Systems and Network Management The Structure of Management Information (SMI) 1 Background The snmp protocol is called “simple” because the protocol itself is quite simple. However, the difficulty is in applying it to actually managing systems and networks. There are many terms and standards involved; it is necessary to understand enough of them to make sense of the mibs that define the objects that you want to monitor and manage. If you can make sense of the mib files, you can identify the objects that you want to monitor. 1.1 Management Information Base (MIB) The mib]s define the objects that you can manage. When you installed the Net snmp software package, you installed some mib files into the directory /usr /share/snmp/mibs/. You can list them all with: $ rpm -ql ucd-snmp | grep snmp/mibs/.*\.txt There are many other mibs that are not included here; you can download others from somewhere such as http://www.simpleweb.org/ietf/ and include them into your Net snmp clients as explained at http: //net-snmp.sourceforge.net/FAQ.html#How_do_ Nick Urbanik ver. 1.1 The Structure of Management Information (SMI) Systems and Network Management 2 I_add_a_MIB_ and at http://net-snmp.sourceforge. net/tutorial/commands/mib-options.html. 1.2 Structure of Management Information smi is a definition of the structure of the mibs, how they are connected together into a tree. See the rfcs below in section 2.1 on page 10. It specifies which part of asn.1 will be used to define mibs. 1.3 Abstract Syntax Notation One (ASN.1) asn.1 is widely used for many things other than snmp. See http://asn1.elibel.tm.fr/en/uses/ for a list of some of the applications of asn.1. There is a web site dedicated to providing information about it at http: //asn1.elibel.tm.fr/. 1.4 Basic Encoding Rules (BER) The basic encoding rules is an iso standard. It describes a method for encoding values of each asn.1 type as a string of octets. 1.5 What we are doing today We will examine the specification for mib-2, on your machine at /usr/share/snmp/mibs/RFC1213-MIB.txt and understand the structure of it. Nick Urbanik ver. 1.1 The Structure of Management Information (SMI) Systems and Network Management 3 1.6 Syntax of a Managed Object Definition Every object definition in smi has the format: name OBJECT-TYPE SYNTAX datatype ACCESS either read-only, read-write, write-on DESCRIPTION "Some text that describes this managed obj ::= { unique object ID that defines this obje We will refer to this later in our activities. 2 The MIB-II Definition Here I will refer to my edited version of RFC1213-MIB.txt, available at http://sysadmin.no-ip.com/snm/lectures/ smi/RFC1213-MIB.txt. RFC1213-MIB DEFINITIONS ::= BEGIN IMPORTS mgmt, NetworkAddress, IpAddress, Counter, TimeTicks FROM RFC1155-SMI OBJECT-TYPE FROM RFC-1212; The first line defines the name of the mib, here RFC1213-MI. The format of this definition is always the same. The IMPORTS section of the mib is sometimes called the linkage section. It lets you import definitions of Nick Urbanik ver. 1.1 The Structure of Management Information (SMI) Systems and Network Management 4 datatypes and oids from other mibs. Here we get the definition of: • mgmt • NetworkAddress • IpAddress • Counter • Gauge • TimeTicks from RFC1155-SMI, the mib from the rfc that defines smiv1. It also imports OBJECT-TYPE from RFC-1212, the Concise MIB Definition, which defines how mib files are written. mib-2 OBJECT IDENTIFIER ::= { mgmt 1 } The line above says that the oid of mib-2 is 1.3.6.1.2.1. RFC1155-SMI defines mgmt as the oid 1.3.6.1.2. -- groups in MIB-II system interfaces at ip icmp tcp udp Nick Urbanik OBJECT OBJECT OBJECT OBJECT OBJECT OBJECT OBJECT IDENTIFIER IDENTIFIER IDENTIFIER IDENTIFIER IDENTIFIER IDENTIFIER IDENTIFIER ::= ::= ::= ::= ::= ::= ::= { { { { { { { mib-2 mib-2 mib-2 mib-2 mib-2 mib-2 mib-2 ver. 1.1 1 2 3 4 5 6 7 } } } } } } } The Structure of Management Information (SMI) Systems and Network Management 5 egp OBJECT IDENTIFIER ::= { mib-2 8 } transmission OBJECT IDENTIFIER ::= { mib-2 10 } snmp OBJECT IDENTIFIER ::= { mib-2 11 } So here the system group is defines as the oid 1.3.6.1.2.1.1, and so on. A comment is a line starting with --. -- the Interfaces table ------- The Interfaces table contains information on th interfaces. Each interface is thought of as be attached to a ‘subnetwork’. Note that this ter not be confused with ‘subnet’ which refers to a addressing partitioning scheme used