draft-ietf-dhc-csr-07.txt   rfc3442.txt 
Network Working Group Ted Lemon
Internet Draft Nominum, Inc.
Obsoletes: draft-ietf-dhc-csr-06.txt Stuart Cheshire
Apple Computer, Inc.
Bernie Volz
Ericsson
July, 2002 Network Working Group T. Lemon
Expires January, 2003 Request for Comments: 3442 Nominum, Inc.
Updates: 2132 S. Cheshire
Category: Standards Track Apple Computer, Inc.
B. Volz
Ericsson
December 2002
The Classless Static Route Option for DHCPv4 The Classless Static Route Option for
<draft-ietf-dhc-csr-07.txt> Dynamic Host Configuration Protocol (DHCP) version 4
Status of this Memo Status of this Memo
This document is an Internet-Draft and is subject to all provisions This document specifies an Internet standards track protocol for the
of Section 10 of RFC2026. Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Internet-Drafts are working documents of the Internet Engineering Official Protocol Standards" (STD 1) for the standardization state
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Abstract Abstract
This document defines a new DHCP option which is passed from the This document defines a new Dynamic Host Configuration Protocol
DHCP Server to the DHCP Client to configure a list of static routes (DHCP) option which is passed from the DHCP Server to the DHCP Client
in the client. The network destinations in these routes are to configure a list of static routes in the client. The network
classless - each routing table entry includes a subnet mask. destinations in these routes are classless - each routing table entry
includes a subnet mask.
Introduction Introduction
This option supersedes the Static Route option (option 33) defined This option obsoletes the Static Route option (option 33) defined in
in RFC2132 [2]. RFC 2132 [4].
The IP protocol [4] uses routers to transmit packets from hosts The IP protocol [1] 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
routing table to determine the IP address of the router that should table to determine the IP address of the router that should be used
be used to forward the packet to the destination host. 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 [8], ICMP
router discovery [6,9] or using the DHCP Router option, defined in
RFC 2132 [4].
^L In a network that already provides DHCP service, using DHCP to update
ways - using a routing information protocol such as RIP [5], ICMP the routing table on a DHCP client has several virtues. It is
router discovery [6,7] or using the DHCP Router option, defined in efficient, since it makes use of messages that would have been sent
RFC2132 [2]. anyway. It is convenient - the DHCP server configuration is already
being maintained, so maintaining routing information, at least on a
In a network that already provides DHCP service, using DHCP to relatively stable network, requires little extra work. If DHCP
update the routing table on a DHCP client has several virtues. It service is already in use, no additional infrastructure need be
is efficient, since it makes use of messages that would have been deployed.
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 RFC2131 [1] and the options defined The DHCP protocol as defined in RFC 2131 [3] and the options defined
in RFC2132 [2] only provide a mechanism for installing a default in RFC 2132 [4] only provide a mechanism for installing a default
route or installing a table of classful routes. Classful routes route or installing a table of classful routes. Classful routes are
are routes whose subnet mask is implicit in the subnet number - see routes whose subnet mask is implicit in the subnet number - see
section 3.2 of RFC791 [4] for details on classful routing. section 3.2 of STD 5, RFC 791 [1] for details on classful routing.
Classful routing is no longer in common use, so the DHCP Static Classful routing is no longer in common use, so the DHCP Static Route
Route option is no longer useful. Currently, classless routing, option is no longer useful. Currently, classless routing [7, 10] is
described in [8] and [9], is the most commonly-deployed form of the most commonly-deployed form of routing on the Internet. In
routing on the Internet. In classless routing, IP addresses classless routing, IP addresses consist of a network number (the
consist of a network number (the combination of the network number combination of the network number and subnet number described in RFC
and subnet number described in [8]) and a host number. 950 [7]) and a host number.
In classful IP, the network number and host number are derived from In classful IP, the network number and host number are derived from
the IP address using a bitmask whose value is determined by the the IP address using a bitmask whose value is determined by the first
first few bits of the IP address. In classless IP, the network few bits of the IP address. In classless IP, the network number and
number and host number are derived from the IP address using a host number are derived from the IP address using a separate
seperate quantity, the subnet mask. In order to determine the quantity, the subnet mask. In order to determine the network to
network to which a given route applies, an IP host must know both which a given route applies, an IP host must know both the network
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 RFC791 [4] and RFC950 determined using the algorithm specified in STD 5, RFC 791 [1] and
[8]. STD 5, RFC 950 [7].
