I've found that when the first inform is not 'aknowledged' by the trap receiving system (after 5 retries), the agent doesn't send any inform to it anymore. After that, this message appears in the log everytime a new inform need to be sent: "snmpd: send_trap: USM unknown security name (no such user exists)"
After digging deeply into the code I've found that the agent only tries to discover the peer engineID once per 'session':
send_v2trap -> send_trap_vars -> send_enterprise_trap_vars -> netsnmp_send_traps -> send_trap_to sess -> snmp_async_send -> snmp_sess_async_send -> _sess_async_send calls to snmpv3_engineID_probe. This function sets the session->flags SNMP_FLAGS_DONT_PROBE to prevent recursion... but if the remote engine doesn't answer, this flag remains set. And, later, if a new inform needs to be sent the agent doesn't retry to obtain the engineID again.
I've detected this behaviour using net-snmp versions 5.3.0.1 and 5.3.1 libraries in a subagent module through agentx (SNMPV3) running on Linux.
Is this behaviour correct? Shouldn't the agent keep trying to obtain the engineID of the receiver if the previous communication failed?
To correct this I've done the following modification (turn off the flag SNMP_FLAG_DONT_PROBE if we have no answer) in function snmpv3_engineID_probe in snmplib/snmp_api.c
if (slp->session->securityEngineIDLen == 0) {
DEBUGMSGTL(("snmp_api",
"unable to determine remote engine ID\n"));
session->flags &= ~SNMP_FLAGS_DONT_PROBE; /* EBP */
return 0;
}
But this modification provoques two effects: 1) the engineID is requested for every inform while we have no answer from the receiver (OK ;)))).
2)The informs are repeated a lot of times when the receiver acknowledge them (BAD!!!).
My snmpd.conf ...
createUser snmp_usr MD5 snmp_pass
trapsess -C i -v 3 -u snmp_usr -a MD5 -A snmp_pass -l authNoPriv 172.16.2.5:162
Any comments would be appreciated..
Thanks in advance.
Enric.
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