netdev - Re: ipv6_addr_type() and mapped IPv4 loopback

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Date:	Tue, 4 Nov 2008 16:33:47 -0500
From:	Chuck Lever <chuck.lever@...cle.com>
To:	Brian Haley <brian.haley@...com>
Cc:	netdev@...r.kernel.org
Subject: Re: ipv6_addr_type() and mapped IPv4 loopback

Hi Brian-

On Nov 4, 2008, at Nov 4, 2008, 4:16 PM, Brian Haley wrote:
> Chuck Lever wrote:
>> Hi Brian-
>> On Nov 3, 2008, at Nov 3, 2008, 9:33 PM, Brian Haley wrote:
>>> Chuck Lever wrote:
>>>> The __ipv6_addr_type() function does not recognize the mapped  
>>>> IPv4 loopback address:
>>>>  ::ffff:7f00:0001
>>>> as type IPV6_ADDR_LOOPBACK.  Is this intentional?
>>>
>>> I would think that since the IPv6 loopback address is ::1, and  
>>> ipv4-mapped is ::ffff:* that this would be IPV6_ADDR_MAPPED.   
>>> That's what RFC 4291 seems to say.
>> So the answer to my original question is then "yes, this is  
>> intentional," correct?
>
> I didn't design the Linux IPv6 stack, but it looks right to me.
>
>> On a system with an AF_INET6 listener, legacy applications use  
>> 127.0.0.1 to contact a local listener.  The incoming source address  
>> will be ::ffff:7f00:0001, not ::1.  This means IPv6-enabled  
>> applications have to perform a separate check for ::ffff:7f00:0001  
>> if they are looking for loopback addresses.
>
> Right, applications have to see if it's really an IPv4 address if  
> necessary.  If you're just looking for loopback and it's in the  
> kernel, it's only about this much code:
>
> 	if (ipv6_addr_type(addr->sin6_addr) == IPV6_ADDR_MAPPED) {
>                __be32 v4addr = addr->sin6_addr.s6_addr32[3];
>                if (ipv4_is_loopback(v4addr)) {
>                        /* it's 127.0.0.* */
>                }
> 	}
>
> There's the IN6_IS_ADDR_V4MAPPED() macro for user-space as well.

Thanks for the steer.  I'll submit this fix for 2.6.29.  I'm hoping to  
introduce AF_INET6-based kernel RPC services (namely NFSD and lockd)  
in that release.

-- 
Chuck Lever
chuck[dot]lever[at]oracle[dot]com
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