Transport Working Group D. Raz
INTERNET-DRAFT Bell-Labs, Lucent
Technologies
Category: Informational B. Sugla
Expire in six months Bell-Labs, Lucent
Technologies
December 1998
An SNMP Application Level Gateway for Payload Address Translation
<draft-ietf-nat-snmp-alg-00.txt>
Status of this Memo
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Preface
The SNMP Application Level Gateway for Payload Address
Translation described in this document is a specific case of an
Application Level Gateway (ALG), as described in [SH 98] and [SE
98]. It includes detailed description of the need and
implementations of such a gateway for SNMP (Simple Network
Management Protocol).
Abstract
The SNMP Application Level Gateway for Payload Address
Translation (SALG-PAT) is a feature by which IP addresses in the
payload of SNMP packets are statically mapped from one group to
another, transparent to management application. This is a
specific case of Application Level Gateway, ALG, as described in
[SH 98] and [SE 98]. When combined with basic NAT, this document
describes a mechanism by which a management device can manage
multiple networks that use conflicting IP addresses.
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1. Introduction
The need for IP address translation arises when a network's
internal IP addresses cannot be used outside the network either
for security reasons or because they are invalid for use outside
the network. Topology outside a local domain can change in many
ways. Customers may change providers, company backbones may be
reorganized, or providers may merge or split. Address
translation allows hosts in such private networks to
transparently access an external network and vice versa.
In many of these cases, there is a need to manage the local
domain from a manager site outside the domain. However, managing
such networks is a big problem. Most available management
devices use SNMP (Simple Network Management Protocol) to retrieve
address information from the network elements. In this case a
router may be queried by the management software about the
addresses of its neighbor elements. This information is then
sent by the router back to the management device as part of the
payload of an SNMP packet. In order to retain consistency in the
view as seen by the management device we need to be able to also
locate and translate IP address related information in the
payload of such packets.
The SNMP Application Level Gateway for Payload Address
Translation, or SALG-PAT, is a technique in which the payload of
SNMP packets (PDUs) is scanned and all IP address related
information is translated if needed. This is a particular
example of Application Level Gateways (ALGs) as described in [SH
98] and [SE 98]: "ALG are application specific translation
agents that allow hosts from one routing realm to connect to
hosts in a different realm. The ALGs may optionally utilize
address/port assignments by NAT and perform translations of
packets pertaining to the application."
In this context SALG-PAT can be an additional component in any
NAT implementation, or be a separate entity, that may reside in
the same gateway or even on a separate node. Note that we deal
with management application, and hence all devices in the network
are assumed to have a fixed IP address. Thus, SALG-PAT should
only be combined with basic NAT that uses static mapping.
2. Terminology and concepts used
In general we adapt the terminology used in [SH 98]. Our main
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concern are packets used by the SNMP protocol. These are
typically UDP packets that contain PDUs - Protocol Data Units.
The notion of flow is less relevant in this case, and hence we
will focus on the information contained in a single packet. The
main definition of SNMP is from RFC 1157. Other RFCs (1155,
1213, 1215) define the structure of the managed information (SMI)
and the management information base (MIB). There are many
versions of SNMP. For simplicity, unless otherwise mentioned, we
refer to SNMP version 1 as SNMP.
The actual encoding of data in SNMP packets is done using BER -
basic encoding rules, which provide the transfer syntax. It uses
part of the ASN.1 to define the abstract syntax of the messages.
These standards are defined in ISO 8824-1, and ISO 8825-1.
As mentioned before, SALG-PAT is a specific example of the
Application Level gateway (ALG) described in [SH 98] and [SE 98].
Application Level Gateways (ALGs) are application specific
translation agents that allow hosts from one routing realm to
connect to hosts in a different realm. The ALGs may optionally
utilize address/port assignments by NAT and perform translation
of packets pertaining to the application.
We also refer in this document to IPv4, and thus we refer to IPv4
addresses as just IP addresses. We also use some terminology
from the IP security domain. In particular we will refer to IP
over IP as defined in RFC 2003, as tunneling.
