SNMPv3

1
SNMPv3

• Wes Hardaker
• Network Associates Laboratories
• 6 August 2002
• Wes_Hardaker_at_nai.com

2
Overview

• About SNMP in General
• SNMPv3 Architecture Breakdown
• User Based Security
• View Based Access Control
• Future of SNMP
• Summary Questions

3
Authors Background

• Past
• Lead developer of the Net-SNMP open source
  project
• Developer of the OpenSNMP toolkit
• Present
• Project lead in a scalable IPSec policy
  management system build on top of SNMPv3.
• Future
• Participating in various IETF working groups
  responsible for continuing development of SNMP
  technologies.
• Apologetic
• Not all the slides in this presentation are in
  your hand out.
• http//www.net-snmp.org/presentations/snmpv3/

4
About SNMP
Simple Network Management Protocol -- RFC
1157 SNMP Can I buy a vowel? --Harrie
Hazewinkel
5
SNMP Overview What is SNMP?

• Treats the network as a distributed database

Computer
Management Station
Router
6
SNMP Database example netsh

• netshgt select ifIndex, ipAdEntIfIndex, ifDescr,
• ipAdEntAddr, ifOperStatus
• from ifTable, ipAddrTable
• where ifIndex ipAdEntIfIndex
• ---------------------------------------------
  ------
• ifIndexipAdEntIfIndexifDescripAdEntAddrifOper
  Status
• ---------------------------------------------
  ------
• 1 1 lo 127.0.0.1
  up
• 2 2 sit0 0.0.0.0
  down
• 3 3 vmnet1172.16.80.1
  up
• ---------------------------------------------
  ------

7
SNMP Overview

• Network monitoring and Information Collection

GET What is in your routing table?
8
SNMP Overview

• Network Configuration

SET Add a route to your routing table
9
SNMP Overview

• Alarms and Event conditions

TRAP or INFORM HELP!!!!
10
Terminology

• SNMP the protocol itself
• MIB the specification defining a particular
  piece of data to be transferred over SNMP
• SMI the language a MIB is written in
• It is not ASN.1, contrary to what many people
  think.
• OID / Object Identifier A hierarchical naming
  scheme using number assignment.
• .1.3.6.1.2.1.1.4.0 You (the system contact for
  a device)
• .iso.org.dod.internet.mgmt.mib-2.system.sysContact
  .0

11
SNMP Standards Timeline

• SNMPv1 created in 1988
• Officially a Full Standard
• SNMPv2c added a few things in 1996
• SNMPv2c MSG Format classified as experimental
• PDUs, etc, classified as a FULL Standard
• SNMPv3 added security in 1999
• Currently a Draft standard
• Approved as a Full Standard, but not published
  yet
• SNMPv2p, SNMPv2u, SNMPv2
• secure protocol versions that didnt make the cut.

12
SNMPv1 and SNMPv2c message format

• SNMPv1/SNMPv2c Message
• version snmpv10 snmpv2c1
• community STRING
• PDU (operation)
• request-id INTEGER
• error-status INTEGER
• error-index INTEGER
  variable-bindings
• OID, VALUE

13
SNMPv1 and SNMPv2c problems

• Community strings for authentication
• Clear Text Passwords
• Community strings also used for context
• Accessing data in a certain context is
  difficult
• No Standardized Method for Access Control
• No consistent method to configure communities
  across a network of many devices.
• Most were devices manually configured, or worse,
  left with their default settings intact.
• kingdom keyspublic, private

14
SNMPv1 example

• snmpget -d -v 1 -c public localhost sysUpTime.0
• Sending 43 bytes to 127.0.0.1
• 0000 30 29 02 01 00 04 06 70 75 62 6C 69 63
  A0 1C 02 0).....public ..
• 0016 04 2C 78 27 BC 02 01 00 02 01 00 30 0E
  30 0C 06 .,x'¼......0.0..
• 0032 08 2B 06 01 02 01 01 03 00 05 00
  ..........
• Received 45 bytes from 127.0.0.1
• 0000 30 2B 02 01 00 04 06 70 75 62 6C 69 63
  A2 1E 02 0.....public..
• 0016 04 2C 78 27 BC 02 01 00 02 01 00 30 10
  30 0E 06 .,x'¼......0.0..
• 0032 08 2B 06 01 02 01 01 03 00 43 02 11 0F
  ........C...
• sysUpTimeInstance Timeticks (4367) 00043.67

15
Goals behind SNMPv3

• Security
• Security
• Security
• Provide modularity in the architecture
• Replacing new elements in the future should be
  easier.
• Modularity many IETF RFCs
• Separate context of the request from the
  authentication

16
Aside CERT Advisery CA-2002-03

• SNMP uses BER encoded packets.
• BER has a bunch of flexible rules which allow
  strangely encoded, but legal packets.
• The failing was in the parsing of the BER
  encoding, NOT in the SNMP protocol.
• SNMPv1 was the only packet format tested.
• Its likely most vendors also fixed SNMPv3
  parsing too.
• The university that produced the results has been
  testing various BER-based protocols.
• Started with LDAP.
• Next? Only they know. Kerberos?

