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rfc:rfc2962

Network Working Group D. Raz Request for Comments: 2962 Lucent Technologies Category: Informational J. Schoenwaelder

                                                        TU Braunschweig
                                                               B. Sugla
                                                           ISPSoft Inc.
                                                           October 2000
 An SNMP Application Level Gateway for Payload Address Translation

Status of this Memo

 This memo provides information for the Internet community.  It does
 not specify an Internet standard of any kind.  Distribution of this
 memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2000).  All Rights Reserved.

IESG Note

 This document describes an SNMP application layer gateway (ALG),
 which may be useful in certain environments.  The document does also
 list the issues and problems that can arise when used as a generic
 SNMP ALG.  Specifically, when using SNMPv3's authentication and
 privacy mechanisms this approach may be very problematic and
 jeopardize the SNMP security.  The reader is urged to carefully
 consider these issues before deciding to deploy this type of SNMP
 ALG.

Abstract

 This document describes the ALG (Application Level Gateway) for the
 SNMP (Simple Network Management Protocol) by which IP (Internet
 Protocol) addresses in the payload of SNMP packets are statically
 mapped from one group to another.  The SNMP ALG is a specific case of
 an Application Level Gateway as described in [15].
 An SNMP ALG allows network management stations to manage multiple
 networks that use conflicting IP addresses.  This can be important in
 environments where there is a need to use SNMP with NAT (Network
 Address Translator) in order to manage several potentially
 overlapping addressing realms.

Raz, et al. Informational [Page 1] RFC 2962 SNMP Payload Address Translation October 2000

 This document includes a detailed description of the requirements and
 limitations for an implementation of an SNMP Application Level
 Gateway.  It also discusses other approaches to exchange SNMP packets
 across conflicting addressing realms.

Table of Contents

 1.  Introduction ..................................................2
 2.  Terminology and Concepts Used  ................................5
 3.  Problem Scope and Requirements ................................5
 3.1 IP Addresses in SNMP Messages  ................................6
 3.2 Requirements ..................................................7
 4.  Translating IP Addresses in SNMP Packets ......................7
 4.1 Basic SNMP Application Level Gateway ..........................8
 4.2 Advanced SNMP Application Level Gateway  ......................8
 4.3 Packet Size and UDP Checksum ..................................9
 5.  Limitations and Alternate Solutions  .........................10
 6.  Security Considerations  .....................................12
 7.  Summary and Recommendations  .................................13
 8.  Current Implementations  .....................................14
 9.  Acknowledgments  .............................................14
 10. References ...................................................14
 11. Authors' Addresses ...........................................16
 12. Description of the Encoding of SNMP Packets  .................17
 13. Full Copyright Statement .....................................20

1. Introduction

 The need for IP address translation arises when a network's internal
 IP addresses cannot be used outside the network.  Using basic network
 address translation allows local hosts on such private networks
 (addressing realms) to transparently access the external global
 Internet and enables access to selective local hosts from the
 outside.  In particular it is not unlikely to have several addressing
 realms that are using the same private IPv4 address space within the
 same organization.
 In many of these cases, there is a need to manage the local
 addressing realm from a manager site outside the domain. However,
 managing such a network presents unique problems and challenges.
 Most available management applications use SNMP (Simple Network
 Management Protocol) to retrieve information from the network
 elements.  For example, a router may be queried by the management
 application about the addresses of its neighboring elements.  This
 information is then sent by the router back to the management

Raz, et al. Informational [Page 2] RFC 2962 SNMP Payload Address Translation October 2000

 station as part of the payload of an SNMP packet. In order to retain
 consistency in the view as seen by the management station we need to
 be able to locate and translate IP address related information in the
 payload of such packets.
 The SNMP Application Level Gateway for Payload Address Translation,
 or SNMP ALG, is a technique in which the payload of SNMP packets
 (PDUs) is scanned and IP address related information is translated if
 needed.  In this context, an SNMP ALG can be an additional component
 in a NAT implementation, or it can be a separate entity, that may
 reside in the same gateway or even on a separate node.  Note that in
 our context of management application all devices in the network are
 assumed to have a fixed IP address.  Thus, SNMP ALG should only be
 combined with NAT that uses static address assignment for all the
 devices in the network.
 A typical scenario where SNMP ALG is deployed as part of NAT is
 presented in figure Figure 1.  A manager device is managing a remote
 stub, with translated IP addresses.
       \ | /              .
 +---------------+  WAN   .        +------------------------------+
 |Regional Router|-----------------|Stub Router w/NAT and SNMP ALG|
 +---------------+        .        +------------------------------+
         |                .                   |
         |                .                   |  LAN
    +----------+          .            ---------------
    | Manager  |    Stub border         Managed network
    +----------+
             Figure 1: SNMP ALG in a NAT configuration

