draft-ietf-dhc-csr-03.txt   draft-ietf-dhc-csr-04.txt 
Network Working Group Ted Lemon Dynamic Host Configuration Working Group Ted Lemon
Internet Draft Nominum, Inc. Internet Draft Nominum, Inc.
Obsoletes: draft-ietf-dhc-csr-03.txt December, 2000 Obsoletes: draft-ietf-dhc-csr-03.txt February, 2001
Expires May, 2001 Expires August, 2001
The Classless Static Route Option for DHCP The Classless Static Route Option for DHCP
<draft-ietf-dhc-csr-02.txt> <draft-ietf-dhc-csr-04.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 43 skipping to change at line 43
This document defines a new DHCP option which is passed from the 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 DHCP Server to the DHCP Client to configure a list of static routes
in the client. This option supersedes the Static Route option in the client. This option supersedes the Static Route option
(option 33) defined in [2]. (option 33) defined in [2].
Introduction Introduction
The IP protocol [4] uses routers to transmit packets from hosts The IP protocol [4] uses routers to transmit packets from hosts
connected to one IP subnet to hosts connected to a different IP connected to one IP subnet to hosts connected to a different IP
subnet. When an IP host (the source host) wishes to transmit a subnet. When an IP host (the source host) wishes to transmit a
packet to another IP host (the destination), it first checks the IP packet to another IP host (the destination), it consults its
address of the destination host to see if it is on a subnet to routing table to determine the IP address of the router that should
which the source host is connected. If the destination host's IP be used to forward the packet to the destination host.
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 The routing table on an IP host can be maintained in a variety of
ways - using a routing information protocol such as RIP [5], ICMP ways - using a routing information protocol such as RIP [5], ICMP
router discovery [6,7] or using the DHCP Router option, defined in router discovery [6,7] or using the DHCP Router option, defined in
[2]. [2].
In a network that already provides DHCP service, using DHCP to In a network that already provides DHCP service, using DHCP to
update the routing table on a DHCP client has several virtues. It update the routing table on a DHCP client has several virtues. It
is efficient, since it makes use of messages that would have been is efficient, since it makes use of messages that would have been
sent anyway. It is convenient - the DHCP server configuration sent anyway. It is convenient - the DHCP server configuration
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and subnet number described in [8]) and a host number. and subnet number described in [8]) and a host number.
In classed IP, the network number and host number are derived from 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 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 few bits of the IP address. In classless IP, the network number
and host number are derived from the IP address using a seperate and host number are derived from the IP address using a seperate
quantity, the subnet mask. In order to determine the network to quantity, the subnet mask. In order to determine the network to
which a given route applies, an IP host must know both the network which a given route applies, an IP host must know both the network
number AND the subnet mask for that network. number AND the subnet mask for that network.
The Static Routes option does not provide a subnet mask for each The Static Routes option (option 33) does not provide a subnet mask
route - it is assumed that the subnet mask is implicit in whatever for each route - it is assumed that the subnet mask is implicit in
network number is specified in each route entry. The Classless whatever network number is specified in each route entry. The
Static Routes option does provide a subnet mask for each entry, so Classless Static Routes option does provide a subnet mask for each
that the subnet mask can be other than what would be determined entry, so that the subnet mask can be other than what would be
using the algorithm specified in [4] and [8]. determined using the algorithm specified in [4] and [8].
Definitions Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY" and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY" and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [3]. document are to be interpreted as described in RFC 2119 [3].
This document also uses the following terms: This document also uses the following terms:
"DHCP client" "DHCP client"
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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 the case where there is more than one IP subnet connected to the
local network, the DHCP server MAY send routes for those subnets local network, the DHCP server MAY send routes for those subnets
that specify an IP destination address of 0.0.0.0. DHCP clients specifying an IP destination address of 0.0.0.0. This statement
that implement this option MUST check for an IP destination address applies strictly to the Classless Static Routes option. The
of 0.0.0.0, and MUST EITHER configure their IP stack to ARP for IP behaviour of the DHCP client in the case that a Routers option
addresses whose routing destination is 0.0.0.0, OR ignore routes contains a destination of 0.0.0.0 is not specified here.
with a destination of 0.0.0.0. DHCP clients that support ARPing
as described here MUST ignore the Router option (option code 3) if
the Router option contains the client's own IP address.
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. DHCP
clients that support this option MUST NOT install the routes clients that support this option MUST NOT install the routes
specified in the Static Routes option (option code 33) if both a specified in the Static Routes option (option code 33) if both a
Static Routes option and the Classless Static Routes option are Static Routes option and the Classless Static Routes option are
provided. 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. 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 a Router option, clients that support If the DHCP server returns both a Router option and a Classless
the Classless Static Routes option MUST use the default route(s) Static Routes option, the DHCP client MUST ignore the Routers
listed in the Router option in addition to the routes listed option.
in the Classless Static Routes 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 each route to
determine if are any bits in the destination network number whose determine if there are any bits in the destination network number
value is one whose corresponding value in the subnet mask is zero, whose value is one whose corresponding value in the subnet mask is
and SHOULD NOT install any routes for which this is the case. For zero, and SHOULD NOT install any routes for which this is the case.
example, the client should not install a route with a destination For example, the client should not install a route with a
of 129.210.377.4 and a subnet mask of 255.255.255.128. 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 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.
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 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 servers to send both a Routers option and a Classless Static Routes
Routes option, and should specify all default routes in the Routers option, and should specify the default router(s) both in the
option, and not specify any default routes in the Classless Routers option and in the Classless Static Routes option.
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.
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.
References References
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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",
draft-ietf-dhc-concat-00.txt, Nominum, Inc., February, 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 May 31, 2001. This document will expire on August 31, 2001.
Full Copyright Statement Full Copyright Statement
Copyright (C) The Internet Society (2000). All Rights Reserved. Copyright (C) The Internet Society (2000-2001). All Rights
Reserved.
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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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included on all such copies and derivative works. However, this included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing document itself may not be modified in any way, such as by removing
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Internet organizations, except as needed for the purpose of Internet organizations, except as needed for the purpose of
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