Date: Sun, 10 Jul 2022 22:45:32 -0500
From: "W. Michael Petullo" <mike@flyn.org>
To: Jakub Jirutka <jakub@jirutka.cz>
Cc: ~alpine/users@lists.alpinelinux.org
Subject: Re: IPv6 firewall blocking router advertisements
Message-ID: <Ysuc3KIlEuzwzOHa@imp.flyn.org>
References: <YstKuMf89j67CIfO@imp.flyn.org>
 <7a1d4e10-02c8-7e48-9e6f-31979f28ed69@jirutka.cz>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <7a1d4e10-02c8-7e48-9e6f-31979f28ed69@jirutka.cz>

>> I am trying to configure an Alpine host to use DHCPv6 to obtain an IPv6
>> address. I have performed this task before using OpenWrt and Fedora.
>> 
>> On Alpine, it appears that the IPv6 firewall (ip6tables) prevents router
>> advertisements from working. I use "dhclient -6 ..."

[...]

> do you have IPFORWARD disabled ("no") in /etc/conf.d/ip6tables? See https://strugglers.net/~andy/blog/2011/09/04/linux-ipv6-router-advertisements-and-forwarding/.

Very interesting!

After some time spent reading the source code for ifupdown-ng, I elected
to remove dhclient and install dhcpcd. The latter can interact with DHCP
and DHCPv6 in the same process, and thus ifupdown-ng suddenly does the
right thing for IPv6. Using dhclient had required a "post-up" kludge in
/etc/network/interfaces. Using dhcpcd seems to simplify things.

Back to Jakub's suggestion: I am still investigating, but the router
advertisements now work even though the host has forwarding on. I am
not sure why this is the case, but I suspect the dhcpcd arrangement is
better at getting things setup before the firewall activates.

I hope to experiment tomorrow to reach a stronger conclusion. Until then,
I would appreciate any input as to why dhcpcd seems to have fixed things
without turning off forwarding.

-- 
Mike

:wq