Raz, et al. Informational [Page 3] RFC 2962 SNMP Payload Address Translation October 2000

 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.
                       +---------------+     +-----------------+
                       | SNMP ALG      |-----|Management device|
                       +---------------+     +-----------------+
                     T1  |           | T1
                         |           |
     Stub A .............|....   ....|............ Stub B
                         |           |
               +---------------+   +----------------+
               |Bi-directional |   |Bi-directional |
               |NAT Router w/  |   |NAT Router w/  |
               |static address |   |static address |
               |mapping        |   |mapping        |
               +---------------+   +---------------+
                 |                         |
                 |  LAN               LAN  |
         -------------             -------------
      192.10.x.y   |                 |  192.10.x.y
                 /____\           /____\
   Figure 2: Using external SNMP ALG 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, the NAT used
 in this case must be a basic NAT with a static one to one mapping.
 An SNMP ALG 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 bi-
 directional NAT, however they both use the same translation tables.
 In many cases the router may be unable to handle SNMP ALG and retain
 acceptable performance. In these cases it may be better to locate the
 SNMP ALG outside the router, as described in Figure 2.

Raz, et al. Informational [Page 4] RFC 2962 SNMP Payload Address Translation October 2000

2. Terminology and Concepts Used

 In general we adapt the terminology defined in [15].  Our main
 concern are SNMP messages exchanged between SNMP engines.  This
 document only discusses SNMP messages that are send over UDP, which
 is the preferred transport mapping for SNMP messages [5].  SNMP
 messages send over other transports can be handled in a similar way.
 Thus, the term SNMP packet is used throughout this document to refer
 to an SNMP message contained in an UDP packet.
 SNMP messages contain SNMP PDUs (Protocol Data Units).  An SNMP PDU
 defines the parameters for a specific SNMP protocol operation.  The
 notion of flow is less relevant in this case, and hence we will focus
 on the information contained in a single SNMP packet.
 There are currently three versions of SNMP. SNMP version 1 (SNMPv1)
 protocol is defined in STD 15, RFC 1157 [2]. The SNMP version 2c
 (SNMPv2c) protocol is defined in RFC 1901 [3], RFC 1905 [4] and RFC
 1906 [5].  Finally, the SNMP version 3 (SNMPv3) protocol is defined
 in RFC 1905 [4], 1906 [5], RFC 2572 [10] and RFC 2574 [12].  See RFC
 2570 [9] for a more detailed overview over the SNMP standards.  In
 the following, unless otherwise mentioned, we use the term SNMP in
 statements that are applicable to all three SNMP versions.
 SNMP uses ASN.1 [13] to define the abstract syntax of the messages.
 The actual encoding of the messages is done by using the Basic
 Encoding Rules (BER) [14], which provide the transfer syntax.
 We refer to packets that go from a management station to the network
 elements as "outgoing", and packets that go from the network elements
 to the management station as "incoming".
 A basic SNMP ALG is an SNMP ALG implementation in which only IP
 address values encoded in the IpAddress type are translated. A basic
 SNMP ALG therefore does not need to be MIB aware.
 An advanced SNMP ALG is an SNMP ALG implementation which is capable
 of handling and replacing IP address values encoded in well known IP
 address data types and instance identifiers derived from those data
 types. This implies that an advanced SNMP ALG is MIB aware.

3. Problem Scope and Requirements

 As mentioned before, in many cases, there is a need to manage a local
 addressing realm that is using NAT, from a manager site outside the
 realm.  A particular important example is the case of network
 management service providers who provide network management services
 from a remote site.  Such providers may have many customers, each

Raz, et al. Informational [Page 5] RFC 2962 SNMP Payload Address Translation October 2000

 using the same private address space. When all these addressing
 realms are to be managed from a single management station address
 collision occurs.  There are two straight forward ways to overcome
 the address collision. One can
 1.  reassign IP addresses to the different addressing realms, or
 2.  use static address NAT to hide the address collisions from the
     network management application.
 The first solution is problematic as it requires both a potentially
 large set of IP addresses, and the reconfiguration of a large portion
 of the network.  The problem with the second solution is that many
 network management applications are currently unaware of NAT, and
 because of the large investment needed in order to make them NAT
 aware are likely to remain so in the near future.
 Hence, there is a need for a solution that is transparent to the
 network management application (but not to the user), and that does
 not require a general reconfiguration of a large portion of the
 network (i.e. the addressing realm).  The SNMP ALG described in this
 memo is such a solution.

