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Johnathan Corgan
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A client process opens a flow to a peer's public key on a chosen port and sends and receives datagrams on a file descriptor the daemon hands it. No IPv6 emulation, no TUN device, no DNS: a datagram travels from key to key. The feature is off by default and is not a stable interface. The wire needs no change and gets none. Every FSP data packet has carried a port pair inside its AEAD envelope since v0.2.0, and port 256 is simply the IPv6 shim. What was missing was a way for a program to ask for a port of its own and be handed the traffic. Addressing is the part worth reading twice, because the obvious design is wrong. The x-only public key is the address. An npub is that key written in bech32, so converting between them is a local encoding rather than a lookup or a name service. The 16-byte node address that travels on the wire is the first half of a SHA-256 of the key: it is a truncated hash, it does not invert, and it appears nowhere a client can see. An earlier iteration of this work reported a peer by that hash and could supply a key only sometimes, which is what treating a wire identifier as an identity produces. An accepted flow therefore always knows its peer. The key is captured where the peer is authenticated rather than looked up when a report is rendered: every inbound datagram passes one call site inside a handler that refuses anything whose session is not established, and the responder has already rejected the session unless the claimed address derives from the key it proved. Reaching for the identity cache instead gives a best-effort answer from a structure that evicts. A listener is a descriptor. The daemon writes one message per arrival to it, carrying the new flow's descriptor and the peer's address, so poll, select and epoll work on a listener and accepting is a recvmsg. That is what lets the API be used from a program that already has an event loop, which a command-and-reply listener could not support: an arrival could not be waited on beside anything else. There is no accept command and no reject command. Refusing a flow is closing the descriptor you were handed. The Rust surface mirrors std::net. FipsStream::connect, FipsListener::bind, incoming, accept, io::Result and an errno mapping rather than a bespoke error type. An address is given as an npub, as a key, or as a pair, through one parameter, the way ToSocketAddrs takes several spellings of one thing. Each type holds its descriptor and copies of what setup told it and nothing else, so a stream that outlives its setup connection is not representable. set_nonblocking, AsFd and the four deadline methods carry the names and signatures std::net uses for the same jobs. They were asked for by a user integrating the API with tokio: AsyncFd requires a non-blocking descriptor, and anything receiving from a peer needs a bounded wait. AsFd is the better of the two descriptor accessors, because the borrow cannot outlive the value that owns the descriptor, so a reactor cannot hold a registration for a descriptor that has since been closed and its number reused by the next open. The non-blocking flag is read, modified and written back rather than assigned, since the flag word carries more than that one bit and a caller may have set O_ASYNC. A zero timeout is refused with EINVAL, because the kernel reads a zero timeval as "wait for ever", which inverts what a caller passing zero means; std::net refuses it for the same reason. The two directions are separate options and stay that way. FipsListener gets no timeout methods, matching TcpListener: bounding an accept is set_nonblocking plus the caller's own poll, which the reactor how-to builds. A flow taken from accept is blocking whatever the listener was set to, because the two are separate sockets and the daemon hands over a fresh one. One rule has no counterpart in Berkeley sockets and a client author must know it: the v1 wire carries no half-close, so nothing peer-driven ever closes a flow. A server written to read until the flow ends waits for a signal that cannot arrive, holding a thread and a flow per peer until its process exits. A program decides its own termination, and the example serves one datagram per flow. The tests reach a live daemon rather than a stand-in. Every public item had a unit test against a hand-written stand-in with canned replies, and the five entry points a program actually calls first, connect, connect_from, connect_at, bind and the SOCKET constant, had no coverage of any kind, because the tests that appear to cover them build a Wire over a socket pair and hand it to the private open and hold, so nothing ever resolved a socket path or mapped its errors. examples/native-surface.rs walks all thirty-eight items against a running daemon and reports the number of assertions it made. The count is read from the recorder rather than written as a literal, and the harness asserts the exit status, the completion marker and the count together, so deleting an assertion fails the check rather than quietly