Dropping IPv6 packets with an unspecified source address

[evpopov]

FreeRTOS-Plus-TCP/source/FreeRTOS_IPv6.c

Lines 501 to 507 in 9f4ebd5

	/* Drop if packet has unspecified IPv6 address (defined in RFC4291 - sec 2.5.2)
	* either in source or destination address. */
	if( ( memcmp( pxDestinationIPAddress->ucBytes, FreeRTOS_in6addr_any.ucBytes, sizeof( IPv6_Address_t ) ) == 0 ) ||
	( memcmp( pxSourceIPAddress->ucBytes, FreeRTOS_in6addr_any.ucBytes, sizeof( IPv6_Address_t ) ) == 0 ) )
	{
	xHasUnspecifiedAddress = pdTRUE;
	}

The comment above references RFC4291 section 2.5.2 which reads:

   The address 0:0:0:0:0:0:0:0 is called the unspecified address.  It
   must never be assigned to any node.  It indicates the absence of an
   address.  One example of its use is in the Source Address field of
   any IPv6 packets sent by an initializing host before it has learned
   its own address.

   The unspecified address must not be used as the destination address
   of IPv6 packets or in IPv6 Routing headers.  An IPv6 packet with a
   source address of unspecified must never be forwarded by an IPv6
   router.

Nowhere in that section does it say that packets with a SOURCE address of :: are not allowed on the network and should be discarded. The text actually gives us a vague example of a use case. At the same time,
RFC3810 section 5.2.13 reads:

An MLDv2 Report MUST be sent with a valid IPv6 link-local source
   address, or the unspecified address (::), if the sending interface
   has not acquired a valid link-local address yet.  Sending reports
   with the unspecified address is allowed to support the use of IP
   multicast in the Neighbor Discovery Protocol [[RFC2461](https://www.rfc-editor.org/rfc/rfc2461)].  For
   stateless autoconfiguration, as defined in [[RFC2462](https://www.rfc-editor.org/rfc/rfc2462)], a node is
   required to join several IPv6 multicast groups, in order to perform
   Duplicate Address Detection (DAD).  Prior to DAD, the only address
   the reporting node has for the sending interface is a tentative one,
   which cannot be used for communication.  Thus, the unspecified
   address must be used.

   On the other hand, routers MUST silently discard a message that is
   not sent with a valid link-local address, without taking any action
   on the contents of the packet.  Thus, a Report is discarded if the
   router cannot identify the source address of the packet as belonging
...

#1019 actually generates these reports that RFC3810 talks about and my code there does set the source address to :: if there is no valid link-local address. I plan on eventually removing the source address check above and just leaving the destination address check. Does anyone see any issues with that proposal?

[tony-josi-aws]

@evpopov

IPv6 packets sent by an initializing host and MLD reports sent from hosts without link local addresses seem like valid examples of why we shouldn't drop packets with source addresses as ::.

To me, removing that source address check seems reasonable.

[evpopov]

@tony-josi-aws Thanks for your input. I don't have immediate plans for a PR that fixes this issue. I will probably eventually get to it in #1019 but it doesn't look like this will be happen any time soon. I also don't want to rush any changes so for now, I'll just leave this issue open.
If you or someone else gets to it before me, so be it. If not, I'll get to it eventually.