3.1 IP Addresses in SNMP Messages

 SNMP messages can contain IP addresses in various places and formats.
 The following four categories have been identified:
 1.  IP version 4 addresses and masks stored in the IpAddress tagged
     ASN.1 data type which are not part of an instance identifier. An
     example is the ipAdEntNetMask object defined in the IP-MIB [6].
 2.  IP version 4 addresses contained in instance identifiers derived
     from index objects using the IpAddress data type.  An example is
     the ipAdEntAddr index object of the IP-MIB [6].
 3.  IP addresses (any version) contained in OCTET STRINGS.  Examples
     include addressMapNetworkAddress object of the RMON2-MIB [7], and
     IP addresses contained in OCTET STRINGS derived from well-known
     textual conventions (e.g. TAddress [5] or Ipv6Address [8] or
     InetAddress [19]).
 4.  IP addresses (any version) contained in instance identifiers
     derived from OCTET STRINGS.  This may derived from well-known
     textual conventions (e.g. TAddress [5] or Ipv6Address [8] or
     InetAddress [19]) like the ipv6AddrAddress index object of the
     IPV6-MIB [8].
 Textual conventions that can contain IP addresses can be further
 divided in NAT friendly and NAT unfriendly ones.  A NAT friendly
 textual convention ensures that the encoding on the wire contains

Raz, et al. Informational [Page 6] RFC 2962 SNMP Payload Address Translation October 2000

 sufficient information that an advanced SNMP ALG which understands
 the textual convention and which has the necessary MIB knowledge can
 do a proper translation.  An example of this type is the Ipv6Address
 textual convention.
 A NAT unfriendly textual convention requires that an SNMP ALG, which
 understands the textual convention and which has the necessary MIB
 knowledge, has access to additional information in order to do a
 proper translation.  Examples of this type are the TAddress and the
 InetAddress textual conventions which require that an additional
 varbind is present in an SNMP packet to determine what type of IP
 address a given value represents.  Such a varbind may or may not be
 present depending on the way a management applications retrieves
 data.

3.2 Requirements

 An SNMP ALG should provide transparent IP address translation to
 management applications.  An SNMP ALG must be compatible with the
 behavior of the SNMP protocol operations as defined by RFC 1157 [2]
 and RFC 1905 [4] and must not have negative impact on the security
 provided by the SNMP protocol.  A fully transparent SNMP ALG must be
 able to translate all categories of IP addresses as described above,
 when provided with the specified OID's and the encoding details.
 The SNMP ALG requires bi-directional NAT devices enroute, that
 support static address mapping for all nodes in the respective
 private realms.  When there are multiple private realms supported by
 a single SNMP ALG, the external addresses assumed by each of the NAT
 devices must not collide with each other.

4. Translating IP Addresses in SNMP Packets

 This section describes several ways to translate IP addresses in SNMP
 packets.
 A general SNMP ALG must be capable to translate IP addresses in
 outgoing and incoming SNMP packets.
 SNMP messages send over UDP may experience fragmentation at the IP
 layer. In an extreme case, fragmentation may cause an IP address type
 to be partitioned into two different fragments.  In order to
 translate IP addresses in SNMP messages, the complete SNMP message
 must be available. As described in [18], fragments of UDP packets do
 not carry the destination/source port number with them.  Hence, an
 SNMP ALG must reassemble IP packets which contain SNMP messages.  The

Raz, et al. Informational [Page 7] RFC 2962 SNMP Payload Address Translation October 2000

 good news is, however, that usually SNMP agents are aware of the MTU,
 and that SNMP packets are usually relatively small.  Some SNMP
 implementations also set the don't fragment (DF) bit in the IP header
 [1] to avoid fragmentation.