3. Overview of the SNMP application level gateway
Using basic address translation allows local hosts on a
private network to transparently access the external global
network and enables access to selective local hosts from the
outside. This solution is becoming widely popular as the range
of IPv4 addresses is limited. In particular it is not unlikely
to have several private networks that are using the same private
IP address space within the same organization.
However, managing such a network presents unique problems and
challenges. Managing devices typically use the SNMP simple
network management protocol, which is an application that
exchanges IP address related information. Thus, in order for the
SNMP application to work properly across NAT, Application Level
Gateways, or ALGs, must be used to perform translation on SNMP
packets. This translation of the payload of SNMP packets is
called an SNMP Application Level Gateway for Payload Address
Translation - or just SALG-PAT.
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A typical scenario where SALG-PAT is deployed as part of NAT is
presented in figure 1. A manager device is managing a remote
stub, with translated IP addresses.
\ | / .
+---------------+ WAN .
+------------------------------+
|Regional Router|-------------------|Stub Router w/NAT and
SALG-PAT|
+---------------+ .
+------------------------------+
| . |
| . | LAN
+----------+ . ---------------
|Manager | Stub border Managed network
+----------+
Figure 1: NAT+SALG-PAT configuration
A similar scenario occurs when several subnetworks with private
(and possibly conflicting) IP addresses are to be managed by the
same management station. This scenario is presented in Figure 2.
\ | /
+-------------------+ +-----------------+
| Access Router|-----|Management device|
|w/ NAT and SALG-PAT| +-----------------+
+-------------------+
T1 | | T1
| |
Stub A .............|.... ....|............ Stub B
| |
| |
+------------+ +------------+
|Stub Router | |Stub Router |
+------------+ +------------+
| |
| LAN LAN |
------------- -------------
192.10.x.y | | 192.10.x.y
/____\ /____\
Figure 2: Using SALG-PAT+NAT to manage two private networks
Since the devices in the managed network are monitored by the
manager device they must obtain a fixed IP address. Therefore,
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the NAT used in this case must be a basic NAT with a static one
to one mapping.
A management payload translator is required to scan all the
payload of SNMP packets, to detect IP address related data, and
to translate this data if needed. This is a much more
computationally involved process than the basic NAT, however they
both use the same translation tables. In many cases the router
may be unable to handle SALG-PAT and retain acceptable
performance. In these cases it may be better to locate the
SALG-PAT outside the router.
3.1 SALG-PAT on a separate machine
As described before, in some cases it may be beneficial to locate
the SALG-PAT functionality in a separate node in the network.
This can be done using tunneling. The use of SALG-PAT as
described in this section can be generalized for any ALG, but
different restrictions and considerations are required, which are
out of the scope of this document.
+-------------+
| SALG-PAT |
+-------------+
\ | / |
+---------------+ | +-----------------+
|Access Router|-----|Management device|
|w/ NAT(and SA) | +-----------------+
+---------------+
T1 | | T1
| |
Stub A .............|.... ....|............ Stub B
| |
+------------+ +------------+
|Stub Router | |Stub Router |
+------------+ +------------+
| |
| LAN LAN |
------------- -------------
192.10.x.y | | 192.10.x.y
/____\ /____\
Figure 3: Using external SALG-PAT to manage two private networks
The idea here is to send the packets that belong to a specific
application (SNMP PDUs in our case) to the appropriate external
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ALG, using tunneling. These packets arrive at the remote ALG,
the appropriate filter is applied to them, and then they are
either injected to the network, or tunneled back to the access
router, and from there to the network.
4.0 Parsing and translating data in SNMP packets
SNMP packets are built using the ASN.1/BER encoding. We will not
cover the full details of this encoding in this document. These
details can be found in the International Standards ISO-8824 and
ISO-8825. A good description of ASN.1/BER can be found in the
book Managing Internetworks with SNMP, by M. A. Miller [Mi 97],
or in Appendix A of the book Understanding SNMP MIBs, by D.
Perkins, and E. McGinnis [PM 97].
