Modify the behaviour of --synth-domain for IPv6.

When deriving a domain name from an IPv6 address, an address
such as 1234:: would become 1234--.example.com, which is
not legal in IDNA2008. Stop using the :: compression method,
so 1234:: becomes
1234-0000-0000-0000-0000-0000-0000-0000.example.com

CHANGELOG

@@ -44,6 +44,13 @@ version 2.91
         returns truncated, do likewise. If upstream is OK, but the
 	answer is too big for downstream, truncate the answer.
 
+	Modify the behaviour of --synth-domain for IPv6.
+	When deriving a domain name from an IPv6 address, an address
+	such as 1234:: would become 1234--.example.com, which is
+	not legal in IDNA2008. Stop using the :: compression method,
+	so 1234:: becomes
+	1234-0000-0000-0000-0000-0000-0000-0000.example.com
+	
 	
 version 2.90
 	Fix reversion in --rev-server introduced in 2.88 which

man/dnsmasq.8

@@ -769,12 +769,11 @@ results in the name internal-0.thekelleys.org.uk. returning 192.168.0.50, intern
 Second, 
 .B --synth-domain=thekelleys.org.uk,192.168.0.0/24,internal- (no *)
 will result in a query for internal-192-168-0-56.thekelleys.org.uk returning
-192.168.0.56 and a reverse query vice versa. The same applies to IPv6,
+192.168.0.56 and a reverse query vice versa. The same applies to IPv6;
-but IPv6 addresses may start with '::'
+the representation doesn't use the :: compression feature or
-but DNS labels may not start with '-' so in this case if no prefix is
+the special representation of V4 mapped IPv6 addresses as these
-configured a zero is added in front of the label. ::1 becomes 0--1.
+can generate illegal domain names, so all domains are of the form
-
+internal-1000-0000-0000-0000-0000-0000-0000-0008.example.com
-V4 mapped IPv6 addresses, which have a representation like ::ffff:1.2.3.4 are handled specially, and become like 0--ffff-1-2-3-4
 
 The address range can be of the form
 <start address>,<end address> or <ip address>/<prefix-length> in both forms of the option. For IPv6 the start and end addresses

src/domain.c

@@ -115,26 +115,15 @@ int is_name_synthetic(int flags, char *name, union all_addr *addrp)
 	  
 	  *p = 0;	
 	  
-	  if (prot == AF_INET6 && strstr(tail, "--ffff-") == tail)
+	  /* swap . or : for - */
-	    {
+	  for (p = tail; *p; p++)
-	      /* special hack for v4-mapped. */
+	    if (*p == '-')
-	      memcpy(tail, "::ffff:", 7);
+	      {
-	      for (p = tail + 7; *p; p++)
+		if (prot == AF_INET)
-		if (*p == '-')
 		  *p = '.';
-	    }
+		else
-	  else
+		  *p = ':';
-	    {
+	      }
-	      /* swap . or : for - */
-	      for (p = tail; *p; p++)
-		if (*p == '-')
-		  {
-		    if (prot == AF_INET)
-		      *p = '.';
-		    else
-		      *p = ':';
-		  }
-	    }
 	  
 	  if (hostname_isequal(c->domain, p+1) && inet_pton(prot, tail, &addr))
 	    found = (prot == AF_INET) ? match_domain(addr.addr4, c) : match_domain6(&addr.addr6, c);
@@ -194,9 +183,8 @@ int is_rev_synth(int flag, union all_addr *addr, char *name)
 
    if ((flag & F_IPV6) && (c = search_domain6(&addr->addr6, daemon->synth_domains))) 
      {
-       char *p;
-       
        *name = 0;
+
        if (c->indexed)
 	 {
 	   u64 index = addr6part(&addr->addr6) - addr6part(&c->start6);
@@ -204,24 +192,17 @@ int is_rev_synth(int flag, union all_addr *addr, char *name)
 	 }
        else
 	 {
+	   int i;
+	   char frag[6];
+
 	   if (c->prefix)
-	     strncpy(name, c->prefix, MAXDNAME - ADDRSTRLEN);
+	     strncpy(name, c->prefix, MAXDNAME);
 	   
-	   inet_ntop(AF_INET6, &addr->addr6, name + strlen(name), ADDRSTRLEN);
+	   for (i = 0; i < 16; i += 2)
-
-	   /* IPv6 presentation address can start with ":", but valid domain names
-	      cannot start with "-" so prepend a zero in that case. */
-	   if (!c->prefix && *name == ':')
 	     {
-	       *name = '0';
+	       sprintf(frag, "%s%02x%02x",  i == 0 ? "" : "-", addr->addr6.s6_addr[i], addr->addr6.s6_addr[i+1]);
-	       inet_ntop(AF_INET6, &addr->addr6, name+1, ADDRSTRLEN);
+	       strncat(name, frag, MAXDNAME);
 	     }
-	   
-	   /* V4-mapped have periods.... */
-	   for (p = name; *p; p++)
-	     if (*p == ':' || *p == '.')
-	       *p = '-';
-	   
 	 }
 
        strncat(name, ".", MAXDNAME);

src/option.c

@@ -2636,8 +2636,11 @@ static int one_opt(int option, char *arg, char *errstr, char *gen_err, int comma
 			  {
 			    if (option != 's')
 			      {
+				/* IPv6 address is longest and represented as
+				   xxxx-xxxx-xxxx-xxxx-xxxx-xxxx-xxxx-xxxx
+				   which is 39 chars */
 				if (!(new->prefix = canonicalise_opt(arg)) ||
-				    strlen(new->prefix) > MAXLABEL - INET_ADDRSTRLEN)
+				    strlen(new->prefix) > (MAXLABEL - 39))
 				  ret_err_free(_("bad prefix"), new);
 			      }
 			    else if (strcmp(arg, "local") != 0)