draft-ietf-dhc-csr-04.txt   draft-ietf-dhc-csr-05.txt 
Dynamic Host Configuration Working Group Ted Lemon
Network Working Group Ted Lemon
Internet Draft Nominum, Inc. Internet Draft Nominum, Inc.
Obsoletes: draft-ietf-dhc-csr-03.txt February, 2001 Obsoletes: draft-ietf-dhc-csr-04.txt July, 2001
Expires August, 2001 Expires January, 2002
The Classless Static Route Option for DHCP The Classless Static Route Option for DHCP
<draft-ietf-dhc-csr-04.txt> <draft-ietf-dhc-csr-05.txt>
Status of this Memo Status of this Memo
This document is an Internet-Draft and is in full conformance with This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. all provisions of Section 10 of RFC2026.
This document is an Internet-Draft. Internet-Drafts are working This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas, documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts. working documents as Internet-Drafts.
skipping to change at line 160 skipping to change at line 160
Subnet number Subnet mask Destination descriptor Subnet number Subnet mask Destination descriptor
0 0 0 0 0 0
10.0.0.0 255.0.0.0 8.10 10.0.0.0 255.0.0.0 8.10
10.17.0.0 255.255.0.0 16.10.17 10.17.0.0 255.255.0.0 16.10.17
10.27.129.0 255.255.255.0 24.10.27.129 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 10.229.0.128 255.255.255.128 25.10.229.0.128
10.198.122.47 255.255.255.255 32.10.198.122.47 10.198.122.47 255.255.255.255 32.10.198.122.47
Local Subnet Routes Local Subnet Routes
In the case where there is more than one IP subnet connected to the In some cases more than one IP subnet may be configured within a
local network, the DHCP server MAY send routes for those subnets given network broadcast domain. In such cases, a host whose IP
specifying an IP destination address of 0.0.0.0. This statement address is in one IP subnet in the broadcast domain could communicate
applies strictly to the Classless Static Routes option. The directly with a host whose IP address is in a different IP subnet in
behaviour of the DHCP client in the case that a Routers option the same broadcast domain. In cases where a client is being
contains a destination of 0.0.0.0 is not specified here. assigned an IP address on an IP subnet in such a broadcast domain,
for each IP subnet in the broadcast domain 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 particular broadcast domain: 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 Client Behavior
DHCP clients that do not support this option MUST ignore it if it DHCP clients that do not support this option MUST ignore it if it
is received from a DHCP server. DHCP clients that support this is received from a DHCP server. DHCP clients that support this
option MUST install the routes specified in the option. DHCP option MUST install the routes specified in the option, except as
clients that support this option MUST NOT install the routes specified in the Local Subnet Routes section. DHCP clients that
specified in the Static Routes option (option code 33) if both a support this option MUST NOT install the routes specified in the
Static Routes option and the Classless Static Routes option are Static Routes option (option code 33) if both a Static Routes
provided. option and the Classless Static Routes option are provided.
DHCP clients that support this option and that send a DHCP DHCP clients that support this option and that send a DHCP
Parameter Request List option MUST request both this option and the Parameter Request List option MUST request both this option and the
Router option [2] in the DHCP Parameter Request List. DHCP clients Router option [2] in the DHCP Parameter Request List. DHCP clients
that support this option and send a parameter request list MUST NOT that support this option and send a parameter request list MUST NOT
request the Static Routes option. The Classless Static Routes request the Static Routes option. The Classless Static Routes
option code SHOULD appear in the parameter request list prior to option code SHOULD appear in the parameter request list prior to
the Routers option code. the Router option code.
If the DHCP server returns both a Router option and a Classless If the DHCP server returns both a Router option and a Classless
Static Routes option, the DHCP client MUST ignore the Routers Static Routes option, the DHCP client MUST ignore the Router
option. option.
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 implement the
Classless Static Routes option MUST 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 found in the Classless Static Routes option that have
a destination of 0.0.0.0.
After deriving a subnet number and subnet mask from each After deriving a subnet number and subnet mask from each
destination descriptor, the DHCP client SHOULD check each route to destination descriptor, the DHCP client SHOULD check the
determine if there are any bits in the destination network number combination of the network number and the subnet mask for validity.
whose value is one whose corresponding value in the subnet mask is If the network number contains nonzero bits beyond the subnet mask,
zero, and SHOULD NOT install any routes for which this is the case. the client SHOULD discard that route. For example, the client
For example, the client should not install a route with a should not install a route with a destination of 129.210.377.4 and
destination of 129.210.377.4 and a subnet mask of 255.255.255.128. a subnet mask of 255.255.255.128.
