Mac OS X link-local address convention

[Fyodor <fyodor () insecure org>]

Here is an interesting (well, maybe to IPv6 wonks anyway) issue which
david discovered and worked around a little bit ago in the way Mac OS
X appears to include interface indexes in link-local addresses in an
unconventional way.  I figured this detailed commit log was worth
forwarding along since nmap-svn isn't archived on seclists.  This at
least lets me link to it from the Nmap CHANGELOG.

Cheers,
-F


----- Forwarded message from commit-mailer () insecure org -----

Date: Sun, 18 Sep 2011 16:49:34 -0700
From: commit-mailer () insecure org
Reply-To: nmap-dev () insecure org
To: nmap-svn () insecure org
Subject: [nmap-svn] r26618 - nmap/libnetutil

Author: david
Date: Sun Sep 18 16:49:34 2011
New Revision: 26618

Log:
Canonicalize interface addresses returned from libdnet.

Mac OS X appears to have a new address convention that I can't find
documented. The link-local address fe80:4::X:X:X:X stands for
fe80::X:X:X:X%en0, if en0 is the interface with index 4. (I.e., it would
be fe80::X:X:X:X%4 on Windows.) The number 4 could be different numbers.

The interface addresses seem to be stored with these pseudo-zone IDs at
a low level, because they appear that way when they come from libdnet.
This is what "nmap --iflist" shows:

lo0  (lo0)   fe80:1::1/64                   loopback    up   16384
en0  (en0)   fe80:4::xxxx:xxxx:xxxx:xxxx/64 ethernet    up   1500  XX:XX:XX:XX:XX:XX

The OS X network tools seem to hide this. This is what "ifconfig" and
"netstat -rn -f inet6" show:

lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> mtu 16384
        inet6 fe80::1%lo0 prefixlen 64 scopeid 0x1
en0: flags=8963<UP,BROADCAST,SMART,RUNNING,PROMISC,SIMPLEX,MULTICAST> mtu 1500
        inet6 fe80::xxxx:xxxx:xxxx:xxxx%en0 prefixlen 64 scopeid 0x4

fe80::1%lo0                             link#1                          UHLI            lo0
fe80::xxxx:xxxx:xxxx:xxxx%en0           XX:XX:XX:XX:XX:XX               UHLI            lo0

(Not the "scopeid 0x4".) Also, if you use one of these addresses, it
magically transforms into one without the pseudo-ID (getaddrinfo does
this):

$ ping6 fe80:4::1234
PING6(56=40+8+8 bytes) fe80::xxxx:xxxx:xxxx:xxxx%en0 --> fe80::1234%en0
$ ping6 fe80:1::1234
PING6(56=40+8+8 bytes) fe80::1%lo0 --> fe80::1234%lo0

This translation is messing up our netmask comparisons for the purpose
of routing. If you use a normal link-local address starting with fe80:0,
then it doesn't compare equal with the fe80:4/64 interface address. If
you try to use the 4, then getaddrinfo turns it into fe80:0 anyway, and
the comparison still fails.

So for this reason I added a canonicalize_address function, which calls
getnameinfo on an address, then calls getaddrinfo on the returned
string, to mangle an address the same way the OS would do it. One would
hope this is a no-op in cases other than the one I have described.

Now "nmap --iflist" shows:
lo0  (lo0)   fe80::1/64                   loopback    up   16384
en0  (en0)   fe80::xxxx:xxxx:xxxx:xxxx/64 ethernet    up   1500  XX:XX:XX:XX:XX:XX

Modified:
   nmap/libnetutil/netutil.cc

Modified: nmap/libnetutil/netutil.cc
==============================================================================
--- nmap/libnetutil/netutil.cc  (original)
+++ nmap/libnetutil/netutil.cc  Sun Sep 18 16:49:34 2011
@@ -1180,10 +1180,48 @@
 
 
 
+/* Convert an address to a string and back again. The first parsing step
+   eliminates magical OS-specific syntax, for example on OS X, fe80:4::X:X:X:X
+   becomes "fe80::X:X:X:X" (the "4" in this case is another way of writing the
+   zone ID, like "%en0"; i.e., in this case en0 is interface number 4. This must
+   be done before e.g. comparing addresses by netmask. */
+static int canonicalize_address(const struct sockaddr_storage *ss,
+  struct sockaddr_storage *output) {
+  char canonical_ip_string[NI_MAXHOST];
+  struct addrinfo hints;
+  struct addrinfo *ai;
+  int rc;
+
+  /* Convert address to string. */
+  rc = getnameinfo((struct sockaddr *) ss, sizeof(*ss),
+    canonical_ip_string, sizeof(canonical_ip_string), NULL, 0, NI_NUMERICHOST);
+  if (rc != 0) {
+    /* Don't care. */
+    *output = *ss;
+    return 0;
+  }
+
+  memset(&hints, 0, sizeof(hints));
+  hints.ai_family = ss->ss_family;
+  hints.ai_socktype = SOCK_DGRAM;
+  hints.ai_flags |= AI_NUMERICHOST;
+
+  rc = getaddrinfo(canonical_ip_string, NULL, &hints, &ai);
+  if (rc != 0 || ai == NULL)
+    return -1;
+  assert(ai->ai_addrlen > 0 && ai->ai_addrlen <= (int) sizeof(*output));
+  memcpy(output, ai->ai_addr, ai->ai_addrlen);
+  freeaddrinfo(ai);
+
+  return 0;
+}
+
 static int collect_dnet_interfaces(const struct intf_entry *entry, void *arg) {
   struct dnet_collector_route_nfo *dcrn = (struct dnet_collector_route_nfo *) arg;
   bool primary_done;
   unsigned int num_aliases_done;
+  struct sockaddr_storage tmpss;
+  int rc;
 
   primary_done = false;
   num_aliases_done = 0;
@@ -1198,13 +1236,21 @@
     /* The first time through the loop we add the primary interface record.
        After that we add the aliases one at a time. */
     if (!primary_done) {
-      if (addr_ntos(&entry->intf_addr, (struct sockaddr *) &dcrn->ifaces[dcrn->numifaces].addr) == -1)
+      if (addr_ntos(&entry->intf_addr, (struct sockaddr *) &tmpss) == -1) {
         dcrn->ifaces[dcrn->numifaces].addr.ss_family = 0;
+      } else {
+        rc = canonicalize_address(&tmpss, &dcrn->ifaces[dcrn->numifaces].addr);
+        assert(rc == 0);
+      }
       dcrn->ifaces[dcrn->numifaces].netmask_bits = entry->intf_addr.addr_bits;
       primary_done = true;
     } else if (num_aliases_done < entry->intf_alias_num) {
-      if (addr_ntos(&entry->intf_alias_addrs[num_aliases_done], (struct sockaddr *) 
&dcrn->ifaces[dcrn->numifaces].addr) == -1)
+      if (addr_ntos(&entry->intf_alias_addrs[num_aliases_done], (struct sockaddr *) &tmpss) == -1) {
         dcrn->ifaces[dcrn->numifaces].addr.ss_family = 0;
+      } else {
+        rc = canonicalize_address(&tmpss, &dcrn->ifaces[dcrn->numifaces].addr);
+        assert(rc == 0);
+      }
       dcrn->ifaces[dcrn->numifaces].netmask_bits = entry->intf_alias_addrs[num_aliases_done].addr_bits;
       num_aliases_done++;
     }

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