draft-ietf-dhc-csr-05.txt   draft-ietf-dhc-csr-06.txt 
Network Working Group Ted Lemon Network Working Group Ted Lemon
Internet Draft Nominum, Inc. Internet Draft Nominum, Inc.
Stuart Cheshire
Apple Computer, Inc.
Bernie Volz
Ericsson
Obsoletes: draft-ietf-dhc-csr-04.txt July, 2001 Obsoletes: draft-ietf-dhc-csr-05.txt October, 2001
Expires January, 2002 Expires April, 2002
The Classless Static Route Option for DHCP The Classless Static Route Option for DHCP
<draft-ietf-dhc-csr-05.txt> <draft-ietf-dhc-csr-06.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 37 skipping to change at line 40
http://www.ietf.org/ietf/1id-abstracts.txt http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html. http://www.ietf.org/shadow.html.
Abstract Abstract
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 RFC2132 [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 consults its packet to another IP host (the destination), it consults its
routing table to determine the IP address of the router that should routing table to determine the IP address of the router that should
be used to forward the packet to the destination host. 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]. RFC2132 [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
is already being maintained, so maintaining routing information, at is already being maintained, so maintaining routing information, at
least on a relatively stable network, requires little extra work. least on a relatively stable network, requires little extra work.
If DHCP service is already in use, no additional infrastructure If DHCP service is already in use, no additional infrastructure
need be deployed. need be deployed.
The DHCP protocol as defined in [1] and the options defined in [2] The DHCP protocol as defined in RFC2131 [1] and the options defined
only provide a mechanism for installing a default route or in RFC2132 [2] only provide a mechanism for installing a default
installing a table of classed routes. Classed routes are routes route or installing a table of classed routes. Classed routes are
whose subnet mask is implicit in the subnet number - see section routes whose subnet mask is implicit in the subnet number - see
3.2 of [4] for details on classed routing. section 3.2 of RFC791 [4] for details on classed routing.
Classed routing is no longer in common use, so the DHCP Static Classed routing is no longer in common use, so the DHCP Static
Route option is no longer useful. Currently, classless routing, Route option is no longer useful. Currently, classless routing,
described in [8] and [9], is the most commonly-deployed form of described in [8] and [9], is the most commonly-deployed form of
routing on the Internet. In classless routing, IP addresses routing on the Internet. In classless routing, IP addresses
consist of a network number (the combination of the network number consist of a network number (the combination of the network number
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
skipping to change at line 88 skipping to change at line 91
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 (option 33) does not provide a subnet mask 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 for each route - it is assumed that the subnet mask is implicit in
whatever network number is specified in each route entry. The whatever network number is specified in each route entry. The
Classless Static Routes option does provide a subnet mask for each Classless Static Routes option does provide a subnet mask for each
entry, so that the subnet mask can be other than what would be entry, so that the subnet mask can be other than what would be
determined using the algorithm specified in [4] and [8]. determined using the algorithm specified in RFC791 [4] and RFC950
[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"
DHCP client or "client" is an Internet host using DHCP to DHCP client or "client" is an Internet host using DHCP to
obtain configuration parameters such as a network address. obtain configuration parameters such as a network address.
"DHCP server" "DHCP server"
A DHCP server or "server" is an Internet host that returns A DHCP server or "server" is an Internet host that returns
configuration parameters to DHCP clients. 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 Classless Route Option Format
The code for this option is TBD, and its minimum length is 5 bytes. 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 This option can contain one or more static routes, each of which
consists of a destination descriptor and the IP address of the consists of a destination descriptor and the IP address of the
router that should be used to reach that destination. router that should be used to reach that destination.
Code Len Destination 1 Router 1 Code Len Destination 1 Router 1
+-----+---+----+-----+----+----+----+----+----+ +-----+---+----+-----+----+----+----+----+----+
| TBD | n | d1 | ... | dN | r1 | r2 | r3 | r4 | | TBD | n | d1 | ... | dN | r1 | r2 | r3 | r4 |
skipping to change at line 130 skipping to change at line 148
Destination 2 Router 2 Destination 2 Router 2
+----+-----+----+----+----+----+----+ +----+-----+----+----+----+----+----+
| d1 | ... | dN | r1 | r2 | r3 | r4 | | d1 | ... | dN | r1 | r2 | r3 | r4 |
+----+-----+----+----+----+----+----+ +----+-----+----+----+----+----+----+
In the above example, two static routes are specified. In the above example, two static routes are specified.
Destination descriptors describe the IP subnet number and subnet Destination descriptors describe the IP subnet number and subnet
mask of a particular destination using a compact encoding. This mask of a particular destination using a compact encoding. This
encoding consists of one octet describing the width of the subnet encoding consists of one octet describing the width of the subnet
mask, followed by all the non-zero octets of the subnet number. 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 width of the subnet mask describes the number of one bits in
the mask, so for example a subnet with a subnet number of 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 10.0.127.0 and a netmask of 255.255.255.0 would have a subnet mask
width of 24. width of 24.
