avocado.core.remoter option to reject remote unknown hosts

When Avocado runs tests on remote machines it always accepts the
host SSH key fingerprint. This could allow credentials to be stolen
if the remote host name or address is spoofed.

This patch adds to Avocado the configuration option to enable/disable
the reject_unknown_hosts option, so user can choose between safe or
flexible.

Notice there is an issue in Paramiko where it does not recognizes
ecdsa keys in known_hosts file. So for reject_unknown_hosts to work,
we currently have to accept the remote host key fingerprint in RSA format.
To do so, the SSH command shouled be like below:

  $ ssh -oHostKeyAlgorithms='ssh-rsa' <host>

Reference: https://trello.com/c/oqyAvZBqSigned-off-by: NAmador Pahim <apahim@redhat.com>

avocado/core/remoter.py

@@ -20,6 +20,7 @@ import getpass
 import logging
 import time
 
+from .settings import settings
 from ..utils import process
 
 LOG = logging.getLogger('avocado.test')
@@ -70,6 +71,14 @@ class Remote(object):
         self.password = password
         self.port = port
         self.quiet = quiet
+        reject_unknown_hosts = settings.get_value('remoter.behavior',
+                                                  'reject_unknown_hosts',
+                                                  key_type=bool,
+                                                  default=True)
+        disable_known_hosts = settings.get_value('remoter.behavior',
+                                                 'disable_known_hosts',
+                                                 key_type=bool,
+                                                 default=False)
         self._setup_environment(host_string=hostname,
                                 user=username,
                                 password=password,
@@ -78,7 +87,9 @@ class Remote(object):
                                 connection_attempts=attempts,
                                 linewise=True,
                                 abort_on_prompts=True,
-                                abort_exception=ConnectionError)
+                                abort_exception=ConnectionError,
+                                reject_unknown_hosts=reject_unknown_hosts,
+                                disable_known_hosts=disable_known_hosts)
 
     @staticmethod
     def _setup_environment(**kwargs):

etc/avocado/avocado.conf

@@ -38,6 +38,19 @@ utf8 =
 # Keep job temporary files after jobs (useful for avocado debugging)
 keep_tmp_files = False
 
+[remoter.behavior]
+# __Insecure__, reject unknown SSH host keys.
+# 'False' will leave you wide open to man-in-the-middle attacks!
+# 'True' will only work with RSA keys (due to a bug in Paramiko).
+# If using 'True', accept the remote host key fingerprint by using:
+#   $ ssh -oHostKeyAlgorithms='ssh-rsa' <host>
+reject_unknown_hosts = False
+# __Insecure__, SSH layer to skip loading the user’s known-hosts
+# file. Useful for avoiding exceptions in situations where a
+# 'known host' changing its host key is actually valid (e.g. cloud
+# servers such as EC2.)
+disable_known_hosts = False
+
 [job.output]
 # Base log level for --show-job-log.
 # Allowed levels: debug, info, warning, error, critical

selftests/functional/test_thirdparty_bugs.py

+import json
+import sys
+from avocado.utils import download
+
+if sys.version_info[:2] == (2, 6):
+    import unittest2 as unittest
+else:
+    import unittest
+
+
+class TestThirdPartyBugs(unittest.TestCase):
+    """
+    Class created to verify third-party known issues
+    """
+
+    def test_paramiko_ecsda_bug(self):
+        # https://github.com/paramiko/paramiko/issues/243
+        # Problems with using ECDSA known_hosts keys when negotiation also
+        # accepts RSA or DSS keys
+        try:
+            issue_url = 'https://api.github.com/repos/paramiko/paramiko/issues/243'
+            issue = json.load(download.url_open(issue_url))
+            self.assertEqual(issue['state'], 'open', 'The issue %s is not open '
+                             'anymore. Please double check and, if already fixed, '
+                             'change the avocado.conf option '
+                             '"reject_unknown_hosts" defaults to True.' %
+                             'https://github.com/paramiko/paramiko/issues/243')
+        except download.urllib2.URLError as details:
+            raise unittest.SkipTest(details)
+
+if __name__ == '__main__':
+    unittest.main()