4.1. General description of the encoding of data in
SNMP PDU's (ASN.1 and BER)
In general, each variable that is referred to in an SNMP packet
has a unique OID (Object Identifier) which is a set of numbers
separated by a dot (for example: 1.2.4.56.12.34). This OID gives
a
unique identification to each variable both in time and space.
Each such an element also has a type (this is not very accurate
but good enough for this level of description). One possible
type is the IP address type. The type of each piece of data, and
its OID are part of the ASN.1/BER encoding. When a value of a
variable is needed by a manager it sends a get-request PDU with
the OID of that variable, and a null value. The managed element
then responds by sending a get-response PDU that has in it the
same OID, the type of the variable, and the current value. Here
is an example of real data in an SNMP get-response PDU
packet:
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+-----------------------------------------+
| IP Header | 45 00 00 5E
| | 47 40 00 00
| | 3F 11 39 00
| | 87 B4 8C CA
| | 87 B4 8C 16
+-----------------------------------------+
| UDP Header | 00 A1 05 F5
| | 00 4A D3 65
+-----------------------------------------+
| PDU | 30 82 00 3E
| version | | 02 01 00 04
| Community = public | 06 70 75 62
| | | 6C 69 63 A2
| PDU Type | | 82 00 2F 02
| Request ID | 04 6C F2 0C
| | Error Status | 5C 02 01 00
| Error Index | SEQUENCE | 02 01 00 30
| of length 31 | SEQUENCE | 82 00 1F 30
| of length 27 | OID | 82 00 1B 06
| length=19 | | 13 2B 06 01
| | 02 01 07 05
| | 01 01 81 40
| | 81 34 81 0C
| | 81 4A 84 08
| IP Type | 135 | 180 | 40 04 87 B4
| 140 | 202 +-------------------+ 8C CA
+---------------------+
The first 20 bytes are the IP header. The next 8 bytes are the
UDP header, with the last two byte in it the UDP checksum (D3 65).
The next four bytes 30 82 00 3E are the beginning of the PDU: 30
is SEQUENCE OF, and 82 00 3E is the length of the payload in
bytes (62). Next come the Version (02 01 00) and the Community
(04 06 .. 63 = public). The next part is the PDU, first item is
the PDU type (A2 82 00 2F = GetResponse), the request ID (02 04
6C F2 0C 5C), the Error Status (02 01 00 = No Error), and the
Error Index (02 01 00). Now come the variables (i.e. the real
data): SEQUENCE of length 31 (30 82 00 1F). The first element
is a SEQUENCE of length 27 (30 82 00 1B). In it, the first
object is an OID of length 19 (06 13), then comes the OID:
.1.3.6.1.2.1.7.5.1.1.192.180.140.202.520. The last 6 octets 40
04 87 B4 8C CA represent an IP address: 40 is the type IP
address, 04 is the length, and the next four octets are the IP
address: 135.180.140.202.
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4.2. Translating IP address type
The basic requirement from SALG-PAT is that it will be able to
detect IP addresses in the SNMP packets payload. Once an IP
address is detected, SALG-PAT should check the translation table
and decide whether this address should be translated. If so, the
4 bytes representing the IPv4 address should be replaced by the
translated address, and the UDP checksum should be adjusted.
Therefore, SALG-PAT should parse the ASN.1/BER encoding, looking
for an IP address type. If it sees a different object type it can
jump to the beginning of the next object, unless the object is a
SEQUENCE Of. In that case the sequence should be parsed as it
may contain an IP address type inside. If an object is of type
IP address, the translation table is checked to see if this
address needs to be translated, and if so what the new value
should be. The translating function then should replace the 4
octets with the new address, and continue to parse the packet.
4.3. General MIB depended translation
For different applications it may be necessary to translate IP
addresses that are not encoded in the standard way. It may be a
part of a proprietary or a private MIB, which uses some other way
to represent an IP address (Integers or ASCII). In that case
some external information is needed, which states the OID of the
objects that are IP addresses and the way they are encoded. The
translation function, then, scans the packet for these specific
OIDs, checks the translation table and replaces the data
if needed. Note that since OIDs do not have a fixed size
this search is much more computationally consuming, and the
lookup operation may be very expensive.