17
SNMPv3 Architecture
The architecture is designed to be modular to
allow the evolution of the Framework over
time. -- RFC 2570 RFC 2571
18
SNMPv3 Standards Breakdown

• RFC2570 SNMPv3 Overview
• RFC2571 Architecture
• RFC2572 Message Processing
• RFC2573 Applications
• RFC2574 User-based Security Model
• RFC2575 View-based Access Control Model
• RFC2576 v1/v2c/v3 Coexistence / Transition
• RFC2578-80 SMIv2 -- Language of the MIBs
• RFC1157 SNMPv1
• RFC1901,5 SNMPv2
• ...

19
SNMPv3 Framework Architecture
Application or Agent
Access Control
VACM
SNMPv3 Engine
...
CG
CR
NR
NG
Message Processing
Security
Dispatcher
SNMPv3 MP
User-based (USM)
Kerberos
SNMPv1
...
...
UDP
TCP
...
Network
20
SNMPv3 Framework Architecture
Application or Agent
Access Control
VACM
...
CG
CR
NR
NG
Message Processing
Security
Dispatcher
SNMPv3 MP
User-based (USM)
Kerberos
SNMPv1
...
...
UDP
TCP
...
Network
21
SNMPv3 Applications (RFC 2573)

• Application types have been formally defined
• CG Command Generator (was Manager)
• CR Command Responder (was Agent)
• NG Notification Generator
• NR Notification Responder
• PF Proxy Forwarder (not shown in diagram)
• An application can be of multiple types
• An agent is typically a CR and a NG
• A mid level manager is likely a CG, CR, NG, and
  NR
• (and maybe a PF)

22
SNMPv3 Protocol Packet Breakdown

• SNMPv3Message
• Version INTEGER
• (snmpv3 3)
• HeaderData
• msgID INTEGER
• MaxSize INTEGER
• Flags STRING
• SecurityModel INTEGER
• (USM 3)
• UsmSecurityParameters STRING
• AuthoritativeEngineID STRING,
• AuthoritativeEngineBoots INTEGER
• AuthoritativeEngineTime INTEGER
• UserName STRING
• AuthenticationParameters STRING
• PrivacyParameters STRING
• ScopedPduData

Dispatcher
MSG Processor
Security Model
Application
23
SNMP Overview

• Network monitoring and Information Collection

GET What is in your routing table?
RESPONSE Its
24
SNMPv3 Example Generating a Request
Application or Agent
Access Control
VACM
...
CG
CR
NR
NG
Message Processing
Security
Dispatcher
SNMPv3 MP
User-based (USM)
Kerberos
SNMPv1
...
...
UDP
TCP
...
Network
25
SNMPv3 Example Receiving a Request
Application or Agent
Access Control
VACM
...
CG
CR
NR
NG
Message Processing
Security
Dispatcher
SNMPv3 MP
User-based (USM)
Kerberos
SNMPv1
...
...
UDP
TCP
...
Network
26
SNMPv3 Example Generating a Response
Application or Agent
Access Control
VACM
...
CG
CR
NR
NG
Message Processing
Security
Dispatcher
SNMPv3 MP
User-based (USM)
Kerberos
SNMPv1
...
...
UDP
TCP
...
Network
27
SNMPv3 Example Receiving a Response
Application or Agent
Access Control
VACM
...
CG
CR
NR
NG
Message Processing
Security
Dispatcher
SNMPv3 MP
User-based (USM)
Kerberos
SNMPv1
...
...
UDP
TCP
...
Network
28
Architecture Components
The major portions of the architecture are an
SNMP engine containing a Message Processing
Subsystem, a Security Subsystem and an Access
Control Subsystem, and possibly multiple SNMP
applications which provide specific functional
processing of management data.. -- RFC 2571
29
The Dispatcher
Application or Agent
Access Control
VACM
...
CG
CR
NR
NG
Message Processing
Security
Dispatcher
SNMPv3 MP
User-based (USM)
Kerberos
SNMPv1
...
...
UDP
TCP
...
Network
30
The Dispatcher (RFC 2572)

• The simplest component of the architecture
• Directs packets to and from the other elements
• Application or agent
• CG, CR, NG, NR
• The Network (through the appropriate transport
  layer)
• The correct message processor
• Makes its decision based on
• What component sent it
• What protocol version is being sent
• What type of PDU is being sent
• Knowledge of it not required by the typical user

31
The Message Processor
Application or Agent
Access Control
VACM
...
CG
CR
NR
NG
Message Processor
Security
Dispatcher
SNMPv3 MP
User-based (USM)
Kerberos
SNMPv1
...
...
UDP
TCP
...
Network
32
The SNMPv3 Message Processor (RFC 2572)

• Encodes and decodes the majority of the packet
• Handles errors and exceptions
• Message too big to fit in a packet
• Parse errors are detected
• ...
• Passes to appropriate security model for
  authentication and encryption support.
• (Currently, the only defined security model is
  the USM)
• Knowledge of it not required by the typical user

