Introduce "wildcard" SNMP engine ID (#297)

This change introduces "wildcard" SNMP engine ID (0x00000000). Right
before deciding on firing up SNMP engine ID discovery and key
localization procedure, originating SNMP engine will check for
the presence of this magical engine ID (5 zeros), if it is present
in LCD along with the user name being used, localized keys from that
entry will be used.

Does this have security implications?

1 files changed, 59 insertions, 24 deletions

diff --git a/pysnmp/proto/secmod/rfc3414/service.py b/pysnmp/proto/secmod/rfc3414/service.py
index 9fdfe707..de0330c9 100644
--- a/pysnmp/proto/secmod/rfc3414/service.py
+++ b/pysnmp/proto/secmod/rfc3414/service.py
@@ -93,6 +93,11 @@ class SnmpUSMSecurityModel(AbstractSecurityModel):
         nopriv.NoPriv.SERVICE_ID: nopriv.NoPriv()
     }
 
+    # If this, normally impossible, SNMP engine ID is present in LCD, we will use
+    # its master/localized keys when preparing SNMP message towards any unknown peer
+    # SNMP engine
+    WILDCARD_SECURITY_ENGINE_ID = pMod.OctetString(hexValue='0000000000')
+
     def __init__(self):
         AbstractSecurityModel.__init__(self)
         self._securityParametersSpec = UsmSecurityParameters()
@@ -370,13 +375,25 @@ class SnmpUSMSecurityModel(AbstractSecurityModel):
         elif securityEngineID:
             # 3.1.1b
             try:
-                (usmUserName, usmUserSecurityName, usmUserAuthProtocol,
-                 usmUserAuthKeyLocalized, usmUserPrivProtocol,
-                 usmUserPrivKeyLocalized) = self._getUserInfo(
-                    snmpEngine.msgAndPduDsp.mibInstrumController,
-                    securityEngineID,
-                    self._sec2usr(snmpEngine, securityName, securityEngineID)
-                )
+                try:
+                    (usmUserName, usmUserSecurityName, usmUserAuthProtocol,
+                     usmUserAuthKeyLocalized, usmUserPrivProtocol,
+                     usmUserPrivKeyLocalized) = self._getUserInfo(
+                        snmpEngine.msgAndPduDsp.mibInstrumController,
+                        securityEngineID,
+                        self._sec2usr(snmpEngine, securityName,
+                                      securityEngineID)
+                    )
+
+                except NoSuchInstanceError:
+                    (usmUserName, usmUserSecurityName, usmUserAuthProtocol,
+                     usmUserAuthKeyLocalized, usmUserPrivProtocol,
+                     usmUserPrivKeyLocalized) = self._getUserInfo(
+                        snmpEngine.msgAndPduDsp.mibInstrumController,
+                        self.WILDCARD_SECURITY_ENGINE_ID,
+                        self._sec2usr(snmpEngine, securityName,
+                                      self.WILDCARD_SECURITY_ENGINE_ID)
+                    )
 
                 debug.logger & debug.FLAG_SM and debug.logger(
                     '__generateRequestOrResponseMsg: found USM user entry '
@@ -881,29 +898,47 @@ class SnmpUSMSecurityModel(AbstractSecurityModel):
                     snmpEngine.msgAndPduDsp.mibInstrumController,
                     msgAuthoritativeEngineId, msgUserName
                 )
+
                 debug.logger & debug.FLAG_SM and debug.logger(
                     'processIncomingMsg: read user info from LCD')
 
             except NoSuchInstanceError:
-                debug.logger & debug.FLAG_SM and debug.logger(
-                    'processIncomingMsg: unknown securityEngineID %r '
-                    'msgUserName %r' % (msgAuthoritativeEngineId, msgUserName))
+                try:
+                    (usmUserName,
+                     usmUserSecurityName,
+                     usmUserAuthProtocol,
+                     usmUserAuthKeyLocalized,
+                     usmUserPrivProtocol,
+                     usmUserPrivKeyLocalized) = self._getUserInfo(
+                        snmpEngine.msgAndPduDsp.mibInstrumController,
+                        self.WILDCARD_SECURITY_ENGINE_ID, msgUserName
+                    )
 
-                usmStatsUnknownUserNames, = mibBuilder.importSymbols(
-                    '__SNMP-USER-BASED-SM-MIB', 'usmStatsUnknownUserNames')
-                usmStatsUnknownUserNames.syntax += 1
+                    debug.logger & debug.FLAG_SM and debug.logger(
+                        'processIncomingMsg: read wildcard user info from LCD')
 
-                raise error.StatusInformation(
-                    errorIndication=errind.unknownSecurityName,
-                    oid=usmStatsUnknownUserNames.name,
-                    val=usmStatsUnknownUserNames.syntax,
-                    securityStateReference=securityStateReference,
-                    securityLevel=securityLevel,
-                    contextEngineId=contextEngineId,
-                    contextName=contextName,
-                    msgUserName=msgUserName,
-                    maxSizeResponseScopedPDU=maxSizeResponseScopedPDU
-                )
+                except NoSuchInstanceError:
+
+                    debug.logger & debug.FLAG_SM and debug.logger(
+                        'processIncomingMsg: unknown securityEngineID '
+                        '%r msgUserName %r' % (msgAuthoritativeEngineId,
+                                               msgUserName))
+
+                    usmStatsUnknownUserNames, = mibBuilder.importSymbols(
+                        '__SNMP-USER-BASED-SM-MIB', 'usmStatsUnknownUserNames')
+                    usmStatsUnknownUserNames.syntax += 1
+
+                    raise error.StatusInformation(
+                        errorIndication=errind.unknownSecurityName,
+                        oid=usmStatsUnknownUserNames.name,
+                        val=usmStatsUnknownUserNames.syntax,
+                        securityStateReference=securityStateReference,
+                        securityLevel=securityLevel,
+                        contextEngineId=contextEngineId,
+                        contextName=contextName,
+                        msgUserName=msgUserName,
+                        maxSizeResponseScopedPDU=maxSizeResponseScopedPDU
+                    )
 
             except PyAsn1Error as exc:
                 debug.logger & debug.FLAG_SM and debug.logger(