nixos/initrd-ssh: switch from Dropbear to OpenSSH

Dropbear lags behind OpenSSH significantly in both support for modern
key formats like `ssh-ed25519`, let alone the recently-introduced
U2F/FIDO2-based `sk-ssh-ed25519@openssh.com` (as I found when I switched
my `authorizedKeys` over to it and promptly locked myself out of my
server's initrd SSH, breaking reboots), as well as security features
like multiprocess isolation. Using the same SSH daemon for stage-1 and
the main system ensures key formats will always remain compatible, as
well as more conveniently allowing the sharing of configuration and
host keys.

The main reason to use Dropbear over OpenSSH would be initrd space
concerns, but NixOS initrds are already large (17 MiB currently on my
server), and the size difference between the two isn't huge (the test's
initrd goes from 9.7 MiB to 12 MiB with this change). If the size is
still a problem, then it would be easy to shrink sshd down to a few
hundred kilobytes by using an initrd-specific build that uses musl and
disables things like Kerberos support.

This passes the test and works on my server, but more rigorous testing
and review from people who use initrd SSH would be appreciated!

10 files changed, 35 insertions, 64 deletions

diff --git a/nixos/tests/initrd-network-ssh/default.nix b/nixos/tests/initrd-network-ssh/default.nix
index 73d9f938e226..017de6882081 100644
--- a/nixos/tests/initrd-network-ssh/default.nix
+++ b/nixos/tests/initrd-network-ssh/default.nix
@@ -3,7 +3,7 @@ import ../make-test-python.nix ({ lib, ... }:
 {
   name = "initrd-network-ssh";
   meta = with lib.maintainers; {
-    maintainers = [ willibutz ];
+    maintainers = [ willibutz emily ];
   };
 
   nodes = with lib; {
@@ -17,9 +17,9 @@ import ../make-test-python.nix ({ lib, ... }:
           enable = true;
           ssh = {
             enable = true;
-            authorizedKeys = [ "${readFile ./openssh.pub}" ];
+            authorizedKeys = [ (readFile ./id_ed25519.pub) ];
             port = 22;
-            hostRSAKey = ./dropbear.priv;
+            hostKeys = [ ./ssh_host_ed25519_key ];
           };
         };
         boot.initrd.preLVMCommands = ''
@@ -42,11 +42,11 @@ import ../make-test-python.nix ({ lib, ... }:
               "${toString (head (splitString " " (
                 toString (elemAt (splitString "\n" config.networking.extraHosts) 2)
               )))} "
-              "${readFile ./dropbear.pub}"
+              "${readFile ./ssh_host_ed25519_key.pub}"
             ];
           };
           sshKey = {
-            source = ./openssh.priv; # dont use this anywhere else
+            source = ./id_ed25519;
             mode = "0600";
           };
         };
@@ -56,7 +56,17 @@ import ../make-test-python.nix ({ lib, ... }:
   testScript = ''
     start_all()
     client.wait_for_unit("network.target")
-    client.wait_until_succeeds("ping -c 1 server")
+
+
+    def ssh_is_up(_) -> bool:
+        status, _ = client.execute("nc -z server 22")
+        return status == 0
+
+
+    with client.nested("waiting for SSH server to come up"):
+        retry(ssh_is_up)
+
+
     client.succeed(
         "ssh -i /etc/sshKey -o UserKnownHostsFile=/etc/knownHosts server 'touch /fnord'"
     )
diff --git a/nixos/tests/initrd-network-ssh/dropbear.priv b/nixos/tests/initrd-network-ssh/dropbear.priv
deleted file mode 100644
index af340535f0a3..000000000000
--- a/nixos/tests/initrd-network-ssh/dropbear.priv
+++ /dev/null
Binary files differdiff --git a/nixos/tests/initrd-network-ssh/dropbear.pub b/nixos/tests/initrd-network-ssh/dropbear.pub
deleted file mode 100644
index 385c625522aa..000000000000
--- a/nixos/tests/initrd-network-ssh/dropbear.pub
+++ /dev/null
@@ -1 +0,0 @@
-ssh-rsa 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 tmtynkky@duuni
diff --git a/nixos/tests/initrd-network-ssh/generate-keys.nix b/nixos/tests/initrd-network-ssh/generate-keys.nix
index 0183e12d7a88..3d7978890ab0 100644
--- a/nixos/tests/initrd-network-ssh/generate-keys.nix
+++ b/nixos/tests/initrd-network-ssh/generate-keys.nix
@@ -1,12 +1,10 @@
 with import ../../.. {};
 
