draft-ietf-dhc-csr-00.txt   draft-ietf-dhc-csr-01.txt 
Network Working Group Ted Lemon Network Working Group Ted Lemon
Internet Draft Nominum, Inc. Internet Draft Nominum, Inc.
Obsoletes: draft-ietf-dhc-csr-00.txt January, 2000
Expires July 2000 Obsoletes: draft-ietf-dhc-csr-00.txt March, 2000
Expires September 2000
The Classless Static Route Option for DHCP The Classless Static Route Option for DHCP
<draft-ietf-dhc-csr-00.txt> <draft-ietf-dhc-csr-01.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 113 skipping to change at line 114
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.
Classless Route Option Format Classless Route Option Format
The code for this option is TBD, and its minimum length is 12 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 network number, a destination subnet mask consists of a destination descriptor and the IP address of the
and the IP address of the router that should be used to reach that router that should be used to reach that destination.
destination.
Code Len Destination 1 Subnet Mask 1 Router 1 Code Len Destination 1 Router 1
+----+---+----+----+----+----+----+----+----+----+----+----+----+----+ +-----+---+----+-----+----+----+----+----+----+
| 33 | n | d1 | d2 | d3 | d4 | m1 | m2 | m3 | m4 | r1 | r2 | r3 | r4 | | TBD | n | d1 | ... | dN | r1 | r2 | r3 | r4 |
+----+---+----+----+----+----+----+----+----+----+----+----+----+----+ +-----+---+----+-----+----+----+----+----+----+
Destination 2 Subnet Mask 2 Router 2 Destination 2 Router 2
+----+----+----+----+----+----+----+----+----+----+----+----+ +----+-----+----+----+----+----+----+
| d1 | d2 | d3 | d4 | m1 | m2 | m3 | m4 | 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
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 24.10.27.129
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
DHCP Client Behavior DHCP Client Behavior
The DHCP client MAY use this option to install a set of static The DHCP client MAY use this option to install a set of static
routes in its routing table. A DHCP client that implements this routes in its routing table. A DHCP client that implements this
option SHOULD use this option in preference to the Static routes option SHOULD use this option in preference to the Static routes
option if both are present in a reply from the DHCP server. option if both are present in a reply from the DHCP server. The
client MAY request both options.
After deriving a subnet number and subnet mask from each
destination descriptor, the DHCP client SHOULD check each route to
determine if 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 255.255.255.0.
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 [2] 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 Server administrator responsibilities
The client's behaviour if both a Routers option and a Classless
Static Routes option default route (network number 0.0.0.0, network
mask 0.0.0.0) are specified is not defined in this document, so as
to avoid placing onerous requirements on the client and server
implementations. Therefore, the DHCP server administrator SHOULD
NOT configure the DHCP server so that it sends both a Routers
option and a Classless Static Routes option containing a default
route. Either no Routers option should be configured (this is
probably preferable in the near term, since only newer DHCP clients
will implement this option), or the Classless Static Routes option
should not contain a default route.
The client's behaviour is also not defined in the case where the
server sends a classless static route in which some bits in the
network number are 1, and corresponding bits in the subnet mask are
zero. Therefore, DHCP server administrators SHOULD NOT configure
the DHCP server to send such a route.
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
 End of changes. 

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