PySFTP failing with "No hostkey for host X found" when deploying Django/Heroku

Question

I'm trying to deploy a Django web application which uses pysftp to access to a SFTP server through some views.

The thing was perfectly working in local development, but when trying the first deployment on Heroku, the traceback below appeared ending with an error. It seems like I need to configure host keys and I believe I also need to set them in known_hosts at Heroku, but I have no idea about how to do that. In local development I was accessing with user/password without a problem, but from Heroku this error shows up:

remote: paramiko.ssh_exception.SSHException: No hostkey for host somehost.myftp.org found

You can see the whole output here:

remote: -----> Compressing...
remote:        Done: 68.8M
remote: -----> Launching...
remote:  !     Release command declared: this new release will not be available until the command succeeds.
remote:        Released v16
remote:        https://somehostonlineproject.herokuapp.com/ deployed to Heroku
remote: 
remote: Verifying deploy... done.
remote: Running release command...
remote: 
remote: ===============> ParseResult(scheme='', netloc='', path='somehost.sytes.net', params='', query='', fragment='')
remote: /app/.heroku/python/lib/python3.7/site-packages/pysftp/__init__.py:61: UserWarning: Failed to load HostKeys from /app/.ssh/known_hosts.  You will need to explicitly load HostKeys (cnopts.hostkeys.load(filename)) or disableHostKey checking (cnopts.hostkeys = None).
remote:   warnings.warn(wmsg, UserWarning)
remote: Traceback (most recent call last):
remote:   File "manage.py", line 31, in <module>
remote:     execute_from_command_line(sys.argv)
remote:   File "/app/.heroku/python/lib/python3.7/site-packages/django/core/management/__init__.py", line 381, in execute_from_command_line
remote:     utility.execute()
remote:   File "/app/.heroku/python/lib/python3.7/site-packages/django/core/management/__init__.py", line 375, in execute
remote:     self.fetch_command(subcommand).run_from_argv(self.argv)
remote:   File "/app/.heroku/python/lib/python3.7/site-packages/django/core/management/base.py", line 323, in run_from_argv
remote:     self.execute(*args, **cmd_options)
remote:   File "/app/.heroku/python/lib/python3.7/site-packages/django/core/management/base.py", line 361, in execute
remote:     self.check()
remote:   File "/app/.heroku/python/lib/python3.7/site-packages/django/core/management/base.py", line 390, in check
remote:     include_deployment_checks=include_deployment_checks,
remote:   File "/app/.heroku/python/lib/python3.7/site-packages/django/core/management/commands/migrate.py", line 65, in _run_checks
remote:     issues.extend(super()._run_checks(**kwargs))
remote:   File "/app/.heroku/python/lib/python3.7/site-packages/django/core/management/base.py", line 377, in _run_checks
remote:     return checks.run_checks(**kwargs)
remote:   File "/app/.heroku/python/lib/python3.7/site-packages/django/core/checks/registry.py", line 72, in run_checks
remote:     new_errors = check(app_configs=app_configs)
remote:   File "/app/.heroku/python/lib/python3.7/site-packages/django/core/checks/urls.py", line 40, in check_url_namespaces_unique
remote:     all_namespaces = _load_all_namespaces(resolver)
remote:   File "/app/.heroku/python/lib/python3.7/site-packages/django/core/checks/urls.py", line 57, in _load_all_namespaces
remote:     url_patterns = getattr(resolver, 'url_patterns', [])
remote:   File "/app/.heroku/python/lib/python3.7/site-packages/django/utils/functional.py", line 80, in __get__
remote:     res = instance.__dict__[self.name] = self.func(instance)
remote:   File "/app/.heroku/python/lib/python3.7/site-packages/django/urls/resolvers.py", line 584, in url_patterns
remote:     patterns = getattr(self.urlconf_module, "urlpatterns", self.urlconf_module)
remote:   File "/app/.heroku/python/lib/python3.7/site-packages/django/utils/functional.py", line 80, in __get__
remote:     res = instance.__dict__[self.name] = self.func(instance)
remote:   File "/app/.heroku/python/lib/python3.7/site-packages/django/urls/resolvers.py", line 577, in urlconf_module