4.4. Forwarding tables, and IP address type as a table index
IP address type is used in the standard MIB-II (as defined in RFC
1213) as the index (or part of the index) of several tables.
Some of the proprietary MIBs may also use it, since this is a very
convenient way to store information related to IP addresses.
The following MIB-II tables have IP address type in their indexes:
atTable, ipAddrTable, ipRouteTable, tcpConnTable, udpConnTable,
egpNeighTable.
The problem now is that if the manager is trying
to retrieve a specific value from one of these tables using the
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IP address as an index, it should use the local address and not
the translated one. If the translated address is used then the
index should be translated by SALG-PAT. However, if the access
to the table is done in order to get the entire table, or the
next entry in the table, such a translation may result in an
unpredicted result. Note that in such cases translation of the
queries is also required. A more detailed discussion of the need
and ability to translate queries can be found in Section 4.5.
The ability to translate IP addresses that are part of the tables
indexes is thus another required feature of SALG-PAT. In this
case the OID of the table should be predefined (by parsing the
MIBs offline). This is a special case of the General MIB
depended translation discussed in the last subsection. In this
case the encoding of the address is known (and different from the
IP address type). For example the IP address 135.180.140.202 is
encoded as 87 B4 8C CA when it is IP address type (each byte is a
number), and 81 40 81 34 81 0C 81 4A as an IP address index to a
table (this is due to the OID encoding scheme).
In this case the function searches for objects with an OID that
matches one of the OIDs in the translating table. If such an
object is found the next four OID numbers (it may be four to
eight bytes, depending on the ranges of the specific IP address)
of the OID are checked in the IP translation tables, and replaced
if needed. This mechanism allows us to replace table entries in
MIB tables indexed by IP addresses. A similar but a bit more
complicated mechanism, can handle tables that are indexed by more
than one IP address (like tcpConnTable).
4.5. Full transparency and forward/backward translation
As described before, making NAT completely transparent to all
management applications may be a very hard task. In some cases
it may also be undesirable. A big part of the manual management
is done by performing a telnet session to the appropriate device,
and changing some of the local configuration data. This data
contains local addresses. Therefore, it is clear that the full
address structure should be available to the administrators of
the network. The main purpose of SALG-PAT is to be a transparent
IP address translation mechanism for the automated discovery and
management tools. This raises the questions of how to use
SALG-PAT, what information to translate, and what not. If we
only need the basic IP address type translation then we only need
to translate packets that are going from the managed network to
the management station (i.e. GetResponse and Trap packets),
since they are the only ones containing IP addresses. However, if
we need to use the general OID translation, and in particular the
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table indexes, we must also translate outgoing packets.
5.0 Package size and UDP checksum
Changing the IP address in the payload should not change the size
of a packet, as we only replace 4 bytes by 4 bytes. General MIB
translation may require a change in the size as a different
number of bytes may be used to encode different IP addresses.
This is highly undesirable.
The BER encoding allows the use of both short and long
length encoding to represent a small index (i.e. smaller than
127). Therefore, in the table index case one can always
translate to long encoding. As a result, the encoded length will
not decrease. However if a byte smaller than 127 in an IP
address is translated to a value bigger than 127, an additional
byte may be required (this depends on the encoding used by the
agent application). This will require additional changes in the
headers (UDP and IP). In any case the UDP checksum should be
adjusted when making an IP translation. We can use the algorithm
from [SE 98], but a small modification must be introduced as the
4 bytes may start on an odd position. The following C code
adjusts the checksum to a replacement of one byte in an odd or
even position:
void checksumbyte(unsigned char *chksum, unsigned char *optr,
unsigned char *nptr, int odd)
/* assuming: unsigned char is 8 bits, long is 32 bits,
we replace one byte by one byte in an odd position.
