SNMP(Simple Network Management Protocol)

Jon Sevy

Geometric and Intelligent Computing Laboratory

Department of Mathematics and Computer Science

Drexel University

http://gicl.mcs.drexel.edu

SNMP Overview

• Define mechanism for remote management of network devices (routers, bridges, etc.)

• Fundamental principle: all device management done by simple variable value manipulation

• Approach:– standard means for specifying quantities recognized by

devices– protocol for requesting, returning, notifying of changes of

values

SNMP Message Protocol

• Messages use UDP, ports 161 (requests/responses) and 162 (notifications)

• Message types:– GetRequest: request values of variables from device– GetNextRequest: request value of variable following the one

supplied– GetResponse: return values– SetRequest: instruct device to set values of variables– Trap: from device - notify monitor / manager of value change

SNMP Communities

• Define “communities” specifying access to specific variable sets (read-write, read only, none)

• Supply community name as “password” in each message– unencrypted in version of SNMP still widely used!!

SNMP Message Encoding

• Encode message as byte stream using ASN.1 BER (Abstract Syntax Notation 1 Basic Encoding Rules)

• Quantites encoded as Type,Length,Value triples• Types

– Subset of basic ASN.1 types used in SNMP: integer, octet string, object identifier (“variable name”), sequence

– SNMP-defined types: gauge, counter, IP address, etc.

• Values– weirdly encoded!! (see ASN.1 specs)

SNMP SMI(Structure of Management Information)

• Variables recognized by device supplied in MIB (Management Information Base)– text file giving variables and data structures defined using

ASN.1– standard variable sets often provided as RFC’s– device-specific sets provided by vendors

• Management stations parse MIB’s to determine variables available for management– obtain both data structure and management information

Example portion of MIB (from RFC 1213)

-- the Interfaces group

-- Implementation of the Interfaces group is mandatory for -- all systems.

ifNumber OBJECT-TYPE SYNTAX INTEGER ACCESS read-only STATUS mandatory DESCRIPTION "The number of network interfaces (regardless of their current state) present on this system." ::= { interfaces 1 }

ASN.1 Object Identifiers

• Variables identified by globally unique strings of digits– ex: 1.3.6.1.4.1.3.5.1.1– name space is hierarchical; tree on next slide

• in above, 1 stands for iso, 3 stands for org, 6 stands for dod, 1 stands for internet, 4 stands for private, etc.

• Variable names are aliases for digit strings (within MIB) – From previous page: ifNumber ::= { interfaces 1 } – interfaces was previously defined in MIB as 1.3.6.1.2.1.2, so

ifNumber = 1.3.6.1.2.1.2.1

Partial ASN.1 Object Identifier Tree

(from: ASN.1 Complete, J. Larmouth, Open Systems Solutions, 1999)

Application: GICL SNMP Monitor

• Java-based SNMP application– Query devices for available MIB variables– Set desired variable values

• Current status– retrieve and display all values from device

– automatically build data structures to hold retrieved values

• Future work– incorporate MIB information via MIB parser

– auto-generate GUI display

– implement device discovery

References

• Internet Standards 15, 16 and 17• ASN.1 Complete, J. Larmouth, Open Systems

Solutions, 1999 (available online)• SNMP : a guide to network management, S. Feit,

McGraw-Hill, 1995