UltradesuandClaude Sonnet 5.5
47c5d38d0c
The exit-node host rules existed only for Linux (iptables + policy routing); macOS reported "only supported on Linux yet". Offering: a pf anchor com.apple/tsunagi-exit-<if> (evaluated by the stock pf.conf) holds a nat rule per offered range and a pass-in rule. pf is enabled if needed and the pfctl -E reference is released when the rules are removed. net.inet.ip.forwarding is only read, never written, as on Linux. Using: macOS has no uidrange policy routing and the agent is root, so the overlay gets 0.0.0.0/1 and 128.0.0.0/1 routes and a pf route-to rule (matched by the agent's uid) sends the agent's own QUIC/relay traffic back out the physical interface, translated to its address. The rules are loaded before the routes, so a failed load never sends traffic into a tunnel it cannot exempt the agent from. IPv6 is blocked with rejecting ::/1 and 8000::/1 routes. ExitHostPlan gains `overlay` (every network range) for the route-to exemption; Linux ignores it. Known limits: the egress is the default route's interface at apply time; other root processes also bypass the tunnel. Forwarding hints in the CLI and tray name the macOS sysctl. The rule text and command order are unit-tested with a scripted host; nothing was run against a live pf/route (needs root), so the route-to + nat second-pass behaviour is unverified. Checks: cargo fmt --check, cargo clippy --locked --workspace --all-targets -D warnings and cargo test --locked --workspace --all-targets, run on macOS only. Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
104 lines
4.8 KiB
Markdown
104 lines
4.8 KiB
Markdown
# 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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tsng 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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tsng exit-node # who offers one
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tsng exit-node <name|id> # send everything through it
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tsng 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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- **IPv6 is blocked while you use one.** There is no IPv6 through an exit node,
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and letting it go the ordinary way would leak it. The same three rules exist
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for IPv6 with an `unreachable default` in the table, so applications get an
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immediate "network unreachable" and fall back to IPv4. LAN, ULA and other
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specific IPv6 routes keep working, and so does the agent's own IPv6. A host
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with no IPv6 has nothing to block and is left alone.
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## macOS
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macOS has no `iptables` and no per-user policy routing, so the same behaviour
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is built from `pf` and `route`. The agent runs as root there.
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- **Offering:** a `pf` anchor `com.apple/tsunagi-exit-<interface>` (the stock
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`/etc/pf.conf` already evaluates `com.apple/*`) holds
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`nat on <egress> from <range> to any -> (<egress>)` and a `pass in` rule.
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`pf` is enabled if it was off, and that reference is released when the rules
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go. Forwarding is only read: `sudo sysctl -w net.inet.ip.forwarding=1`, which
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macOS forgets at reboot. The egress is the default route's interface **at the
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time the rules are applied**; after it changes (Wi-Fi to Ethernet) switch the
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offer off and on.
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- **Using:** `0.0.0.0/1` and `128.0.0.0/1` routes through the overlay interface,
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and a `pf` `route-to` rule that sends the agent's own traffic (matched by its
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user id, root) back out the physical interface, with translation to that
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interface's address. Anything else you run as root also bypasses the tunnel.
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IPv6 is blocked with rejecting `::/1` and `8000::/1` routes.
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- Not verified on a live host when this was written: run `tsng exit-node` and
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check `sudo pfctl -a com.apple/tsunagi-exit-utunN -s rules -s nat`.
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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 `tsng 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-only destinations are unreachable** meanwhile; carrying IPv6 through
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the exit node is not implemented. DNS is untouched.
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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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