Fabric currently makes use of a pure-Python SSH re-implementation for managing
connections, meaning that there are occasionally spots where it is limited by
that library’s capabilities. Below are areas of note where Fabric will exhibit
behavior that isn’t consistent with, or as flexible as, the behavior of the
ssh command-line program.
SSH’s host key tracking mechanism keeps tabs on all the hosts you attempt to
connect to, and maintains a
~/.ssh/known_hosts file with mappings between
identifiers (IP address, sometimes with a hostname as well) and SSH keys. (For
details on how this works, please see the OpenSSH documentation.)
paramiko library is capable of loading up your
and will then compare any host it connects to, with that mapping. Settings are
available to determine what happens when an unknown host (a host whose username
or IP is not found in
known_hosts) is seen:
- Reject: the host key is rejected and the connection is not made. This results in a Python exception, which will terminate your Fabric session with a message that the host is unknown.
- Add: the new host key is added to the in-memory list of known hosts, the
connection is made, and things continue normally. Note that this does not
modify your on-disk
- Ask: not yet implemented at the Fabric level, this is a
paramikolibrary option which would result in the user being prompted about the unknown key and whether to accept it.
Whether to reject or add hosts, as above, is controlled in Fabric via the
env.reject_unknown_hosts option, which is False
by default for convenience’s sake. We feel this is a valid tradeoff between
convenience and security; anyone who feels otherwise can easily modify their
fabfiles at module level to set
env.reject_unknown_hosts = True.
Known hosts with changed keys¶
The point of SSH’s key/fingerprint tracking is so that man-in-the-middle
attacks can be detected: if an attacker redirects your SSH traffic to a
computer under his control, and pretends to be your original destination
server, the host keys will not match. Thus, the default behavior of SSH (and
its Python implementation) is to immediately abort the connection when a host
previously recorded in
known_hosts suddenly starts sending us a different
In some edge cases such as some EC2 deployments, you may want to ignore this
potential problem. Our SSH layer, at the time of writing, doesn’t give us
control over this exact behavior, but we can sidestep it by simply skipping the
known_hosts – if the host list being compared to is empty, then
there’s no problem. Set env.disable_known_hosts to
True when you want this behavior; it is False by default, in order to preserve
default SSH behavior.
Enabling env.disable_known_hosts will leave you wide open to man-in-the-middle attacks! Please use with caution.