4.1 Basic SNMP Application Level Gateway

 A basic SNMP ALG is an SNMP ALG implementation in which only IP
 address values encoded in the IpAddress base type are translated.  A
 basic SNMP ALG implementation parses an ASN.1/BER encoded SNMP packet
 looking for elements that are encoded using the IpAddress base type.
 Appendix A contains a more detailed description of the structure and
 encoding used by SNMP.
 An IpAddress value can be identified easily by its tag value (0x40).
 Once an IpAddress has been detected, the SNMP ALG checks the
 translation table and decides whether the address should be
 translated. If the address needs translation, the 4 bytes
 representing the IPv4 address are replaced with the translated IPv4
 address and the UDP checksum is adjusted.  Section 4.3 describes an
 efficient algorithm to adjust the UDP checksum without recalculating
 it.
 The basic SNMP ALG does not require knowledge of any MIBs since it
 relies on the ASN.1/BER encoding of SNMP packets.  It is therefore
 easy to implement.  A basic SNMP ALG does not change the overall
 messages size and hence it does not cause translated messages to be
 lost due to message size constraints.
 However, a basic SNMP ALG is only able to translate IPv4 addresses in
 objects that use the IpAddress base type. Furthermore, a basic SNMP
 ALG is not capable to translate IP addresses in objects that are
 index components of conceptual tables.  This is especially
 problematic on index components that are not accessible.  Hence, the
 basic SNMP ALG is restricted to the first out of the four possible
 ways to represent IP addresses in SNMP messages (see Section 3.1).

4.2 Advanced SNMP Application Level Gateway

 An advanced SNMP ALG is an SNMP ALG implementation which is capable
 of handling and replacing IP address values encoded in well known IP
 address data types and instance identifiers derived from those data
 types.  Hence, an advanced SNMP ALG may be able to transparently map
 IP addresses that are in the format 1-4 as described in Section 3.1.
 This implies that an advanced SNMP ALG must be MIB aware.

Raz, et al. Informational [Page 8] RFC 2962 SNMP Payload Address Translation October 2000

 An advanced SNMP ALG must maintain an OBJECT IDENTIFIER (OID)
 translation table in order to identify IP addresses that are not
 encoded in an IpAddress base type.  The OID translation table needs
 to maintain information about the OIDs where translation may be
 needed.  Furthermore, the translation table needs to keep information
 about instance identifiers for conceptual tables that contain IP
 addresses.  Such an OID translation table may be populated offline by
 using a MIB compiler which loads the MIBs used within an addressing
 realm and searches for types, textual conventions and table indexes
 that may contain IP addresses.
 The translation function 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 expensive.
 The ability to translate IP addresses that are part of the index of a
 conceptual table is a required feature of an advanced SNMP ALG.  IP
 addresses embedded in an instance identifier are ASN.1/BER encoded
 according to the OID encoding rules. For example, the OID for the
 10.1.2.3 instance of the ipAdEntIfIndex object of the IP-MIB [6] is
 encoded as 06 0D 2B 06 01 02 01 04 14 01 02 0A 01 02 03.  Replacing
 the embedded private IPv4 address with 135.180.140.202 leads to the
 OID 06 11 2B 06 01 02 01 04 14 01 02 81 07 81 34 81 0C 81 4A.  This
 example shows that an advanced SNMP ALG may change the overall packet
 size since IP addresses embedded in an OID can change the size of the
 ASN.1/BER encoded OID.
 Another effect of an advanced SNMP ALG is that it changes the
 lexicographic ordering of rows in conceptual tables as seen by the
 SNMP manager.  This may have severe side-effects for management
 applications that use lexicographic ordering to retrieve only parts
 of a conceptual table.  Many SNMP managers check lexicographic
 ordering to detect loops caused by broken agents. Such a manager will
 incorrectly report agents behind an advanced SNMP ALG as broken SNMP
 agents.

4.3 Packet Size and UDP Checksum

 Changing an IpAddress value in an SNMP packet does not change the
 size of the SNMP packet.  A basic SNMP ALG does therefore never
 change the size of the underlying UDP packet.
 An advanced SNMP ALG may change the size of an SNMP packet since a
 different number of bytes may be needed to encode a different IP
 address.  This is highly undesirable but unavoidable in the general
 case.  A change of the SNMP packet size requires additional changes
 in the UDP and IP headers.  Increasing packet sizes are especially

Raz, et al. Informational [Page 9] RFC 2962 SNMP Payload Address Translation October 2000

 problematic with SNMPv3.  The SNMPv3 message header contains the
 msgMaxSize field so that agents can generate Response PDUs for
 GetBulkRequest PDUs that are close to the maximum message size the
 receiver can handle.  An SNMP ALG which increases the size of an SNMP
 packet may have the effect that the Response PDU can not be processed
 anymore.  Thus, an advanced SNMP ALG may cause some SNMPv3
 interactions to fail.
 In both cases, the UDP checksum must be adjusted when making an IP
 address translation.  We can use the algorithm from [18], but a small
 modification must be introduced as the modified bytes may start on an
 odd position.  The C code shown in Figure 3 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;
      }