The non-zero portion of the subnet number is simply all of the The significant portion of the subnet number is simply all of the
octets of the subnet number, with the least significant octets that octets of the subnet number where the corresponding octet in the
are zero omitted. For a subnet mask width of between 25 and 32, subnet mask is non-zero. The number of significant octets is the
the subnet number will be four octets. Mask widths of between 17 width of the subnet mask divided by eight, rounding up, as shown
and 24 indicate a three-octet subnet number; between 9 and 16 in the following table:
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: 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 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
10.0.0.0 255.255.255.0 24.10.0.0
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 some cases more than one IP subnet may be configured within a In some cases more than one IP subnet may be configured on a link.
given network broadcast domain. In such cases, a host whose IP In such cases, a host whose IP address is in one IP subnet in the
address is in one IP subnet in the broadcast domain could communicate link could communicate directly with a host whose IP address is in
directly with a host whose IP address is in a different IP subnet in a different IP subnet on the same link. In cases where a client is
the same broadcast domain. In cases where a client is being being assigned an IP address on an IP subnet on such a link,
assigned an IP address on an IP subnet in such a broadcast domain, for each IP subnet in the link other than the IP subnet on which
for each IP subnet in the broadcast domain other than the IP subnet the client has been assigned the DHCP server MAY be configured to
on which the client has been assigned the DHCP server MAY be specify a router IP address of 0.0.0.0.
configured to specify a router IP address of 0.0.0.0.
For example, consider the case where there are three IP subnets For example, consider the case where there are three IP subnets
configured on a particular broadcast domain: 10.0.0/24, configured on a link: 10.0.0/24, 192.168.0/24, 10.0.21/24. If the
192.168.0/24, 10.0.21/24. If the client is assigned an IP address client is assigned an IP address of 10.0.21.17, then the server
of 10.0.21.17, then the server could include a route with a could include a route with a destination of 10.0.0/24 and a router
destination of 10.0.0/24 and a router address of 0.0.0.0, and also address of 0.0.0.0, and also a route with a destination of
a route with a destination of 192.168.0/24 and a router address of 192.168.0/24 and a router address of 0.0.0.0.
0.0.0.0.
A DHCP client whose underlying TCP/IP stack does not provide this A DHCP client whose underlying TCP/IP stack does not provide this
capability MUST ignore routes in the Classless Static Routes option capability MUST ignore routes in the Classless Static Routes option
whose router IP address is 0.0.0.0. Please note that the behavior whose router IP address is 0.0.0.0. Please note that the behavior
described here only applies to the Classless Static Routes option, described here only applies to the Classless Static Routes option,
not to the Static Routes option nor the Router 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, except as option MUST install the routes specified in the option, except as
specified in the Local Subnet Routes section. DHCP clients that specified in the Local Subnet Routes section. DHCP clients that
support this option MUST NOT install the routes specified in the support this option MUST NOT install the routes specified in the
Static Routes option (option code 33) if both a Static Routes Static Routes option (option code 33) if both a Static Routes
option and the Classless Static Routes option are 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.
that support this option and send a parameter request list MUST NOT
request the Static Routes option. The Classless Static Routes DHCP clients that support this option and send a parameter request
option code SHOULD appear in the parameter request list prior to list MAY also request the Static Routes option, for compatibility
the Router option code. 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 If the DHCP server returns both a Router option and a Classless
Static Routes option, the DHCP client MUST ignore the Router Static Routes option, the DHCP client MUST ignore the Router
option. option.
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 the destination descriptor, the DHCP client MUST set any bits in the
combination of the network number and the subnet mask for validity. subnet number that are zero in the subnet mask to zero. For
If the network number contains nonzero bits beyond the subnet mask, example, if the server sends a route with a destination of
the client SHOULD discard that route. For example, the client 129.210.177.132 (hexadecimal 81D4B184) and a subnet mask of
should not install a route with a destination of 129.210.377.4 and 255.255.255.128 (hexadecimal FFFFFF80), the client will install a
a subnet mask of 255.255.255.128. route with a destination of 129.210.177.128 (hexadecimal
81D4B180).
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 at least
the interface that the client is configuring. If the client the MTU of the interface that the client is configuring. The
supports UDP fragmentation, it MAY set the value of this option to client MAY set the value of this option higher, up to the size of
the size of the largest UDP packet it is prepared to accept. 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 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 Router 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
Router 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 Router option and the When a DHCP client requests the Classless Static Routes option and
Classless Static Routes option, and the DHCP server is configured also requests either or both of the Router option and the Static
with both a Classless Static Routes option and a Router option Routes option, and the DHCP server is sending Classless Static
that applies to the client, the DHCP server MAY exclude the Router Routes options to that client, the server SHOULD NOT include the
option from its response. Router or Static Routes options.
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.
IANA Considerations
This DHCP option will require the allocation of an option code in
the list DHCP option codes that the IANA maintains.
References References
[1] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, [1] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
Bucknell University, March 1997. Bucknell University, March 1997.
[2] Alexander, S. and Droms, R., "DHCP Options and BOOTP Vendor [2] Alexander, S. and Droms, R., "DHCP Options and BOOTP Vendor
Extensions", RFC 2132, Silicon Graphics, Inc., Bucknell Extensions", RFC 2132, Silicon Graphics, Inc., Bucknell
University, March 1997. University, March 1997.
[3] Bradner, S., "Key words for use in RFCs to indicate requirement [3] Bradner, S., "Key words for use in RFCs to indicate requirement
levels", RFC 2119, Harvard University, March 1997. levels", RFC 2119, Harvard University, March 1997.
[4] Postel, J., "Internet Protocol", RFC 791, USC/Information [4] Postel, J., "Internet Protocol", RFC 791, USC/Information
skipping to change at line 280 skipping to change at line 311
[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., Cheshire, S., "Encoding Long DHCP Options",
draft-ietf-dhc-concat-01.txt, Nominum, Inc., July, 2001. draft-ietf-dhc-concat-02.txt, Nominum, Inc., October, 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
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 Expiration
This document will expire on January 31, 2002. This document will expire on April 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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