Add a FAQ entry about two nodes seeing each other

See: #2431
See: #2174

Author: nblock

docs/about/faq.md

@@ -121,3 +121,17 @@ help to the community.
 ## Can I use headscale and tailscale on the same machine?
 
 Running headscale on a machine that is also in the tailnet can cause problems with subnet routers, traffic relay nodes, and MagicDNS. It might work, but it is not supported.
+
+
+## Why do two nodes see each other in their status, even if an ACL allows traffic only in one direction?
+
+A frequent use case is to allow traffic only from one node to another, but not the other way around. For example, the
+workstation of an administrator should be able to connect to all nodes but the nodes themselves shouldn't be able to
+connect back to the administrator's node. Why do all nodes see the administrator's workstation in the output of
+`tailscale status`?
+
+This is essentially how Tailscale works. If traffic is allowed to flow in one direction, then both nodes see each other
+in their output of `tailscale status`. Traffic is still filtered according to the ACL, with the exception of `tailscale
+ping` which is always allowed in either direction.
+
+See also <https://tailscale.com/kb/1087/device-visibility>.