[lug] sendto(2) problem with ipv4/ipv6 addresses...

[Alan Robertson]

I wrote a little test program that just did the sendto with a variety of 
ipv4 and ipv6 addresses.  It works fine :-D.

Now I have something working to compare my "real" code to.  [Should have 
done this earlier]

It uses IF_INET6 - which makes sense since it _is_ and IPv6-format 
address (that is, 16 bytes and not 4).

So the good news is I'm doing it _almost_ right.


On 04/02/2012 01:32 PM, John Hernandez wrote:
> Alan,
>
> I think in the case of the IPv4-sourced packet, you want
>
> sin6_family = AF_INET
>
> (not AF_INET6)
>
> -John
>
> On Mon, Apr 2, 2012 at 12:38 PM, Alan Robertson<alanr at unix.sh>  wrote:
>> As part of the Assimilation Monitoring project (assimmon.org), I've been
>> writing some UDP network code to send packets.  It seems to work fine when
>> sending to the ipv6 loopback address (::1), but not when sending to ipv4
>> addresses.  I'm sending to ipv4 addresses by using the ipv4 part of the ipv6
>> address space ("::ffff:aa.bb.cc.dd").  Tcpdump is showing that nothing is
>> being sent - so the kernel is blocking this call before it goes out.
>>
>> The code being invoked is exactly the same (except for the address) for the
>> working and failing cases - so it must be something in the data...
>>
>> The failing line of code is simple:
>>      rc = sendto(self->getfd(self),  packet, (size_t)length, flags, (const
>> struct sockaddr*)&v6addr, sizeof(v6addr));
>>      g_return_if_fail(rc == length);
>>
>> When the v6addr structure refers to "::1" it works fine.  But when it is set
>> to "::ffff:10.10.10.4" (my local address) it fails with ENETUNREACH
>>
>>
>> When I watch it with strace you see this system call when it succeeds:
>> sendto(4,
>> "\0\1\0*\0\0\0\1\0\"\1\2\337?a\230\4\251/\333 at W\31-\304=\327H\352w\212\334"...,
>> 48, 0, {sa_family=AF_INET6, sin6_port=htons(1984), inet_pton(AF_INET6,
>> "::1",&sin6_addr), sin6_flowinfo=0, sin6_scope_id=0}, 28) = 48
>>
>> When I watch it with strace you see this system call when it fails:
>> sendto(4,
>> "\0\1\0*\0\0\0\1\0\"\1\2\337?a\230\4\251/\333 at W\31-\304=\327H\352w\212\334"...,
>> 48, 0, {sa_family=AF_INET6, sin6_port=htons(1984), inet_pton(AF_INET6,
>> "::ffff:10.10.10.5",&sin6_addr), sin6_flowinfo=0, sin6_scope_id=0}, 28) =
>> -1 ENETUNREACH (Network is unreachable)
>>
>> When I look at the struct sockaddr_in6 in gdb in the failing case, I see
>> this:
>> (gdb) print v6addr
>> $1 = {sin6_family = 10, sin6_port = 49159, sin6_flowinfo = 0, sin6_addr =
>> {__in6_u = {
>>        __u6_addr8 =
>> "\000\000\000\000\000\000\000\000\000\000\377\377\n\n\n\005", __u6_addr16 =
>> {0, 0, 0, 0, 0, 65535, 2570, 1290},
>>        __u6_addr32 = {0, 0, 4294901760, 84544010}}}, sin6_scope_id = 0}
>>
>>
>> Any suggestions?
>>
>> --
>>      Alan Robertson<alanr at unix.sh>  - @OSSAlanR
>>
>> "Openness is the foundation and preservative of friendship...  Let me claim
>> from you at all times your undisguised opinions." - William Wilberforce
>>
>>
>> _______________________________________________
>> Web Page:  http://lug.boulder.co.us
>> Mailing List: http://lists.lug.boulder.co.us/mailman/listinfo/lug
>> Join us on IRC: irc.hackingsociety.org port=6667 channel=#hackingsociety
> _______________________________________________
> Web Page:  http://lug.boulder.co.us
> Mailing List: http://lists.lug.boulder.co.us/mailman/listinfo/lug
> Join us on IRC: irc.hackingsociety.org port=6667 channel=#hackingsociety
>


-- 
     Alan Robertson<alanr at unix.sh>  - @OSSAlanR

"Openness is the foundation and preservative of friendship...  Let me claim from you at all times your undisguised opinions." - William Wilberforce