5. Limitations and Alternate Solutions

 Making SNMP ALGs completely transparent to all management
 applications is not an achievable task.  The basic SNMP ALG described
 in Section 4.1 only translates IP addresses encoded in the IpAddress
 base type.  Such an SNMP ALG achieves only very limited transparency
 since IP addresses are frequently used as part of an index into a
 conceptual table.  A management application will therefore see both
 the translated as well as the original address, which can lead to

Raz, et al. Informational [Page 10] RFC 2962 SNMP Payload Address Translation October 2000

 confusion and erroneous behavior of management applications.
 However, a certain class of management applications like e.g.
 network discovery tools may work pretty well across NATs with a basic
 SNMP ALG in place.
 An advanced SNMP ALG described in Section 4.2 achieves better
 transparency.  However, an advanced SNMP ALG can only claim to be
 transparent for the set of data types (textual conventions)
 understood by the advanced SNMP ALG implementation and for a given
 set of MIB modules.  The price paid for better transparency is
 additional complexity, potentially increased SNMP packet sizes and
 mixed up lexicographic ordering.  Especially with SNMPv3, there is an
 opportunity that communication fails due to increased packet sizes.
 Management applications that rely on lexicographic ordering will show
 erroneous behavior.
 Both, basic and advanced SNMP ALGs, introduce problems when using
 SNMPv3 security features.  The SNMPv3 authentication mechanism
 protects the whole SNMP message against modifications while the
 SNMPv3 privacy mechanism protects the payload of SNMPv3 messages
 against unauthorized access.  Thus, an SNMP ALG must have access to
 all localized keys in use in order to modify SNMPv3 messages without
 invalidating them.  Furthermore, the SNMP ALG must track any key
 changes in order to function.  More details on the security
 implications of using SNMP ALGs can be found in Section 6.
 Finally, an SNMP ALG only deals with SNMP traffic and does not modify
 the payload of any other protocol.  However, management systems
 usually use a set of protocols to manage a network.  In particular
 the telnet protocol is often used to configure or troubleshoot
 managed devices.  Hence, a management system and the human network
 operator must generally be aware that a network address translation
 is occurring, even in the presence of an SNMP ALG.
 A possible alternative to SNMP ALGs are SNMP proxies, as defined in
 RFC 2573 [11].  An SNMP proxy forwarder application forwards SNMP
 messages to other SNMP engines according to the context, and
 irrespective of the specific managed object types being accessed.
 The proxy forwarder also forwards the response to such previously
 forwarded messages back to the SNMP engine from which the original
 message was received.  Such a proxy forwarder can be used in a NAT
 environment to address SNMP engines with conflicting IP addresses.
 (Just replace the box SNMP ALG with a box labeled SNMP PROXY in
 Figure 2.)  The deployment of SNMP proxys has the advantage that
 different security levels can be used inside and outside of the
 conflicting addressing realms.

Raz, et al. Informational [Page 11] RFC 2962 SNMP Payload Address Translation October 2000

 The proxy solution, which is structurally preferable, requires that
 the management application is aware of the proxy situation.
 Furthermore, management applications have to use internal data
 structures for network elements that allow for conflicting IP
 addresses since conflicting IP addresses are not translated by the
 SNMP proxy.  Deployment of proxies may also involve the need to
 reconfigure network elements and management stations to direct their
 traffic (notifications and requests) to the proxy forwarder.

6. Security Considerations

 SNMPv1 and SNMPv2c have very week security services based on
 community strings. All management information is sent in cleartext
 without encryption and/or authentication. In such an environment,
 SNMP messages can be modified by any intermediate node and management
 application are not able to verify the integrity of SNMP messages.
 Furthermore, an SNMP ALG does not need to have knowledge of the
 community strings in order to translate embedded IP addresses.  Thus,
 deployment of SNMP ALGs in an SNMPv1/SNMPv2c environment introduces
 no additional security problems.
 SNMPv3 supports three security levels: no authentication and no
 encryption (noAuth/noPriv), authentication and no encryption
 (auth/noPriv), and authentication and encryption (auth/priv).  SNMPv3
 messages without authentication and encryption (noAuth/noPriv) are
 send in cleartext.  In such a case the usage of SNMP ALGs introduces
 no additional security problems.
 However, the usage of SNMP ALG introduces new problems when SNMPv3
 authentication and optionally encryption is used.  First, SNMPv3
 messages with authentication and optionally encryption (auth/noPriv
 and auth/priv) can only be processed by an SNMP ALG which supports
 the corresponding cryptographic algorithms and which has access to
 the keys in use.  Furthermore, as keys may be updated, the SNMP ALG
 must have a mechanism that tracks key changes (either by analyzing
 the key change interactions or by propagating key changes by other
 mechanisms).  Second, the computational complexity of processing SNMP
 messages may increase dramatically.  The message has to be decrypted
 before the translation takes place.  If any translation is done the
 hash signature used to authenticate the message and to protect its
 integrity must be recomputed.
 In general, key exchange protocols are complicated and designing an
 SNMP ALG which maintains the keys for a set of SNMP engines is a
 non-trivial task. The User-based Security Model for SNMPv3 [12]
 defines a mechanism which takes a password and generates localized

