draft-ietf-dhc-dhcp-dns-05.txt   draft-ietf-dhc-dhcp-dns-06.txt 
Network Working Group Yakov Rekhter Network Working Group Yakov Rekhter
Internet Draft Cisco Systems Internet Draft Cisco Systems
Expiration Date: May 1998 November 1997 Expiration Date: July 1998 January 1997
Interaction between DHCP and DNS Interaction between DHCP and DNS
draft-ietf-dhc-dhcp-dns-05.txt draft-ietf-dhc-dhcp-dns-06.txt
1. Status of this Memo 1. Status of this Memo
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.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
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However, the autoconfiguration provided by DHCP does not include However, the autoconfiguration provided by DHCP does not include
updating DNS, and specifically updating the name to address and updating DNS, and specifically updating the name to address and
address to name mappings maintained by DNS. address to name mappings maintained by DNS.
This document specifies how DHCP clients and servers should use the This document specifies how DHCP clients and servers should use the
Dynamic DNS Updates mechanism to update the DNS name to address and Dynamic DNS Updates mechanism to update the DNS name to address and
address to name mapping, so that the mappings for DHCP clients would address to name mapping, so that the mappings for DHCP clients would
be consistent with the IP addresses that the clients acquire via be consistent with the IP addresses that the clients acquire via
DHCP. DHCP.
3. Interaction between DHCP and DNS 3. Terminology
Throughout this document, the words that are used to define the
significance of particular requirements are capitalized. These words
are:
- "MUST"
This word or the adjective "REQUIRED" means that the item is an
absolute requirement of this specification.
- "MUST NOT"
This phrase means that the item is an absolute prohibition of
this specification.
- "SHOULD"
This word or the adjective "RECOMMENDED" means that there may
exist valid reasons in particular circumstances to ignore this
item, but the full implications should be understood and the
case carefully weighed before choosing a different course.
- "SHOULD NOT"
This phrase means that there may exist valid reasons in
particular circumstances when the listed behavior is acceptable
or even useful, but the full implications should be understood
and the case carefully weighed before implementing any behavior
described with this label.
- "MAY"
This word or the adjective "OPTIONAL" means that this item is
truly optional. One vendor may choose to include the item
because a particular marketplace requires it or because it
enhances the product, for example; another vendor may omit the
same item.
4. Interaction between DHCP and DNS
DNS [RFC1034, RFC1035] maintains (among other things) the information DNS [RFC1034, RFC1035] maintains (among other things) the information
about mapping between hosts' Fully Qualified Domain Names (FQDNs) about mapping between hosts' Fully Qualified Domain Names (FQDNs)
[RFC1594] and IP addresses assigned to the hosts. The information is [RFC1594] and IP addresses assigned to the hosts. The information is
maintained in two types of Resource Records (RRs): A and PTR. The A maintained in two types of Resource Records (RRs): A and PTR. The A
RR contains mapping from a FQDN to an IP address; the PTR RR contains RR contains mapping from a FQDN to an IP address; the PTR RR contains
mapping from an IP address to a FQDN. mapping from an IP address to a FQDN.
DHCP [RFC1541] provides a mechanism by which a host (a DHCP client) DHCP [RFC1541] provides a mechanism by which a host (a DHCP client)
could acquire certain configuration information, and specifically its could acquire certain configuration information, and specifically its
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The Dynamic DNS Update protocol can be used to maintain consistency The Dynamic DNS Update protocol can be used to maintain consistency
between the information stored in the A and PTR RRs and the actual between the information stored in the A and PTR RRs and the actual
address assignment done via DHCP. When a host with a particular FQDN address assignment done via DHCP. When a host with a particular FQDN
acquires its IP address via DHCP, the A RR associated with the host's acquires its IP address via DHCP, the A RR associated with the host's
FQDN would be updated (by using the Dynamic DNS Updates protocol) to FQDN would be updated (by using the Dynamic DNS Updates protocol) to
reflect the new address. Likewise, when an IP address gets assigned reflect the new address. Likewise, when an IP address gets assigned
to a host with a particular FQDN, the PTR RR associated with this to a host with a particular FQDN, the PTR RR associated with this
address would be updated (using the Dynamic DNS Updates protocol) to address would be updated (using the Dynamic DNS Updates protocol) to
reflect the new FQDN. reflect the new FQDN.
