Network Working Group Ted Lemon
Internet Draft Nominum, Inc.
Obsoletes: draft-ietf-dhc-csr-06.txt Stuart Cheshire
Apple Computer, Inc.
Bernie Volz
Ericsson
July, 2002
Expires January, 2003
The Classless Static Route Option for DHCPv4
<draft-ietf-dhc-csr-07.txt>
Status of this Memo
This document is an Internet-Draft and is subject to all provisions
of Section 10 of RFC2026.
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Abstract
This document defines a new DHCP option which is passed from the
DHCP Server to the DHCP Client to configure a list of static routes
in the client. The network destinations in these routes are
classless - each routing table entry includes a subnet mask.
Introduction
This option supersedes the Static Route option (option 33) defined
in RFC2132 [2].
The IP protocol [4] uses routers to transmit packets from hosts
connected to one IP subnet to hosts connected to a different IP
subnet. When an IP host (the source host) wishes to transmit a
packet to another IP host (the destination), it consults its
routing table to determine the IP address of the router that should
be used to forward the packet to the destination host.
The routing table on an IP host can be maintained in a variety of
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ways - using a routing information protocol such as RIP [5], ICMP
router discovery [6,7] or using the DHCP Router option, defined in
RFC2132 [2].
In a network that already provides DHCP service, using DHCP to
update the routing table on a DHCP client has several virtues. It
is efficient, since it makes use of messages that would have been
sent anyway. It is convenient - the DHCP server configuration
is already being maintained, so maintaining routing information, at
least on a relatively stable network, requires little extra work.
If DHCP service is already in use, no additional infrastructure
need be deployed.
The DHCP protocol as defined in RFC2131 [1] and the options defined
in RFC2132 [2] only provide a mechanism for installing a default
route or installing a table of classful routes. Classful routes
are routes whose subnet mask is implicit in the subnet number - see
section 3.2 of RFC791 [4] for details on classful routing.
Classful routing is no longer in common use, so the DHCP Static
Route option is no longer useful. Currently, classless routing,
described in [8] and [9], is the most commonly-deployed form of
routing on the Internet. In classless routing, IP addresses
consist of a network number (the combination of the network number
and subnet number described in [8]) and a host number.
In classful IP, the network number and host number are derived from
the IP address using a bitmask whose value is determined by the
first few bits of the IP address. In classless IP, the network
number and host number are derived from the IP address using a
seperate quantity, the subnet mask. In order to determine the
network to which a given route applies, an IP host must know both
the network number AND the subnet mask for that network.
The Static Routes option (option 33) does not provide a subnet mask
for each route - it is assumed that the subnet mask is implicit in
whatever network number is specified in each route entry. The
Classless Static Routes option does provide a subnet mask for each
entry, so that the subnet mask can be other than what would be
determined using the algorithm specified in RFC791 [4] and RFC950
[8].
Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY" and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [3].
This document also uses the following terms:
"DHCP client"
DHCP client or "client" is an Internet host using DHCP to
obtain configuration parameters such as a network address.
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"DHCP server"
A DHCP server or "server" is an Internet host that returns
configuration parameters to DHCP clients.
"link"
Any set of all network attachment points that will recieve
a link-layer broadcast sent on any one of the attachment
points. This term is used in DHCP because in some cases
more than one IP subnet may be configured on a link. DHCP
uses a local-network (all-ones) broadcast, which is not
subnet-specific, and will therefore reach all nodes
connected to the link, regardless of the IP subnet or
subnets on which they are configured.
A "link" is sometimes referred to as a broadcast domain or
physical network segment.
Classless Route Option Format
The code for this option is TBD, and its minimum length is 5 bytes.
This option can contain one or more static routes, each of which
consists of a destination descriptor and the IP address of the
router that should be used to reach that destination.
