There are three specific requirements for Funnel to work:
1) They must accept an invite.
2) They must enable HTTPS.
3) The "funnel" node attribute must be appropriately set up in the ACLs.
Signed-off-by: Shayne Sweeney <shayne@tailscale.com>
Current behavior is broken. tailscale serve text / "" returns no error
and shows up in tailscale serve status but requests return a 500
"empty handler".
Adds an error if the user passes in an empty string for the text
handler.
Closes#6405
Signed-off-by: Shayne Sweeney <shayne@tailscale.com>