4. Models of operations 5. Models of operations
When a DHCP client acquires a new address, both the A RR (for the When a DHCP client acquires a new address, both the A RR (for the
client's FQDN) and the PTR RR (for the acquired address) have to be client's FQDN) and the PTR RR (for the acquired address) have to be
updated. Therefore, we have two separate Dynamic DNS Update updated. Therefore, we have two separate Dynamic DNS Update
transactions. Acquiring an address via DHCP involves two entities: a transactions. Acquiring an address via DHCP involves two entities: a
DHCP client and a DHCP server. In principle each of these entities DHCP client and a DHCP server. In principle each of these entities
could perform none, one, or both of the transactions. However, upon could perform none, one, or both of the transactions. However, upon
some introspection one could realize that not all permutations make some introspection one could realize that not all permutations make
sense. This document covers the possible design permutations: sense. This document covers the possible design permutations:
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client's FQDN) and the PTR RR (for the acquired address) have to be client's FQDN) and the PTR RR (for the acquired address) have to be
updated. Therefore, we have two separate Dynamic DNS Update updated. Therefore, we have two separate Dynamic DNS Update
transactions. Acquiring an address via DHCP involves two entities: a transactions. Acquiring an address via DHCP involves two entities: a
DHCP client and a DHCP server. In principle each of these entities DHCP client and a DHCP server. In principle each of these entities
could perform none, one, or both of the transactions. However, upon could perform none, one, or both of the transactions. However, upon
some introspection one could realize that not all permutations make some introspection one could realize that not all permutations make
sense. This document covers the possible design permutations: sense. This document covers the possible design permutations:
(1) DHCP client updates the A RR, DHCP server updates the PTR (1) DHCP client updates the A RR, DHCP server updates the PTR
RR RR
(2) DHCP server updates both the A and the PTR RRs (2) DHCP server updates both the A and the PTR RRs
One could observe that the only difference between these two cases is One could observe that the only difference between these two cases is
whether the FQDN to IP address mapping is updated by a DHCP client or whether the FQDN to IP address mapping is updated by a DHCP client or
by a DHCP server. The IP address to FQDN mapping is updated by a DHCP by a DHCP server. The IP address to FQDN mapping is updated by a DHCP
server in both cases. server in both cases.
4.1. Client FQDN Option 5.1. Client FQDN Option
To update the IP address to FQDN mapping a DHCP server needs to know To update the IP address to FQDN mapping a DHCP server needs to know
FQDN of the client to which the server leases the address. To allow FQDN of the client to which the server leases the address. To allow
the client to convey its FQDN to the server this document defines a the client to convey its FQDN to the server this document defines a
new option, called "Client FQDN". new option, called "Client FQDN".
The code for this option is 81. Its minimum length is 4. The code for this option is 81. Its minimum length is 4.
Code Len Flags RCODE1 RCODE2 Domain Name Code Len Flags RCODE1 RCODE2 Domain Name
+------+------+------+------+------+------+-- +------+------+------+------+------+------+--
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server to indicate to a DHCP client that the server assumes the server to indicate to a DHCP client that the server assumes the
responsibility for updating the FQDN to IP address mapping, even if responsibility for updating the FQDN to IP address mapping, even if
the client wants to be responsible for this update (if Flags is set the client wants to be responsible for this update (if Flags is set
to 3). to 3).
The RCODE1 and RCODE2 fields are used by a DHCP server to indicate to The RCODE1 and RCODE2 fields are used by a DHCP server to indicate to
a DHCP client the Response Code from Dynamic DNS Updates. a DHCP client the Response Code from Dynamic DNS Updates.
The Domain Name part of the option carries FQDN of a client. The Domain Name part of the option carries FQDN of a client.
