Re: Source address fib invalidation on IPv6

[David Ahern]

On 11/11/16 12:29 PM, Jason A. Donenfeld wrote:
> Hi folks,
> 
> If I'm replying to a UDP packet, I generally want to use a source
> address that's the same as the destination address of the packet to
> which I'm replying. For example:
> 
> Peer A sends packet: src = 10.0.0.1,  dst = 10.0.0.3
> Peer B replies with: src = 10.0.0.3, dst = 10.0.0.1
> 
> But let's complicate things. Let's say Peer B has multiple IPs on an
> interface: 10.0.0.2, 10.0.0.3. The default route uses 10.0.0.2. In
> this case what do you think should happen?
> 
> Case 1:
> Peer A sends packet: src = 10.0.0.1,  dst = 10.0.0.3
> Peer B replies with: src = 10.0.0.2, dst = 10.0.0.1
> 
> Case 2:
> Peer A sends packet: src = 10.0.0.1,  dst = 10.0.0.3
> Peer B replies with: src = 10.0.0.3, dst = 10.0.0.1
> 
> Intuition tells me the answer is "Case 2". If you agree, keep reading.
> If you disagree, stop reading here, and instead correct my poor
> intuition.
> 
> So, assuming "Case 2", when Peer B receives the first packet, he notes
> that packet's destination address, so that he can use it as a source
> address next. When replying, Peer B sets the stored source address and
> calls the routing function:
> 
>     struct flowi4 fl = {
>        .saddr = from_daddr_of_previous_packet,
>        .daddr = from_saddr_of_previous_packet,
>     };
>     rt = ip_route_output_flow(sock_net(sock), &fl, sock);
> 
> What if, however, by the time Peer B chooses to reply, his interface
> no longer has that source address? No problem, because
> ip_route_output_flow will return -EINVAL in that case. So, we can do
> this:
> 
>     struct flowi4 fl = {
>        .saddr = from_daddr_of_previous_packet,
>        .daddr = from_saddr_of_previous_packet,
>     };
>     rt = ip_route_output_flow(sock_net(sock), &fl, sock);
>     if (unlikely(IS_ERR(rt))) {
>         fl.saddr = 0;
>         rt = ip_route_output_flow(sock_net(sock), &fl, sock);
>     }
> 
> And then all is good in the neighborhood. This solution works. Done.
> 
> But what about IPv6? That's where we get into trouble:
> 
>     struct flowi6 fl = {
>        .saddr = from_daddr_of_previous_packet,
>        .daddr = from_saddr_of_previous_packet,
>     };
>     ret = ipv6_stub->ipv6_dst_lookup(sock_net(sock), sock, &dst, &fl);
> 
> In this case, IPv6 returns a valid dst, when no interface has the
> source address anymore! So, there's no way to know whether or not the
> source address for replying has gone stale. We don't have a means of
> falling back to inaddr_any for the source address.

What do you mean by 'valid dst'? ipv6 returns net->ipv6.ip6_null_entry on lookup failures so yes dst is non-NULL but that does not mean the lookup succeeded.

For example take a look at ip6_dst_lookup_tail():
        if (!*dst)
                *dst = ip6_route_output_flags(net, sk, fl6, flags);

        err = (*dst)->error;
        if (err)
                goto out_err_release;


perhaps I should add dst->error to the fib tracepoints ...

> 
> Primary question: is this behavior a bug? Or is this some consequence
> of a fundamental IPv6 difference with v4? Or is something else
> happening here?
> 
> Thanks,
> Jason
>