Raz, et al. Informational [Page 12] RFC 2962 SNMP Payload Address Translation October 2000

 keys for every SNMP engine.  The localized keys have the property
 that a compromised single localized key does not automatically give
 an attacker access to other SNMP engines, even if the key for other
 SNMP engines is derived from the same password.
 An SNMP ALG implementation which maintains lists of (localized) keys
 is a potential target to attack the security of all the systems which
 use these keys.  An SNMP ALG implementation which maintains passwords
 in order to generate localized keys is a potential target to attack
 the security of all systems that use the same password.  Hence, an
 SNMP ALG implementation must be properly secured so that people who
 are not authorized to access keys or passwords can not access them.
 Finally, SNMP ALGs do not allow a network operator to use different
 security levels on both sides of the NAT.  Using a secure SNMP
 version outside of a private addressing realm while the private
 addressing realm runs an unsecured version of SNMP may be highly
 desirable in many scenarios, e.g. management outsourcing scenarios.
 The deployment of SNMPv3 proxies instead of SNMP ALGs should be
 considered in these cases since SNMP proxies can be configured to use
 different security levels and parameters on both sides of the
 proxies.

7. Summary and Recommendations

 Several approaches to address SNMP agents across NAT devices have
 been discussed in this memo.
 1.  Basic SNMP ALGs as described in Section 4.1 provide very limited
     transparency since they only translate IPv4 addresses encoded in
     the IpAddress base type.  They are fast and efficient and may be
     sufficient to execute simple management applications (e.g.
     topology discovery applications) in a NAT environment. However,
     other management applications are likely to fail due to the
     limited transparency provided by a basic SNMP ALG.  Basic SNMP
     ALGs are problematic in a secure SNMP environment since they need
     to maintain lists of keys or passwords in order to function.
 2.  Advanced SNMP ALGs as described in Section 4.2 provide better
     transparency.  They can be transparent for the set of data types
     they understand and for a given set of MIB modules.  However, an
     advanced SNMP ALG is much more complex and less efficiency than a
     basic SNMP ALG. An advanced SNMP ALG may break the lexicographic
     ordering when IP addresses are used to index conceptual tables
     and it may change the SNMP packet sizes.  Especially with SNMPv3,
     there is an opportunity that communication fails due to increased
     message sizes.  Advanced SNMP ALGs are problematic in a secure
     SNMP environment, since they need to maintain lists of keys or
     passwords in order to function.

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 3.  SNMP proxies as described in RFC 2573 [11] allow management
     applications to access SNMP agents with conflicting IP addresses.
     No address translation is performed on the SNMP payload by an
     SNMP proxy forwarder.  Hence, management applications must be
     able to deal with network elements that have conflicting IP
     addresses.  This solution requires that management applications
     are aware of the proxy situation.  Deployment of proxies may also
     involve the need to reconfigure network elements and management
     stations to direct their traffic (notifications and requests) to
     the proxy forwarder.  SNMP proxies have the advantage that they
     allow to use different security levels inside and outside of a
     given addressing realm.
 It is recommended that network operators who need to manage networks
 in a NAT environment make a careful analysis before deploying a
 solution.  In particular, it must be analyzed whether the management
 applications will work with the transparency and the side-effects
 provided by SNMP ALGs.  Furthermore, it should be researched whether
 the management applications are able to deal with conflicting IP
 addresses for network devices.  Finally, the additional complexity
 introduced to the over all management system by using SNMP ALGs must
 be compared to the complexity introduced by the structurally
 preferable SNMP proxy forwarders.