4.2. DHCP Client behavior 5.2. DHCP Client behavior
If a client wants to be responsible for updating the FQDN to IP If a client wants to be responsible for updating the FQDN to IP
address mapping for the FQDN and address(es) used by the client, then address mapping for the FQDN and address(es) used by the client, then
the client shall include the Client FQDN option in the DHCPREQUEST the client MUST include the Client FQDN option in the DHCPREQUEST
message originated by the client. The Flags field in the option shall message originated by the client. The Flags field in the option MUST
be set to 0. Once the client's DHCP configuration is completed (the be set to 0. Once the client's DHCP configuration is completed (the
client receives a DHCPACK message, and successfully completed a final client receives a DHCPACK message, and successfully completed a final
check on the parameters passed in the message), the client shall check on the parameters passed in the message), the client MUST
originate an update for the A RR (associated with the client's FQDN). originate an update for the A RR (associated with the client's FQDN).
The update shall be originated following the procedures described in The update MUST be originated following the procedures described in
[DynDNS]. [DynDNS].
If a client does not want to be responsible for updating the FQDN to If a client does not want to be responsible for updating the FQDN to
IP address mapping for the FQDN and address(es) used by the client, IP address mapping for the FQDN and address(es) used by the client,
then the client shall include the Client FQDN option in the then the client MUST include the Client FQDN option in the
DHCPREQUEST message originated by the client. The Flags field in the DHCPREQUEST message originated by the client. The Flags field in the
option shall be set to 1. option MUST be set to 1.
A client that delegates the responsibility for updating the FQDN to A client that delegates the responsibility for updating the FQDN to
IP address mapping to a server may not receive any indications IP address mapping to a server MAY not receive any indications
(either positive or negative) from the server whether the server was (either positive or negative) from the server whether the server was
able to perform the update. In this case the client may use DNS query able to perform the update. In this case the client SHOULD use DNS
to check whether the mapping is updated. query to check whether the mapping is updated.
A client should set the RCODE1 and RCODE2 fields in the Client FQDN A client MUST set the RCODE1 and RCODE2 fields in the Client FQDN
option to 0 when sending the option. option to 0 when sending the option.
Whether the client wants to be responsible for updating the FQDN to
IP address mapping, or whether the client wants to delegate this
responsibility to a server is a local to the client matter. The
choice between the two alternatives may be based on a particular
security model that is used with the Dynamic DNS Update protocol
(e.g., only a client may have sufficient credentials to perform
updates to the FQDN to IP address mapping for its FQDN).
If a client releases its address lease prior to the lease expiration If a client releases its address lease prior to the lease expiration
time, and the client is responsible for updating its A RR(s), the time, and the client is responsible for updating its A RR(s), the
client should delete the A RR (following the procedures described in client SHOULD delete the A RR (following the procedures described in
[DynDNS]) associated with the leased address before sending DHCP [DynDNS]) associated with the leased address before sending DHCP
RELEASE message. RELEASE message.
4.3. DHCP Server behavior 5.3. DHCP Server behavior
When a server receives a DHCPREQUEST message from a client, if the When a server receives a DHCPREQUEST message from a client, if the
message contains the Client FQDN option, and the server replies to message contains the Client FQDN option, and the server replies to
the message with a DHCPACK message, the server may originate an the message with a DHCPACK message, the server SHOULD originate an
update for the PTR RR (associated with the address leased to the update for the PTR RR (associated with the address leased to the
client). The update shall be originated following the procedures client). The update MUST be originated following the procedures
described in Section 4.4. The server may originate the update before described in Section 4.4. The server MAY originate the update before
the server sends the DHCPACK message to the client. In this case the the server sends the DHCPACK message to the client. In this case the
RCODE from the update [DynDNS] shall be carried to the client in the RCODE from the update [DynDNS] MUST be carried to the client in the
RCODE1 field of the Client FQDN option in the DHCPACK message and the RCODE1 field of the Client FQDN option in the DHCPACK message and the
RCODE2 field shall be set to 0. Alternatively, the server may send RCODE2 field MUST be set to 0. Alternatively, the server MAY send the
the DHCPACK message to the client without waiting for the update to DHCPACK message to the client without waiting for the update to be
be completed. In this case the RCODE1 field of the Client FQDN completed. In this case the RCODE1 field of the Client FQDN option
option in the DHCPACK message shall be set to 255, and the RCODE2 in the DHCPACK message MUST be set to 255, and the RCODE2 field MUST
field shall be set to 0. The choice between the two alternatives is be set to 0. The choice between the two alternatives is a local to a
a local to a DHCP server matter. DHCP server matter.
