# Networking ARP, IPv4, ICMP, UDP, TCP and DHCP, over a driver-agnostic interface class, reached through the same socket syscalls as Unix domain sockets. Verified end to end rather than by inspection: `bootcheck -Net` starts a listener and a UDP responder on the host, tells the guest to talk to them, and asserts on **both** ends — one side alone cannot tell a working stack from one that completes a handshake and drops every byte. --- ## The four layers ``` socket() bind() connect() send() recv() syscalls, one set ┌────────────────┴───────────────────┐ │ AF_UNIX AF_INET │ socknew() dispatches on domain │ kernel/net/unix.c kernel/net/inet.c └────────────────┬───────────────────┘ │ lwIP raw API ┌────────────────┴───────────────────┐ │ lwIP 2.2.0, NO_SYS=1 │ ports/lwip, compiled into the kernel │ port layer: kernel/net/lwip/ │ ~400 lines: a lock, a clock, a netif └────────────────┬───────────────────┘ │ struct netdev_ops ┌────────────────┴───────────────────┐ │ e1000, and any future NIC driver │ └────────────────────────────────────┘ ``` ## The netdev class A driver publishes a [`struct netdev_ops`](../../include/netdev.h) and is then finished with the network stack forever: it never mentions lwIP, and the stack never mentions it. ```c struct netdev_ops { int (*transmit)(struct device *, const void *frame, int len); int (*receive) (struct device *, void *out, int cap); const unsigned char *(*mac)(struct device *); int (*link_up) (struct device *); // optional }; ``` `transmit` returning -1 is **congestion, not failure** — the ring is full and the caller is expected to retry. `receive` never blocks; it returns 0 when nothing is waiting, and the caller decides how to wait. This class was added late, and the reason it had to be is instructive: the stack had reached straight into the driver. `net.c` called `e1000_present()`, the lwIP glue was named `netif_e1000.c`, and the e1000 shipped with `.ops = 0` and a comment saying so. A second NIC driver would have meant editing the protocol stack to add itself. lwIP now gets one netif per bound device, `/proc/netif` lists them all, and the e1000 drives up to four cards rather than refusing the second — verified with two attached, each with its own MAC and its own DHCP lease. ### `netdev_selftest()` `echo n > /proc/sysrq` sends a hand-built ARP request and waits for the reply. It proves the whole driver path in one exchange — descriptor formats, the DD handshake, tail bookkeeping, DMA addresses, the receive ring hand-back, and the MAC filter, because an ARP reply is addressed to this station and does not arrive if the filter is wrong. It was written *before* there was any protocol stack, deliberately: a stack on an unproven driver fails in ways indistinguishable from its own bugs. It belongs to the class rather than to the e1000, because it is the check every new NIC driver will want first. ## lwIP lwIP 2.2.0, fetched by `ports/lwip` and **compiled into the kernel** — it is the one port that installs nothing into the sysroot, because it is kernel code. `NO_SYS=1`, so lwIP has no threads, mailboxes or semaphores of its own. The port layer is `kernel/net/lwip/`: an architecture header, a lock, a clock, an Ethernet netif, and a set of shim headers standing in for the libc lwIP expects. ### Why lwIP and not a BSD stack The licence was not the deciding factor — FreeBSD's `sys/netinet` is BSD-3 too. **It is that FreeBSD's stack is not a library.** `tcp_input.c` alone is 122 KB and reaches into mbufs, `epoch(9)`, netisr, VNET, `uma(9)`, sockbufs and inpcb hash tables. Taking it means porting FreeBSD's kernel infrastructure first and finding the TCP somewhere underneath — the same conclusion already reached about FreeBSD's USB stack. NetBSD was ruled out on licence: its `tcp_input.c` and `tcp_output.c` carry the WIDE Project's 4-clause advertising clause, which Berkeley's rescission does not reach. See [../licensing.md](../licensing.md) §3. lwIP was written for exactly the case SchOS is in: a small operating system with a raw Ethernet driver and no infrastructure to speak of. **The reason not to write one, kept because it is the reason to resist later too:** it is not the packet formats that are hard. It is congestion control, retransmission, window management and twenty years of corner cases that only appear against a real peer. ## The network thread `kernel/net/net.c`. One kernel thread, started by `netinit()`, feeds lwIP received frames and runs its timers. **A thread rather than the interrupt handler, and the reason is the lock:** a call into lwIP can reach the driver, the driver can sleep, and an interrupt handler cannot. So the frame is always *pulled*, by this thread, through `.receive`. The interrupt's only job is to end the thread's wait early. `netdev_rx_signal()` is what a driver calls from its handler; it delivers no frame and reads no ring. Without it a frame that arrived just after a poll sat there until the next clock tick — up to 10 ms, and the largest fixed latency in the system. **The wait stays on the timer tick rather than moving