[Libguestfs] [PATCH nbdkit v2] ssh: Improve the error message when all authentication methods fail

[Richard W.M. Jones]

The current error message:

  nbdkit: ssh[1]: error: all possible authentication methods failed

is confusing and non-actionable.  It's hard even for experts to
understand the relationship between the authentication methods offered
by a server and what we require.

Try to improve the error message in some common situations, especially
where password authentication on the server side is disabled but the
client supplied a password=... parameter.  After this change, you will
see an actionable error:

  nbdkit: ssh[1]: error: the server does not offer password
  authentication but you tried to use a password; if you have root
  access to the server, try editing 'sshd_config' and setting
  'PasswordAuthentication yes'; otherwise try setting up public key
  authentication

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=2158300
Thanks: Laszlo Ersek
---
 plugins/ssh/ssh.c | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/plugins/ssh/ssh.c b/plugins/ssh/ssh.c
index aaa7c2b9f..5a132d8f2 100644
--- a/plugins/ssh/ssh.c
+++ b/plugins/ssh/ssh.c
@@ -361,6 +361,28 @@ authenticate (struct ssh_handle *h)
     if (rc == SSH_AUTH_SUCCESS) return 0;
   }
 
+  /* All compatible methods were tried and none worked.  Come up with
+   * an actionable diagnostic message if we recognise the problem.
+   */
+  if (!(method & SSH_AUTH_METHOD_PUBLICKEY) && password == NULL) {
+    nbdkit_error ("the server does not offer public key authentication; "
+                  "try using the password=... parameter");
+    return -1;
+  }
+  if ((method & SSH_AUTH_METHOD_PASSWORD) && password != NULL) {
+    nbdkit_error ("password authentication failed, "
+                  "is the username and password correct?");
+    return -1;
+  }
+  if (!(method & SSH_AUTH_METHOD_PASSWORD) && password != NULL) {
+    nbdkit_error ("the server does not offer password authentication "
+                  "but you tried to use a password; if you have root access "
+                  "to the server, try editing 'sshd_config' and setting "
+                  "'PasswordAuthentication yes'; otherwise try setting up "
+                  "public key authentication");
+    return -1;
+  }
+
   nbdkit_error ("all possible authentication methods failed");
   return -1;
 }
-- 
2.37.3