| Description | This article describes how to troubleshoot the SNMPv3 debug error 'USM decryption error'. |
| Scope | FortiGate. |
| Solution |
Debugging SNMP:
diagnose debug reset
After enabling the above commands, initiate connectivity from the SNMP manager/server.
To stop the debugging, run the following command:
Sample error message in debug:
snmpd: v3 recv: parse failed. errno=-48 (USM decryption error) 2023-08-26 14:26:20 snmpd: </msg> 0 2023-08-26 14:26:25 snmpd: checking if community "snmpcom" is valid
Capture the SNMPv3 packets:
diagnose sniffer packet any "host 172.20.4.8 and (port 161 or port 162)" 6 0 l
After enabling the above commands, initiate connectivity from the SNMP manager/server.
Decrypt the SNMPv3 packets:
From the decrypted packets, validate the status of both authentication and privacy parameters. If any errors are observed as below, make sure the authentication and privacy algorithms and the respective passwords are correctly configured on both ends. If the connectivity is still down afterwards, validate the supported encryption types on both devices. 
In this example, the authentication is successful. However, the privacy parameters show a warning even though the packets are decrypted with the correct algorithm and password.
The error 'USM decryption error' occurs when the SNMP server uses a different method from what FortiGate supports to extend the size of the localized Key of the SNMPv3 encryption protocol.
Configure the correct algorithm supported on both devices.
An example output of a successful communication:
Related article: |