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 BCP 14, RFC 2119 [2].
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.
^L
"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" "link"
Any set of all network attachment points that will recieve Any set of network attachment points that will all receive a
a link-layer broadcast sent on any one of the attachment link-layer broadcast sent on any one of the attachment points.
points. This term is used in DHCP because in some cases This term is used in DHCP because in some cases more than one
more than one IP subnet may be configured on a link. DHCP IP subnet may be configured on a link. DHCP uses a local-
uses a local-network (all-ones) broadcast, which is not network (all-ones) broadcast, which is not subnet-specific, and
subnet-specific, and will therefore reach all nodes will therefore reach all nodes connected to the link,
connected to the link, regardless of the IP subnet or regardless of the IP subnet or subnets on which they are
subnets on which they are configured. configured.
A "link" is sometimes referred to as a broadcast domain or A "link" is sometimes referred to as a broadcast domain or
physical network segment. 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 121, 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
router that should be used to reach that destination. 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 | | 121 | n | d1 | ... | dN | r1 | r2 | r3 | r4 |
+-----+---+----+-----+----+----+----+----+----+ +-----+---+----+-----+----+----+----+----+----+
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
mask of a particular destination using a compact encoding. This of a particular destination using a compact encoding. This encoding
encoding consists of one octet describing the width of the subnet consists of one octet describing the width of the subnet mask,
mask, followed by all the significant octets of the subnet number. 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
the mask, so for example a subnet with a subnet number of mask, so for example a subnet with a subnet number of 10.0.127.0 and
10.0.127.0 and a netmask of 255.255.255.0 would have a subnet mask a netmask of 255.255.255.0 would have a subnet mask width of 24.
width of 24.
The significant 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 where the corresponding octet in the octets of the subnet number where the corresponding octet in the
subnet mask is non-zero. The number of significant octets is the subnet mask is non-zero. The number of significant octets is the
width of the subnet mask divided by eight, rounding up, as shown width of the subnet mask divided by eight, rounding up, as shown in
the following table:
^L
in the following table:
Width of subnet mask Number of significant octets Width of subnet mask Number of significant octets
0 0 0 0
1- 8 1 1- 8 1
9-16 2 9-16 2
17-24 3 17-24 3
25-32 4 25-32 4
The following table contains some examples of how various subnet The following table contains some examples of how various subnet
number/mask combinations can be encoded: 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 10.0.0.0 255.255.255.0 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 on a link. In some cases more than one IP subnet may be configured on a link.
In such cases, a host whose IP address is in one IP subnet in the In such cases, a host whose IP address is in one IP subnet in the
link could communicate directly with a host whose IP address is in link could communicate directly with a host whose IP address is in a
a different IP subnet on the same link. In cases where a client is different IP subnet on the same link. In cases where a client is
being assigned an IP address on an IP subnet on such a link, being assigned an IP address on an IP subnet on such a link, for each
for each IP subnet in the link other than the IP subnet on which IP subnet in the link other than the IP subnet on which the client
the client has been assigned the DHCP server MAY be configured to has been assigned the DHCP server MAY be configured to specify a
specify a router IP address of 0.0.0.0. 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 link: 10.0.0/24, 192.168.0/24, 10.0.21/24. If the configured on a link: 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 client is assigned an IP address of 10.0.21.17, then the server could
could include a route with a destination of 10.0.0/24 and a router include a route with a destination of 10.0.0/24 and a router address
address of 0.0.0.0, and also a route with a destination of of 0.0.0.0, and also a route with a destination of 192.168.0/24 and a
192.168.0/24 and a router address of 0.0.0.0. router address of 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
is received from a DHCP server. DHCP clients that support this received from a DHCP server. DHCP clients that support this option
option MUST install the routes specified in the option, except as MUST install the routes specified in the option, except as specified
specified in the Local Subnet Routes section. DHCP clients that in the Local Subnet Routes section. DHCP clients that support this
support this option MUST NOT install the routes specified in the option MUST NOT install the routes specified in the Static Routes
Static Routes option (option code 33) if both a Static Routes option (option code 33) if both a Static Routes option and the
option and the Classless Static Routes option are provided. Classless Static Routes option are provided.
