Bug#916457: grub2-common: Grub refuses to re-enable IPv6 traffic

[Alan Reding]

Hello Colin

Thanks for your response.

Following your reply, I did a test which consisted of the following steps:

1. I ran sudo gedit /etc/default/grub, deleted/removed GRUB_CMDLINE_LINUX_DEFAULT="ipv6.disable=1 quiet", ran sudo update-grub and rebooted my machine.

2. Immediately after the reboot, I issued the command nano /proc/cmdline and below is the result:

BOOT_IMAGE=/boot/vmlinuz-4.18.0-0.bpo.1-amd64 root=UUID=[string of alphanumeric characters] ro ipv6.disable=1 quiet

3. Next, I ran the command sudo grep -RF ipv6.disable /etc and below is the result:

/etc/default/grub.ucf-old:#GRUB_CMDLINE_LINUX_DEFAULT="ipv6.disable=1 quiet"
/etc/default/grub:GRUB_CMDLINE_LINUX_DEFAULT="ipv6.disable=1 quiet"

I am puzzled why ipv6.disable=1 quiet appears in the above results of (2) and (3) despite the fact that I had deleted/removed GRUB_CMDLINE_LINUX_DEFAULT="ipv6.disable=1 quiet" from /etc/default/grub.

Regards.

Alan Reding


--------------------------------------------
On Fri, 12/14/18, Colin Watson <cjwatson at debian.org> wrote:

 Subject: Re: Bug#916457: grub2-common: Grub refuses to re-enable IPv6 traffic
 To: "Alan Reding" <gajUph4pre at yahoo.com>, 916457 at bugs.debian.org
 Date: Friday, December 14, 2018, 10:56 PM
 
 On Fri, Dec 14, 2018 at 05:23:35PM +0000, Alan Reding
 wrote:
 > To block IPv6 traffic, I do the following:
 > 
 > 1. Add the following line to /etc/default/grub
 > 
 > GRUB_CMDLINE_LINUX_DEFAULT="ipv6.disable=1 quiet"
 > 
 > 2. Run update-grub
 > 
 > 3. In /etc/hosts, add # in front of all lines that
 mention IPv6 hosts
 > 
 > 3. Reboot machine
 > 
 > To re-enable IPv6 traffic, I do the following:
 [...]
 
 GRUB just passes the stuff in GRUB_CMDLINE_LINUX_DEFAULT
 straight
 through to the kernel; it has no other involvement in how
 the operating
 system's networking stack is set up.  This could only
 possibly be a GRUB
 bug if you're finding that update-grub isn't correctly
 transferring
 GRUB_CMDLINE_LINUX_DEFAULT into /boot/grub/grub.cfg, or if
 the kernel
 parameters are somehow not actually being passed to the
 kernel (which
 you can check after boot by looking in /proc/cmdline). 
 Otherwise, this
 cannot possibly be a GRUB bug, and I'd appreciate it if you
 could either
 close it or figure out some other suitable package to
 reassign it to, as
 appropriate.
 
 > Method A
 > 
 > 4. Add # in front of the line that contains
 > GRUB_CMDLINE_LINUX_DEFAULT="ipv6.disable=1 quiet" as in
 below:
 > 
 > #GRUB_CMDLINE_LINUX_DEFAULT="ipv6.disable=1 quiet"
 > 
 > 5. Run update-grub
 > 
 > 6. In /etc/hosts, remove # from all lines that mention
 IPv6 hosts
 > 
 > 7. Reboot machine
 > 
 > Result: IPv6 traffic is still blocked
 > 
 > Method B
 > 
 > 8. In /etc/default/grub, I delete the line
 > GRUB_CMDLINE_LINUX_DEFAULT="ipv6.disable=1 quiet"
 > 
 > 9. Run update-grub
 > 
 > 10. In /etc/hosts, I ensure that # does not appear in
 front of lines that
 > mention IPv6 hosts
 > 
 > 11. Reboot machine
 > 
 > Result: IPv6 traffic is still blocked
 > 
 > Method C
 > 
 > 12. In /etc/default/grub, I change the value of
 ipv6.disable=0 as in:
 > 
 > GRUB_CMDLINE_LINUX_DEFAULT="ipv6.disable=0 quiet"
 > 
 > 13. Run update-grub
 > 
 > 14. In /etc/hosts, I ensure that # does not appear in
 front of lines that
 > mention IPv6 hosts
 > 
 > 15. Reboot machine
 > 
 > Result: IPv6 traffic is NOT blocked. IPv6 traffic is
 re-enabled
 
 These symptoms indicate to me that the equivalent of
 ipv6.disable=1 is
 being set somewhere else *as well*, which causes things to
 only work
 properly if you explicitly pass the kernel argument
 ipv6.disable=0.  I'd
 suggest looking around in /etc/, perhaps /etc/modprobe.d/.
 
 -- 
 Colin Watson             
                
          [cjwatson at debian.org]
 
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