Run the server side and it prints one string: the server's public key plus a pointer to a DERP relay [3]. You hand that to the client over some other channel, the two ends meet at the relay, and Tailscale's magicsock layer tries to punch a direct WireGuard path over UDP underneath [3][4]. When the punch fails, DERP keeps carrying the bytes [4]. Nothing else negotiates, because possession of a Tailcat address is an unlisted connection capability by default [7].
There are two knobs. A server can restrict access to one named client public key, and saved keypairs make a connection reusable [7]. Distributing those tokens and keys is your problem [7]. Publisher runtimewire.com states the deleted layer plainly: what goes with the control plane is the part that answered who may connect, which devices belong to whom, and which policies apply [20].
The userspace property is the part worth copying. No TUN device, no edit to the host routing table, no root [5]. An embedded TCP/IP stack terminates connections inside the Tailcat process, which is why the capability list reads file transfer, port forward, SSH session and temporary SOCKS proxy instead of "this host is now on your network" [6]. Treat that as a limit rather than a rough edge. A daemon already running on the box does not see the tunnel unless you point it at the SOCKS proxy [6].
The provenance explains the shape. Brad Fitzpatrick wrote the first version as derpcat on a 10-hour flight in September 2023, the repository history records repeated bit rot as Tailscale's internals moved, and the tool was revived when customers turned up with use cases that fit [14][15][16]. That is roughly 35 months from flight to launch [22]. The repo carried 38 stars in a snapshot five days before release [12], which is about what an extracted internal utility looks like before anyone writes the post. Shipping it as BSD three-clause code with an explicit refusal to promise anything about the CLI, the Go API or the wire format is the honest version of this release [8][11].
Now the transfer question. Fitzpatrick's launch account describes giving an AI agent access to several generations of Raspberry Pi, letting an EC2 sandbox reboot another EC2 instance during a UEFI port, and creating and destroying Hyper-V virtual machines while debugging Go [18]. Every one of those is a disposable or isolated machine with someone in the loop at handoff time, which is the class runtimewire.com says the design suits [19]. For the number to carry into a fleet of long-lived hosts, you would need a token distribution story, key rotation and some record of which device is whose. That is the control plane, and you would be writing it.
The dependency you did not remove is the relay. Public DERP is rate-limited, carries no uptime commitment, and may be withdrawn [9]. Teams that need dedicated relays or support pay Tailscale [10]. So the free tier covers encryption and transport, and reliability of the fallback path stays on someone else's balance sheet, which is exactly the commercial boundary runtimewire.com says the release makes explicit [21]. In my context that is a good trade for agent sandboxes and short-lived hardware, where a session dying is a retry rather than an incident. It is a bad trade for anything a customer depends on.