^L DHCP clients that support this option and that send a DHCP Parameter
DHCP clients that support this option and that send a DHCP Request List option MUST request both this option and the Router
Parameter Request List option MUST request both this option and the option [4] in the DHCP Parameter Request List.
Router option [2] in the DHCP Parameter Request List.
DHCP clients that support this option and send a parameter request DHCP clients that support this option and send a parameter request
list MAY also request the Static Routes option, for compatibility list MAY also request the Static Routes option, for compatibility
with older servers that don't support Classless Static Routes. with older servers that don't support Classless Static Routes. The
The Classless Static Routes option code MUST appear in the Classless Static Routes option code MUST appear in the parameter
parameter request list prior to both the Router option code and the request list prior to both the Router option code and the Static
Static Routes option code, if present. Routes option code, if present.
If the DHCP server returns both a Router option and a Classless If the DHCP server returns both a Classless Static Routes option and
Static Routes option, the DHCP client MUST ignore the Router a Router option, the DHCP client MUST ignore the Router option.
option.
After deriving a subnet number and subnet mask from each Similarly, if the DHCP server returns both a Classless Static Routes
destination descriptor, the DHCP client MUST set any bits in the option and a Static Routes option, the DHCP client MUST ignore the
subnet number that are zero in the subnet mask to zero. For Static Routes option.
example, if the server sends a route with a destination of
129.210.177.132 (hexadecimal 81D4B184) and a subnet mask of After deriving a subnet number and subnet mask from each destination
255.255.255.128 (hexadecimal FFFFFF80), the client will install a descriptor, the DHCP client MUST zero any bits in the subnet number
route with a destination of 129.210.177.128 (hexadecimal where the corresponding bit in the mask is zero. In other words, the
subnet number installed in the routing table is the logical AND of
the subnet number and subnet mask given in the Classless Static
Routes option. For example, if the server sends a route with a
destination of 129.210.177.132 (hexadecimal 81D4B184) and a subnet
mask of 255.255.255.128 (hexadecimal FFFFFF80), the client will
install a route with a destination of 129.210.177.128 (hexadecimal
81D4B180). 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
a Maximum DHCP Message Size [2] option if the DHCP client's TCP/IP Maximum DHCP Message Size [4] option if the DHCP client's TCP/IP
stack is capable of reassembling fragmented IP datagrams. In this stack is capable of receiving larger IP datagrams. In this case, the
case, the client SHOULD set the value of this option to at least client SHOULD set the value of this option to at least the MTU of the
the MTU of the interface that the client is configuring. The interface that the client is configuring. The client MAY set the
client MAY set the value of this option higher, up to the size of value of this option higher, up to the size of the largest UDP packet
the largest UDP packet it is prepared to accept. (Note that the it is prepared to accept. (Note that the value specified in the
value specified in the Maximum DHCP Message Size option is the Maximum DHCP Message Size option is the total maximum packet size,
total maximum packet size, including IP and UDP headers.) including IP and UDP headers.)
DHCP servers sending this option MUST use the technique described DHCP clients requesting this option, and DHCP servers sending this
in [10] for sending options larger than 255 bytes when storing this option, MUST implement DHCP option concatenation [5]. In the
option in outgoing DHCP packets. DHCP clients supporting this terminology of RFC 3396 [5], the Classless Static Route Option is a
option MUST support the technique described in [10] when reading concatenation-requiring option.
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
Router option and in the Classless Static Routes option. option and in the Classless Static Routes option.
DHCP Server Considerations
^L
When a DHCP client requests the Classless Static Routes option and When a DHCP client requests the Classless Static Routes option and
also requests either or both of the Router option and the Static also requests either or both of the Router option and the Static
Routes option, and the DHCP server is sending Classless Static Routes option, and the DHCP server is sending Classless Static Routes
Routes options to that client, the server SHOULD NOT include the options to that client, the server SHOULD NOT include the Router or
Router or Static Routes options. Static Routes options.