33
Security Model
Application or Agent
Access Control
VACM
...
CG
CR
NR
NG
Message Processing
Security
Dispatcher
SNMPv3 MP
User-based (USM)
Kerberos
SNMPv1
...
...
UDP
TCP
...
Network
34
Security Model

• Authenticates packets to ensure/verify origin
• Current authentication methods
• Provides message encryption/decryption support
• The ScopedPDU can be protected by encryption
• Makes its decisions based on packet data
• EngineID, Engine Boots, Engine Time
• User Name
• Authentication field
• Privacy field
• Currently only one standard security model exists
• The User Based Security Model, or USM

35
User Based Security USM (RFC 2574)

• The User Based Security model provides
• Authentication via MD5 or SHA1 hash
• The hash verifies the authenticity of the entire
  v3 message.
• Modified or forged packets will be rejected
• Encryption via DES encryption
• The ScopedPDU is encrypted (basically, the
  payload)
• 3 levels of security noAuthNoPriv, authNoPriv,
  authPriv
• It does not provide protection against
• Denial of Service
• Traffic Analysis

36
USM About EngineIDs, etc...

• EngineIDs are
• A unique string of data
• Generally defined from one of
• IPv4 address
• IPv6 address
• MAC address
• Administratively defined strings
• Implementation dependent
• EngineBoots number of reboots
• EngineTime Time since last initialized
• Information is automatically probed by protocol.

37
USM A User is...

• A USM User is defined by
• The EngineID of the authoritative engine
• The SecurityName of the user (i.e., user name)
• The authentication type (MD5 or SHA1) and key
• The privacy type (DES) and key
• A user is modifiable via SNMP SET operations
• Authentication and privacy keys can be changed.
• Encryption types and authentication types can not
  be changed without deleting and recreating the
  user

38
USM The Authoritative Engine

• Only one side of a transaction is authoritative
• Authoritative side where the master user key
  exists
• Typically this means the SNMP agents are
  authoritative
• The authoritative side is defined by whether the
  packet being sent is expecting a response or not.
• An odd effect of this is
• The engine receiving SNMPv3 INFORMs, which expect
  a I got it response, are authoritative.
• The engine sending SNMPv3 TRAPs, which dont
  require a response, are authoritative.
• Ick.

39
USM Keys

• USM Keys used to authenticate and encrypt
  messages are generated
• A password hashed using the authentication
  algorithm (maybe)
• The resulting hash is then re-hashed after mixing
  it with the authoritative engineID.
• This means
• All user keys are different on each host
• Pro A cracked systems keys cant be used to
  gain access to other systems.
• Con Distributing keys to many systems is
  difficult

40
USM Keys
Management Application
Password
Ku Master Key
Kul1 Local Key 1
Kul2 Local Key 2
Kul3 Local Key 3
41
USM The math behind the keys

• PassLong repeat(password) till 1Mb long
• Ku hash(PassLong)
• Kul hash(Ku authEngineID Ku)
• Notes
• Passwords must be at least 8 characters long
• Ku need not be generated from a password, but can
  be generated randomly instead.
• Protects against brute-forcing low entropy
  passwords
• Hash is currently one of MD5, SHA1

42
Access Control
Application or Agent
Access Control
VACM
...
CG
CR
NR
NG
Message Processing
Security
Dispatcher
SNMPv3 MP
User-based (USM)
Kerberos
SNMPv1
...
...
UDP
TCP
...
Network
43
Access Control

• Decides if a particular object instance may be
  accessed or not.
• Consulted by the CR and NG application types
• Currently, only one access control model exists
• The View-Based Access Control Module (VACM)

44
View Based Access Control (RFC 2575)

• Bases access control decisions on
• Operation type (read, write, notify)
• Security model
• User performing the action
• Users are placed into a group and the groups
  are assigned the rights, not the user.
• Security level of the transaction (authenticated?
  encrypted?)
• Object OID being accessed (and the context
  information)
• OID ranges are included or excluded from view
• I wish
• Value of the object being accessed
• Users could be assigned to multiple groups
  (unix-like)

45
VACM Access based on OID tree
1
Include .1.2 Exclude .1.2.2 Include .1.2.2.3 Ob
jects excluded are simply not visible
2
3
4
2
1
3
3
2
1
1
2
Simple Wild-carding not shown, but possible
46
The Future of SNMP

• Other security work
• AES encryption for the USM
• USM alternatives kerberos,
• New protocol PDU operations (EoS WG)
• Better bulk retrieval
• Better write transaction handling
• Better data compression
• Easier row operations (creation, deletion, )
• A new SMI (SMIng WG)
• Hierarchical Data Definition Language
• Easier to write
• Easier to read

47
Summary

• SNMPv1, v2c bad, no security
• SNMPv3 better, has security
• Start switching to SNMPv3 now.
• Almost FULL standard
• Most vendors support it
• SNMPv3 is modular in design

48
Conclusion

• Questions?
• Wes Hardaker ltWes_Hardaker_at_nai.comgt