In addition, if the Client FQDN option carried in the DHCPREQUEST In addition, if the Client FQDN option carried in the DHCPREQUEST
message has its Flags field set to 1, then the server shall originate message has its Flags field set to 1, then the server MUST originate
an update for the A RR (associated with the FQDN carried in the an update for the A RR (associated with the FQDN carried in the
option). The update shall be originated following the procedures option). The update MUST be originated following the procedures
described in Section 4.4. The server may originate the update before described in Section 4.4. The server MAY originate the update before
the server sends the DHCPACK message to the client. In this case the the server sends the DHCPACK message to the client. In this case the
RCODE from the update [DynDNS] shall be carried to the client in the RCODE from the update [DynDNS] MUST be carried to the client in the
RCODE2 field of the Client FQDN option in the DHCPACK message. RCODE2 field of the Client FQDN option in the DHCPACK message.
Alternatively the server may send the DHCPACK message to the client Alternatively the server MAY send the DHCPACK message to the client
without waiting for the update to be completed. In this case the without waiting for the update to be completed. In this case the
RCODE2 field of the Client FQDN option in the DHCKACK message shall RCODE2 field of the Client FQDN option in the DHCKACK message MUST be
be set to 255. The choice between the two alternatives is a local to set to 255. The choice between the two alternatives is a local to the
the server matter. server matter.
Even, if the Client FQDN option carried in the DHCPREQUEST message Even, if the Client FQDN option carried in the DHCPREQUEST message
has its Flags field set to 0 (indicating that the client wants to has its Flags field set to 0 (indicating that the client wants to
update the A RR), the server could (under configuration control) update the A RR), the server MAY (under configuration control) update
update the A RR. The update shall be originated following the the A RR. The update MUST be originated following the procedures
procedures described in Section 4.4. The server may originate the described in Section 4.4. The server MAY originate the update before
update before the server sends the DHCPACK message to the client. In the server sends the DHCPACK message to the client. In this case the
this case the RCODE from the update [DynDNS] shall be carried to the RCODE from the update [DynDNS] MUST be carried to the client in the
client in the RCODE2 field of the Client FQDN option in the DHCPACK RCODE2 field of the Client FQDN option in the DHCPACK message, and
message, and the Flags field in the Client FQND option shall be set the Flags field in the Client FQND option MUST be set to 3.
to 3. Alternatively, the server may send the DHCPACK message to the Alternatively, the server MAY send the DHCPACK message to the client
client without waiting for the update to be completed. In this case without waiting for the update to be completed. In this case the
the RCODE2 field of the Client FQDN option in the DHCKACK message RCODE2 field of the Client FQDN option in the DHCKACK message MUST be
shall be set to 255, and the Flags field in the Client FQDN option set to 255, and the Flags field in the Client FQDN option MUST be set
shall be set to 3. The choice between the two alternatives is a local to 3. The choice between the two alternatives is a local to the
to the server matter. server matter.
Whether a DHCP server is always responsible for updating the FQDN to
IP address mapping (in addition to updating the IP to FQDN mapping),
regarless of the wishes of a DHCP client, is a local to the server
matter. The choice between the two alternatives may be based on a
particular security model.
When a server receives a DHCPREQUEST message from a client, and the When a server receives a DHCPREQUEST message from a client, and the
message contains the Client FQDN option, the server shall ignore the message contains the Client FQDN option, the server MUST ignore the
value carried in the RCODE1 and RCODE2 fields of the option. value carried in the RCODE1 and RCODE2 fields of the option.
When a DHCP server sends the Client FQDN option to a client in the When a DHCP server sends the Client FQDN option to a client in the
DHCPACK message, the server shall copy the Domain Name fields from DHCPACK message, the server MUST copy the Domain Name fields from the
the Client FQDN option that the client sent to the server in the Client FQDN option that the client sent to the server in the
DHCPREQUEST message. DHCPREQUEST message.
