first draft

random6.c

@@ -0,0 +1,143 @@
+// vim:sw=4:ts=4:et:
+
+/*
+ * Copyright (C) 2021 Andrei Belov (@defanator on github)
+ *
+ */
+
+#include <sys/types.h>
+#include <sys/socket.h>
+#include <netinet/in.h>
+#include <arpa/inet.h>
+#include <inttypes.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <time.h>
+#include <unistd.h>
+
+
+/* 16 octets in binary form + dots between octets + trailing zero */
+#define INET6_ADDRBINSTRLEN ((8 * 16) + (1 * 15) + 1UL)
+
+#ifndef INET6_ADDRSTRLEN
+#define INET6_ADDRSTRLEN sizeof("ffff:ffff:ffff:ffff:ffff:ffff:255.255.255.255")
+#endif
+
+char * sprintb(char *dst, void const * const ptr, size_t const size)
+{
+    int i, j;
+    char *dp = dst;
+    unsigned char byte;
+    unsigned char *b = (unsigned char*) ptr;
+
+    for (i = size - 1; i >= 0; i--) {
+        for (j = 7; j >= 0; j--) {
+            byte = (b[i] >> j) & 1;
+            *dp++ = byte ? '1' : '0';
+        }
+    }
+
+    *dp = 0x0;
+    return dst;
+}
+
+char * sprintb_addr6(char *dst, struct in6_addr *in6) {
+    char *dp = dst;
+    u_char *p = in6->s6_addr;
+    int i;
+
+    for (i = 0; i < 16; i++) {
+        sprintb(dp, &p[i], sizeof(u_char));
+        dp += 8;
+        if (i < 15) *dp++ = '.';
+    }
+
+    return dst;
+}
+
+struct in6_addr ipv6_rand(char *net,uint8_t prefixlen)
+{
+    int i, j, s;
+    uint8_t bits, shift;
+    u_char *addr, *mask;
+    struct in6_addr addr6, mask6,rand6;
+    //char straddr6[INET6_ADDRSTRLEN];
+    //char binaddr6[INET6_ADDRBINSTRLEN];
+
+    char *ip6net = net;
+
+    if (inet_pton(AF_INET6, ip6net, &addr6) < 1) {
+        printf("incorrect IPv6 address/net: \"%s\"\n", ip6net);
+        exit(1);
+    }
+
+    addr = addr6.s6_addr;
+    mask = mask6.s6_addr;
+    shift = prefixlen;
+    bits = 128 - shift;
+
+    for (i = 0; i < 16; i++) {
+        s = (shift > 8) ? 8 : shift;
+        shift -= s;
+
+        mask[i] = (u_char) (0xffu << (8 - s));
+
+        if (addr[i] != (addr[i] & mask[i])) {
+            addr[i] &= mask[i];
+        }
+    }
+
+
+    //inet_ntop(AF_INET6, &addr6, straddr6, INET6_ADDRSTRLEN);
+    //printf("network: %s/%d\n", straddr6, prefixlen);
+
+    //inet_ntop(AF_INET6, &mask6, straddr6, INET6_ADDRSTRLEN);
+    //printf("netmask: %s (free bits=%d)\n", straddr6, bits);
+
+    //printf("%s\n", sprintb_addr6(binaddr6, &addr6));
+    //printf("%s\n", sprintb_addr6(binaddr6, &mask6));
+    //printf("----\n");
+
+    srand(((unsigned) getpid() << 16) ^ time(NULL));
+
+    uint32_t rv = rand();
+    int k = 0;
+
+    shift = bits;
+    rand6 = addr6;
+    addr = rand6.s6_addr;
+
+    for (j = 15; j >= 0; j--) {
+        s = (shift > 8) ? 8 : shift;
+        shift -= s;
+
+        addr[j] = addr[j] ^ ((addr[j] ^ rv) & ~mask[j]);
+
+        if (shift == 0) break;
+
+        rv >>= 8;
+
+        /*
+            * Note that the MSB of the first octet in random value rv
+            * will always be 0 as RAND_MAX=0x7FFFFFFF, i.e. if we refresh
+            * rv after using all the 4 octets from uint32_t, the leading
+            * bit in octets 4, 8, 12, 16 in generated IPv6 address
+            * will _always_ be 0.
+            *
+            * While it seems legit for e.g. ::ffff:0:0/96 (IPv4-mapped
+            * addresses), there may be a better way of handling this
+            * (e.g. refresh rv after using 3 of 4 octets or reverse
+            * bits in first octet before applying).
+            *
+            */
+        //if (++k > 2) {
+        if (++k > 3) {
+            rv = rand();
+            k = 0;
+        }
+    }
+    //inet_ntop(AF_INET6, &rand6, straddr6, INET6_ADDRSTRLEN);
+    //printf("%s [%s]\n", sprintb_addr6(binaddr6, &rand6), straddr6);
+
+    return rand6;
+}