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CHAPTER 16 TELNET AND SSH
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We will learn about all of these kinds of channels in the following sections.
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When an SSH client first connects to a remote host, they exchange temporary public keys that let them encrypt the rest of their conversation without revealing any information to any watching third parties. Then, before the client is willing to divulge any further information, it demands proof of the remote server's identity. This makes good sense as a first step: if you are really talking to a hacker who has temporarily managed to grab the remote server's IP, you do not want SSH to divulge even your username much less your password! As we saw in 6, one answer to the problem of machine identity on the Internet is to build a public-key infrastructure. First you designate a set of organizations called certificate authorities that can issue certs; then you install a list of their public keys in all of the web browsers and other SSL clients in existence; then those organizations charge you money to verify that you really are google.com and that you deserve to have your google.com SSL certificate signed; and then, finally, you can install the certificate on your web server, and everyone will trust that you are really google.com. There are many problems with this system from the point of view of SSH. While it is true that you can build a public-key infrastructure internal to an organization, where you distribute your own signing authority's certificates to your web browsers or other applications and then can sign your own server certificates without paying a third party, a public-key infrastructure is still considered too cumbersome a process for something like SSH; server administrators want to set up, use, and tear down servers all the time, without having to talk to a central authority first. So SSH has the idea that each server, when installed, creates its own random public-private key pair that is not signed by anybody. Instead, one of two approaches is taken to key distribution: A system administrator writes a script that gathers up all of the host public keys in an organization, creates an ssh_known_hosts listing them all, and places this file in the /etc/sshd directory on every system in the organization. They might also make it available to any desktop clients, like the PuTTY command under Windows. Now every SSH client will know about every SSH host key before they even connect for the first time. Abandon the idea of knowing host keys ahead of time, and instead memorize them at the moment of first connection. Users of the SSH command line will be very familiar with this: the client says it does not recognize the host to which you are connecting, you reflexively answer yes, and its key gets stored in your ~/.ssh/known_hosts file. You actually have no guarantee that you are really talking to the host you think it is; but at least you will be guaranteed that every subsequent connection you ever make to that machine is going to the right place, and not to other servers that someone is swapping into place at the same IP address. (Unless, of course, they have stolen your host keys!)
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The familiar prompt from the SSH command line when it sees an unfamiliar host looks like this: $ ssh asaph.rhodesmill.org The authenticity of host 'asaph.rhodesmill.org (74.207.234.78)' can't be established. RSA key fingerprint is 85:8f:32:4e:ac:1f:e9:bc:35:58:c1:d4:25:e3:c7:8c. Are you sure you want to continue connecting (yes/no) yes Warning: Permanently added 'asaph.rhodesmill.org,74.207.234.78' (RSA) to the list of known hosts.
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CHAPTER 16 TELNET AND SSH
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That yes answer buried deep on the next-to-last full line is the answer that I typed giving SSH the go-ahead to make the connection and remember the key for next time. If SSH ever connects to a host and sees a different key, its reaction is quite severe: $ ssh asaph.rhodesmill.org @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ @ WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED! @ @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY! Someone could be eavesdropping on you right now (man-in-the-middle attack)! This message will be familiar to anyone who has ever had to re-build a server from scratch, and forgets to save the old SSH keys and lets new ones be generated by the re-install. It can be painful to go around to all of your SSH clients and remove the offending old key so that they will quietly learn the new one upon reconnection. The paramiko library has full support for all of the normal SSH tactics surrounding host keys. But its default behavior is rather spare: it loads no host-key files by default, and will then, of course, raise an exception for the very first host to which you connect because it will not be able to verify its key! The exception that it raises is a bit un-informative; it is only by looking at the fact that it comes from inside the missing_host_key() function that I usually recognize what has caused the error: >>> import paramiko >>> client = paramiko.SSHClient() >>> client.connect('my.example.com', username='test') Traceback (most recent call last): ... File ".../paramiko/client.py", line 85, in missing_host_key raise SSHException('Unknown server %s' % hostname) paramiko.SSHException: Unknown server my.example.com To behave like the normal SSH command, load both the system and the current user's known-host keys before making the connection: >>> client.load_system_host_keys() >>> client.load_host_keys('/home/brandon/.ssh/known_hosts') >>> client.connect('my.example.com', username='test') The paramiko library also lets you choose how you handle unknown hosts. Once you have a client object created, you can provide it with a decision-making class that is asked what to do if a host key is not recognized. You can build these classes yourself by inheriting from the MissingHostKeyPolicy class: >>> class AllowAnythingPolicy(paramiko.MissingHostKeyPolicy): ... def missing_host_key(self, client, hostname, key): ... return ... >>> client.set_missing_host_key_policy(AllowAnythingPolicy()) >>> client.connect('my.example.com', username='test') Note that, through the arguments to the missing_host_key() method, you receive several pieces of information on which to base your decision; you could, for example, allow connections to machines on your own server subnet without a host key, but disallow all others. Inside paramiko there are also several decision-making classes that already implement several basic host-key options: paramiko.AutoAddPolicy: Host keys are automatically added to your user host-key store (the file ~/.ssh/known_hosts on Unix systems) when first encountered, but any change in the host key from then on will raise a fatal exception.
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