If the DHCPREQUST message received by a DHCP server from a DHCP If the DHCPREQUST message received by a DHCP server from a DHCP
client doesn't carry the Client FQDN option, and the DHCP client client doesn't carry the Client FQDN option, and the DHCP client
acquires its FQDN from a DHCP server (as part of a normal DHCP acquires its FQDN from a DHCP server (as part of a normal DHCP
transaction), then the server may be configured to update both A and transaction), then the server MAY be configured to update both A and
PTR RRs. In this scenario the DHCPOFFER message originated by the PTR RRs. In this scenario the DHCPOFFER message originated by the
server shall carry the Domain Name option, and the client server MUST carry the Domain Name option, and the client acknowledges
acknowledges the use of the FQDN carried in this option by including the use of the FQDN carried in this option by including the option
the option (with the FQDN) in the DHCPREQUEST originated by the (with the FQDN) in the DHCPREQUEST originated by the client. The
client. The updates shall be originated following the procedures updates MUST be originated following the procedures described in
described in Section 4.4. Section 4.4.
If a server originates updates for both the A and PTR RRs, then the If a server originates updates for both the A and PTR RRs, then the
order in which the updates are generated is not significant. order in which the updates are generated is not significant.
If a server detects that a lease on an address that the server leases If a server detects that a lease on an address that the server leases
to a client expires, the server should delete the PTR RR associated to a client expires, the server SHOULD delete the PTR RR associated
with the address. In addition, if the client authorized the server to with the address. In addition, if the client authorized the server to
update its A RR, the server should also delete the A RR. The deletion update its A RR, the server SHOULD also delete the A RR. The deletion
should follow the procedures described in [DynDNS]. MUST follow the procedures described in [DynDNS].
If a server terminates a lease on an address prior to the lease If a server terminates a lease on an address prior to the lease
expiration time, the server should delete the PTR RR associated with expiration time, the server SHOULD delete the PTR RR associated with
the address. In addition, if the client (that leased the address) the address. In addition, if the client (that leased the address)
authorized the server to update its A RR, the server should also authorized the server to update its A RR, the server SHOULD also
delete the A RR. The deletion should follow the procedures described delete the A RR. The deletion MUST follow the procedures described in
in [DynDNS]. [DynDNS].
4.4. Procedures for performing DNS updates 5.4. Procedures for performing DNS updates
When a DHCP server needs to update the PTR RR for a particular IP When a DHCP server needs to update the PTR RR for a particular IP
address, the server just adds a new PTR RR for that address. address, the server just adds a new PTR RR for that address.
When a DHCP server needs to update the A RR for a particular FQDN, When a DHCP server needs to update the A RR for a particular FQDN,
the server first has to delete all the A RRs associated with that the server first has to delete all the A RRs associated with that
FQDN, and then add a new A RR for that FQDN. Note that this rule FQDN, and then add a new A RR for that FQDN. Note that this rule
precludes the ability to support multi-homed hosts in the scenario precludes the ability to support multi-homed hosts in the scenario
where A RRs are updated by a DHCP server. Therefore, multi-homed where A RRs are updated by a DHCP server. Therefore, multi-homed
hosts should perform updates to their A RRs by themselves. hosts SHOULD perform updates to their A RRs by themselves.
Procedures for deleting and adding RRs are described in [DynDNS]. Procedures for deleting and adding RRs are described in [DynDNS].
5. Updating other RRs 6. Updating other RRs
The procedures described in this document cover updates only to the A The procedures described in this document cover updates only to the A
and PTR RRs. Updating other types of RRs is outside the scope of this and PTR RRs. Updating other types of RRs is outside the scope of this
document. document.
6. Security Considerations 7. Security Considerations
Security issues are not discussed in this document. Whether the client wants to be responsible for updating the FQDN to
IP address mapping, or whether the client wants to delegate this
responsibility to a server is a local to the client matter. The
choice between the two alternatives may be based on a particular
security model that is used with the Dynamic DNS Update protocol
(e.g., only a client may have sufficient credentials to perform
updates to the FQDN to IP address mapping for its FQDN).
