75 lines
3.2 KiB
Markdown
75 lines
3.2 KiB
Markdown
 
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# Exit nodes
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A member can offer itself as an **exit node** in a network. Other members can
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then send *all* their internet traffic through it, the way a commercial VPN
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or Tailscale's exit nodes work. Linux only for now.
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## Using it
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The offering side, per network, off by default:
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```sh
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tsunagi network exit-node <network> on # or off
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```
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Rules are installed even when the kernel does not forward yet, and `status`
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says so. Turning forwarding on is left to you, on purpose:
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```sh
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sudo sysctl -w net.ipv4.ip_forward=1
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```
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The using side:
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```sh
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tsunagi exit-node # who offers one
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tsunagi exit-node <name|id> # send everything through it
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tsunagi exit-node off
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```
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The tray does the same: a checkbox on the network tile to offer, and an icon on
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each device in the devices window that offers one, with a confirmation.
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One device has one default route, so it uses **one** exit node at a time;
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choosing another replaces the first. Offering is per network, so a device can
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be an exit node in one network and not in another.
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## How it works
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- An offering agent says so in its control-plane `Announce` (`exit_node`), and
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only once its rules are in place. Members see which peers offer one.
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- **Server:** `iptables` masquerades the overlay range of that network out of
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any other interface and allows forwarding for it. Rules carry the tag
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`tsunagi-exit:<interface>:<range>` and are removed when the offer ends, the
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network stops, and when the agent exits. Only networks that offer have
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rules, so the others' traffic is not forwarded.
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- **Client:** policy routing, with no change to the main table. Table `28787`
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holds `default dev <interface>`, and three rules sit after Tailscale's:
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`5280 uidrange <agent uid> lookup main` (the agent's own connections never
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go through the tunnel, which would be a loop), `5290 lookup main
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suppress_prefixlength 0` (LAN and other specific routes keep working) and
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`5300 lookup 28787`.
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- Packets for addresses outside every overlay range are sent to the exit node
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by the router; the exit node's replies are accepted from any source. A member
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that is not your exit node cannot do that.
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## What it does not do
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- **The agent must run as a system service** (its own user). The uid exemption
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would otherwise cover everything you run. A per-user agent refuses to use an
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exit node and says why.
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- **If the exit node disappears the rules stay** and traffic is dropped until
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it returns or you run `tsunagi exit-node off`. This is deliberate: falling
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back to the direct path would leak traffic nobody agreed to send in the
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clear. The agent logs it once, `status` and the tray say so loudly.
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- IPv6 and DNS are untouched. IPv6 traffic still leaves the ordinary way.
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- Kernel forwarding is only read, never changed.
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## Trust
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Anyone who knows the network's secret can pick your exit node if you offer
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one, and you carry what they send: it leaves from your address and is yours to
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answer for. The exit node sees every destination and, for plain traffic, its
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contents. Choosing an exit node means trusting its owner completely. Offer one
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only in networks whose members you trust, and only from a machine you are
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willing to have that traffic come from.
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