- chksum points to the chksum in the packet
- optr points to the old byte in the packet
- nptr points to the new byte in the packet
- odd is 1 if the byte is in an odd position 0 otherwise
*/
{ long x, old, new;
x=chksum[0]*256+chksum[1];
x=~x & 0xFFFF;
if (odd) old=optr[0]*256; else old=optr[0];
x-=old & 0xFFFF;
if (x<=0) { x--; x&=0xFFFF; }
if (odd) new=nptr[0]*256; else new=nptr[0];
x+=new & 0xFFFF;
if (x & 0x10000) { x++; x&=0xFFFF; }
x=~x & 0xFFFF;
chksum[0]=x/256; chksum[1]=x & 0xff;}
Unlike TCP, the UDP checksum can be set to 0, which makes all the
applications ignore it. This can be used by SALG-PAT if the
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computational resources are limited.
6.0. Limitations of SALG-PAT
As described before, making the address translation completely
transparent to all management application is not an achievable
task. Many times system administrators use the telnet utility
in order to configure the managed devices in the managed domain.
In such cases address translation cannot be done, and
the administrator must be aware that a translation takes place.
6.1. Privacy, security, and debugging considerations
We assume that all the management information is sent on the
clear, i.e. without encryption and/or authentication. If such
encryption tools are used, then the SALG-PAT must have access to
the keys/protocols in order to be able to perform the translation
and/or to verify authentication. This should not be a problem
since in general there is only one source for the management
applications (i.e. these type of applications are not run by
general users). However, the complexity and resources needed to
perform the translation under these conditions will be much
higher.
6.2. Translation of fragmented UDP packets
As described in [SE 98], fragments of UDP packets do not carry the
destination/source port number with them. In order to parse an
SNMP packet the complete PDU must be built, and then sent to the
translation function. Note that in an extreme case,
fragmentation may cause an IP address type to be partitioned into
two different fragments. The good news is, however, that usually
the SNMP agents are aware of the MTU, and the SNMP packets are
usually relatively small.
7. SNMP versions
In this section we briefly describe some of the most significant
changes related to the newer versions of SNMP. A very good
description of the different versions can be found in:
http://www.simple-times.org/pub/simple-times/issues/5-1.html#alternative.
7.1 SNMP Version 2
The name SNMP Version 2 refers to a set of 12 documents (RFCs
1441-1452), that describe a new and enriched version of SNMP.
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However, some of the new features were not commonly accepted,
which resulted in various subversions (such as SNMPv2c, SNMPv2u,
and SNMPv2*). The consensus of the SNMPv2 working group was
published in RFCs 1902-1908 in 1996 and is referred to as
SNMPv2c.
The basic encoding of the PDU, including the use of the IP
address type, is very much similar to SNMPv1. Therefore, the
principles of SALG-PAT as described in Sections 4 and 5 are valid
for SNMPv2c.
One of the main purposes of this version was to add security
features, such as authentication, and privacy to SNMP. When
presented, these security features can prevent the use of
SALG-PAT as described in Section 6. It turns out that the
specifications of these features were very controversial, and
this was one of the main problems that prevented SNMPv2 from
becoming widely accepted.
7.2 SNMPv3
SNMP Version 3 (SNMPv3) is a new standard being proposed. This
version of the protocol is a combination of user-based security,
the protocol operations and data types from SNMPv2p, and support
for proxies. The security is based on the one found in SNMPv2u
and SNMPv2*, and updated after much review. The documents
defining this protocol are described in the RFCs 2261-2265. The
use of SALG-PAT for SNMPv3 is not covered by this document.
8. Current implementations
The SNMP application level gateway for payload address
translation was implemented in Bell-Labs. The C code is running
on a Solaris machine. The solution described in Figure 3, where
SALG-PAT was combined with the NAT implementation of Lucent's
RABU, was deployed in a large network management service
organization.
9. Acknowledgments
We thank Brett A. Denison for his contribution to the work that
led to this document. We also thank Pyda Srisuresh, for the
support, encouragement, and advice through out the work on this
document.
