ISSUE

When using the # /opt/nessus_agent/sbin/nessuscli agent status  command, you get the following error:

[error] [agent] Link failed with error 401: You need to log in to perform this request

CAUSE

You may receive this error if the Nessus Agent was once linked to a Manager (Nessus Manager or Tenable.io), but that Manager no longer considers the Agent as linked.  This occurs when an agent is unlinked from the Manager, either by manual unlinking or from the, "Unlink inactive agents" setting, if enabled. Unlinking from the Manager does not communicate to the Agent that it is being unlinked. You may also see the agent offline in the Nessus Manager User Interface.

RESOLUTION

To resolve this issue, you may need to unlink and re-link the Nessus Agent, from the Agent host. 

Linux

From a root-level command prompt, run the following commands:

# service nessusagent stop
# /opt/nessus_agent/sbin/nessuscli agent unlink (--force)
# /opt/nessus_agent/sbin/nessuscli agent link --key=<key> --host=<host> --port=<port>
# service nessusagent start

Windows

As an administrator, from a command prompt, run the following commands:
Based on your operating system, use either C:\Program Files\Tenable\Nessus Agent or C:\ProgramData\Tenable\Nessus Agent.

> net stop "Tenable Nessus Agent"
> "C:\Program Files\Tenable\Nessus Agent\nessuscli" agent unlink (--force)
> "C:\Program Files\Tenable\Nessus Agent\nessuscli" agent link --key=<key> --host=<host> --port=<port>
> net start "Tenable Nessus Agent"

Mac OS

From a command prompt, run the following commands:

$ sudo launchctl stop com.tenablesecurity.nessusagent
$ /Library/NessusAgent/run/sbin/nessuscli agent unlink (--force)
$ /Library/NessusAgent/run/sbin/nessuscli agent link --key=<key> --host=<host> --port=<port>
$ sudo launchctl start com.tenablesecurity.nessusagent

To verify connection to managing host, you can performing a nessuscli agent status in the command line/terminal of the agent target. 
More on that here: https://docs.tenable.com/nessus/Content/NessusCLIAgent.htm

If unlinking and re-linking your agent does not resolve this issue, check your firewall settings or your certificates.

ADDITIONAL RESOURCES

For a complete list of Nessus and Nessus Agent nessuscli commands and syntax, refer to the Nessus Command Line Reference Guide.

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