SNMP讯息格式

1、SNMP 訊息格式IP 標頭UDP 標頭SNMP 訊息SNMP 標頭PDU 標頭PDU 資料1SNMP HeaderSNMP MessageVersion IdentifierCommunity Name ( “public”)Protocol Data UnitThe length of SNMP messages should not exceed 484 octets.Message := SEQUENCE versionINTEGER version-1(0),communityOCTET STRING,data ANY VersionCommunitySNMP PDU2SNMP 區域

2、名稱CommunityRelationship between an Agent and Managers.Community NameUsed to validate the SNMP messages.A device can participate in more than one community (password).A community name (password) defines the scope of control that a manager has over elements in a community. Default community name: “pub

3、lic”.3PDU Header可分成兩種形式針對 GetRequest , GetNextRequest , SetRequest , GetReponse針對 Trap 而言PDU 型態要求代號錯誤狀況錯誤索引PDU typeenterpriseagent-addrgeneric-trapvariable-bindingsspecific-traptime-stamp4PDU 型態五種 SNMP-PDUs 型態SNMP-PDUs:= CHOICE getrequest GetRequest-PDU, get-next-request GetNextRequest-PDU, get-resp

4、onse GetResponse-PDU, set-request SetRequest-PDU, trap Trap-PDU5SNMP PDURequest-idError-statusError-indexPDU := SEQUENCE request-id INTEGER,error-status INTEGER noError(0),tooBig(1),noSuchName(2),badValue(3),readOnly(4)genErr(5),error-index INTEGER,variable-bindingsSEQUENCE OF nameObjectName,valueOb

5、jectSyntax 6SNMP PDUs (cont.)GetRequest, GetNextRequest, SetRequestGetResponsePDU typerequest-id00variable-bindingsPDU typerequest-iderror-statuserror-indexvariable-bindings7Trap PDUEnterprise: Type of Object generating trap.Agent Address:Address of object generating trap.Generic Trap:Generic trap t

6、ype.Specific Trap:Enterprise specific trap.Time Stamp:表示該 Agent 啟動後，到發出 trap的時間Variable Bindings“Interesting” informationTrap-PDU := 4IMPLICIT SEQUENCE enterprise OBJECT IDENTIFIER,agent-addr NetworkAddress, generic-trapINTEGER coldStart(0), warmStart(1), linkDown(2), linkUp(3), authenticationFailur

7、e(4), egpNeighborLoss(5), enterpriseSpecific(6),specific-trapINTEGER,time-stampTimeTicks,variable-bindingsVarBindList 8PDU 資料Variable binding VarBindListVarBindList:=SEQUENCE OF VarBindVarBind:=SEQUENCE name ObjectName, value ObjectSyntaxObject NameObject SyntaxObject NameObject Syntax9完整 SNMP 訊息格式S

8、NMP 標頭PDU 標頭PDU 資料10BER 基本編碼方式BER 主要是將 ASN.1所描述的網管物件轉換成 a string of octets 的格式，包含以下三個欄位：Type : 表示該網管物件的資料型態Length : 表示該網管物件內容的長度，以 Byte為單位Value : 表示該網管物件內容值11BER 基本編碼方式(cont.)INTEGER (0 x02)OCTET STRING (0 x04)OBJECT IDENTIFIER (0 x06)NULL (0 x05)SEQUENCE/SEQUENCE OF (Ox30)GetRequest-PDU (0 xA0)GetN

9、extRequest-PDU (0 xA1)12BER 基本編碼方式(cont.)GetResponse-PDU (0 xA2)SetRequest-PDU (0 xA3)Trap-PDU (0 xA4)13Example 1Question：Manager 利用 SNMP protocol 去詢問 Agent 的 sysName (.) 經過 BER 的編碼過後的 SNMP MessageSNMP 標頭PDU 標頭PDU 資料14Example 1 (cont.)0:30 29 02 01 00 04 06 70 75 62 6C 69 63 a0 1c 02 16:04 05 ae 56

10、02 02 01 00 02 01 00 30 0e 30 0c 06 32:08 2b 06 01 02 01 01 05 00 05 00 紅色：資料型態 黃色：長度 白色：內容INTEGER (0 x02)OCTET STRING (0 x04)OBJECT IDENTIFIER (0 x06)NULL (0 x05)SEQUENCE/SEQUENCE OF (Ox30)GetRequest-PDU (0 xA0)15Object ID 編碼方式針對 object ID 的前面兩個位數表示方法：例如：.Content:the first two numbers, x and y, enc

11、oded as 40 x+y in 1 byte1.3 - 2B16Example 2 當 Agent 收到 Manager 傳送過來的 Request message 時，立刻作 get Response message 的動作，如下：0:30 3e 02 01 00 04 06 70 75 62 6C 69 63 a2 31 02 16:04 05 ae 56 02 02 01 00 02 01 00 30 23 30 21 06 32:08 2b 06 01 02 01 01 05 00 04 15 INTEGER (0 x02)OCTET STRING (0 x04)OBJECT ID

12、ENTIFIER (0 x06)NULL (0 x05)SEQUENCE/SEQUENCE OF (Ox30)GetRequest-PDU (0 xA0)17Example 330 2C02 01 0004 07 69 6E 74 65 72 6F 70A0 1E02 04 23 41 3D 3E02 01 0002 01 0030 1030 0E06 0A 2B 06 01 02 01 02 02 01 0B 0105 00Message := SEQUENCE (0 x30)versionINTEGER (0 x02)communityOCTET STRING (0 x04)GetRequest-PDU := 0 (10 1 00000 = 0 xA0)request-idRequestID := INTEGER (0 x02)error-statusErrorStatus := INTEGER (0 x02)error-indexErrorIndex := INTEGER (0 x02)variable-bindings VarBindList := SEQUENCE OF (0 x30)VarBind := SEQUENCE (0 x30)nameObjectNa