to a channel of its own**, and that is the design rather than an accident. lwIP's timers must still run when the link is silent, so the clock has to remain a floor under the wait, and `sleep()` cannot wait on two things at once. Joining the tick's existing wakeup costs nothing and leaves one wait with one predicate. The flag lives under `tickslock` for the same reason it does not get a lock of its own: `sleep()` releases `tickslock` atomically, so a driver setting the flag and waking under that lock cannot lose the wakeup by arriving between the predicate test and the sleep. A private lock would not be held across the sleep, and that window would be back. `/proc/netif` reports `wake_rx` and `wake_tick` so this is observable rather than assumed — if `wake_rx` stays at zero while frames arrive, the interrupt is not reaching the stack and the thread is running on the clock exactly as it used to. Measured across three pings: `rx_frames` and `wake_rx` both went 2 → 6. `netinit()` runs after `sinit()` because `kthread_create` needs a usable process table, and it returns immediately: everything happens in that thread, which cannot run until the scheduler is entered. DHCP obtains an address at boot. `/proc/netif` reports each interface's address, netmask, gateway, MAC and frame counts, taken at the lwIP seam. > It is `/proc/netif` and not `/proc/net` deliberately: on Linux `/proc/net` is a > directory of many files, and a single file by that name would be a lie about a > well-known path. ## Sockets One set of syscalls, at their real Linux numbers. `socknew()` dispatches on the domain, and **no syscall changes shape** between the two families. ### AF_UNIX `kernel/net/unix.c`. `SOCK_STREAM`, `socketpair`, `shutdown`. A connected pair is **two buffers, not one**: writing appends to the peer's buffer and reading drains your own, which is what makes a socket bidirectional where a pipe is not, without needing two of anything. **One lock covers the whole table.** A pipe gets away with a lock per pipe because every operation touches one object; a socket operation touches two, and two-object locking needs an ordering rule every path obeys or it deadlocks. Worse, the wakeup races are subtle — a reader testing `peer_closed` under its own lock while the closer sets it under the peer's has no interlock at all. **Names live in a table here, not in the filesystem.** Binding creates no directory entry. The table gives the behaviour that matters — a client finds a server by a name agreed in advance — and what is lost is `ls -l` showing an `s`, and `unlink()` of a socket path working. Both want a VFS change rather than a socket change. ### AF_INET `kernel/net/inet.c`, on lwIP's **raw** API rather than lwIP's own sockets layer, which would need `NO_SYS=0` and a second socket abstraction alongside this kernel's own. **Everything is protected by the lwIP lock and nothing else**, and that is not a shortcut: lwIP's callbacks run in the network thread with that lock held and are the only writers to a socket's receive buffer. A syscall reading it takes the same lock. There is no second lock because there is no second writer. #### How a blocking socket waits Worth reading before touching, because the obvious implementation does not work. A blocking call cannot `sleep(&socket, &lwip_lock)`: `sleep()` wants a spinlock and the lwIP lock is a *sleeplock* — it has to be, because a call into lwIP can reach the driver. And a waiter cannot hold that lock while sleeping, or the network thread could never deliver the thing being waited for. So the wait is on a **counter** with a spinlock of its own: ``` reader: lwip_lock; check condition; s = net_seq(); lwip_unlock; net_wait(s) waker: (inside a callback, lwIP lock held) net_wake() ``` No wakeup can be lost. `s` is read while the reader still holds the lwIP lock, so anything changing the condition must do so afterwards and therefore bumps the counter after the read; `net_wait` re-checks under its own spinlock and does not sleep. One counter and one channel for **all** inet sockets rather than one each. At this scale waking sixteen sockets costs a handful of instructions in the ones with nothing to do, and a per-socket channel would need careful teardown in `close()` against callbacks that can still arrive. Before this, a blocked socket woke on every timer tick and looked — costing a wakeup per socket per 10 ms and answering up to 10 ms late even when the data was already there. `tcp_sent` had to be registered for it as well, or a write blocked on a full send buffer slept with nobody left to wake it. #### ICMP, and ping sockets `socket(AF_INET, SOCK_DGRAM, IPPROTO_ICMP)` — Linux's *ping socket*, which gives an unprivileged process the one thing raw sockets were used for. The caller reads and writes the ICMP message alone; the kernel owns the IP header. Built on lwIP's raw API: `raw_new(IP_PROTO_ICMP)`, with the receive callback stripping the variable-length IP header before queueing the message on the same datagram queue UDP uses. **Only echo replies are consumed**, and