Code Len Destination 1 Router 1
+-----+---+----+-----+----+----+----+----+----+
| TBD | n | d1 | ... | dN | r1 | r2 | r3 | r4 |
+-----+---+----+-----+----+----+----+----+----+
Destination 2 Router 2
+----+-----+----+----+----+----+----+
| d1 | ... | dN | r1 | r2 | r3 | r4 |
+----+-----+----+----+----+----+----+
In the above example, two static routes are specified.
Destination descriptors describe the IP subnet number and subnet
mask of a particular destination using a compact encoding. This
encoding consists of one octet describing the width of the subnet
mask, followed by all the significant octets of the subnet number.
The width of the subnet mask describes the number of one bits in
the mask, so for example a subnet with a subnet number of
10.0.127.0 and a netmask of 255.255.255.0 would have a subnet mask
width of 24.
The significant portion of the subnet number is simply all of the
octets of the subnet number where the corresponding octet in the
subnet mask is non-zero. The number of significant octets is the
width of the subnet mask divided by eight, rounding up, as shown
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in the following table:
Width of subnet mask Number of significant octets
0 0
1- 8 1
9-16 2
17-24 3
25-32 4
The following table contains some examples of how various subnet
number/mask combinations can be encoded:
Subnet number Subnet mask Destination descriptor
0 0 0
10.0.0.0 255.0.0.0 8.10
10.0.0.0 255.255.255.0 24.10.0.0
10.17.0.0 255.255.0.0 16.10.17
10.27.129.0 255.255.255.0 24.10.27.129
10.229.0.128 255.255.255.128 25.10.229.0.128
10.198.122.47 255.255.255.255 32.10.198.122.47
Local Subnet Routes
In some cases more than one IP subnet may be configured on a link.
In such cases, a host whose IP address is in one IP subnet in the
link could communicate directly with a host whose IP address is in
a different IP subnet on the same link. In cases where a client is
being assigned an IP address on an IP subnet on such a link,
for each IP subnet in the link other than the IP subnet on which
the client has been assigned the DHCP server MAY be configured to
specify a router IP address of 0.0.0.0.
For example, consider the case where there are three IP subnets
configured on a link: 10.0.0/24, 192.168.0/24, 10.0.21/24. If the
client is assigned an IP address of 10.0.21.17, then the server
could include a route with a destination of 10.0.0/24 and a router
address of 0.0.0.0, and also a route with a destination of
192.168.0/24 and a router address of 0.0.0.0.
A DHCP client whose underlying TCP/IP stack does not provide this
capability MUST ignore routes in the Classless Static Routes option
whose router IP address is 0.0.0.0. Please note that the behavior
described here only applies to the Classless Static Routes option,
not to the Static Routes option nor the Router option.
DHCP Client Behavior
DHCP clients that do not support this option MUST ignore it if it
is received from a DHCP server. DHCP clients that support this
option MUST install the routes specified in the option, except as
specified in the Local Subnet Routes section. DHCP clients that
support this option MUST NOT install the routes specified in the
Static Routes option (option code 33) if both a Static Routes
option and the Classless Static Routes option are provided.
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DHCP clients that support this option and that send a DHCP
Parameter Request List option MUST request both this option and the
Router option [2] in the DHCP Parameter Request List.
DHCP clients that support this option and send a parameter request
list MAY also request the Static Routes option, for compatibility
with older servers that don't support Classless Static Routes.
The Classless Static Routes option code MUST appear in the
parameter request list prior to both the Router option code and the
Static Routes option code, if present.
If the DHCP server returns both a Router option and a Classless
Static Routes option, the DHCP client MUST ignore the Router
option.
After deriving a subnet number and subnet mask from each
destination descriptor, the DHCP client MUST set any bits in the
subnet number that are zero in the subnet mask to zero. For
example, if the server sends a route with a destination of
129.210.177.132 (hexadecimal 81D4B184) and a subnet mask of
255.255.255.128 (hexadecimal FFFFFF80), the client will install a
route with a destination of 129.210.177.128 (hexadecimal
81D4B180).
