Provide ability to cross-compile to mips-*-linux-musl for openwrt

[infinity0]

This would be useful for networks where some adapters are connected only to the router, and not reachable to any end-user machines. Then I can e.g. have this run on the router as part of some health checks.

Openwrt supports a lot of different architecture targets. The ones I looked at (e.g. ath79/generic toolchain) all use the GCC target triple mips-24kc-linux-musl but there might be others. I'm not sure if it's feasible to provide binary packages for all the ones they support, but being able to cross-compile it yourself would be sufficient.

[infinity0]

As a temporary workaround I am running pla-util on my Debian box, capturing the packets, then re-sending them as raw ethernet packets on the router using socat(1). I've tested this and it seems the packet contents are the same across successive runs of the same command, so hopefully this works. (This also suggests that the underlying crypto protocol is susceptible to replay attacks, which someone could exploit if they have physical access to my power network, but oh well, too late to fix this I guess.)

[infinity0]

Openwrt supports a lot of different architecture targets.

Oh, here is a full list of the compiler triplets they use: https://downloads.openwrt.org/releases/24.10.4/packages/ (everything is linux-musl)

[infinity0]

[..] hopefully this works. (This also suggests that the underlying crypto protocol is susceptible to replay attacks [..]

Actually all the packets are plaintext, the encryption only applies to the data between the PLA units. So yeah, not sure what their threat model is, it's a bit weird to think that someone can snoop my power line but can't snoop my LAN (e.g. to send arbitrary control commands to the PLA unit including change the key).

Anyway ignoring all of that, the principle works. Tested on OpenWRT ath79/generic:

#!/bin/sh
# Checks for the presence of a minimum number of PLA units and restarts one of
# them if there are any missing.
# 
# Requires `opkg install socat tcpdump coreutils-timeout`
set -e -o pipefail # POSIX:2022

# Main variables
IFINDEX="${IFINDEX:-02}"
MAC_OWN="${MAC_OWN:-11:11:11:11:11:11}"
MAC_PLA="${MAC_PLA:-22:22:22:22:22:01}"
REQUIRED_UNITS="${REQUIRED_UNITS:-2}"

# Secondary variables
EXTRA_BPF="${EXTRA_BPF:-}" # e.g. "vlan and" if your interface is a VLAN

abort() {
	local x="$1"
	shift
	echo >&2 "$@"
	exit "$x"
}

debugcat() {
	local lv="$1"; shift
	if [ "${debug:-0}" -ge "$lv" ]; then "$@"; else cat; fi
}

debugtrue() {
	local lv="$1"; shift
	if [ "${debug:-0}" -ge "$lv" ]; then "$@"; else true; fi
}

test "$(uname -m)" = "mips" || abort 2 "machine is not mips, aborting"
which socat >/dev/null
which tcpdump >/dev/null
which timeout >/dev/null

pkt_discover="01:70:a0:00:00:00:1f:84:01:a3:97:a2:55:53:be:f1:fc:f9:79:6b:52:14:13:e9:e2:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00"
pkt_restart="02:20:a0:00:00:00:1f:84:02:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00"

send_raw() {
	local ifidx="$1" # hex
	local src="$2"
	local dst="$3"
	local payload="$4"
	echo -n -e "$(echo ":${dst}:${src}:89:12:${payload}" | sed -e 's/:/\\x/g')" \
	  | socat - SOCKET-SENDTO:17:3:0:x0003x000000"${ifidx}"x0000x00x06x"$(echo "$dst" | tr -d :)"0000
# explanation from https://superuser.com/a/1557083, tweaked for big-endian mips
cat >/dev/null <<'EOF'
echo
  -e (enable interpretation of backslash escapes, each byte needs a \x)
  -n (do not output the trailing newline)
  "\xab\xcd\xef\x01\x23\x45 (dst_mac)
   \x01\x23\x45\xab\xcd\xef (src_mac)
   \x89\x12         (ethertype 0x8912, homeplug protocol)
   \x01\x70(etc)    (payload ("discover" in this case))
  "
domain   - 17 (PF_PACKET, from `procan -c`)
type     -  3 (SOCK_RAW, from `procan -c`)
protocol -  0
address  - (data representation of a sockaddr structure, only the beginning needs an 'x', the rest are optional)
  sockaddr_ll
    family   - (without family)
    protocol - x0003 (ETH_P_ALL (unsigned short), from if_ether.h)
    ifindex  - x00000054 (interface to use for sending, from /sys/class/net/eth0/ifindex (int))
    hatype   - x0000 (0 on send (unsigned short))
    pkttype  - x00   (0 on send (unsigned char))
    halen    - x06   (ETH_ALEN (unsigned char), from if_ether.h)
    addr     - xabcdef0123450000 (dest MAC address on the same segment as ifindex, (unsigned char[8]))
network-order and mips architecture-order are both big-endian, so numbers are represented as-is e.g. 3 is x0003
EOF
}

# tcpdump takes a while to start up
tcpdump_startup=1
observe_raw() {
	# -s 72 to save performance, our packets are all very small
	timeout $((tcpdump_startup + 2)) tcpdump -n -t -q -s 72 $EXTRA_BPF ether proto 0x8912 2>/dev/null || true
}

observe_begin() {
	( x="$(observe_raw | egrep -v '^\s*$' | debugcat 2 tee /dev/stderr | "$@")"; exit "$x" ) & srv=$!
	sleep "$tcpdump_startup"
}

observe_wait() {
	wait "$srv" || x=$?
}

observe_begin wc -l
debugtrue 1 echo "send_raw <discover>"
send_raw "$IFINDEX" "$MAC_OWN" "ff:ff:ff:ff:ff:ff" "$pkt_discover"
observe_wait
#sample trace
cat >/dev/null <<EOF
11:11:11:11:11:11 > ff:ff:ff:ff:ff:ff, Unknown Ethertype (0x8912), length 64:
22:22:22:22:22:01 > 11:11:11:11:11:11, Unknown Ethertype (0x8912), length 64:
22:22:22:22:22:02 > 11:11:11:11:11:11, Unknown Ethertype (0x8912), length 64:
EOF
debugtrue 1 echo "packets observed: $x" # should be 1 + REQUIRED_UNITS

if [ "$x" -lt "$((1 + REQUIRED_UNITS))" ]; then
	observe_begin wc -l
	debugtrue 1 echo "send_raw <restart> $MAC_PLA"
	send_raw "$IFINDEX" "$MAC_OWN" "$MAC_PLA" "$pkt_restart"
	observe_wait
	debugtrue 1 echo "packets observed: $x" # should be 2: send & reply
else
	debugtrue 1 echo "assuming ok, exit success"
fi

pkt_discover and pkt_restart are extracted from packet captures of running pla-util.