|Title||The Classless Static Route Option for Dynamic Host Configuration
Protocol (DHCP) version 4
|Author||T. Lemon, S. Cheshire, B. Volz
Network Working Group T. Lemon
Request for Comments: 3442 Nominum, Inc.
Updates: 2132 S. Cheshire
Category: Standards Track Apple Computer, Inc.
The Classless Static Route Option for
Dynamic Host Configuration Protocol (DHCP) version 4
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright (C) The Internet Society (2002). All Rights Reserved.
This document defines a new Dynamic Host Configuration Protocol
(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.
This option obsoletes the Static Route option (option 33) defined in
RFC 2132 .
The IP protocol  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
ways - using a routing information protocol such as RIP , ICMP
router discovery [6,9] or using the DHCP Router option, defined in
RFC 2132 .
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
The DHCP protocol as defined in RFC 2131  and the options defined
in RFC 2132  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 STD 5, RFC 791  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 [7, 10] 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 RFC
950 ) 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 separate
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 STD 5, RFC 791  and
STD 5, RFC 950 .
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 BCP 14, RFC 2119 .
This document also uses the following terms:
DHCP client or "client" is an Internet host using DHCP to
obtain configuration parameters such as a network address.
A DHCP server or "server" is an Internet host that returns
configuration parameters to DHCP clients.
Any set of network attachment points that will all receive 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
A "link" is sometimes referred to as a broadcast domain or
physical network segment.
Classless Route Option Format
The code for this option is 121, 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
| 121 | 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 in
the following table:
Width of subnet mask Number of significant octets
1- 8 1
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
10.0.0.0 255.255.255.0 126.96.36.199
10.17.0.0 255.255.0.0 16.10.17
10.27.129.0 255.255.255.0 188.8.131.52
10.229.0.128 255.255.255.128 184.108.40.206.128
10.198.122.47 255.255.255.255 220.127.116.11.47
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.
DHCP clients that support this option and that send a DHCP Parameter
Request List option MUST request both this option and the Router
option  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 Classless Static Routes option and
a Router option, the DHCP client MUST ignore the Router option.
Similarly, if the DHCP server returns both a Classless Static Routes
option and a Static Routes option, the DHCP client MUST ignore the
Static Routes option.
After deriving a subnet number and subnet mask from each destination
descriptor, the DHCP client MUST zero any bits in the subnet number
where the corresponding bit in the mask is zero. In other words, the
subnet number installed in the routing table is the logical AND of
the subnet number and subnet mask given in the Classless Static
Routes option. For example, if the server sends a route with a
destination of 18.104.22.168 (hexadecimal 81D4B184) and a subnet
mask of 255.255.255.128 (hexadecimal FFFFFF80), the client will
install a route with a destination of 22.214.171.124 (hexadecimal
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  option if the DHCP client's TCP/IP
stack is capable of receiving larger 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 clients requesting this option, and DHCP servers sending this
option, MUST implement DHCP option concatenation . In the
terminology of RFC 3396 , the Classless Static Route Option is a
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.
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.
Potential exposures to attack in the DHCP protocol are discussed in
section 7 of the DHCP protocol specification  and in
Authentication for DHCP Messages .
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 invalid, 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 RFC 2132  exhibit the same vulnerability.
This DHCP option has been allocated the option code 121 in the list
of DHCP option codes that the IANA maintains.
 Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981.
 Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
 Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
 Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
Extensions", RFC 2132, March 1997.
 Lemon, T. and S. Cheshire, "Encoding Long Options in the Dynamic
Host Configuration Protocol (DHCPv4)", RFC 3396, November 2002.
 Postel, J., "Internet Control Message Protocol", STD 5, RFC 792,
 Mogul, J. and J. Postel, "Internet Standard Subnetting
Procedure", STD 5, RFC 950, August 1985.
 Hedrick, C., "Routing Information Protocol", RFC 1058, June
 Deering, S., "ICMP Router Discovery Messages", RFC 1256,
 Pummill, T. and B. Manning, "Variable Length Subnet Table For
IPv4", RFC 1878, December 1995.
 Droms, R. and W. Arbaugh, "Authentication for DHCP Messages",
RFC 3118, June 2001.
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