So, I found a workaround that wasn't as well documented as I had hoped. Describing it here in case some other hapless sysadmin ever has the same problem with debian installer not attempting dhcp over an interface other than eth0 and doesn't know what to do
If preseed.cfg exists in the root of the initrd, debian-installer will execute any commands listed there before it tries to download the preseed file listed in the pxeboot config. So, if that file happens to contain preseed commands to statically configure an IP address, guess what, it will do that. Of course, this means having an IP config pre-generated in the initrd, right? You're right back to step 1 with needing to know network configuration for the host. Nope If you execute `dhclient` in the init script, before it executes `busybox init`, it will broadcast across all configurable network interfaces. If it gets a valid response, it will get an IP address. I mentioned this earlier as my first attempt at bypassing the netcfg dhcp autoconfig that is the root of all these problems. netcfg SO VERY HELPFULLY clears all running IP configurations when it detects interfaces, whether they be configured via dhcp or statically assigned with `ip addr add ...`. What netcfg DOES NOT clear is the contents of `/var/lib/dhcp.dhclient.leases`, which contains an interface name, ip address, hostname, gateway, nameservers, domain name, etc, that the dhcp server responded with. >From this file, one can parse out information needed for preseed/netcfg static IP configuration; which can be written as answers in preseed.cfg. So, I made the following changes to the initrd: - add `dhclient -v` early in the init script - created a custom program to parse the contents of /var/lib/dhcp/dhclient.leases and generate preseed netcfg answers that get written to preseed.cfg in the initrd root. It can be written in whatever you want, I found it easiest to write it in golang and put the compiled binary in the initrd's /bin - added a line to init to execute this program immediately after the dhcp client finishes - removed /media in the initrd That last step may seem perplexing. One really helpful behavior I discovered is that debian-installer will mount /dev/sda1 to /media if it reads a preseed.cfg file stored in the root of the initrd. I assume it does this because why would you want a preseed file on disk, unless you're on a liveCD, which will probably have more data in /media that is essential. This wouldn't be a problem if the partitioner didn't default to using /dev/sda, because why would anyone anywhere want to install their OS into the first or only disk present in a system? So, if /dev/sda1 exists and has a filesystem, it gets mounted to /media, and no combination of di late commands or partman options to forcibly unmount stuff seemed to work. It would always prompt me to unmount the partition before proceeding. Deleting the /media mountpoint means that it can't mount /dev/sda1 there. In a netboot initrd, /media is empty anyway.