shrinking it. Watchdogs turn a hang into a named failure, which several of the walked behaviours would otherwise produce. The shared Docker image is built once for every integration leg, so the new binary is staged at all ten places the existing one is, the interop builder included, which gets a stub because those images exercise the wire between daemon versions and older refs do not carry the example. The platform gating was tested rather than reasoned about: flipping all eleven gates so the native API is excluded leaves the crate compiling clean across the workspace, every target and the profiling feature. The shipped docs tree gains what only the LaTeX manual under design/ had, which is not published with the daemon. A reference entry covers the whole surface: addressing and the port tiers, the Berkeley mapping, every method on FipsAddr, FipsStream, FipsListener and Incoming, the errno table, the ceilings, the four places data disappears with nothing reported, the line protocol and the command reference. The errno table gives names rather than numbers, since the client maps each name onto the libc constant for the platform it was built for and the supported platforms disagree on the numbers. A tutorial side trip stands up two throwaway nodes on one machine, peered over loopback UDP with no TUN and no DNS, then writes a listening program and a connecting program against them; it needs neither the public mesh nor root, because the native path is the one that does not go through the IPv6 adapter. The obligations a client in another language carries are a how-to of their own, since they are a task rather than a description: reading the setup connection with recvmsg, associating a descriptor with the last complete line, telling an empty datagram from a close, and six others. Serving many peers from one poll loop is another, with the whole program, because the straightforward listener spawns a thread per flow and that is wrong at the node's ceiling of 256. The drop causes are a table mapping each of the seven texts DropReason::as_str produces to the counter it increments, with drop_oversize called out as the ninth counter that is not in the table. What a daemon restart costs is a section of its own: every flow and listener ends, descriptors do not survive, there is no resumption, and datagrams sent but not yet forwarded are lost through a window nothing bounds. A stack comparison diagram places the interface against the stack a reader already knows: the same application over HTTP, TLS, TCP, IP and Ethernet on one side, and over its own format, FSP, FMP and a FIPS transport on the other, aligned so each row is one concern. The two columns are not alternatives and are not drawn as such. An unmodified IPv6 program's packets reach fips0, and the adapter hands each one to FSP as a payload, so the left stack runs inside the right one; the left column ends at a fork, eth0 for the ordinary internet and fips0 for the mesh, and an arrow leaves fips0 and runs back up into FSP's input. The row where TCP would be is empty on purpose and names Reliable Object Delivery, which is where that capability is expected to land. ROD is a v2 capability, the box is dashed because none of it exists yet, and the design entry says the part a reader needs most: nothing on the surface anticipates it, so a program written today should assume it does not exist. Both endpoints carry a scheme and a worked port, https://<npub>.fips:443 and fips://<npub>:443, with a footnote saying the two ports are not the same kind of thing, a TCP port inside the tunnel on the left and an FSP port on the right. The fips:// form is a coinage: nothing in the tree parses it, nothing registers the scheme, and the API takes a key and a port as separate arguments rather than a URL. The diagram also says where the right column stops, since FIPS over UDP still rides IP and Ethernet beneath. It appears in fips-concepts.md and fips-ipv6-adapter.md, which were making its argument in prose without a picture, and deliberately not in fips-architecture.md, which already carries the OSI mapping and makes the same point about the transport row. The gateway's control socket moves onto the same bind policy this API uses, which is the one change here that touches deployed behaviour: fips-gateway now tightens /run/fips to 0750. That is unreachable under the packaged deployment, where fips.service has already created the directory at that mode, and reachable for a source build or a container that starts the gateway alone. One changelog entry under Added, describing the released state: what a client opens and reads, the addressing and why the node address is not it, the listener being a descriptor, the std::net shape of the Rust surface, and the one rule Berkeley sockets have no counterpart for. It says in as many words that the wire is unchanged.
Tutorials
If you have just installed FIPS, this is where to start. The tutorials below take you from a freshly-installed daemon to a node that:
- Has joined the public test mesh and can reach other nodes on it.
- Carries a stable identity that other operators can address.