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REFERENCES
[SH 98] P. Srisuresh, and M. Holdrege, "The IP Network Address
Translator (NAT) terminology and considerations",
<draft-ietf-nat-terminology-01.txt> - Work in progress
[SE 98] P. Srisuresh, and K. Egevang, "Traditional IP Network
Address Translator (Traditional NAT)",
<draft-ietf-nat-traditional-00.txt> - Work in progress
[RFC-1631] P. Srisuresh, and K. Egevang, "The IP Network Address
Translator (NAT)", RFC 1631 February 1998 or its
successor.
[RFC-1066] McCloghrie K., and M. Rose, "Management Information
Base
for Network Management of TCP/IP-based internets", RFC
1066 August 1988 or its successor.
[RFC-1067] Case, J., M. Fedor, M. Schoffstall, and J. Davin,
"The
Simple Network Management Protocol", RFC 1067, August
1988
or its successor.
[RFC-1466] E. Gerich, "Guidelines for Management of IP Address
Space,
RFC 1466, May 1993 or its successor.
[RFC-768] J. Postel, "User Datagram Protocol (UDP)", RFC 768 or
its
successor.
[RFC-950] J. Mogul, J. Postel, "Internet Standard Subnetting
Procedure", RFC 950 or its successor.
[RFC 1157] J. Case, M. Fedor, M. Schoffstall, and J. Davin,
"The
Simple Network Management Protocol", RFC 1157, May
1990.
[RFC 1213] K. McCloghrie, and M.T. Rose, "Management Information
Base for Network
Management of TCP/IP-based internets:MIB-II", RFC 1213
Mars 1991, or its successor.
[RFC 1215] M.T. Rose, "Convention for defining traps for use with
the SNMP",
RFC 1215 Mars 1991, or its successor.
[RFC 1155] K. McCloghrie, and M.T. Rose, "Structure and
identification of
management information for TCP/IP-based internets", RFC
1155
May 1990, or its successor.
[RFC 2003] C. Perkins, "IP Encapsulation within IP", RFC 2003,
October
1996 or its successor.
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[RFC 2261] Harrington, D., Presuhn, R., and B. Wijnen, "An
Architecture for describing SNMP Management
Frameworks",
RFC 2261, January 1998.
[RFC 2262] Case, J., Harrington, D., Presuhn, R., and B. Wijnen,
"Message Processing and Dispatching for the Simple
Network
Management Protocol (SNMP)", RFC 2262, January 1998.
[RFC 2263] SNMPv3 Applications. D. Levi, P. Meyer, B. Stewart. RFC
2263,
January 1998.
[RFC 2264] User-based Security Model (USM) for version 3 of the
Simple
Network Management Protocol (SNMPv3). U. Blumenthal, B.
Wijnen.
RFC 2264, January 1998.
[RFC 2265] Wijnen, B., Presuhn, R., and K. McCloghrie,
"View-based
Access Control Model for the Simple Network Management
Protocol (SNMP)", RFC 2265, January 1998.
[ISO-8824] International Organization for Standardization,
Information Technology: Abstract Syntax Notation One
(ASN.1): Specification of Basic Notation, ISO/IEC
8824-1:
1995.
[ISO-8825] International Organization for Standardization,
Information Technology: ASN.1 Encoding Rules:
Specification of Basic Encoding Rules (BER), Canonical
Encoding Rules (CER) and Distinguished Encoding Rules
(DER), ISO/IEC 8825-1: 1995.
[Mi 97] M. A. Miller, Managing Internetworks with SNMP,
M&T Books,1997.
[PM 97] D. Perkins, and E. McGinnis, Understanding SNMP MIBs,
Prentice-Hall, 1997.
Authors' Addresses
Danny Raz
Bell Labs, Lucent Technologies
Room 4G-637
101 Crawfords Corner Rd
Holmdel, NJ 07733-3030
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U.S.A.
Voice: (732) 949-6712
Fax: (732) 949-0399
EMail: raz@lucent.com
Binay Sugla
Bell Labs, Lucent Technologies
Room 4F-621
101 Crawfords Corner Rd
Holmdel, NJ 07733-3030
U.S.A.
Voice: (732) 949-0850
Fax: (732) 949-0399
EMail: sugla@lucent.com
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