RFC 1027: Using ARP to implement transparent subnet gateways

Network Working Group Smoot Carl-Mitchell
Request for Comments: 1027 Texas Internet Consulting
 John S. Quarterman
 Texas Internet Consulting
 October 1987


 Using ARP to Implement Transparent Subnet Gateways


Status of this Memo

 This RFC describes the use of the Ethernet Address Resolution
 Protocol (ARP) by subnet gateways to permit hosts on the connected
 subnets to communicate without being aware of the existence of
 subnets, using the technique of "Proxy ARP" [6]. It is based on
 RFC-950 [1], RFC-922 [2], and RFC-826 [3] and is a restricted subset
 of the mechanism of RFC-925 [4]. Distribution of this memo is
 unlimited.

Acknowledgment

 The work described in this memo was performed while the authors were
 employed by the Computer Sciences Department of the University of
 Texas at Austin.

Introduction

 The purpose of this memo is to describe in detail the implementation
 of transparent subnet ARP gateways using the technique of Proxy ARP.
 The intent is to document this widely used technique.

1. Motivation

 The Ethernet at the University of Texas at Austin is a large
 installation connecting over ten buildings. It currently has more
 than one hundred hosts connected to it [5]. The size of the
 Ethernet and the amount of traffic it handles prohibit tying it
 together by use of repeaters. The use of subnets provided an
 attractive alternative for separating the network into smaller
 distinct units.

 This is exactly the situation for which Internet subnets as
 described in RFC-950 are intended. Unfortunately, many vendors had
 not yet implemented subnets, and it was not practical to modify the
 more than half a dozen different operating systems running on hosts
 on the local networks.




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RFC 1027 ARP and Transparent Subnet Gateways October 1987


 Therefore a method for hiding the existence of subnets from hosts
 was highly desirable. Since all the local area networks supported
 ARP, an ARP-based method (commonly known as "Proxy ARP" or the "ARP
 hack") was chosen. In this memo, whenever the term "subnet" occurs
 the "RFC-950 subnet method" is assumed.

2. Design

2.1 Basic method

 On a network that supports ARP, when host A (the source) broadcasts
 an ARP request for the network address corresponding to the IP
 address of host B (the target), host B will recognize the IP address
 as its own and will send a point-to-point ARP reply. Host A keeps
 the IP-to-network-address mapping found in the reply in a local
 cache and uses it for later communication with host B.

 If hosts A and B are on different physical networks, host B will not
 receive the ARP broadcast request from host A and cannot respond to
 it. However, if the physical network of host A is connected by a
 gateway to the physical network of host B, the gateway will see the
 ARP request from host A. Assuming that subnet numbers are made to
 correspond to physical networks, the gateway can also tell that the
 request is for a host that is on a different physical network from
 the requesting host. The gateway can then respond for host B,
 saying that the network address for host B is that of the gateway
 itself. Host A will see this reply, cache it, and send future IP
 packets for host B to the gateway. The gateway will forward such
 packets to host B by the usual IP routing mechanisms. The gateway
 is acting as an agent for host B, which is why this technique is
 called "Proxy ARP"; we will refer to this as a transparent subnet
 gateway or ARP subnet gateway.

 When host B replies to traffic from host A, the same algorithm
 happens in reverse: the gateway connected to the network of host B
 answers the request for the network address of host A, and host B
 then sends IP packets for host A to gateway. The physical networks
 of host A and B need not be connected to the same gateway. All that
 is necessary is that the networks be reachable from the gateway.

 With this approach, all ARP subnet handling is done in the ARP
 subnet gateways. No changes to the normal ARP protocol or routing
 need to be made to the source and target hosts. From the host point
 of view, there are no subnets, and their physical networks are
 simply one big IP network. If a host has an implementation of
 subnets, its network masks must be set to cover only the IP network
 number, excluding the subnet bits, for the system to work properly.




