OK, I'm not happy with this answer, but after much testing it's all I have.
The _audit data comes isn't from a "normal" input, it seems. Try to find an inputs.conf stanza that points data to _audit , or a transform that shuffles data into _audit . As best I can tell, they don't exist. I imagine this is because the audittrail data comes from within splunkd and is never logged to a file, and doesn't use anything like a scripted or modular input to push it to splunk. Hence it bypasses props/transforms entirely.
The only workaround I could come up with is to take the opposite approach: forward everything by default via outputs.conf, and use a [default] stanza in props.conf to skip forwarding for everything that matches (which won't happen for audittrail). This method prevents you from further filtering on "login attempt", but I can't find a way to selectively match audittrail events (as per the above paragraph's conclusion).
Here is the config that will forward only audittrail, leaving everything else to only index locally:
outputs.conf
[indexAndForward]
index = true
[syslog]
defaultGroup = syslog_server
[syslog:syslog_server]
server = x.x.x.x:514
props.conf
[default]
# _audit seems to not match [default] so we do this in reverse and send everything that matches to "nowhere"
TRANSFORMS-routing = dont_forward_anything
transforms.conf
[dont_forward_anything]
REGEX = .
DEST_KEY = _SYSLOG_ROUTING
FORMAT = nothing
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