title: What exactly does `ssh-keygen -A` do?
body: <pre><code>$ ssh-keygen --help
  ssh-keygen: unrecognized option: -
  usage: ssh-keygen [-q] [-b bits] [-t dsa | ecdsa | ed25519 | rsa]
                [-N new_passphrase] [-C comment] [-f output_keyfile]
     ssh-keygen -p [-P old_passphrase] [-N new_passphrase] [-f keyfile]
     ssh-keygen -i [-m key_format] [-f input_keyfile]
     ssh-keygen -e [-m key_format] [-f input_keyfile]
     ssh-keygen -y [-f input_keyfile]
     ssh-keygen -c [-P passphrase] [-C comment] [-f keyfile]
     ssh-keygen -l [-v] [-E fingerprint_hash] [-f input_keyfile]
     ssh-keygen -B [-f input_keyfile]
     ssh-keygen -D pkcs11
     ssh-keygen -F hostname [-f known_hosts_file] [-l]
     ssh-keygen -H [-f known_hosts_file]
     ssh-keygen -R hostname [-f known_hosts_file]
     ssh-keygen -r hostname [-f input_keyfile] [-g]
     ssh-keygen -G output_file [-v] [-b bits] [-M memory] [-S start_point]
     ssh-keygen -T output_file -f input_file [-v] [-a rounds] [-J num_lines]
                [-j start_line] [-K checkpt] [-W generator]
     ssh-keygen -s ca_key -I certificate_identity [-h] [-n principals]
                [-O option] [-V validity_interval] [-z serial_number] file ...
     ssh-keygen -L [-f input_keyfile]
     ssh-keygen -A
     ssh-keygen -k -f krl_file [-u] [-s ca_public] [-z version_number]
                file ...
     ssh-keygen -Q -f krl_file file ...
  </code></pre>
  
  <p>You may notice that <code>ssh-keygen -A</code> is conspicuously missing documentation. </p>
  
  <pre><code>$ ssh-keygen -A
  ssh-keygen: generating new host keys: RSA DSA ECDSA ED25519 
  </code></pre>
  
  <p>It appears to be generating (A)ll the key files, but I don't see any keys in <code>/root/.ssh/</code>. Just to confirm, I ran <code>ssh-keygen</code> with no options, entered through all the prompts, and I had keys as expected.</p>
  
  <p>So the question is, "What exactly is happening?"</p>