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RFC 1027 ARP and Transparent Subnet Gateways October 1987


2.2 Routing

 As part of the implementation of subnets, it is expected that the
 elements of routing tables will include network numbers including
 both the IP network number and the subnet bits, as specified by the
 subnet mask, where appropriate. When an ARP request is seen, the
 ARP subnet gateway can determine whether it knows a route to the
 target host by looking in the ordinary routing table. If attempts
 to reach foreign IP networks are eliminated early (see Sanity Checks
 below), only a request for an address on the local IP network will
 reach this point. We will assume that the same network mask applies
 to every subnet of the same IP network. The network mask of the
 network interface on which the ARP request arrived can then be
 applied to the target IP address to produce the network part to be
 looked up in the routing table.

 In 4.3BSD (and probably in other operating systems), a default route
 is possible. This default route specifies an address to forward a
 packet to when no other route is found. The default route must not
 be used when checking for a route to the target host of an ARP
 request. If the default route were used, the check would always
 succeed. But the host specified by the default route is unlikely to
 know about subnet routing (since it is usually an Internet gateway),
 and thus packets sent to it will probably be lost. This special
 case in the routing lookup method is the only implementation change
 needed to the routing mechanism.

 If the network interfaces on which the request was received and
 through which the route to the target passes are the same, the
 gateway must not reply. In this case, either the target host is on
 the same physical network as the gateway (and thus the host should
 reply for itself), or this gateway is not on the most direct path to
 the desired network, i.e., there is another gateway on the same
 physical network that is on a more direct path and the other gateway
 should respond.

 RFC-925 [4] describes a general mechanism for dynamic subnet routing
 using Proxy ARP and routing caches in the gateways. Our technique
 is restricted subset of RFC-925, in which we use static subnet
 routes which are determined administratively. As a result, our
 transparent subnet gateways require no new network routing table
 entries nor ARP cache entries; the only tables which are affected
 are the ARP caches in the host.

 In our implementation, routing loops are prevented by proper
 administration of the subnet routing tables in the gateways.





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RFC 1027 ARP and Transparent Subnet Gateways October 1987


2.3 Multiple gateways

 The simplest subnet organization to administer is a tree structure,
 which cannot have loops. However, it may be desirable for
 reliability or traffic accommodation to have more than one gateway
 (or path) between two physical networks. ARP subnet gateways may be
 used in such a situation: a requesting host will use the first ARP
 response it receives, even if more than one gateway supplies one.
 This may even provide a rudimentary load balancing service, since if
 two gateways are otherwise similar, the one most lightly loaded is
 the more likely to reply first.

 More complex mechanisms could be built in the form of gateway-to-
 gateway protocols, and will no doubt become necessary in networks
 with large numbers of subnets and gateways, in the same way that
 gateway-to-gateway protocols are generally necessary among IP
 gateways.

2.4 Sanity checks

 Care must be taken by the network and gateway administrators to keep
 the network masks the same on all the subnet gateway machines. The
 most common error is to set the network mask on a host without a
 subnet implementation to include the subnet number. This causes the
 host to fail to attempt to send packets to hosts not on its local
 subnet. Adjusting its routing tables will not help, since it will
 not know how to route to subnets.

 If the IP networks of the source and target hosts of an ARP request
 are different, an ARP subnet gateway implementation should not
 reply. This is to prevent the ARP subnet gateway from being used to
 reach foreign IP networks and thus possibly bypass security checks
 provided by IP gateways.

 An ARP subnet gateway implementation must not reply if the physical
 networks of the source and target of an ARP request are the same.
 In this case, either the target host is presumably either on the
 same physical network as the source host and can answer for itself,
 or the target host lies in the same direction from the gateway as
 does the source host, and an ARP reply from the would cause a loop.

 An ARP request for a broadcast address must elicit no reply,
 regardless of the source address or physical networks involved. If
 the gateway were to respond with an ARP reply in this situation, it
 would be inviting the original source to send actual traffic to a
 broadcast address. This could result in the "Chernobyl effect"
 wherein every host on the network replies to such traffic, causing
 network "meltdown".



