Hi all,
I am continuing debugging the ipv6 guestfwd feature, and I would like to
understand the behavior of slirp better.
Progress I've made:
Let QEMU take parameter like following:
guestfwd=tcp:[fec0::105]:54322-tcp:[::1]:6655
For slirp side, I basically searched for the appearance of gfwd_list and
made all code traverse the fwd list compatible with ipv6.
With these change, now I can see the packets coming out of the guest OS to
the assigned guest server port via tcpdump:
```
00:38:18.831831 IP6 fdb5:481:10ce:0:8c41:aaff:fea9:f674.52190 >
fec0::105.54322: tcp 0
0x0000: 600a 1f94 0028 0640 fdb5 0481 10ce 0000 `....(.@........
0x0010: 8c41 aaff fea9 f674 fec0 0000 0000 0000 .A.....t........
0x0020: 0000 0000 0000 0105 cbde d432 df6d 8332 ...........2.m.2
0x0030: 0000 0000 a0*02* fd20 535f 0000 0204 05a0 ........S_......
0x0040: 0402 080a b87b fd3b 0000 0000 0103 0307 .....{.;........
```
02 == SYN so it looks good. But both tcpdump and wireshark (looking into
packet dump provided by QEMU invocation) didn't see any response and this
packet never reached the host.
I added multiple prints inside slirp and confirmed the ipv6 version of [1]
was reached.
in tcp_output function [2], I got following print:
qemu-system-aarch64: info: Slirp: AF_INET6 out dst ip =
fdb5:481:10ce:0:8c41:aaff:fea9:f674, port = 52190
qemu-system-aarch64: info: Slirp: AF_INET6 out src ip = fec0::105, port =
54322
It looks like there should be something being sent back to the guest,
unless my understanding of tcp_output is wrong.
To understand the datapath of guestfwd better, I have the following
questions:
1. What's the meaning of tcp_input and tcp_output? My guess is the
following graph, but I would like to confirm.
tcp_input tcp_output
QEMU --------> slirp ----------> host
<-------- <----------
tcp_output tcp_input
2. I don't see port 6655 in the above process. How does slirp know 6655 is
the port that needs to be visited on the host side?
Thanks in advance, Felix
[1].
https://gitlab.freedesktop.org/slirp/libslirp/-/blob/master/src/tcp_input.c#L630
[2].
https://gitlab.freedesktop.org/slirp/libslirp/-/blob/master/src/tcp_output.c#L477
On Mon, Jun 26, 2023 at 3:08 AM Samuel Thibault <samuel.thiba...@gnu.org>
wrote:
> Hello,
>
> Felix Wu <f...@google.com> wrote:
> > 2. I want to understand what ip I should use. Currently I have following
> > formats for the QEMU invocation in ipv6:
> > ```
> > guestfwd=tcp:[::1]:1234-tcp:[my:host:ip:from:ifconfig]:22
> > ```
> > I know the general form is `guestfwd=tcp:server:port-dev`, where
> > server:port is for guest,
>
> Yes, the address to be used within the guest network. So it needs to be
> within the guest network.
>
> > Is the aforementioned invocation correct?
>
> No, because ::1 isn't in the guest network.
>
> > Or in this case [::1] is the local host address and I should put qemu
> > address for it instead?
>
> You can use whatever IP you want, as long as it's in the guest network.
> e.g. [fec0::1234] if you're with the default fec0::/64 network.
>
> > 3. Is there a default ipv6 address for QEMU instance? I think I need it
> in
> > the invocation.
>
> By default it's address 2 within the prefix, i.e. fec0::2 with the
> default fec0::/64 network.
>
> Samuel
>
