Re: [PATCH v1] net: add per-connection guestfwd unix support

[Samuel Thibault <samuel.thibault@gnu.org>]

Hello,

Daniel P. Berrangé, le jeu. 12 nov. 2026 15:21:20 +0000, a ecrit:
> On Fri, Nov 06, 2026 at 11:42:25PM -0500, Gavin Li wrote:
> > For -netdev user,guestfwd=..., add support for unix-per-connection: in
> > addition to the currently supported tcp: and cmd:. With
> > unix-per-connection:, a new connection is opened to the given unix
> > socket.  This is an improvement over the current unix: support, which
> > does not support 1:1 guest:host connections.
> 
> I'm not familiar with guestfwd, but from the manpage, I would
> rather assume that it always opens a new connection to the host
> socket for each guest connection.

No, qemu opens the unix socket at initialization and just forwards data
to it.

> What are its current semantics when two guest connections are
> initiated on the same port, if that's not happening ?

It's just mixing the data coming from the connections into the unix
connection, and sending all data from the unix connection to one of the
tcp connection.

> > diff --git a/net/slirp.c b/net/slirp.c
> > index 451c0fd1ba..b0b03b2bf5 100644
> > --- a/net/slirp.c
> > +++ b/net/slirp.c
> > @@ -1159,6 +1159,12 @@ static int slirp_guestfwd(SlirpState *s, const char *config_str, Error **errp)
> >                         "forwarding rule '%s'", config_str);
> >              return -1;
> >          }
> > +    } else if (g_str_has_prefix(p, "unix-per-connection:")) {
> 
> This is rather a mouthful as a parameter name,

It seemed a bit long to me too.

> and including '-' in the name is undesirable, since we're already
> using '-' as a general separator.

That's indeed a no-go.

> Could it be just 'unix' to match the use of 'unix' in hostfwd ?

That would prevent from being able to get the one-unix-connection
behavior.

Samuel

> > +        if (slirp_add_unix(s->slirp, &p[20], &server, port) < 0) {
> > +            error_setg(errp, "Conflicting/invalid host:port in guest "
> > +                       "forwarding rule '%s'", config_str);
> > +            return -1;
> > +        }
> >      } else {
> >          Error *err = NULL;
> >          /*
> > diff --git a/qemu-options.hx b/qemu-options.hx
> > index 5fe9d73eaa..062cd39aa3 100644
> > --- a/qemu-options.hx
> > +++ b/qemu-options.hx
> > @@ -3553,6 +3553,15 @@ SRST
> >              # and connect the TCP stream to its stdin/stdout
> >              |qemu_system| -nic  'user,id=n1,guestfwd=tcp:10.0.2.100:1234-cmd:netcat 10.10.1.1 4321'
> >  
> > +    ``guestfwd=[tcp]:server:port-unix-per-connection:path``
> > +        With ``unix-per-connection:``, a new connection is opened to the
> > +        given unix socket per TCP connection made by the guest. The unix socket
> > +        need not exist at startup time.
> 
> I think the existing docs probably benefit from an update to
> better show how the new parmeter is different from what I
> would think was already the semantics.
> 
> > +
> > +        .. parsed-literal::
> > +
> > +            |qemu_system| -nic user,guestfwd=tcp:10.0.2.100:1234-unix-per-connection:/tmp/socket
> > +
> >  ``-netdev tap,id=id[,fd=h][,ifname=name][,script=file][,downscript=dfile][,br=bridge][,helper=helper]``
> >      Configure a host TAP network backend with ID id.
> >  
> > 
> > base-commit: 71f25c75417d31c214ba2cd26b273c114fc99987
> > -- 
> > 2.55.0
> >