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RFC 1027 ARP and Transparent Subnet Gateways October 1987


2.5 Multiple logical subnets per physical network

 The most straightforward way to assign subnet numbers is one to one
 with physical networks. There are, however, circumstances in which
 multiple logical subnets per physical network are quite useful. One
 of the more common is when it is planned that a group of
 workstations will be put on their own physical network but the
 gateway to the new physical network needs to be tested first. (A
 repeater might be used when the gateway was not usable). If a rule
 of one subnet per physical network is enforced, the addresses of the
 workstations must be changed every time the gateway is tested. If
 they may be assigned addresses using a new subnet number while they
 are still on the old physical network, no further address changes
 are needed.

 To permit multiple subnets per physical network, an ARP subnet
 gateway must use the physical network interface, not the subnet
 number to determine when to reply to an ARP request. That is, it
 should send a proxy ARP reply only when the source network interface
 differs from the target network interface. In addition, appropriate
 routing table entries for these "phantom" subnets must be added to
 the subnet gateway routing tables.

2.6 Broadcast addresses

 There are two kinds of IP broadcast addresses: main IP directed
 network broadcast and subnet broadcast. An IP network broadcast
 address consists of the network number plus a well-known value in
 the rest (local part) of the address. An IP subnet broadcast is
 similar, except both the IP network number and the subnet number
 bits are included. RFC-922 standardized the use of all ones in the
 local part, but there were two conventions in use before that: all
 ones and all zeros. For example, 4.2BSD used all zeros, and 4.3BSD
 uses all ones. Thus there are four kinds of IP directed broadcast
 addresses still currently in use on many networks.

 With transparent subnetting a subnet gateway must not issue an IP
 broadcast using the subnet broadcast address, e.g., 128.83.138.255.
 Hosts on the physical network that receive the broadcast will not
 understand such an address as a broadcast address, since they will
 not have subnets enabled (or will not have subnet implementations).
 In fact, 4.2BSD hosts (with or without subnet implementations) will
 instead treat an address with all ones in the local part as a
 specific host address and try to forward the packet. Since there is
 no such target host, there will be no entry in the forwarding host's
 ARP tables and it will generate an ARP request for the target host.
 This presents the scenario (actually observed) of a 4.3BSD gateway
 running the rwho program, which broadcasts a packet once a minute,



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RFC 1027 ARP and Transparent Subnet Gateways October 1987


 causing every 4.2BSD host on the local physical network to generate
 an ARP request at the same time. The same problem occurs with any
 subnet broadcast address, whether the local part is all zeros or all
 ones.

 Thus a subnet gateway in a network with hosts that do not understand
 subnets must take care not to use subnet broadcast addresses:
 instead it must use the IP network directed broadcast address
 instead.

 Finally, since many hosts running out-of-date software will still be
 using (and expecting) old-style all-zeros IP network broadcast
 addresses, the gateway must send its broadcast addresses out in that
 form, e.g., 128.83.0.0. It might be safe to also send a duplicate
 packet with all ones in the local part, e.g., 128.83.255.255. It is
 not clear whether the local network broadcast address of all ones,
 255.255.255.255, will cause ill effects, but it is very likely that
 it will not be recognized by many hosts that are running older
 software.

3. Implementation in 4.3BSD

 Subnet gateways using ARP have been implemented by a number of
 different people. The particular method described in this memo was
 first implemented in 4.2BSD on top of retrofitted beta-test 4.3BSD
 subnet code, and has since been reimplemented as an add-on to the
 distributed 4.3BSD sources. The latter implementation is described
 here.

 Most of the new kernel code for the subnet ARP gatewaying function
 is in the generic Ethernet interface module, netinet/if_ether.c. It
 consists of eight lines in in_arpinput that perform a couple of
 quick checks (to ensure that the facility is enabled on the source
 interface and that the source and target addresses are on different
 subnets), call a new routine, if_subarp, for further checks, and
 then build the ARP response if all checks succeed. This code is
 only reached when an ARP request is received, and does nothing if
 the facility is not enabled on the source interface. Thus
 performance of the gateway should be very little degraded by this
 addition. (Performance of the requesting host should also be
 similar to the latter case, as the only difference there is between
 efficiency of the ARP cache and of the routing tables).

