Foundations of Python Network Programming 978-1-4302-3004-5

CHAPTER 16 ■ TELNET AND SSH
» » return
client = paramiko.SSHClient()
client.set_missing_host_key_policy(AllowAnythingPolicy())
client.connect('127.0.0.1', username='test') # password='')
def read_until_EOF(fileobj):
» s = fileobj.readline()
» while s:
» » print s.strip()
» » s = fileobj.readline()
out1 = client.exec_command('echo One;sleep 2;echo Two;sleep 1;echo Three')[1]
out2 = client.exec_command('echo A;sleep 1;echo B;sleep 2;echo C')[1]
thread1 = threading.Thread(target=read_until_EOF, args=(out1,))
thread2 = threading.Thread(target=read_until_EOF, args=(out2,))
thread1.start()
thread2.start()
thread1.join()
thread2.join()
client.close()
In order to be able to process these two streams of data simultaneously, we are kicking off two
threads, and are handing each of them one of the channels from which to read. They each print out each
line of new information as soon as it arrives, and finally exit when the readline() command indicates
end-of-file by returning an empty string. When run, this script should return something like this:
$ python ssh_threads.py
One
A
B
Two
Three
C
So there you have it: SSH channels over the same TCP connection are completely independent, can
each receive (and send) data at their own pace, and can close independently when the particular
command that they are talking to finally terminates.
The same is true of the features we are about to look at—file transfer and port forwarding—so keep
in mind as you read our last two examples that all of these kinds of communications can happen
simultaneously without your having to open more than one SSH connection to hold all of the channels
of data.
SFTP: File Transfer Over SSH
Version 2 of the SSH protocol includes a sub-protocol called the “SSH File Transfer Protocol” (SFTP) that
lets you walk the remote directory tree, create and delete directories and files, and copy files back and
forth from the local to the remote machine. The capabilities of SFTP are so complex and complete, in
fact, that they support not only simple file-copy operations, but can power graphical file browsers and
can even let the remote filesystem be mounted locally! (Google for the sshfs system for details.)
The SFTP protocol is an incredible boon to those of us who once had to copy files using brittle
scripts that tried to send data across Telnet through very careful escaping of binary data! And instead of
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