 runCommand "gen-keys" {
-    buildInputs = [ dropbear openssh ];
+    buildInputs = [ openssh ];
   }
   ''
     mkdir $out
-    dropbearkey -t rsa -f $out/dropbear.priv -s 4096 | sed -n 2p > $out/dropbear.pub
-    ssh-keygen -q -t rsa -b 4096 -N "" -f client
-    mv client $out/openssh.priv
-    mv client.pub $out/openssh.pub
+    ssh-keygen -q -t ed25519 -N "" -f $out/ssh_host_ed25519_key
+    ssh-keygen -q -t ed25519 -N "" -f $out/id_ed25519
   ''
diff --git a/nixos/tests/initrd-network-ssh/id_ed25519 b/nixos/tests/initrd-network-ssh/id_ed25519
new file mode 100644
index 000000000000..f914b3f712fc
--- /dev/null
+++ b/nixos/tests/initrd-network-ssh/id_ed25519
@@ -0,0 +1,7 @@
+-----BEGIN OPENSSH PRIVATE KEY-----
+b3BlbnNzaC1rZXktdjEAAAAABG5vbmUAAAAEbm9uZQAAAAAAAAABAAAAMwAAAAtzc2gtZW
+QyNTUxOQAAACAVcX+32Yqig25RxRA8bel/f604wV0p/63um+Oku/3vfwAAAJi/AJZMvwCW
+TAAAAAtzc2gtZWQyNTUxOQAAACAVcX+32Yqig25RxRA8bel/f604wV0p/63um+Oku/3vfw
+AAAEAPLjQusjrB90Lk3996G3AbtTeK+XweNgxaegYnml/A/RVxf7fZiqKDblHFEDxt6X9/
+rTjBXSn/re6b46S7/e9/AAAAEG5peGJsZEBsb2NhbGhvc3QBAgMEBQ==
+-----END OPENSSH PRIVATE KEY-----
diff --git a/nixos/tests/initrd-network-ssh/id_ed25519.pub b/nixos/tests/initrd-network-ssh/id_ed25519.pub
new file mode 100644
index 000000000000..40de4a8ac602
--- /dev/null
+++ b/nixos/tests/initrd-network-ssh/id_ed25519.pub
@@ -0,0 +1 @@
+ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIBVxf7fZiqKDblHFEDxt6X9/rTjBXSn/re6b46S7/e9/ nixbld@localhost
diff --git a/nixos/tests/initrd-network-ssh/openssh.priv b/nixos/tests/initrd-network-ssh/openssh.priv
deleted file mode 100644
index 816d65435fd7..000000000000
--- a/nixos/tests/initrd-network-ssh/openssh.priv
+++ /dev/null
@@ -1,51 +0,0 @@
------BEGIN RSA PRIVATE KEY-----
-MIIJKAIBAAKCAgEA7+9A2PCPOTAlFmrablrUWA+VZdAuLfM6JXeHsOF7ZbC2F6lv
-WmvDM925DQqhiAjcgWnt5WHWS5Y+b7lGnuzT7fyKegXd80nCRmqlpSG3srX0/lxR
-aQAJLzfoDjcsF+ceswQo6GSsYnCHVxMNs007gbbVY3f7o+sWZtLdxJPD2iHvl5Zr
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-IAJ8+uqaYfGAyn9q8fjZiRHxLmKDq90tKoCUL5r/2dmEIE+t8T/3PfHoq1QzZts9
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-s3KV+sWFN9KOnakTjj/6iQhXZRhgeAK39F2XTk5Ms5Y+BRSStnMoMZA2grIV+jHi
-1zbWokVqXPI5YRo5isR/PgtKAV6FfNWumcYoFJ9F40pMHQ6hJVEmtrCBx7EApSl3
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-jvVPEAh6YSu/ExglWGJIxTsRUIblxvTxdjEnl/p+rlM0RNJnA6vpo1J51BXA7CdF
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-1zLnaPAD44LZ/0Fjqj+PimQGT6K7CNXPllmYh7MvoU52g3SVPf6rHlIR0Nc=
------END RSA PRIVATE KEY-----
diff --git a/nixos/tests/initrd-network-ssh/openssh.pub b/nixos/tests/initrd-network-ssh/openssh.pub
deleted file mode 100644
index 5b72b8085f27..000000000000
--- a/nixos/tests/initrd-network-ssh/openssh.pub
+++ /dev/null
@@ -1 +0,0 @@
-ssh-rsa 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 tmtynkky@duuni
diff --git a/nixos/tests/initrd-network-ssh/ssh_host_ed25519_key b/nixos/tests/initrd-network-ssh/ssh_host_ed25519_key
new file mode 100644
index 000000000000..f1e29459b7a3
--- /dev/null
+++ b/nixos/tests/initrd-network-ssh/ssh_host_ed25519_key
@@ -0,0 +1,7 @@
+-----BEGIN OPENSSH PRIVATE KEY-----
+b3BlbnNzaC1rZXktdjEAAAAABG5vbmUAAAAEbm9uZQAAAAAAAAABAAAAMwAAAAtzc2gtZW
+QyNTUxOQAAACDP9Mz6qlxdQqA4omrgbOlVsxSGONCJstjW9zqquajlIAAAAJg0WGFGNFhh
+RgAAAAtzc2gtZWQyNTUxOQAAACDP9Mz6qlxdQqA4omrgbOlVsxSGONCJstjW9zqquajlIA
+AAAEA0Hjs7LfFPdTf3ThGx6GNKvX0ItgzgXs91Z3oGIaF6S8/0zPqqXF1CoDiiauBs6VWz
+FIY40Imy2Nb3Oqq5qOUgAAAAEG5peGJsZEBsb2NhbGhvc3QBAgMEBQ==
+-----END OPENSSH PRIVATE KEY-----
diff --git a/nixos/tests/initrd-network-ssh/ssh_host_ed25519_key.pub b/nixos/tests/initrd-network-ssh/ssh_host_ed25519_key.pub
new file mode 100644
index 000000000000..3aa1587e1dce
--- /dev/null
+++ b/nixos/tests/initrd-network-ssh/ssh_host_ed25519_key.pub
@@ -0,0 +1 @@
+ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIM/0zPqqXF1CoDiiauBs6VWzFIY40Imy2Nb3Oqq5qOUg nixbld@localhost