 The routine if_subarp (about sixty lines) ensures that the source
 and target addresses are on the same IP network and that the target
 address is none of the four kinds of directed broadcast address. It
 then attempts to find a path to the target either by finding a
 network interface with the desired subnet or by looking in the



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RFC 1027 ARP and Transparent Subnet Gateways October 1987


 routing tables. Even if a network interface is found that leads to
 the target, for a reply to be sent the ARP gateway must be enabled
 on that interface and the target and source interfaces must be
 different.

 The file netinet/route.c has a static routing entry structure
 definition added, and modifications of about eight lines are made to
 the main routing table lookup routine, rtalloc, to recognize a
 pointer to that structure (when passed by if_subarp) as a direction
 to not use the default route in this routing check. The processor
 priority level (critical section protection) around the inner
 routing lookup check is changed to a higher value, as the routine
 may now be called from network interface interrupts as well as from
 the internal software interrupts that drive processing of IP and
 other high level protocols. This raised processor priority could
 conceivably slow the whole kernel somewhat if there are many routing
 checks, but since the critical section is fast, the effect should be
 small.

 A key kernel modification is about fifteen lines added to the
 routine ip_output in netinet/ip_output.c. It changes subnet
 broadcast addresses in packets originating at the gateway to IP
 network broadcast addresses so that hosts without subnet code (or
 with their network masks set to ignore subnets) will recognize them
 as broadcast addresses. This section of code is only used if the
 ARP gateway is turned on for the outgoing interface, and only
 affects subnet broadcast addresses.

 A new routine, in_mainnetof, of about fifteen lines, is added to
 netinet/in.c to return the IP network number (without subnet number)
 from an IP address. It is called from if_subarp and ip_output.

 Two kernel parameter files have one line added to each: net/if.h
 has a definition of a bit in the network interface structure to
 indicate whether subnet ARP gateways are enabled, and netinet/in.h
 refers to in_mainnetof.

 In addition to these approximately 110 lines of kernel source
 additions, there is one user-level modification. The source to the
 command ifconfig, which is used to set addresses and network masks
 of network interfaces, has four lines added to allow it to turn the
 subnet ARP gateway facility on or off, for each interface. This is
 documented in eleven new lines in the manual entry for that command.








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RFC 1027 ARP and Transparent Subnet Gateways October 1987


4. Availability

 The 4.3BSD implementation is currently available by anonymous FTP
 (login anonymous, password guest) from sally.utexas.edu as
 pub/subarp, which is a 4.3BSD "diff -c" listing from the 4.3BSD
 sources that were distributed in September 1986.

 This implementation was not included in the 4.3BSD distribution
 proper because U.C. Berkeley CSRG thought that that would reduce the
 incentive for vendors to implement subnets per RFC-950. The authors
 concur. Nonetheless, there are circumstances in which the use of
 transparent subnet ARP gateways is indispensable.

References

 1. Mogul, J., and J. Postel, "Internet Standard Subnetting
 Procedure", RFC-950, Stanford University and USC/Information
 Sciences Institute, August 1985.

 2. Mogul, J., "Broadcasting Internet Datagrams in the Presence of
 Subnets", RFC-922, Computer Science Department, Stanford
 University, October 1984.

 3. Plummer, D., "An Ethernet Address Resolution Protocol or
 Converting Network Protocol Addresses to 48-bit Ethernet
 Addresses for Transmission on Ethernet Hardware", RFC-826,
 Symbolics, November 1982.

 4. Postel, J., "Multi-LAN Address Resolution", RFC-925,
 USC/Information Sciences Institute, October 1984.

 5. Carl-Mitchell, S., and J. S. Quarterman, "Nameservers in a Campus
 Domain", SIGCUE Outlook, Vol.19, No.1/2, pp.78-88, ACM SIG
 Computer Uses in Education, P.O. Box 64145, Baltimore, MD 21264,
 Spring/Summer 1986.

 6. Braden, R., and J. Postel, "Requirements for Internet Gateways",
 RFC-1009, USC/Information Sciences Institute, June 1987.













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RFC 1027: Using ARP to implement transparent subnet gateways