net: unbreak ping against hosts that intermittently drop ICMP

Pinging sl0p.foo through gbhub looked hung: DNS resolved, then nothing.
Packet-tracing showed the echo request leaving the hub's TUN and eth0
correctly NATed every time - but 80.78.19.56 blackholes ICMP for all
ids in windows of tens of seconds (provider rate limiting). One lost
reply wedged ping for minutes because net_recv's pump-counted timeout
is effectively unbounded at native emulation speed.

Two fixes:

- NET_RECVNB (op 8): non-blocking recv - one RX pump, $FE if nothing
  buffered, same delivery/EOF semantics as NET_RECV otherwise. ping now
  waits <=~2s per seq (net_recv_nb + msleep loop), prints 'seq=N
  timeout' and moves on, like real ping.

- ICMP echo id was hardcoded $1234 for every GB, every boot, so all
  sessions produced byte-identical flows - hostile to NAT conntrack
  (keyed on icmp id). wNetEchoId is now our host octet + rDIV timing
  noise sampled at DHCP lease, distinct per GB and per boot.

Verified: 8 back-to-back native-speed gbhub runs, zero hangs; a run
that hit a blackhole window printed seq=1 timeout then recovered to
3/4 received. GB<->GB ping and DNS/DHCP unaffected.

1 files changed, 14 insertions, 1 deletions

diff --git a/c/ping.c b/c/ping.c
index f8d5bf7..8671318 100644
--- a/c/ping.c
+++ b/c/ping.c
@@ -14,6 +14,19 @@ static void put_ip(unsigned char *ip) {
     for (i = 0; i < 4; i++) { putu(ip[i]); if (i < 3) putc('.'); }
 }
 
+/* wait up to ~2s for an echo reply without blocking in the kernel: a lost
+   reply (some hosts rate-limit ICMP) must cost one "timeout" line, not
+   minutes wedged inside net_recv's pump-counted wait. */
+static unsigned char wait_reply(unsigned char s) {
+    unsigned char t, r;
+    for (t = 0; t < 40; t++) {
+        r = net_recv_nb(s, rbuf, (unsigned char)sizeof(rbuf));
+        if (r != 0xFE) return r;       /* reply (or error) */
+        msleep(50);
+    }
+    return 0xFF;                       /* ~2s with no reply */
+}
+
 void main(void) {
     char *arg;
     unsigned char s, seq, got = 0, i;
@@ -35,7 +48,7 @@ void main(void) {
     puts("PING "); put_ip(dst); nl();
     for (seq = 1; seq <= 4; seq++) {
         net_send(s, payload, 4);
-        if (net_recv(s, rbuf, (unsigned char)sizeof(rbuf)) != 0xFF) {
+        if (wait_reply(s) != 0xFF) {
             got++;
             puts("  reply from "); put_ip(dst); puts(" seq="); putu(seq); nl();
         } else {