in the Inte of protocols. ifTable OBJECT-TYPE SYNTAX SEQUENCE OF IfEntry ACCESS not-accessible STATUS mandatory DESCRIPTION "A list of interface entries. The num entries is given by the value of ifNum ::= { interfaces 2 } This is the first managed object shown here. ifTable represents a table of network interfaces on a managed device. Notice that object names are defined with mixed case, the first letter is lowercase. Notice that this follows the format of an OBJECT-TYPE in section 1.6 on page 3. Nick Urbanik ver. 1.1 The Structure of Management Information (SMI) Systems and Network Management 6 The SYNTAX of ifTable is SEQUENCE OF IfEntry. The object is not-accessible, which means that you cannot query the agent for the value of this object. It has a STATUS of mandatory, which means that if an agent complies wiht the mibb-ii specification, then it must implement this object. The DESCRIPTION tells you what this object is. The unique oid is 1.3.6.1.2.1.2.2, or iso.org.dod.internet.mgmnt.interfaces.2. Next, let’s look at the SEQUENCE definition, which is used with the SEQUENCE OF type in the ifTable definition. IfEntry ::= SEQUENCE { ifIndex INTEGER, ifDescr DisplayString, ifType INTEGER, ifMtu INTEGER, ifSpeed Gauge, ifPhysAddress PhysAddress, ifAdminStatus INTEGER, ifOperStatus INTEGER, Nick Urbanik ver. 1.1 The Structure of Management Information (SMI) Systems and Network Management 7 ifLastChange TimeTicks, ifInOctets Counter, ifInUcastPkts Counter, ifInNUcastPkts Counter, ifInDiscards Counter, ifInErrors Counter, ifInUnknownProtos Counter, ifOutOctets Counter, ifOutUcastPkts Counter, ifOutNUcastPkts Counter, ifOutDiscards Counter, ifOutErrors Counter, ifOutQLen Gauge, ifSpecific OBJECT IDENTIFIER } Nick Urbanik ver. 1.1 The Structure of Management Information (SMI) Systems and Network Management 8 The name of the SEQUENCE (IfEntry) is mixed-case, but the first letter is capitalised, which is different from the object definition for ifTable. A SEQUENCE is a list of objects that go into one row of a table. After this, we must have OBJECT-TYPE definitions that define each of these variables. A table can have any number of rows. Tha agent manages the number of rows. An nms can also add rows to a table using a set operation. IfEntry is the data type; rather like a struct definition in the C language. Let’s look at ifEntry, the definition of what we find in the table, the actual rows of the table themselves. It looks almost the same as the definition for ifTable, except that it has a new clause, INDEX. The index is a unique value that identifies a single row in the table, like an array index. A table is rather like an array of structs in C. The agent assigns these index values. If a router has eight interfaces, then ifTable will contain eight rows. ifEntry OBJECT-TYPE SYNTAX IfEntry ACCESS not-accessible STATUS mandatory DESCRIPTION "An interface entry containing objects subnetwork layer and below for a parti interface." INDEX { ifIndex } ::= { ifTable 1 } Nick Urbanik ver. 1.1 The Structure of Management Information (SMI) Systems and Network Management 9 Here we now look at the definition for ifIndex, the first item in IfEntry. Notice that indexes start from 1. ifIndex OBJECT-TYPE SYNTAX INTEGER ACCESS read-only STATUS mandatory DESCRIPTION "A unique value for each interface. I ranges between 1 and the value of ifNu value for each interface must remain c least from one re-initialization of th network management system to the next initialization." ::= { ifEntry 1 } This object is read-only, which means that you can see the value, but not change it. Here is the last object we look at from this table: ifDescr OBJECT-TYPE SYNTAX DisplayString (SIZE (0..255)) ACCESS read-only STATUS mandatory DESCRIPTION "A textual string containing informati interface. This string should include the manufacturer, the product name and of the hardware interface." ::= { ifEntry 2 } Nick Urbanik ver. 1.1 The Structure of Management Information (SMI) Systems and Network Management 10 END ifDescr is just a textual description of the interface. The mib definition finishes with END. 2.1 Where can I get the standards documents from? The standard for smiv1 can be downloaded from ftp: //ftp.rfc-editor.org/in-notes/rfc1155.txt, and for —SMIv2 at ftp://ftp.rfc-editor.org/in-notes/ rfc2578.txt. The standards for asn.1 and ber can be downloaded from http://asn1.elibel.tm.fr/en/ standards/. Nick Urbanik ver. 1.1