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Johnathan Corgan
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macOS does not implement SOCK_SEQPACKET for AF_UNIX, so the listener was gated to Linux and FreeBSD and a Mac got no API at all. It now uses SOCK_DGRAM there, which macOS does implement and which keeps the message boundaries the API's contract with its clients rests on. Both kernels were measured rather than reasoned about, and the Linux answer alone refuted the replacement the source had proposed. On Linux 6.8 a connected SOCK_DGRAM pair reports a closed peer not at all: revents stays empty and recv returns EAGAIN, which is exactly what an idle socket with a live peer does. SOCK_SEQPACKET on the same kernel sets POLLHUP and returns a zero-byte read, which is what the receive path keyed on. Darwin does report the close, with ECONNRESET, errno 54, and does not set POLLHUP. So the receive path treats ECONNRESET as end of file alongside the existing POLLHUP rule. One rule accepting either signal is correct on both kernels, where a rule split by platform would be silently wrong on whichever one it guessed at. EAGAIN is deliberately not in that company: it means the socket is empty and the peer alive, so it stays an error and the caller waits again. The measurements are asserted rather than only written down, so a kernel that gains or loses the signal reds a test and reopens the question instead of leaving a stale comment behind. Each carries its result in the assertion message, since a passing test prints nothing and a negative result would otherwise be as uninformative as no result: the close probe reports the poll return, the whole revents bitmask broken out by flag, the recv result and the errno, which is enough to write the real rule without another round trip. A portable test walks datagram sizes upward, because Darwin bounds a unix-domain datagram with the net.local.dgram.maxdgram sysctl, whose default is small and which is a system tunable rather than something this process controls, while the API advertises 1362 bytes to its clients. Every test recv in the seqpacket suite is bounded in time. This is not tidying. Simulating the Darwin configuration on a kernel that does not report the close, the end-of-file test hung for over ten minutes rather than failing, and a hang is not a red: it would have wedged the macOS runner with no diagnostic instead of naming the assertion. The same simulation now fails by name in five seconds. Three things in the native tree compiled on one platform only, all of them in code that had never been built for Darwin before. suseconds_t is i64 on Linux and i32 there, so the timeval microseconds field is a cast, matching the tv_sec line above it; it cannot truncate, because subsec_micros is below 1_000_000 by construction, and the cast is the only form that compiles on both, since From does not exist for the narrower width and try_from is a clippy error on the wider one. MSG_CMSG_CLOEXEC does not exist on Apple, so the recvmsg flags are chosen per platform and each received descriptor is marked close-on-exec with fcntl where there is no flag to pass; a failure to set it is reported rather than ignored, since the descriptor is live either way and the caller must not be told the receive was clean. socketpair takes SOCK_CLOEXEC in its type argument on Linux and FreeBSD and rejects it on macOS, so Darwin sets FD_CLOEXEC with a second fcntl. Both windows between a call and its fcntl are stated in the code rather than closed, since the daemon spawns no child on this path, and a test asserts both halves of a pair are close-on-exec on every platform, because the failure is a silent descriptor leak into a child and nothing else would report it. Every libc item the native tree uses was then checked against the crate's own Apple definitions rather than from memory, and those two constants are the only ones absent. The close difference turned out to be unhandled in six further places, and the whole native API agrees on it now. Darwin reports a closed AF_UNIX SOCK_DGRAM peer as ECONNRESET, and a later send on the disconnected survivor as EDESTADDRREQ, where Linux SOCK_SEQPACKET gives EPIPE on a write and a zero-byte read plus POLLHUP on a read. Each site below promised one of those spellings and saw another. - The client's recv and send passed ECONNRESET through, so a closed daemon half surfaced as errno 54 against the EPIPE the documentation promises. The translation is in one function in seqpacket rather than at each call site, since only this one condition has two spellings. - accept propagated the same errno instead of its documented EPIPE. The listener's read reports a closed peer as the empty chunk both of its callers already read as the far end going away, which leaves accept's contract true on both platforms without either caller knowing which it is on. - why() classified a failed hand-off by BrokenPipe alone, so every ordinary macOS listener close was counted under the counter an operator reads to find a client that stopped reading. It recognises all three errnos now, with a test over each. - A full client buffer ended a flow's only writer. On Linux that never arrives, because the send reports EAGAIN and waits for the client to drain; Darwin has no sender-side queue to wait on and reports ENOBUFS on the send itself. Returning left the registration, the port and the reader alive while every later inbound datagram was counted as a full queue for the rest of the flow's life, and