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docs: describe the host side of the relay as shipped #23

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15 changes: 9 additions & 6 deletions docs/RELAY.md
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# Egress relay (`vprox relay`)

The relay is the vprox side of the macOS static-IP userspace dataplane. A
macOS host runs a root `egress-proxy` daemon that receives VM traffic via pf
`rdr`, opens **one TLS 1.3 connection per VM** to the relay, and multiplexes
that VM's TCP and UDP flows over it with yamux. The relay dials every
The relay is the vprox side of the macOS static-IP dataplane. On a macOS
host, pf `rdr` steers each static-IP VM's off-subnet TCP and UDP to a proxy
that runs inside the Blacksmith agent (`agent/egressproxy` in
FastActions/fa); a small root `pf-helper` child per VM performs the two
privileged pf ioctls and opens the relay connection on the agent's behalf.
The proxy holds **one TLS 1.3 connection per VM** to the relay and
multiplexes that VM's flows over it with yamux. The relay dials every
destination from the static IP the session was granted, so the VM's egress
address is the static IP without any tunnel or `route-to`.

```
VM -> pf rdr -> egress-proxy (mac, root) -> TLS1.3/yamux -> vprox relay -> dst
bind(static_ip)
VM -> pf rdr -> egressproxy (inside the mac agent; root pf-helper for NATLOOK/state kill/dial)
-> TLS 1.3 + yamux, one session per VM -> vprox relay -> bind(static_ip) -> destination
```

## Running
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