8. Current Implementations

 A basic SNMP ALG as described in Section 4.1 was implemented for
 SNMPv1 at Bell-Labs, running on a Solaris Machine.  The solution
 described in Figure 2, where SNMP ALG was combined with the NAT
 implementation of Lucent's PortMaster3, was deployed successfully in
 a large network management service organization.

9. Acknowledgments

 We thank Pyda Srisuresh, for the support, encouragement, and advice
 throughout the work on this document.  We also thank Brett A. Denison
 for his contribution to the work that led to this document.
 Additional useful comments have been made by members of the NAT
 working group.

10. References

 [1]  Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981.
 [2]  Case, J., Fedor, M., Schoffstall, M. and J. Davin, "A Simple
      Network Management Protocol (SNMP)", STD 15, RFC 1157, May 1990.

Raz, et al. Informational [Page 14] RFC 2962 SNMP Payload Address Translation October 2000

 [3]  Case, J., McCloghrie, K., Rose, M. and S. Waldbusser,
      "Introduction to Community-based SNMPv2", RFC 1901, January
      1996.
 [4]  Case, J., McCloghrie, K., Rose, M. and S. Waldbusser, "Protocol
      Operations for Version 2 of the Simple Network Management
      Protocol (SNMPv2)", RFC 1905, January 1996.
 [5]  Case, J., McCloghrie, K., Rose, M. and S. Waldbusser, "Transport
      Mappings for Version 2 of the Simple Network Management Protocol
      (SNMPv2)", RFC 1906, January 1996.
 [6]  McCloghrie, K., "SNMPv2 Management Information Base for the
      Internet Protocol using SMIv2", RFC 2011, November 1996.
 [7]  Waldbusser, S., "Remote Network Monitoring Management
      Information Base Version 2 using SMIv2", RFC 2021, January 1997.
 [8]  Haskin, D. and S. Onishi, "Management Information Base for IP
      Version 6: Textual Conventions and General Group", RFC 2465,
      December 1998.
 [9]  Case, J., Mundy, R., Partain, D. and B. Stewart, "Introduction
      to Version 3 of the Internet-standard Network Management
      Framework", RFC 2570, April 1999.
 [10] Case, J., Harrington, D., Presuhn, R. and B. Wijnen, "Message
      Processing and Dispatching for the Simple Network Management
      Protocol (SNMP)", RFC 2572, April 1999.
 [11] Levi, D., Meyer, P. and B. Stewart, "SNMP Applications", RFC
      2573, April 1999.
 [12] Blumenthal, U. and B. Wijnen, "User-based Security Model (USM)
      for version 3 of the Simple Network Management Protocol
      (SNMPv3)", RFC 2574, April 1999.
 [13] ISO, "Information processing systems - Open Systems
      Interconnection - Specification of Abstract Syntax Notation One
      (ASN.1)", International Standard 8824, December 1987.
 [14] ISO, "Information processing systems - Open Systems
      Interconnection - Specification of Basic Encoding Rules for
      Abstract Syntax Notation One (ASN.1)", International Standard
      8825, December 1987.
 [15] Srisuresh, P. and M. Holdrege, "IP Network Address Translator
      (NAT) Terminology and Considerations", RFC 2663, August 1999.

Raz, et al. Informational [Page 15] RFC 2962 SNMP Payload Address Translation October 2000

 [16] Miller, M., "Managing Internetworks with SNMP", MT Books, 1997.
 [17] Perkins, D. and E. McGinnis, "Understanding SNMP MIBs", Prentice
      Hall, ISBN 0-13-437708-7, 1997.
 [18] Srisuresh, P. and K. Egevang, "Traditional IP Network Address
      Translator (Traditional NAT)", Work in Progress.
 [19] Daniele, M., Haberman, B., Routhier, S. and J. Schoenwaelder,
      "Textual Conventions for Internet Network Addresses", RFC 2851,
      June 2000.