Requirements to avoid sizing constraints Requirements to avoid sizing constraints
Because a full routing table can be quite large, the standard 576 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 octet maximum size for a DHCP message may be too short to contain
some legitimate Classless Static Route options. Because of this, some legitimate Classless Static Route options. Because of this,
clients implementing the Classless Static Route option SHOULD send clients implementing the Classless Static Route option SHOULD send
a Maximum DHCP Message Size [2] option if the DHCP client's TCP/IP a Maximum DHCP Message Size [2] option if the DHCP client's TCP/IP
stack is capable of reassembling fragmented IP datagrams. In this stack is capable of reassembling fragmented IP datagrams. In this
case, the client SHOULD set the value of this option to the MTU of case, the client SHOULD set the value of this option to the MTU of
the interface that the client is configuring. the interface that the client is configuring. If the client
supports UDP fragmentation, it MAY set the value of this option to
the size of the largest UDP packet it is prepared to accept.
DHCP servers sending this option MUST use the technique described DHCP servers sending this option MUST use the technique described
in [10] for sending options larger than 255 bytes when storing this in [10] for sending options larger than 255 bytes when storing this
option in outgoing DHCP packets. DHCP clients supporting this option in outgoing DHCP packets. DHCP clients supporting this
option MUST support the technique described in [10] when reading option MUST support the technique described in [10] when reading
this option from incoming DHCP packets. this option from incoming DHCP packets.
DHCP Server administrator responsibilities DHCP Server administrator responsibilities
Many clients may not implement the Classless Static Routes option. Many clients may not implement the Classless Static Routes option.
DHCP server administrators should therefore configure their DHCP DHCP server administrators should therefore configure their DHCP
servers to send both a Routers option and a Classless Static Routes servers to send both a Router option and a Classless Static Routes
option, and should specify the default router(s) both in the option, and should specify the default router(s) both in the
Routers option and in the Classless Static Routes option. Router option and in the Classless Static Routes option.
DHCP Server Considerations DHCP Server Considerations
When a DHCP client requests both the Routers option and the When a DHCP client requests both the Router option and the
Classless Static Routes option, and the DHCP server is configured Classless Static Routes option, and the DHCP server is configured
with both a Classless Static Routes option and a Routers option with both a Classless Static Routes option and a Router option
that applies to the client, the DHCP server MAY exclude the Routers that applies to the client, the DHCP server MAY exclude the Router
option from its response. option from its response.
Security Considerations Security Considerations
DHCP currently provides no authentication or security mechanisms. DHCP currently provides no authentication or security mechanisms.
Potential exposures to attack are discussed in section 7 of the DHCP Potential exposures to attack are discussed in section 7 of the DHCP
protocol specification [1]. The Classless Static Routes option can protocol specification [1]. The Classless Static Routes option can
be used to misdirect network traffic by providing incorrect IP be used to misdirect network traffic by providing incorrect IP
addresses for routers. addresses for routers.
skipping to change at line 267 skipping to change at line 277
Sciences Institute, September 1981. Sciences Institute, September 1981.
[5] Hedrick, C.L., "Routing Information Protocol", RFC 1058, [5] Hedrick, C.L., "Routing Information Protocol", RFC 1058,
Rutgers University, June 1, 1988. Rutgers University, June 1, 1988.
[6] Deering, S., "ICMP Router Discovery Messages", RFC 1256, [6] Deering, S., "ICMP Router Discovery Messages", RFC 1256,
Xerox PARC, September 1991. Xerox PARC, September 1991.
[7] Postel, J., "Internet Control Message Protocol", RFC 792, [7] Postel, J., "Internet Control Message Protocol", RFC 792,
USC/Information Sciences Institute, September 1981. USC/Information Sciences Institute, September 1981.
[8] Mogul, J., Postel, J., "Internet Standard Subnetting [8] Mogul, J., Postel, J., "Internet Standard Subnetting
Procedure", RFC950, Stanford University, USC/Information Procedure", RFC950, Stanford University, USC/Information
Sciences Institute, August 1985. Sciences Institute, August 1985.
[9] Pummill, T., Manning, B., "Variable Length Subnet Table For [9] Pummill, T., Manning, B., "Variable Length Subnet Table For
IPv4", RFC1878, Alantec, USC/Information Sciences Institute, IPv4", RFC1878, Alantec, USC/Information Sciences Institute,
December, 1995. December, 1995.
[10] Lemon, T., "Encoding Long DHCP Options", [10] Lemon, T., "Encoding Long DHCP Options",
draft-ietf-dhc-concat-00.txt, Nominum, Inc., February, 2001. draft-ietf-dhc-concat-01.txt, Nominum, Inc., July, 2001.
Author Information Author Information
Ted Lemon Ted Lemon
Nominum, Inc. Nominum, Inc.
950 Charter Street 950 Charter Street
Redwood City, CA 94043 Redwood City, CA 94043
email: Ted.Lemon@nominum.com email: Ted.Lemon@nominum.com
Expiration Expiration
This document will expire on August 31, 2001. This document will expire on January 31, 2002.
Full Copyright Statement Full Copyright Statement
Copyright (C) The Internet Society (2000-2001). All Rights Copyright (C) The Internet Society (2000-2001). All Rights
Reserved. Reserved.
This document and translations of it may be copied and furnished to This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any and distributed, in whole or in part, without restriction of any
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