SNMP - An overview provided by Wireshark

Simple Network Management Protocol (SNMP)

SNMP is used to monitor and manage devices on networks.

History

For an overview on SNMP, read this http://en.wikipedia.org/wiki/Simple_Network_Management_Protocol

Protocol dependencies

Typically, SNMP uses UDP as its transport protocol. The well known UDP ports for SNMP traffic are 161 (SNMP) and 162 (SNMPTRAP). It can also run over TCP, Ethernet, IPX, and other protocols. ATM uses SNMP as its ILMI (Integrated Local Management Interface) protocol.

Wireshark

The SNMP dissector is fully functional. If properly configured allows to decrypt encrypted SNMPv3 packets.

MIB files

If built with the libsmi libraries, Wireshark uses those libraries to resolve numeric OIDs (e.g. 1.3.6.1.2.1.2.2.1.6.1) into human readable format (e.g. IF-MIB::ifPhysAddress.1). The default installation only contains some common MIB files so Wireshark won't be able to resolve all possible OIDs.

You can configure which MIB files are loaded by using the preference setting mentioned below. You can also specify "ALL" in the preferences, resulting in all MIBs being loaded.

When specifying the MIB modules to load, use a colon separator. Note that the MIB module name is not necessarily the name of the file itself. The MIB name to use may be discovered by looking for the DEFINITIONS keyword in the MIB file.

For Unix systems, whether your version of Wireshark is built with the libsmi libraries can be determined by opening the "About Wireshark" window with the "Help ==> About Wireshark" menu item. The third paragraph in the "Wireshark" tab of that window (which is the tab that is opened by default) will include the phrase "with SMI", followed by the version number of the libsmi libraries, if Wireshark is built with libsmi, and will include the phrase "without SMI" if Wireshark is built without libsmi. The OS X binaries available from the Wireshark Web site are built with the libsmi libraries. If Wireshark is built with libsmi, it assumes the MIB files are stored in directories under /usr/local/share/mibs.

For Windows, the binaries available from the Wireshark Web site are built with the libsmi libraries. The MIB files are stored e.g. in C:\Program Files\Wireshark\snmp\mibs.

In versions of Wireshark built with libsmi, in the the "Preferences" dialog opened by the "Edit ==> Preferences" menu item, under the topic "Name resolution", you will find a button "SMI (MIB amd PIB) Modules" which opens an editing window where you can add/remove modules. The name of the file you add to the "mibs" directory mentioned above must be the same as the module name itself (found in the MIB itself at the start, just before "DEFINITIONS ::=BEGIN").

XXX - are all MIB files in these dirs are inspected and only the "right" files loaded?

Many network-related MIBs definitions can be downloaded from http://bytesphere.com/mibs/detail.html or http://www.mibdepot.com.

Which MIB's do I need?

If you see any unresolved OID's you may need to add a MIB file to the Net-SNMP libs.

The following will give an example to add the missing information to display SampleCaptures/b6300a.cap correctly.

In packet 7 you see: SNMPv2-SMI::enterprise.253.8.64.4.2.1.5.10.14150900 resp. 1.3.6.1.4.1.253.8.64.4.2.1.5.10.14150900.

What's missing now is the enterprise with the id 253. The IANA Private Enterprise Numbers list tells us that this is Xerox.

Get hold of the Xerox MIBs from one of the links above and place them in the mibs directory (you need the printer-MIB as well) and change the file ending from .mib to .txt.

When loading a specified MIB module failed a warning message like: Cannot find module (IP-MIB): At line 0 in (none) will be shown on the console at Wireshark startup. WIN32: To have a console window already open at that time, set the preference setting "Open a console window" to "Always", Save the Preferences and restart Wireshark. Otherwise the console will be open too late and you'll see nothing.

Preference Settings

Wireshark's SNMP protocol preferences let you control the display of the OID in the info column, desegmentation of SNMP over TCP, and which MIB modules to load (see above).

The USMuserTable file preference allows the user to choose a file with the engine-ids, usernames and passwords in order to allow decryption of encrypted packets. The file is formated as follows:

Please note: the order of the parameters in the file will vary, depending on your version of wireshark! If the sample below doesn't work for you, go into WireShark -> Edit -> Preferences -> Protocols -> SNMP, and add the settings through the GUI; afterwards, look at the created snmp_users file to determine the order of the columns.

Recent versions of Wireshark seem to use the following format for snmp decryption (~/.wireshark/snmp_users)

# This file is automatically generated, DO NOT MODIFY.
,"admin","MD5","myauth","DES","mysecretkey"

The following example seems to be outdated (fix me, I don't want to delete it in case it offers value to someone).

