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

[Alan Robertson]

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

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