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block: better document SSH host key fingerprint checking 

The docs still illustrate host key fingerprint checking using the old
md5 hashes which are considered insecure and obsolete. Change it to
illustrate using a sha256 hash. Also show how to extract the hash
value from the known_hosts file.

Reviewed-by: Hanna Reitz <hreitz@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
This commit is contained in:
Daniel P. Berrangé 2021-09-13 17:55:58 +01:00
parent ad38520bde
commit ab458750d0
1 changed files with 26 additions and 4 deletions
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30
docs/system/qemu-block-drivers.rst.inc
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@ -778,10 +778,32 @@ The optional *HOST_KEY_CHECK* parameter controls how the remote
host's key is checked. The default is ``yes`` which means to use
the local ``.ssh/known_hosts`` file. Setting this to ``no``
turns off known-hosts checking. Or you can check that the host key
matches a specific fingerprint:
``host_key_check=md5:78:45:8e:14:57:4f:d5:45:83:0a:0e:f3:49:82:c9:c8``
(``sha1:`` can also be used as a prefix, but note that OpenSSH
tools only use MD5 to print fingerprints).
matches a specific fingerprint. The fingerprint can be provided in
``md5``, ``sha1``, or ``sha256`` format, however, it is strongly
recommended to only use ``sha256``, since the other options are
considered insecure by modern standards. The fingerprint value
must be given as a hex encoded string::
host_key_check=sha256:04ce2ae89ff4295a6b9c4111640bdcb3297858ee55cb434d9dd88796e93aa795
The key string may optionally contain ":" separators between
each pair of hex digits.
The ``$HOME/.ssh/known_hosts`` file contains the base64 encoded
host keys. These can be converted into the format needed for
QEMU using a command such as::
$ for key in `grep 10.33.8.112 known_hosts | awk '{print $3}'`
do
echo $key | base64 -d | sha256sum
done
6c3aa525beda9dc83eadfbd7e5ba7d976ecb59575d1633c87cd06ed2ed6e366f -
12214fd9ea5b408086f98ecccd9958609bd9ac7c0ea316734006bc7818b45dc8 -
d36420137bcbd101209ef70c3b15dc07362fbe0fa53c5b135eba6e6afa82f0ce -
Note that there can be multiple keys present per host, each with
different key ciphers. Care is needed to pick the key fingerprint
that matches the cipher QEMU will negotiate with the remote server.
Currently authentication must be done using ssh-agent. Other
authentication methods may be supported in future.