NOTES.md: Document some sshd stuff

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+# Notes
+
+## SSH Custom Authentication
+
+This doesn't work yet because
+
+* `ForceCommand` appears to take precedence over `command=` supplied by
+  AuthorizedKeysCommand
+* `AuthorizedKeysCommand` appears to not run at all when logging in via an
+  empty password is available
+
+
+
+### User creation
+
+The `forge` user should have a restricted shell (?). The second field in its
+`/etc/passwd` and `/etc/shadow` entries shall be empty.
+
+### `/etc/ssh/sshd_config`
+
+```
+Match User forge
+	AuthorizedKeysFile none
+	AuthorizedKeysCommand /usr/libexec/lindenii/forge/ssh_authorized_keys_command %C %D %f %h %k %t %U %u
+	AuthorizedKeysCommandUser root
+	PasswordAuthentication yes
+	PermitEmptyPasswords yes
+	DisableForwarding yes
+	PermitTTY no
+	ForceCommand /usr/libexec/lindenii/forge/ssh_shell
+```
+
+### `/usr/libexec/lindenii/forge/ssh_authorized_keys_command`
+
+This file and its corresponding directory must be owned by root and must not be
+writable by group or other.
+
+```sh
+#!/bin/sh
+
+# Allows any key to log in
+
+# Prototype, so let's not care about race conditions and whatever in logging
+printf 'Endpoints: %s\n' "$1" > /var/log/lindenii/authkeys.last
+printf 'Routing domain: %s\n' "$2" >> /var/log/lindenii/authkeys.last
+printf 'Fingerprint: %s\n' "$3" >> /var/log/lindenii/authkeys.last
+printf 'Home: %s\n' "$4" >> /var/log/lindenii/authkeys.last
+printf 'Key/cert base64: %s\n' "$5" >> /var/log/lindenii/authkeys.last
+printf 'Key/cert type: %s\n' "$6" >> /var/log/lindenii/authkeys.last
+printf 'UID: %s\n' "$7" >> /var/log/lindenii/authkeys.last
+printf 'Username: %s\n' "$8" >> /var/log/lindenii/authkeys.last
+
+[ "$8" != "forge" ] && exit
+
+# BUG: validate that key/cert type and base64 are sanitized and will not lead to injections
+printf 'command="/usr/libexec/lindenii/forge/ssh_shell --ssh-key-type %s --ssh-key-base64 %s" %s %s\n' "$6" "$5" "$6" "$5"
+```
+
+Now... uh, if I change the last line to `printf 'command="/usr/libexec/lindenii/forge/ssh_shell --ssh-key-type %s --ssh-key-base64 %s" %s %s\n' "$6" "$5" "$6" "$5"`, I can't log in any more and I get `debug1: /usr/libexec/lindenii/forge/ssh_authorized_keys_command %C %D %f %h %k %t %U %u:1: bad key options: unknown key option` in the debug log?
+
+### Alternatives to the mess above
+
+`ExposeAuthInfo`?