remote:     return import_module(self.urlconf_name)
remote:   File "/app/.heroku/python/lib/python3.7/importlib/__init__.py", line 127, in import_module
remote:     return _bootstrap._gcd_import(name[level:], package, level)
remote:   File "<frozen importlib._bootstrap>", line 1006, in _gcd_import
remote:   File "<frozen importlib._bootstrap>", line 983, in _find_and_load
remote:   File "<frozen importlib._bootstrap>", line 967, in _find_and_load_unlocked
remote:   File "<frozen importlib._bootstrap>", line 677, in _load_unlocked
remote:   File "<frozen importlib._bootstrap_external>", line 728, in exec_module
remote:   File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
remote:   File "/app/config/urls.py", line 27, in <module>
remote:     path("browse/", include("django_sftpbrowser.urls", namespace="sftpbrowser-root"), name='browse_option'),
remote:   File "/app/.heroku/python/lib/python3.7/site-packages/django/urls/conf.py", line 34, in include
remote:     urlconf_module = import_module(urlconf_module)
remote:   File "/app/.heroku/python/lib/python3.7/importlib/__init__.py", line 127, in import_module
remote:     return _bootstrap._gcd_import(name[level:], package, level)
remote:   File "<frozen importlib._bootstrap>", line 1006, in _gcd_import
remote:   File "<frozen importlib._bootstrap>", line 983, in _find_and_load
remote:   File "<frozen importlib._bootstrap>", line 967, in _find_and_load_unlocked
remote:   File "<frozen importlib._bootstrap>", line 677, in _load_unlocked
remote:   File "<frozen importlib._bootstrap_external>", line 728, in exec_module
remote:   File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
remote:   File "/app/django_sftpbrowser/urls.py", line 2, in <module>
remote:     from .views import browse_page
remote:   File "/app/django_sftpbrowser/views.py", line 9, in <module>
remote:     srv = pysftp.Connection(settings.SOMEHOST_SFTP_SERVER_URL, username='madtyn', password=settings.SFTP_PASSWORD)
remote:   File "/app/.heroku/python/lib/python3.7/site-packages/pysftp/__init__.py", line 132, in __init__
remote:     self._tconnect['hostkey'] = self._cnopts.get_hostkey(host)
remote:   File "/app/.heroku/python/lib/python3.7/site-packages/pysftp/__init__.py", line 71, in get_hostkey
remote:     raise SSHException("No hostkey for host %s found." % host)
remote: paramiko.ssh_exception.SSHException: No hostkey for host somehost.myftp.org found.
remote: Exception ignored in: <function Connection.__del__ at 0x7fd94274b950>
remote: Traceback (most recent call last):
remote:   File "/app/.heroku/python/lib/python3.7/site-packages/pysftp/__init__.py", line 1013, in __del__
remote:     self.close()
remote:   File "/app/.heroku/python/lib/python3.7/site-packages/pysftp/__init__.py", line 784, in close
remote:     if self._sftp_live:
remote: AttributeError: 'Connection' object has no attribute '_sftp_live'
remote: Waiting for release... failed.
To https://git.heroku.com/somehostonlineproject.git
 * [new branch]      deployment -> master

Answer 1

For a general discussion about the "No hostkey for host ... found" , see:
Verify host key with pysftp

---

Regarding the implementation on Heroku: I'm not familiar with it, but afaik, and as you as well commented, it does not have a persistent file storage.

For this reason, using an implementation that has the host key hard-coded is appropriate. Two solutions from my answer to the above question suit that need:

1. If you do not want to use an external file, you can also use

   from base64 import decodebytes # ... keydata = b"""AAAAB3NzaC1yc2EAAAADAQAB...""" key = paramiko.RSAKey(data=decodebytes(keydata)) cnopts = pysftp.CnOpts() cnopts.hostkeys.add('example.com', 'ssh-rsa', key) with pysftp.Connection(host, username, password, cnopts=cnopts) as sftp:

2. If you need to verify the host key using its fingerprint only, see Python - pysftp / paramiko - Verify host key using its fingerprint .

---

This is also relevant (while about Paramiko directly, not about pysftp wrapper):
Paramiko SSH failing with "Server '...' not found in known_hosts" when run on web server