Re: [PATCH 6/6] net: Warn when processes listen on AF_INET sockets

[David Woodhouse]

On Wed, 2026-04-01 at 02:11 -0700, Eric Dumazet wrote:
> On Wed, Apr 1, 2026 at 12:45 AM David Woodhouse <dwmw2@xxxxxxxxxxxxx> wrote:
> > 
> > From: David Woodhouse <dwmw@xxxxxxxxxxxx>
> > 
> > There is no need to listen on AF_INET sockets; a modern application can
> > listen on IPv6 (without IPV6_V6ONLY) and will accept connections from
> > the 20th century via IPv4-mapped addresses (::ffff:x.x.x.x) on the IPv6
> > socket.
> > 
> > Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
> > ---
> >  net/ipv4/af_inet.c | 3 +++
> >  1 file changed, 3 insertions(+)
> > 
> > diff --git a/net/ipv4/af_inet.c b/net/ipv4/af_inet.c
> > index dc358faa1647..3838782a8437 100644
> > --- a/net/ipv4/af_inet.c
> > +++ b/net/ipv4/af_inet.c
> > @@ -240,6 +240,9 @@ int inet_listen(struct socket *sock, int backlog)
> >         struct sock *sk = sock->sk;
> >         int err = -EINVAL;
> > 
> > +       pr_warn_once("process '%s' (pid %d) is listening on an AF_INET socket. Consider using AF_INET6 with IPV6_V6ONLY=0 instead.\n",
> > +                    current->comm, task_pid_nr(current));
> > +
> 
> Some kernels are built without CONFIG_IPV6, so this warning would be
> quite misleading.

Maybe on this date next year, we could make it not possible to build
the kernel *without* IPv6... ?

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