openssh: Enable DSA host/client keys

This applies a patch from Fedora to make HostKeyAlgorithms do the
right thing, fixing the issue described in
401782cb67.

nixos/modules/services/networking/ssh/sshd.nix

@@ -52,6 +52,8 @@ let
     ));
   in listToAttrs (map mkAuthKeyFile usersWithKeys);
 
+  supportOldHostKeys = !versionAtLeast config.system.stateVersion "15.07";
+
 in
 
 {
@@ -177,7 +179,7 @@ in
         default =
           [ { type = "rsa"; bits = 4096; path = "/etc/ssh/ssh_host_rsa_key"; }
             { type = "ed25519"; path = "/etc/ssh/ssh_host_ed25519_key"; }
-          ] ++ optionals (!versionAtLeast config.system.stateVersion "15.07")
+          ] ++ optionals supportOldHostKeys
           [ { type = "dsa"; path = "/etc/ssh/ssh_host_dsa_key"; }
             { type = "ecdsa"; bits = 521; path = "/etc/ssh/ssh_host_ecdsa_key"; }
           ];
@@ -347,6 +349,15 @@ in
         ${flip concatMapStrings cfg.hostKeys (k: ''
           HostKey ${k.path}
         '')}
+
+        # Allow DSA client keys for now. (These were deprecated
+        # in OpenSSH 7.0.)
+        PubkeyAcceptedKeyTypes +ssh-dss
+
+        # Re-enable DSA host keys for now.
+        ${optionalString supportOldHostKeys ''
+          HostKeyAlgorithms +ssh-dss
+        ''}
       '';
 
     assertions = [{ assertion = if cfg.forwardX11 then cfgc.setXAuthLocation else true;