11. Authors' Addresses

 Danny Raz
 Lucent Technologies
 101 Crawfords Corner Rd
 Holmdel, NJ  07733-3030
 USA
 Phone: +1 732 949-6712
 Fax:   +1 732 949-0399
 EMail: raz@lucent.com
 URI:   http://www.bell-labs.com/
 Juergen Schoenwaelder
 TU Braunschweig
 Bueltenweg 74/75
 38106 Braunschweig
 Germany
 Phone: +49 531 391-3266
 Fax:   +49 531 391-5936
 EMail: schoenw@ibr.cs.tu-bs.de
 URI:   http://www.ibr.cs.tu-bs.de/
 Binay Sugla
 ISPSoft Inc.
 106 Apple Street
 Tinton Falls, NJ  07724
 USA
 Phone: +1 732 936-1763
 EMail: sugla@ispsoft.com
 URI:   http://www.ispsoft.com/

Raz, et al. Informational [Page 16] RFC 2962 SNMP Payload Address Translation October 2000

12. Appendix A. Description of the Encoding of SNMP Packets

 SNMP packets use 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 [13] and ISO-8825 [14].
 A good description of ASN.1/BER can be found in the book "Managing
 Internetworks with SNMP", by M. A. Miller [16], or in Appendix A of
 the book "Understanding SNMP MIBs", by D. Perkins, and E. McGinnis
 [17].
 Each variable that is referred to in an SNMP packet is uniquely
 identified by an OID (Object Identifier), usually written as a
 sequence of numbers separated by dots (e.g. 1.3.6.1.2.1.1.3.0).  Each
 variable also has an associated base type (this is not very accurate
 but good enough for this level of description).  One possible base
 type is the IpAddress type. The base type of each variable and its
 OID are conveyed by the ASN.1/BER encoding.  Note that it is possible
 to associate additional type information with a variable by using
 textual conventions.  The additional type semantics provided by
 textual conventions are not conveyed by 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
 contains the same OID, the base type of the variable, and the current
 value. Figure 4 shows an example of real data contained in an SNMPv1
 GetResponse PDU.
 The first 20 bytes contain the IPv4 4 header. The next 8 bytes
 contain the UDP header.  The last two bytes of the UDP header contain
 the UDP checksum (D3 65).  The next four bytes 30 82 00 3E are the
 beginning of the SNMP message: 30 is SEQUENCE, and 82 00 3E is the
 length of the data in the SEQUENCE in bytes (62).  The data in the
 SEQUENCE is the version (02 01 00) and the community string (04 06 70
 75 62 6C 69 63).  The last element in the SEQUENCE of the SNMPv1
 message is the SNMP PDU.

Raz, et al. Informational [Page 17] RFC 2962 SNMP Payload Address Translation October 2000

    +-----------------------------------------+
    |       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
    +-----------------------------------------+
    |       SNMP Message                      |     30 82 00 3E
    |  Version                     |          |     02 01 00 04
    |  Community                              |     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                          | SEQUENCE |     82 00 1F 30
    |                              |   OID    |     82 00 1B 06
    |           |                             |     13 2B 06 01
    |                                         |     02 01 07 05
    |                                         |     01 01 81 40
    |                                         |     81 34 81 0C
    |                                         |     81 4A 84 08
    |  IpAddress          | 135    | 180      |     40 04 87 B4
    |  140      | 202     +-------------------+     8C CA
    +---------------------+
 The SNMP PDU itself is a tagged SEQUENCE: A2 indicates a GetResponse
 PDU and 82 00 2F is the length of the data in the GetResponse PDU in
 bytes (47).  The data in the GetResponse PDU is the request-id (02 04
 6C F2 0C 5C), the error-status (02 01 00), and the error-index (02 01
 00).  Now follow the variables which contain the real payload: A
 SEQUENCE OF of length 31 (30 82 00 1F) containing a SEQUENCE of
 length 27 (30 82 00 1B).  In it, the first object is an OID of length
 19 (06 13) with the value 1.3.6.1.2.1.7.5.1.1.192.180.140.202.520.
 The last 6 bytes 40 04 87 B4 8C CA represent an IpAddress: 40 is the
 identification of the base type IpAddress, 04 is the length, and the
 next four bytes are the IP address value (135.180.140.202).
 The example also shows an IP address embedded in an OID.  The OID
 prefix resolves to the udpLocalAddress of the UDP-MIB, which is
 indexed by the udpLocalAddress 192.180.140.202 (81 40 81 34 81 0C 81

Raz, et al. Informational [Page 18] RFC 2962 SNMP Payload Address Translation October 2000

 4A) and the udpLocalPort 520 (84 08). The SNMP packet actually shows
 an internal contradiction caused by a basic SNMP ALG since the
 udpLocalAddress encoded in the OID (192.180.140.202) is not equal to
 the value of the udpLocalAddress object instance (135.180.140.202).

Raz, et al. Informational [Page 19] RFC 2962 SNMP Payload Address Translation October 2000

13. Full Copyright Statement

 Copyright (C) The Internet Society (2000).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Raz, et al. Informational [Page 20]

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