NetworkDynamic Host Configuration Working Group Ted Lemon
Internet Draft Nominum, Inc.
Obsoletes: draft-ietf-dhc-csr-03.txt December, 2000 February, 2001
Expires May, August, 2001
The Classless Static Route Option for DHCP
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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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. This option supersedes the Static Route option
(option 33) defined in .
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 first checks the IP
address of the destination host to see if it is on a subnet to
which the source host is connected. If the destination host's IP
address is not on a subnet to which the source host is connected,
then the source host 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 [6,7] or using the DHCP Router option, defined in
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  and the options defined in 
only provide a mechanism for installing a default route or
installing a table of classed routes. Classed routes are routes
whose subnet mask is implicit in the subnet number - see section
3.2 of  for details on classed routing.
Classed routing is no longer in common use, so the DHCP Static
Route option is no longer useful. Currently, classless routing,
described in  and , 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 ) and a host number.
In classed 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  and .
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 .
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.
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 non-zero 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 non-zero portion of the subnet number is simply all of the
octets of the subnet number, with the least significant octets that
are zero omitted. For a subnet mask width of between 25 and 32,
the subnet number will be four octets. Mask widths of between 17
and 24 indicate a three-octet subnet number; between 9 and 16
indicate a two-octet subnet number, between 1 and 8 indicate a
one-octet number. As a special case, the default route may be
represented by a zero width, with no following subnet number.
Host routes are represented by a mask width of 32, followed by four
octets containing the IP address of the host.
The following table contains some examples:
Subnet number Subnet mask Destination descriptor
0 0 0
10.0.0.0 255.0.0.0 8.10
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 the case where there is more than one IP subnet connected to the
local network, the DHCP server MAY send routes for those subnets
specifying an IP destination address of 0.0.0.0. DHCP clients
that implement this option MUST check for an IP destination address
of 0.0.0.0, and MUST EITHER configure their IP stack This statement
applies strictly to ARP for IP
addresses whose routing destination is 0.0.0.0, OR ignore routes
with a destination the Classless Static Routes option. The
behaviour of 0.0.0.0. DHCP clients that support ARPing
as described here MUST ignore the Router option (option code 3) if DHCP client in the Router case that a Routers option
contains the client's own IP address. a destination of 0.0.0.0 is not specified here.
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. 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
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 MUST NOT
request the Static Routes option. The Classless Static Routes
option code SHOULD appear in the parameter request list prior to
the Routers option code.
If the DHCP server returns both a Router option and a Classless
Static Routes option, the DHCP client MUST ignore the Routers
Some TCP/IP stacks can be configured to send ARP request messages
on an interface for IP addresses that are on subnets not configured
for that interface. Consequently, DHCP clients that support implement the
Classless Static Routes option MUST use the default route(s)
listed in the Router option in addition check each route to see if the
IP destination is 0.0.0.0, and MUST EITHER configure their IP stack
to ARP for IP addresses whose routing destination is 0.0.0.0, OR
ignore routes listed found in the Classless Static Routes option. option that have
a destination of 0.0.0.0.
After deriving a subnet number and subnet mask from each
destination descriptor, the DHCP client SHOULD check each route to
determine if there are any bits in the destination network number
whose value is one whose corresponding value in the subnet mask is
zero, and SHOULD NOT install any routes for which this is the case.
For example, the client should not install a route with a
destination of 129.210.377.4 and a subnet mask of 255.255.255.128.
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 reassembling fragmented IP datagrams. In this
case, the client SHOULD set the value of this option to the MTU of
the interface that the client is configuring.
DHCP servers sending this option MUST use the technique described
in  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  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 Routers option and a Classless Static Routes
option, and should specify all the default routes router(s) both in the
option, option and not specify any default routes in the Classless Static Routes option.
DHCP Server Considerations
When a DHCP client requests both the Routers option and the
Classless Static Routes option, and the DHCP server is configured
with both a Classless Static Routes option and a Routers option
that applies to the client, the DHCP server MAY exclude the Routers
option from its response.
DHCP currently provides no authentication or security mechanisms.
Potential exposures to attack are discussed in section 7 of the DHCP
protocol specification . The Classless Static Routes option can
be used to misdirect network traffic by providing incorrect IP
addresses for routers.
 Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
Bucknell University, March 1997.
 Alexander, S. and Droms, R., "DHCP Options and BOOTP Vendor
Extensions", RFC 2132, Silicon Graphics, Inc., Bucknell
University, March 1997.
 Bradner, S., "Key words for use in RFCs to indicate requirement
levels", RFC 2119, Harvard University, March 1997.
 Postel, J., "Internet Protocol", RFC 791, USC/Information
Sciences Institute, September 1981.
 Hedrick, C.L., "Routing Information Protocol", RFC 1058,
Rutgers University, June 1, 1988.
 Deering, S., "ICMP Router Discovery Messages", RFC 1256,
Xerox PARC, September 1991.
 Postel, J., "Internet Control Message Protocol", RFC 792,
USC/Information Sciences Institute, September 1981.
 Mogul, J., Postel, J., "Internet Standard Subnetting
Procedure", RFC950, Stanford University, USC/Information
Sciences Institute, August 1985.
 Pummill, T., Manning, B., "Variable Length Subnet Table For
IPv4", RFC1878, Alantec, USC/Information Sciences Institute,
December, 1995 1995.
 Lemon, T., "Encoding Long DHCP Options",
draft-ietf-dhc-concat-00.txt, Nominum, Inc., February, 2001.
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Redwood City, CA 94043
This document will expire on May August 31, 2001.
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