Graphics Programs Reference
In-Depth Information
However, there are two different protocols of SSH—SSH1 and SSH2—
each with separate host fingerprints.
iz@tetsuo:~ $ rm ~/.ssh/known_hosts
iz@tetsuo:~ $ ssh -1 jose@192.168.42.72
The authenticity of host '192.168.42.72 (192.168.42.72)' can't be established.
RSA1 key fingerprint is e7:c4:81:fe:38:bc:a8:03:f9:79:cd:16:e9:8f:43:55.
Are you sure you want to continue connecting (yes/no)? no
Host key verification failed.
iz@tetsuo:~ $ ssh -2 jose@192.168.42.72
The authenticity of host '192.168.42.72 (192.168.42.72)' can't be established.
RSA key fingerprint is ba:06:7f:d2:b9:74:a8:0a:13:cb:a2:f7:e0:10:59:a0.
Are you sure you want to continue connecting (yes/no)? no
Host key verification failed.
iz@tetsuo:~ $
The banner presented by the SSH server describes which SSH protocols
it understands (shown in bold below):
iz@tetsuo:~ $ telnet 192.168.42.72 22
Trying 192.168.42.72...
Connected to 192.168.42.72.
Escape character is '^]'.
SSH-1.99-OpenSSH_3.9p1
Connection closed by foreign host.
iz@tetsuo:~ $ telnet 192.168.42.1 22
Trying 192.168.42.1...
Connected to 192.168.42.1.
Escape character is '^]'.
SSH-2.0-OpenSSH_4.3p2 Debian-8ubuntu1
Connection closed by foreign host.
i
z@tetsuo:~ $
The banner from 192.168.42.72 (loki) includes the string
SSH-1.99
, which,
by convention, means that the server speaks both protocols 1 and 2. Often, the
SSH server will be configured with a line like
Protocol 2,1
, which also means
the server speaks both protocols and tries to use SSH2 if possible. This is to
retain backward compatibility, so SSH1-only clients can still connect.
In contrast, the banner from 192.168.42.1 includes the string
SSH-2.0
,
which shows that the server only speaks protocol 2. In this case, it's obvious
that any clients connecting to it have only communicated with SSH2 and
therefore only have host fingerprints for protocol 2.
The same is true for loki (192.168.42.72); however, loki also accepts SSH1,
which has a different set of host fingerprints. It's unlikely that a client will
have used SSH1, and therefore doesn't have the host fingerprints for this
protocol yet.
If the modified SSH daemon being used for the MitM attack forces the
client to communicate using the other protocol, no host fingerprint will be
found. Instead of being presented with a lengthy warning, the user will simply