# this is an example snmp_users_file (e.g. ~/.wireshark/snmp_users)

# comments start with a # and finish at the end of line
# empty lines are ignored

# each record has the following fileds
# engine-id username auth_pass priv_pass auth_proto priv_proto
# fileds are separated by whitespace
# engine-id, username and passwds can be either "double quoted" or hex
# double quoted strings are c-style strings and take escape chars
# a '*' as engine-id matches any any given engine-id
# auth_proto can be either MD5 or SHA1 (or just SHA). Defaults to MD5 if not given.
# priv_proto can be either DES or AES. Defaults to DES if not given.
# protocols are case-insensitive (md5 == MD5)
# first match wins (place localized entries before those for any engine)

80001F888059DC486145A26322 "pippo" "plutoxxx" "PLUTOxxx" MD5 DES

# pippo for any engine will use DES and MD5
* "pippo" "plutoxxx" "PLUTOxxx"


# SHA + DES
80001F888059DC486145A26322 "pippo2" "plutoxxx" "PLUTOxxx" SHA

# SHA + AES
80001F888059DC486145A26322 "pippo3" "plutoxxx" "PLUTOxxx" SHA AES

# MD5 + AES
80001F888059DC486145A26322 "pippo4" "plutoxxx" "PLUTOxxx" AES

Example traffic

SampleCaptures/b6300a.cap A bunch of GETs and RESPONSEs

Display Filter

A complete list of SNMP display filter fields can be found in the display filter reference

Show only the SNMP based traffic:
```
 snmp
```

Capture Filter

You cannot directly filter SNMP protocols while capturing. However, if you know the UDP ports used (see above), you can filter on that ones.

Capture SNMP traffic over the default ports (161 and 162):
```
 udp port 161 or udp port 162
```

External links

SGMP (an ancestor of SNMP):

RFC 1028 A Simple Gateway Monitoring Protocol

SNMPv1

RFC 1155 Structure and Identification of Management Information for TCP/IP-based Internets
RFC 1156 Management Information Base for Network Management of TCP/IP-based internets
RFC 1157 A Simple Network Management Protocol (SNMP)
RFC 1158 Management Information Base for Network Management of TCP/IP-based internets: MIB-II

SNMPv2

RFC 1441 Introduction to version 2 of the Internet-standard Network Management Framework
RFC 1442 Structure of Management Information for version 2 of the Simple Network Management Protocol (SNMPv2)
RFC 1443 Textual Conventions for version 2 of the Simple Network Management Protocol (SNMPv2)
RFC 1444 Conformance Statements for version 2 of the Simple Network Management Protocol (SNMPv2)
RFC 1445 Administrative Model for version 2 of the Simple Network Management Protocol (SNMPv2)
RFC 1446 Security Protocols for version 2 of the Simple Network Management Protocol (SNMPv2)
RFC 1447 Party MIB for version 2 of the Simple Network Management Protocol (SNMPv2)
RFC 1448 Protocol Operations for version 2 of the Simple Network Management Protocol (SNMPv2)
RFC 1449 Transport Mappings for version 2 of the Simple Network Management Protocol (SNMPv2)
RFC 1450 Management Information Base for version 2 of the Simple Network Management Protocol (SNMPv2)
RFC 1451 Manager-to-Manager Management Information Base
RFC 1452 Coexistence between version 1 and version 2 of the Internet-standard Network Management Framework

SNMPv2 (Community based)

RFC 1901 Introduction to Community-based SNMPv2
RFC 1902 Structure of Management Information for Version 2 of the Simple Network Management Protocol (SNMPv2)
RFC 1903 Textual Conventions for Version 2 of the Simple Network Management Protocol (SNMPv2)
RFC 1904 Conformance Statements for Version 2 of the Simple Network Management Protocol (SNMPv2)
RFC 1905 Protocol Operations for Version 2 of Simple Network Management Protocol (SNMPv2)
RFC 1906 Transport Mappings for Version 2 of the Simple Network Management Protocol (SNMPv2)
RFC 1907 Management Information Base for Version 2 of the Simple Network Management Protocol (SNMPv2)
RFC 1908 Coexistence between Version 1 and Version 2 of the Internet-standard Network Management Framework

SNMPv3

RFC 2271 An Architecture for Describing SNMP Management Frameworks
RFC 2272 Message Processing and Dispatching for the Simple Network Management Protocol (SNMP)
RFC 2273 SNMPv3 Applications
RFC 2275 View-based Access Control Model (VACM) for the Simple Network Management Protocol (SNMP)
RFC 3414 User-based Security Model (USM) for version 3 of the Simple Network Management Protocol (SNMPv3)
RFC 3826 The Advanced Encryption Standard (AES) Cipher Algorithm in the SNMP User-based Security Model

RMON

RFC 1271 Remote Network Monitoring Management Information Base
RFC 1513 Token Ring Extensions to the Remote Network Monitoring MIB
RFC 1757 Remote Network Monitoring Management Information Base (obsoletes 1271)
RFC 2021 Remote Network Monitoring Management Information Base - Version 2 - using SMIv2
RFC 2613 Remote Network Monitoring MIB Extensions for Switched Networks Version 1.0
RFC 2819 Remote Network Monitoring Management Information Base (obsoletes 1757)
RFC 2895 Remote Network Monitoring MIB Protocol Identifier Reference
RFC 2896 Remote Network Monitoring MIB Protocol Identifier Macros
RFC 3144 Remote Monitoring MIB Extensions for Interface Parameters Monitoring
RFC 3273 Remote Network Monitoring Management Information Base for High Capacity Networks
RFC 3287 Remote Monitoring MIB Extensions for Differentiated Services
RFC 3395 Remote Network Monitoring MIB Protocol Identifier Reference Extensions
RFC 3434 Remote Monitoring MIB Extensions for High Capacity Alarms

SMI

RFC 2578 Structure of Management Information Version 2 (SMIv2)
RFC 2579 Textual Conventions for SMIv2
RFC 2580 Conformance Statements for SMIv2