether   00:40:05:49:77:01   C     *               eth0
x.y.z.67                ether   08:00:20:0B:79:47   C     *               eth0
x.y.z.5                 ether   00:00:3B:80:18:E5   C     *               eth1
x.y.z.64                ether   00:40:96:20:CD:D2   CMP eth1

   Alternatively, you can examine the /proc/net/arp file with eg. cat(1).
   The last line is the proxy entry for the subnet. The CMP flags
   indicate that it is a static (Manually entered) entry and that it is
   to be Published. The entry is only going to reply to ARP requests on
   eth1 where the requested IP number, once masked, matches the network
   number, also masked. Note that arp(8) has automatically determined the
   hardware address of eth1 and inserted this for the address to use (the
   -Ds option).
   Likewise, it is probably prudent to check that the routing table has
   been set up correctly. Here is mine (again, the names are changed to
   protect the innocent):
#/bin/netstat -rn
Kernel routing table
Destination     Gateway         Genmask         Flags Metric Ref Use    Iface
x.y.z.64 U     0      0       71 eth0
x.y.z.0   U     0      0      389 eth1       U     0      0        7 lo         x.y.z.1         UG    1      0      573 eth1

   Alternatively, you can examine the /proc/net/route file with eg.
   Note that the first entry is a proper subset of the second, but the
   routing table has ranked them in netmask order, so the eth0 entry will
   be checked before the eth1 entry.
5. Other alternatives to Proxy ARP with subnetting

   There are several other alternatives to using Proxy ARP with
   subnetting in this situation, apart from the ones mentioned about
   (bridging and straight routing):
     * IP-Masquerading (see the IP-Masquerade mini-HOWTO), in which
       network 0 is "hidden" behind machine A from the rest of the
       Internet. As machines on network 0 attempt to connect outside
       through machine A, it re-addresses the source address and port
       number of the packets and makes them look like they are coming
       from itself, rather than from the machine on the hidden network 0.
       This is an elegant solution, although it prevents any machine on
       network 1 from initiating a connection to any machine on network
       0, as the machines on network 0 effectively don't exist outside of
       network 0. This effectively increases security of the machines on
       network 0, but is also means that servers on network 1 cannot
       check the identity of clients on network 0 using IP numbers (eg.
       NFS servers use IP hostnames for access to mountable file
     * Another option is IP in IP tunneling, which isn't supported on all
       platforms (such as Macs and Windoze machines) so I opted not to go
       this way.
     * Use Proxy ARP without subnetting. This is certainly possible, it
       just means that a separate entry needs to be created for each
       machine on network 0, instead of a single entry for all machines
       (current and future) on network 0.
     * Possibly IP Aliasing might also be useful here, but I haven't
       looked into this at all.
6. Other Applications of Proxy ARP with subnetting

   There is only one other application that I know about that uses Proxy
   ARP with subnetting, also here at the Australian National University.
   It is the one that Andrew Tridgell originally wrote the subnetting
   extensions to Proxy ARP for. However, Andrew reliably informs me that
   there are, in fact, several other sites around the world using it as
   well (I don't have any details).
   The other A.N.U. application involves a teaching lab set up to teach
   students how to configure machines to use TCP/IP, including setting up
   the gateway. The network used is a Class C network, and Andrew needed
   to "subnet" it for security, traffic control and the educational
   reason mentioned above. He did this using Proxy ARP, and then decided
   that a single entry in the ARP cache for the whole subnet would be
   faster and cleaner than one for each host on the subnet. Voila...Proxy
   ARP with subnetting!
7. Copying conditions

   Copyright 1997 by Bob Edwards <[9]>
   Voice: (+61) 2 6249 4090
   Unless otherwise stated, Linux HOWTO documents are copyrighted by
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   1. Proxy-ARP-Subnet.html#INTRO
   2. Proxy-ARP-Subnet.html#WHY
   3. Proxy-ARP-Subnet.html#HOW
   4. Proxy-ARP-Subnet.html#SETUP
   5. Proxy-ARP-Subnet.html#ALTERNATIVES
   6. Proxy-ARP-Subnet.html#APPLICATIONS
   7. Proxy-ARP-Subnet.html#COPYING

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