lib/devos: bake bootstaping link-local network into live iso

Replace default networking with a specially configured systemd-networkd
network which is responsible for setting up DHCP and also provide a well
known ipv6 link-local address as well as enable multicastDNS to expose
the hostname on the local link over the reserved `local.` top level domain.

doc/start/iso.md

@@ -10,11 +10,68 @@ dd bs=4M if=result/iso/*.iso of=/dev/$your_installation_device \
 
 This works for any file matching `hosts/*.nix` excluding `default.nix`.
 
+## Remote access to the live installer
+
+The iso live installer comes preconfigured with a network configuration
+which announces it's hostname via [MulticastDNS][mDNS] as `hostname.local`, 
+that is `NixOS.local` in the above example.
+
+In the rare case that [MulticastDNS][mDNS] is not availabe or turned off
+in your network, there is a static link-local IPv6 address configured to
+`fe80::47`(mnemonic from the letter's position in the english alphabet:
+`n=14 i=9 x=24; 47 = n+i+x`).
+
+Provided that you have added your public key to the authorized keys of the
+`nixos` user:
+
+```nix
+{ ... }:
+{
+  users.users.nixos.openssh.authorizedKeys.keyFiles = [
+    ../secrets/path/to/key.pub
+  ];
+}
+```
+
+You can then ssh into the live installer through one of the 
+following options:
+
+```console
+ssh [email protected]
+
+ssh nixos@fe80::47%eno1  # where eno1 is your network interface on which you are linked to the target
+```
+
+_Note: the [static link-local IPv6 address][staticLLA] and [MulticastDNS][mDNS] is only
+configured on the live installer. If you wish to enable [MulticastDNS][mDNS]
+for your environment, you ought to configure that in a regular [profile](../../profiles)._
+
+## EUI-64 LLA & Host Identity
+
+The iso's IPv6 Link Local Address (LLA) is configured with a static 64-bit Extended
+Unique Identifiers (EUI-64) that is derived from the host interface's Message
+Authentication Code (MAC) address.
+
+After a little while (a few seconds), you can remotely disvover this unique and host
+specific address over [NDP][NDP] for example with:
+
+```console
+ip -6 neigh show # also shows fe80::47
+```
+
+***This LLA is stable for the host, unless you need to swap that particular network card.*** 
+Under this reservation, though, you may use this EUI-64 to wire up a specific
+(cryptographic) host identity.
+
 ## Bootstrap Target Machine
 
+_Note: nothing prevents you from remotely exceuting the boostrapping process._
+
 Once your target host has booted into the live iso, you need to partion
 and format your disk according to the [official manual][manual].
 
+### Mount partitions
+
 Then properly mount the formatted partitions at `/mnt`, so that you can
 install your system to those new partitions.
 
@@ -27,6 +84,8 @@ $ mkdir -p /mnt/boot && mount /dev/disk/by-label/boot /mnt/boot # UEFI only
 $ swapon /dev/$your_swap_partition
 ```
 
+### Install
+
 Install using the `flk` wrapper baked into the iso off of a copy of devos 
 from the time the iso was built:
 
@@ -36,9 +95,14 @@ $ nix develop
 $ flk install NixOS --impure # use same host as above
 ```
 
-_Note: You _could_ install another machine than the one your iso was built for,
-but the iso doesn't necesarily already carry all the necesary build artifacts._
-
 <!-- TODO: find out why --impure is necesary / PRs welcome! -->
 
+_Note: You _could_ install another machine than the one your iso was built for,
+but the iso doesn't carry all the necesary build artifacts so the target would
+start to build the missing parts on demand instead of substituting them from
+the iso itself._
+
 [manual]: https://nixos.org/manual/nixos/stable/index.html#sec-installation-partitioning
+[mDNS]: https://en.wikipedia.org/wiki/Multicast_DNS
+[NDP]: https://en.wikipedia.org/wiki/Neighbor_Discovery_Protocol
+[staticLLA]: https://tools.ietf.org/html/rfc7404

lib/devos/devosSystem.nix

@@ -22,6 +22,34 @@ lib.nixosSystem (args // {
               networking.networkmanager.enable = lib.mkForce false;
               # confilcts with networking.wireless
               networking.wireless.iwd.enable = lib.mkForce false;
+              # Set up a link-local boostrap network
+              # See also: https://github.com/NixOS/nixpkgs/issues/75515#issuecomment-571661659
+              networking.usePredictableInterfaceNames = lib.mkForce true; # so prefix matching works
+              networking.useNetworkd = lib.mkForce true;
+              networking.useDHCP = lib.mkForce false;
+              networking.dhcpcd.enable = lib.mkForce false;
+              systemd.network = {
+                # https://www.freedesktop.org/software/systemd/man/systemd.network.html
+                networks."boostrap-link-local" = {
+                  matchConfig = {
+                    Name = "en* wl* ww*";
+                  };
+                  networkConfig = {
+                    Description = "Link-local host bootstrap network";
+                    MulticastDNS = true;
+                    LinkLocalAddressing = "ipv6";
+                    DHCP = "yes";
+                  };
+                  address = [
+                    # fall back well-known link-local for situations where MulticastDNS is not available
+                    "fe80::47" # 47: n=14 i=9 x=24; n+i+x
+                  ];
+                  extraConfig = ''
+                    # Unique, yet stable. Based off the MAC address.
+                    IPv6LinkLocalAddressGenerationMode = "eui64"
+                  '';
+                };
+              };
             })
           ];
         })).config;