a client that resumed reading never recovered. The datagram is dropped instead, which is what a datagram API does when the far end cannot take it. - The flow pair was never sized, and the two kernels charge a queued message to different ends: Linux to the sender's SO_SNDBUF, BSD to the receiver's so_rcv. Sizing only the sender, as the listener pair does, left the flow pair bounded on Darwin by a system default small enough that a batch held for an arriving client could not fit, and the whole flow was destroyed before its client ever saw it. Both halves are sized now. - peer_hung_up polled with an empty events field, on the rule that POLLHUP is reported whether or not it is requested. That holds on Linux, where it was measured, and not on Darwin, where a poll requesting nothing registers no filter. Nothing observable depended on it, because ECONNRESET arrives first and both callers act on it earlier. The cost was elsewhere: three assertions written as tripwires for a change in Darwin's behaviour could not fail there, which is a guard that executes and proves nothing. Requesting POLLIN fixes the function and the guards together. One difference is not an errno at all, and reading the kernel source rather than a manual page is what found it. Darwin's unp_disconnect sets SS_CANTRCVMORE and runs soisdisconnected on both ends for SOCK_STREAM. For SOCK_DGRAM it removes the reflink, clears SS_ISCONNECTED and stops: no sorwakeup, no socantrcvmore, no soisdisconnected. The closing peer deposits ECONNRESET in the survivor's so_error and wakes no knote. The registration is edge-triggered and was made while the socket was healthy, so nothing re-evaluates it, and recv awaited readiness before its syscall, which left the ECONNRESET arm sitting behind an await that never returns. A client closing its descriptor left the daemon's reader parked for ever, and the flow's port and registry entry held for the node's lifetime. recv reads before it waits now, because the latched error is visible to a syscall and only to a syscall, so the attempt that precedes the wait is what sees a close that has already happened. A close can also land while the task is parked, which no first attempt can catch, so on Darwin the wait is bounded and the syscall retried; the error is latched until a read consumes it, so the bound sets how long a dead flow holds its port rather than deciding whether the close is seen at all. On Linux this is one extra recv returning EAGAIN before the wait and changes nothing else, and everywhere else the readiness is authoritative and the wait stays unbounded. The three tests this predicted are the three that had failed: end of file on a closed client half, a listener's port unbound on close, and one flow's port freed while its connection stays open. One test asserted a delivery detail rather than the rule it exists to guard. a_descriptor_lands_on_the_last_complete_line_of_the_read_that_carried_it asserted that a plain write and the sendmsg following it arrive in one recvmsg. Linux coalesces them, so the read returns both lines and the descriptor together; Darwin stops a stream read at the ancillary boundary, so the plain line arrives by itself and the descriptor-bearing line comes on the next read. The rule the module rests on is unaffected, and Darwin satisfies it more easily than Linux, because the read it arrives on holds nothing later. The test fills until both lines are queued and asserts the rule instead of the number of reads it took. The client compiled in /run/fips/api.sock on every platform, and macOS has no /run for that path to be in. The daemon never had this problem: it resolves its socket at startup by looking for a directory, and its macOS branch lands on /var/run/fips. The constant is conditional the same way now, so a client that is told nothing looks where a packaged daemon on its own platform actually is. The reference documentation described that branch as FreeBSD-only and describes both. Windows stays excluded and cannot be included: it has no SCM_RIGHTS, so there is no way to pass a descriptor to another process at all, which is the whole mechanism rather than a detail of it. The platform statements in the source and in the shipped documentation all named Linux and FreeBSD and name macOS now, including the configuration reference, the security reference, the how-to and the walkthrough. The how-to also states how far the testing goes, because the person who would meet the gap first is the one enabling the API on a Mac. The end-to-end suite drives a client container against a node container over a shared volume, which is a Linux arrangement, so the socket lifecycle, the descriptor hand-off across a process boundary and the reclaiming of a port when a client exits are covered on macOS by unit tests rather than by anything that runs a daemon and a client as two real processes. That is a gap in testing and not a known defect, and it is a coverage gap rather than a discharged risk. The same place names the socket-type difference, since a reader who knows the descriptor is SOCK_DGRAM there can make sense of a close arriving as a different errno than the Linux documentation elsewhere describes. The changelog entry for the API is revised rather than followed by a second one: it now names the socket type each platform uses and the two end-of-file signals the receive path accepts. The entry describes what the release ships rather than the order the commits landed in.
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.