that restriction is load-bearing. `raw_input` runs *before* lwIP's own `icmp_input`, so a callback returning "eaten" for every ICMP message would swallow the echo **requests** too and this machine would silently stop answering pings from outside. Returning 0 leaves the packet for lwIP, which is what should happen to every ICMP message nobody asked for. There is no per-socket echo-id filtering. Linux rewrites the id and matches on it; here a reply goes to every open ICMP socket, which is invisible with one ping running and means a second one may see a sequence number it never sent. ### AF_NETLINK, and why it exists `getifaddrs(3)` is how every program asks *what interfaces does this machine have, and what addresses do they carry* — and on Linux there is exactly one answer: a `NETLINK_ROUTE` socket. musl implements the libc call over it, so what looks like a syscall is really a small protocol conversation, and without this side of it every caller fails identically. `kernel/net/netlink.c` is a **responder**, not a general socket family. There is no kernel-to-userspace event stream, no multicast groups, nothing subscribes to link state. A request arrives, the whole answer is generated into the socket's buffer, and reads drain it. That shape is deliberate. `getifaddrs` sends a dump request and reads until `NLMSG_DONE` with `MSG_DONTWAIT`, so generating the answer at send time means a read never blocks and the non-blocking flag is honoured for free. Two message types are served — `RTM_GETLINK` returns one `RTM_NEWLINK` per interface (name, flags, MAC), `RTM_GETADDR` returns one `RTM_NEWADDR` per address (address, prefix length, label). Anything else gets a bare `NLMSG_DONE`. **The data is not netlink's.** Interfaces come from the netdev class and addresses from lwIP's netifs, exactly as `/proc/netif` reads them. Nothing in the file knows what an e1000 is. Interface indices matter more than they look: `getifaddrs` matches address records to link records *by index*, so the two dumps have to agree. Loopback is 1 and the netdevs follow from 2. What it produces, read back through musl: ``` lo flags=0x49 fam=-1 eth0 flags=0x1043 fam=17 (AF_PACKET, the MAC) lo flags=0x49 fam=2 addr=127.0.0.1 mask=255.0.0.0 eth0 flags=0x1043 fam=2 addr=10.0.2.15 mask=255.255.255.0 ``` ### Loopback `127.0.0.1` is a real destination: `LWIP_HAVE_LOOPIF` creates the interface and `lwip_init()` brings it up. `LWIP_NETIF_LOOPBACK` is the half that is easy to miss — without it, a packet addressed to an interface's *own* address goes out of the wire and never comes back. With `NO_SYS=1` there is no thread to drain the loopback queue, so the network thread calls `netif_poll_all()` alongside `sys_check_timeouts()`. ### Datagrams UDP has a **datagram queue**, not a byte ring. It shared TCP's ring at first, and two datagrams arriving together came back as one — which is the bug that had to be fixed before `recvfrom` could report a source address at all. --- ## What it does not do - **`ping` is a first-party program**, `cmd/ping`, not toybox's. toybox's reads replies with `recvmsg(2)` and asks for the TTL through an `IP_RECVTTL` control message, and neither exists — `recvmsg` is item 1 of [../plan/foundations.md](../plan/foundations.md), where it is wanted for `SCM_RIGHTS` anyway. Ours has no `-I`, no flood mode and no TTL display. - **No resolver reachable from userland.** lwIP's DNS client is compiled but nothing exposes it, so `ping` takes a dotted quad. `localhost` is special-cased in the program rather than resolved. - **No routing table**, and no `/proc/net/route` to report one. With two interfaces up, the first is the default route and the second is reachable only by an address on its own subnet. That is a rule rather than an accident, and it is not routing. The missing *file* is the more immediately visible half: it is how a program asks which interface carries the default route, and without it anything that filters on "the default one" sees nothing. - **Netlink serves two message types.** No `RTM_GETROUTE`, no `RTM_NEWROUTE`, no event stream, no multicast groups. A program that expects to *subscribe* to link changes gets a socket that answers dumps and never speaks again. - **No `getsockname`/`getpeername`**, and `accept()` does not report the peer's address for an AF_INET socket. A server that wants to log who connected cannot. - **`setsockopt` covers six options**, and refuses the rest with `ENOPROTOOPT` rather than pretending: `SO_REUSEADDR`, `SO_KEEPALIVE`, `SO_BROADCAST` and `TCP_NODELAY` reach lwIP's pcb; `SO_SNDBUF`/`SO_RCVBUF` are accepted and report the real size on read. Notably absent: `SO_RCVTIMEO`/`SO_SNDTIMEO`. - **`SOCK_DGRAM` over AF_UNIX does not exist** — stream only. - **AF_UNIX names are not filesystem paths**, so nothing can `stat()` or `unlink()` a socket. - **One NIC driver exists.** The class is proven with two e1000 cards, not with two different drivers. - **No IPv6**, no DNS resolver in the kernel (lwIP's is compiled but unused by anything), and no firewall or packet filter of any kind.