- Discovers peers — and is discoverable — over Nostr.
- Hosts and consumes real services across the mesh.
Each tutorial is a complete, working session at the keyboard. You configure something, restart the daemon, watch it come up, and verify the result. The point is to build muscle memory, not to cover every option.
Read them in order. Each tutorial assumes the state the previous one left you in. If you skip ahead, the cross-references that lead you back may not match what you have on disk.
The new-user progression
| # | Tutorial | What you'll do |
|---|---|---|
| 1 | join-the-test-mesh.md | Add one public test peer to your config, watch the link come up, ping that peer and a second mesh node it routes you to. The starting point for everything else. |
| 2 | persistent-identity.md | Pin your daemon to a stable Nostr keypair so your address stops changing on every restart. Other operators can now add you to their peers: lists; the services you host get a fixed name. |
| 3 | resolve-peers-via-nostr.md | Stop hard-coding peer addresses. Drop the address line from your peer entry and let the daemon look up the current endpoint from public Nostr relays at dial time. |
| 4 | advertise-your-node.md | Publish your own UDP endpoint to Nostr so any operator who knows your npub can reach you, with a short final section on udp:nat best-effort hole-punching for nodes without a directly reachable UDP endpoint. |
| 5 | open-discovery.md | Switch to policy: open and let your peer list populate itself from the ambient fips-overlay-v1 namespace. Hands-off mesh participation. |
| 6 | reach-mesh-services.md | Drive ordinary IPv6 tools — ping6, nc, traceroute6, curl, ssh — at mesh nodes by <npub>.fips. Get a feel for the daemon's IPv6 adapter, which makes unmodified IPv6 software work over the mesh. |
| 7 | host-a-service.md | Bring up an HTTP server bound to fips0 so mesh nodes can reach it, with a deliberate exposure decision (mesh-only vs every interface), and the mesh firewall as a default-deny baseline. The peer ACL (a separate, transport-layer control over which npubs may peer with your node) is briefly mentioned alongside. |
| 8 | ground-up-mesh.md | Bring up a second deployment mode: two devices joined by Ethernet (or WiFi, or BLE) with no IP infrastructure between them. The mesh emerges from layer 2 up. Coexists with overlay peers — the same daemon can carry both. |
After tutorial 8 you have a fully participating mesh node that reaches services hosted by other mesh nodes and hosts services of its own, with identity, discovery, reachability, an explicit exposure policy, and an understanding of both deployment modes — overlay on top of existing IP, and ground-up where the mesh is the network.
There are also two side trips you can take:
-
ipv6-adapter-walkthrough.md — trace one
sshfrom DNS query through session setup to the far-side TUN, usingfipstopandfipsctlto watch each step. Optional, but if you like seeing how the pieces fit together, this is the doc that shows you. Take it any time after tutorial 1. -
native-api-walkthrough.md — write a program against the experimental native datagram API, addressing a peer by public key and port with no IPv6 emulation and no TUN. Runs two throwaway nodes on one machine, so it needs no mesh and no root, and you can take it without doing the tutorials first.
Advanced
These are not part of the new-user progression. They assume you have already worked through the tutorials above and now want to fold FIPS into a wider network deployment.
- deploy-fips-gateway.md — Stand up a
fips-gatewayon an OpenWrt access point so unmodified LAN hosts can reach<npub>.fipsdestinations through a DNS- allocated virtual IPv6 pool and kernel nftables NAT, with no per-host FIPS install. Also walks through one inbound port forward exposing a LAN service to mesh peers. Aimed at operators bridging a LAN segment into the overlay from the edge router. For a non-OpenWrt host the same deployment is in ../how-to/deploy-gateway.md.
When to use the how-to guides instead
The tutorials here walk through one specific path each. The how-to guides under ../how-to/ are the operator recipes — alternative provisioning paths, less-common configurations, troubleshooting techniques. Once you have the shape of FIPS in your head from these tutorials, the how-tos are where you'll go to look up "how do I do X?" without being walked through the surrounding context.