7. References Whether a DHCP server is always responsible for updating the FQDN to
IP address mapping (in addition to updating the IP to FQDN mapping),
regarless of the wishes of a DHCP client, is a local to the server
matter. The choice between the two alternatives may be based on a
particular security model.
The client SHOULD use some form of data origin authentication
procedures (e.g., DNSSEC [DNSSEC]) when performing DNS updates.
While the DHCP client SHOULD be the one to update the DNS A record,
in certain specialized cases a DHCP server MAY do so instead. In
this case, the DHCP server MUST be sure of both the name to use for
the client, as well as the identity of the client.
In the general case, both of these conditions are not satisfied --
one needs to be mindful of the possibility of MAC address spoofing in
a DHCP packet. It is not difficult for a DHCP server to know
unambiguously the DNS name to use for a client, but only in certain
relatively unusual circumstances will the DHCP server know for sure
the identity of the client. One example of such a circumstance is
where the DHCP client is connected to a network through an MCNS cable
modem, and the CMTS (headend) of the cable modem ensures that MAC
address spoofing simply does not occur.
Another example where the DHCP server would know the identity of the
client would be in a case where it was interacting with a remote
access server which encoded a client identification into the DHCP
client-id option. In this case, the remote access server as well as
the DHCP server would be operating within a trusted environment, and
the DHCP server could trust that the user authentication and
authorization procedure of the remote access server was sufficient,
and would therefore trust the client identification encoded within
the DHCP client-id.
In either of these cases, a DHCP server would be able to correctly
enter the DNS A record on behalf of a client, since it would know the
name associated with a client (through some administrative procedure
outside the scope of this protocol), and it would also know the
client's identity in a secure manner.
8. References
[RFC1034] P. Mockapetris, "Domain names - concepts and facilities", [RFC1034] P. Mockapetris, "Domain names - concepts and facilities",
RFC1034, 11/01/1987 RFC1034, 11/01/1987
[RFC1035] P. Mockapetris, "Domain names - implementation and [RFC1035] P. Mockapetris, "Domain names - implementation and
specification", RFC1035, 11/01/1987 specification", RFC1035, 11/01/1987
[RFC1541] R. Droms, "Dynamic Host Configuration Protocol", RFC1541, [RFC1541] R. Droms, "Dynamic Host Configuration Protocol", RFC1541,
10/27/1993 10/27/1993
[RFC1594] A. Marine, J. Reynolds, G. Malkin, "FYI on Questions and [RFC1594] A. Marine, J. Reynolds, G. Malkin, "FYI on Questions and
Answer Answers to Commonly asked ``New Internet User'' Questions", Answer Answers to Commonly asked ``New Internet User'' Questions",
RFC1594, 03/11/1994 RFC1594, 03/11/1994
[DNSSEC]
[DynDNS] P. Vixie, S. Thomson, Y. Rekhter, J. Bound, "Dynamic Updates [DynDNS] P. Vixie, S. Thomson, Y. Rekhter, J. Bound, "Dynamic Updates
in the Domain Name System (DNS UPDATE)", RFC2136, April 1997 in the Domain Name System (DNS UPDATE)", RFC2136, April 1997
8. Acknowledgements 9. Acknowledgements
Many thanks to Mark Beyer, Jim Bound, Ralph Droms, Peter Ford, Edie Many thanks to Mark Beyer, Jim Bound, Ralph Droms, Peter Ford, Edie
Gunter, Stuart Kwan, Ted Lemon, Michael Lewis, Michael Patton, Mark Gunter, Kim Kinnear, Stuart Kwan, Ted Lemon, Michael Lewis, Michael
Stapp, and Glenn Stump for their review and comments. Patton, Mark Stapp, and Glenn Stump for their review and comments.
9. Author Information 10. Author Information
Yakov Rekhter Yakov Rekhter
cisco Systems, Inc. cisco Systems, Inc.
170 Tasman Dr. 170 Tasman Dr.
San Jose, CA 95134 San Jose, CA 95134
Phone: (914) 235-2128 Phone: (914) 235-2128
email: yakov@cisco.com email: yakov@cisco.com
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