Re: [PATCH wireguard] wireguard: prevent ipv6 addrconf via IFF_NO_ADDRCONF flag

["Jason A. Donenfeld" <Jason@zx2c4.com>]

On Sun, May 03, 2026 at 09:18:18PM +0200, Jason A. Donenfeld wrote:
> On Sat, Mar 21, 2026 at 08:20:53PM +0100, Valentin Spreckels wrote:
> > Hi Jason,
> > 
> > On 11/03/2026 23:59, Jason A. Donenfeld wrote:
> > > Hi Valentin,
> > > 
> > > On Sun, Feb 08, 2026 at 06:05:45PM +0100, Valentin Spreckels wrote:
> > >> Use the flag introduced in commit 8a321cf7becc6 ("net: add
> > >> IFF_NO_ADDRCONF and use it in bonding to prevent ipv6 addrconf")
> > >> instead of mangling the addr_gen_mode to prevent ipv6 addrconf.
> > > 
> > > Can you give some more context here? Why was IFF_NO_ADDRCONF added when
> > > the IN6_ADDR_GEN_MODE_NONE method has been working fine? What's the
> > > difference between these approaches? I don't doubt that your patch is
> > > correct, but I would like to better understand this.
> > 
> > Only wireguard configures addr_gen_mode inside the kernel, otherwise it 
> > is only set by userspace; userspace is also able to overwrite the 
> > IFF_NO_ADDRCONF set by wireguard.
> > 
> > Commit 8a321cf7becc ("net: add IFF_NO_ADDRCONF and use it in bonding to 
> > prevent ipv6 addrconf") introduces the private interface flag 
> > IFF_NO_ADDRCONF, which isn't accessible by userspace.
> > 
> > Thus use the IFF_NO_ADDRCONF flag in wireguard.
> > 
> > 
> > Does that answer your questions? If yes, I will submit a v2 with this as 
> > commit message.
> 
> I applied this here:
> https://git.zx2c4.com/wireguard-linux/commit/?id=88427bcbe5bd3711de387b1c1f6540ef6fc05a78
> 
> Sorry for the delay! Patch looks good as-is, once I looked into the
> internal mechanism.

I'm backing this patch out for now. It seems to break the selftests:

    [+] NS2: ping6 -c 10 -f -W 1 fd00::1
    ping6: connect: Network unreachable

Try it yourself with:

    $ make -C tools/testing/selftests/wireguard/qemu -j$(nproc) 

I assume it's because of:

        case NETDEV_UP:
        case NETDEV_CHANGE:
                if (idev && idev->cnf.disable_ipv6)
                        break;

                if (dev->priv_flags & IFF_NO_ADDRCONF) {
			[...]
                        break;
                }

Feel free to submit a v2 if you think this is fixable or if the tests
themselves are wrong.

Jason