net: real IP stack, milestone A - ICMP echo (you can ping a Game Boy)

Start of an actual TCP/IP stack on gbos (TLS stays in a proxy). netd is a
userland IP responder over SLIP: our address is 10.0.0.2, the SLIP peer
10.0.0.1. It parses IPv4 headers, answers ICMP echo requests, and rebuilds
the packet with correct IP + ICMP checksums (RFC 1071 one's-complement sum,
carry-folded - works fine on the SM83).

c/netd.c + register; tools/gateway.py gains --mode ping: it crafts ICMP echo
requests over SLIP and verifies the replies.

Verified: `netd` answers 4 pings, gateway reports reply from 10.0.0.2 with
cksum=ok for each. Next: UDP, then TCP.

1 files changed, 47 insertions, 0 deletions

diff --git a/c/netd.c b/c/netd.c
new file mode 100644
index 0000000..36435d4
--- /dev/null
+++ b/c/netd.c
@@ -0,0 +1,47 @@
+#include "netlib.h"
+/* netd - a minimal IP stack over SLIP. Milestone A: answer ICMP echo (ping).
+   Our address is 10.0.0.2; the SLIP peer (gateway) is 10.0.0.1.
+
+   Everything is done on raw byte arrays so endianness is explicit (network
+   byte order = big-endian): field[hi], field[lo]. */
+
+/* Internet checksum (RFC 1071): 16-bit one's-complement sum with carry fold. */
+static unsigned int cksum(unsigned char *d, unsigned char len) {
+    unsigned int sum = 0, w;
+    unsigned char i = 0;
+    while ((unsigned char)(i + 1) < len) {
+        w = ((unsigned int)d[i] << 8) | d[i + 1];
+        sum += w; if (sum < w) sum++;
+        i += 2;
+    }
+    if (len & 1) { w = (unsigned int)d[len - 1] << 8; sum += w; if (sum < w) sum++; }
+    return ~sum;
+}
+
+void main(void) {
+    unsigned char pkt[128];
+    unsigned char len, ihl, i, t;
+    unsigned int c;
+    puts("netd: IP/ICMP up (10.0.0.2)"); nl();
+    for (;;) {
+        len = slip_recv((char *)pkt, 127);
+        if (len < 28) continue;                 /* IP(20) + ICMP(8) minimum */
+        if ((pkt[0] >> 4) != 4) continue;       /* IPv4 only */
+        ihl = (pkt[0] & 0x0f) << 2;
+        if (pkt[9] != 1) continue;              /* protocol 1 = ICMP */
+        if (pkt[ihl] != 8) continue;            /* ICMP type 8 = echo request */
+
+        /* turn the request into a reply, in place */
+        for (i = 0; i < 4; i++) { t = pkt[12 + i]; pkt[12 + i] = pkt[16 + i]; pkt[16 + i] = t; }
+        pkt[8] = 64;                            /* reset TTL */
+        pkt[ihl] = 0;                           /* ICMP type -> echo reply */
+        pkt[ihl + 2] = 0; pkt[ihl + 3] = 0;     /* recompute ICMP checksum */
+        c = cksum(pkt + ihl, (unsigned char)(len - ihl));
+        pkt[ihl + 2] = (unsigned char)(c >> 8); pkt[ihl + 3] = (unsigned char)c;
+        pkt[10] = 0; pkt[11] = 0;               /* recompute IP header checksum */
+        c = cksum(pkt, ihl);
+        pkt[10] = (unsigned char)(c >> 8); pkt[11] = (unsigned char)c;
+
+        slip_send((char *)pkt, len);
+    }
+}