Hi there, I work at Tailscale.

Part of the reason that we don't (currently) let you do this is that a hostname is a user-reported field, and can change over time; it's not a durable form of identity that you can write ACLs on. One could imagine, for example:

1. Creating an ACL rule that allows hostname "webserver" to hostname "db".

2. (time passes)

3. Hostname "webserver" is deleted/changed to "web"/etc.

4. Someone can now register a user device with the system hostname set to "webserver"

Should they be allowed to inherit the pre-existing ACL rule?

However, you can accomplish something very close to what you're asking for, I think, by defining a "host" in the policy file (https://tailscale.com/docs/reference/syntax/policy-file#host...) that points to a single Tailscale IP. Since we don't allow non-admins to change their Tailscale IP, this uniquely identifies a single device even if the hostname changes, and thus you can write a policy similar to:

  "hosts": {
    "myhost": "100.64.1.2",
  },
  "grants": [
    {
      "src": ["myhost"],
      "dst": ["tag:db"],
    },
  ]