When netcfg is finished configuring the interface, it will call
kill_dhcp_client() (from netcfg_activate_dhcp()), which will send
SIGTERM and then SIGKILL to dhcp6c. When dhcp6c gets a SIGTERM, it will
send a DHCP6 Release message and remove the IP from the interface, which
means that the installer will have no (public) IP anymore, and
installation from network will fail.

This happens both in an IPv6-only and in a dual-stack configuration
(though in a dual-stack configuration there still will be an IPv4
address, which means the installation will probably succeed).

A possible workaround is to switch to another console with Alt+F3 after
the DHCP6 configuration was done but before netcfg finishes and kill the
dhcp6c process with -9 (to prevent it from releasing the IP).

Hi,
Just submitted a fix on Salsa: https://salsa.debian.org/installer-team/netcfg/-/merge_requests/18

Dear Maintainer,
I would like to add my own anecdotal experience with this bug and the trixie
net installer iso,
and also add the keywords that should cause this report to show up in web
search results (for those affected):
        The mirror does not support the specified release (trixie).

   * What led up to the situation?
        I agree with the original report that this situation is most prominent
when the system
        can have connectivity via IPv6 only (and DHCP6).

   * What exactly did you do (or not do) that was effective (or ineffective)?
        Effective: Applying the IPv6 manually to the system after the network
detection step.
        Effective: Running the installer with IPv4 connectivity.
        Ineffective: Trying different debian mirrors.
        Ineffective: Tampering with the APT sources (as web search results
often recommended).

   * What was the outcome of this action?
        The net installer reaches the step where it would be downloading
packages to set up the
        system, but then it fails with the message:
                The mirror does not support the specified release (trixie).

   * What outcome did you expect instead?
        The installer would show the progress of downloading and installing
system packages.

Note: This bug does not exist in Debian 12 (Bookworm)'s installer when testing
with the same exact hardware configuration.

Hi,

As noted in
<https://lists.debian.org/msgid-search/angHe8EQxyEqN3Yi@mail.bitfolk.com>
I just wanted to confirm that this problem still exists in the trixie
installer (also tested with the daily installer image).

On an IPv6-only network with no SLAAC, gateway provided by router
solicitation, d-i attempts DHCPv6 which succeeds and networking is
initially functional.

After the host name and domain name questions are answered, netcfg does
still issue a kill -TERM to the dhcp6c process. This causes dhcp6c to
send a DHCP RELEASE packet and de-configure the interface. d-i does not
request a lease again later, so it's not possible to continue the
install.

The environment is a bit unusual in that there's no SLAAC and no router
advertisements; the prefix is /127 (just this host and its gateway).
This is a valid configuration though and it does work fine from an
installed Debian using the "dhcp" method in /etc/network/interfaces.

The patch provided earlier in this bug, at
<https://salsa.debian.org/installer-team/netcfg/-/merge_requests/18>
does avoid the issue by doing kill -KILL.

A workaround is to switch to the installer shell after dhcp6c has been
killed and launch it again:

# dhcp6c -c /var/lib/netcfg/dhcp6c.conf eno1

Or, one can hack in the patch's -KILL behaviour in kill-all-dhcp before
network configuration happens:

# sed -i 's/-TERM/-KILL/' /usr/bin/kill-all-dhcp

Possibly placing a patched /usr/bin/kill-all-dhcp file there could be
done in a preseeded early_cmd if that's easier than deploying the patch.

Without this, DHCPv6 is not feasible; install on an IPv6-only network
can only be carried out when there is SLAAC or static networking
configuration.

Thanks,
Andy