howtos: expand using the nixos installer and fixups for using own kexec image

Author: Mic92

docs/howtos.md

@@ -8,41 +8,99 @@
 
 [Using nixos-anywhere without flakes](#using-nixos-anywhere-without-flakes)
 
-TODO: Add more topics
-
 ## Installing on a machine with no operating system
 
-TODO: Still to be documented
+If your machine does not run any operating system, you can boot the standard
+NixOS installer using a USB or netboot. Detailed instructions on how to do this
+can be found in the
+[NixOS installation guide](https://nixos.org/manual/nixos/stable/index.html#sec-booting-from-usb).
+
+When you're using `nixos-anywhere`, it will detect a NixOS installer by checking
+if the `/etc/os-release` file contains the identifier `VARIANT=installer`
+(available since NixOS 23.05). If an installer is detected, `nixos-anywhere`
+will not attempt to kexec into its own image. This is particularly useful for
+targets that do not have enough RAM for kexec or do not support kexec.
+
+NixOS starts an SSH server on the installer by default, but you need to set a
+password in order to access it. To set a password for the `nixos` user, run the
+following command in a terminal on the NixOS machine:
+
+```
+passwd
+```
+
+If you don't know the IP address of the installer on your network, you can find
+it by running the following command:
+
+```
+$ ip addr
+1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
+    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
+    inet 127.0.0.1/8 scope host lo
+       valid_lft forever preferred_lft forever
+    inet6 ::1/128 scope host
+       valid_lft forever preferred_lft forever
+2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000
+    link/ether 52:54:00:12:34:56 brd ff:ff:ff:ff:ff:ff
+    altname enp0s3
+    altname ens3
+    inet 10.0.2.15/24 brd 10.0.2.255 scope global dynamic noprefixroute eth0
+       valid_lft 86385sec preferred_lft 75585sec
+    inet6 fec0::5054:ff:fe12:3456/64 scope site dynamic mngtmpaddr noprefixroute
+       valid_lft 86385sec preferred_lft 14385sec
+    inet6 fe80::5054:ff:fe12:3456/64 scope link
+       valid_lft forever preferred_lft forever
+```
+
+This will display the IP addresses assigned to your network interface(s),
+including the IP address of the installer. In the example output below, the
+installer's IP address is `10.0.2.15`, `fec0::5054:ff:fe12:3456`, and
+`fe80::5054:ff:fe12:3456%eth0`:
+
+To test if you can connect and your password works, you can use the following
+SSH command (replace the IP address with your own):
+
+```
+ssh -v nixos@fec0::5054:ff:fe12:3456
+```
 
-Include:
+You can then use the IP address and other parameters in nixos-anywhere, like
+this:
 
-`nixos-anywhere` can detect a nixos installer if it contains the
-identifier `VARIANT=installer` in its `/etc/os-release` file. This is the case
-for the nixos-unstable installer and will be also part of nixos 23.05. If an
-installer is detected `nixos-anywhere` will not try to kexec into its own image.
+```
+nixos-anywhere --flake '.#' nixos@fec0::5054:ff:fe12:3456
+```
 
 ## Using your own kexec image
 
-By default `nixos-anywhere` will download the kexec image
-from [here](https://github.com/nix-community/nixos-images#kexec-tarballs). It is
-also possible to provide your own by using the command line switch `--kexec` to
-specify the image file. The image will then be uploaded prior to executing.
+By default, nixos-anywhere downloads the kexec image from the off
+[NixOS images repository](https://github.com/nix-community/nixos-images#kexec-tarballs).
+However, you can provide your own kexec image file if desired. To do this, use
+the `--kexec` command line switch followed by the path to your image file. The
+image will be uploaded prior to execution.
+
+Here's an example command that demonstrates how to use a custom kexec image with
+`nixos-anywhere` for aarch64 instead of the default `x86_64` architecture:
 
 ```
 nixos-anywhere \
-  --kexec "$(nix build --print-out-paths github:nix-community/nixos-images#packages.x86_64-linux.kexec-installer-noninteractive-nixos-unstable)/nixos-kexec-installer-noninteractive-x86_64-linux.tar.gz" \
+  --kexec "$(nix build --print-out-paths github:nix-community/nixos-images#packages.aarch64-linux.kexec-installer-noninteractive-nixos-unstable)/nixos-kexec-installer-noninteractive-aarch64-linux.tar.gz" \
   --flake 'github:your-user/your-repo#your-system' \
   root@yourip
 ```
 
-This is particularly useful for distributions like aarch64-linux, where there is
-no pre-build image. The following example assumes that your local machine can
-build for aarch64-linux either natively or through a remote builder
+Make sure to replace `github:your-user/your-repo#your-system` with the
+appropriate Flake URL representing your NixOS configuration.
 
-## Using nixos-anywhere without flakes
+This is particularly useful for architectures other than `x86_64`, where there
+is no pre-build image.
 
-TODO: Add content
-
-```
+The example above assumes that your local machine can build for aarch64 either
+natively, through a remote builder or by emulating the architecture with qemu
+with the following NixOS configuration:
 
+```nix
+{
+  boot.binfmt.emulatedSystems = [ "aarch64-linux" ];
+}
 ```