Security Considerations Security Considerations
Potential exposures to attack in the DHCP protocol are discussed in Potential exposures to attack in the DHCP protocol are discussed in
section 7 of the DHCP protocol specification [1] and in section 7 of the DHCP protocol specification [3] and in
Authentication for DHCP Messages [5]. Authentication for DHCP Messages [11].
The Classless Static Routes option can be used to misdirect network The Classless Static Routes option can be used to misdirect network
traffic by providing incorrect IP addresses for routers. This can traffic by providing incorrect IP addresses for routers. This can be
be either a Denial of Service attack, where the router IP address either a Denial of Service attack, where the router IP address given
given is simply valid, or can be used to set up a man-in-the-middle is simply invalid, or can be used to set up a man-in-the-middle
attack by providing the IP address of a potential snooper. This is attack by providing the IP address of a potential snooper. This is
not a new problem - the existing Router and Static Routes options not a new problem - the existing Router and Static Routes options
defined in RFC2132 [2] exhibit the same vulnerability. defined in RFC 2132 [4] exhibit the same vulnerability.
IANA Considerations IANA Considerations
This DHCP option will require the allocation of an option code in This DHCP option has been allocated the option code 121 in the list
the list of DHCP option codes that the IANA maintains. of DHCP option codes that the IANA maintains.
References Normative References
[1] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, [1] Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981.
Bucknell University, March 1997.
[2] Alexander, S. and Droms, R., "DHCP Options and BOOTP Vendor
Extensions", RFC 2132, Silicon Graphics, Inc., Bucknell
University, March 1997.
[3] Bradner, S., "Key words for use in RFCs to indicate requirement
levels", RFC 2119, Harvard University, March 1997.
[4] Postel, J., "Internet Protocol", RFC 791, USC/Information
Sciences Institute, September 1981.
[5] Hedrick, C.L., "Routing Information Protocol", RFC 1058,
Rutgers University, June 1, 1988.
[6] Deering, S., "ICMP Router Discovery Messages", RFC 1256,
Xerox PARC, September 1991.
[7] Postel, J., "Internet Control Message Protocol", RFC 792,
USC/Information Sciences Institute, September 1981.
[8] Mogul, J., Postel, J., "Internet Standard Subnetting
Procedure", RFC950, Stanford University, USC/Information
Sciences Institute, August 1985.
[9] Pummill, T., Manning, B., "Variable Length Subnet Table For
IPv4", RFC1878, Alantec, USC/Information Sciences Institute,
December, 1995.
[10] Lemon, T., Cheshire, S., "Encoding Long DHCP Options",
draft-ietf-dhc-concat-05.txt, Nominum, Inc., Apple Computer,
Inc., July, 2002.
^L [2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
Author Information [3] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
March 1997.
Ted Lemon [4] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
Nominum, Inc. Extensions", RFC 2132, March 1997.
2385 Bay Road
Redwood City, CA 94063
email: Ted.Lemon@nominum.com
Stuart Cheshire [5] Lemon, T. and S. Cheshire, "Encoding Long Options in the Dynamic
Apple Computer, Inc. Host Configuration Protocol (DHCPv4)", RFC 3396, November 2002.
1 Infinite Loop
Cupertino
California 95014
USA
Phone: +1 408 974 3207
EMail: rfc@stuartcheshire.org
Bernie Volz Informative References
Ericsson
959 Concord Street
Framingham, MA, 01701
Phone: +1 508 875 3162
EMail: bernie.volz@ericsson.com
Expiration [6] Postel, J., "Internet Control Message Protocol", STD 5, RFC 792,
September 1981.
This document will expire on December 31, 2002. [7] Mogul, J. and J. Postel, "Internet Standard Subnetting
Procedure", STD 5, RFC 950, August 1985.
[8] Hedrick, C., "Routing Information Protocol", RFC 1058, June
1988.
[9] Deering, S., "ICMP Router Discovery Messages", RFC 1256,
September 1991.
[10] Pummill, T. and B. Manning, "Variable Length Subnet Table For
IPv4", RFC 1878, December 1995.
[11] Droms, R. and W. Arbaugh, "Authentication for DHCP Messages",
RFC 3118, June 2001.
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Authors' Addresses
Ted Lemon
Nominum, Inc.
2385 Bay Road
Redwood City, CA 94063
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
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^L
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"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
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BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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