Requirements to avoid sizing constraints
Because a full routing table can be quite large, the standard 576
octet maximum size for a DHCP message may be too short to contain
some legitimate Classless Static Route options. Because of this,
clients implementing the Classless Static Route option SHOULD send
a Maximum DHCP Message Size [2] option if the DHCP client's TCP/IP
stack is capable of reassembling fragmented IP datagrams. In this
case, the client SHOULD set the value of this option to at least
the MTU of the interface that the client is configuring. The
client MAY set the value of this option higher, up to the size of
the largest UDP packet it is prepared to accept. (Note that the
value specified in the Maximum DHCP Message Size option is the
total maximum packet size, including IP and UDP headers.)
DHCP servers sending this option MUST use the technique described
in [10] for sending options larger than 255 bytes when storing this
option in outgoing DHCP packets. DHCP clients supporting this
option MUST support the technique described in [10] when reading
this option from incoming DHCP packets.
DHCP Server administrator responsibilities
Many clients may not implement the Classless Static Routes option.
DHCP server administrators should therefore configure their DHCP
servers to send both a Router option and a Classless Static Routes
option, and should specify the default router(s) both in the
Router option and in the Classless Static Routes option.
DHCP Server Considerations
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When a DHCP client requests the Classless Static Routes option and
also requests either or both of the Router option and the Static
Routes option, and the DHCP server is sending Classless Static
Routes options to that client, the server SHOULD NOT include the
Router or Static Routes options.
Security Considerations
Potential exposures to attack in the DHCP protocol are discussed in
section 7 of the DHCP protocol specification [1] and in
Authentication for DHCP Messages [5].
The Classless Static Routes option can be used to misdirect network
traffic by providing incorrect IP addresses for routers. This can
be either a Denial of Service attack, where the router IP address
given is simply valid, or can be used to set up a man-in-the-middle
attack by providing the IP address of a potential snooper. This is
not a new problem - the existing Router and Static Routes options
defined in RFC2132 [2] exhibit the same vulnerability.
IANA Considerations
This DHCP option will require the allocation of an option code in
the list of DHCP option codes that the IANA maintains.
References
[1] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
Bucknell University, March 1997.
[2] Alexander, S. and Droms, R., "DHCP Options and BOOTP Vendor
Extensions", RFC 2132, Silicon Graphics, Inc., Bucknell
University, March 1997.
[3] Bradner, S., "Key words for use in RFCs to indicate requirement
levels", RFC 2119, Harvard University, March 1997.
[4] Postel, J., "Internet Protocol", RFC 791, USC/Information
Sciences Institute, September 1981.
[5] Hedrick, C.L., "Routing Information Protocol", RFC 1058,
Rutgers University, June 1, 1988.
[6] Deering, S., "ICMP Router Discovery Messages", RFC 1256,
Xerox PARC, September 1991.
[7] Postel, J., "Internet Control Message Protocol", RFC 792,
USC/Information Sciences Institute, September 1981.
[8] Mogul, J., Postel, J., "Internet Standard Subnetting
Procedure", RFC950, Stanford University, USC/Information
Sciences Institute, August 1985.
[9] Pummill, T., Manning, B., "Variable Length Subnet Table For
IPv4", RFC1878, Alantec, USC/Information Sciences Institute,
December, 1995.
[10] Lemon, T., Cheshire, S., "Encoding Long DHCP Options",
draft-ietf-dhc-concat-05.txt, Nominum, Inc., Apple Computer,
Inc., July, 2002.
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Author Information
Ted Lemon
Nominum, Inc.
2385 Bay Road
Redwood City, CA 94063
email: Ted.Lemon@nominum.com
Stuart Cheshire
Apple Computer, Inc.
1 Infinite Loop
Cupertino
California 95014
USA
Phone: +1 408 974 3207
EMail: rfc@stuartcheshire.org
Bernie Volz
Ericsson
959 Concord Street
Framingham, MA, 01701
Phone: +1 508 875 3162
EMail: bernie.volz@ericsson.com
Expiration
This document will expire on December 31, 2002.
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