scp – Made by Mikal

Paramiko doesn’t provide a scp implementation, so I’ve been using my own for a while. http://blogs.sun.com/janp/entry/how_the_scp_protocol_works provides good documentation about the scp protocol, but it missed out on one detail I needed — how to send more than one file in a given session. In the end I implemented a simple scp logger to see what the protocol was doing during the copying of files. My logger said this:

>>> New command invocation: /usr/bin/scp -d -t /tmp
O: \0
I: C0644 21 a\n
O: \0
I: file a file a file a\n\0
O: \0
I: C0644 21 b\n
O: \0
I: file b file b file b\n\0
O: \0
>>>stdin closed
>>> stdout closed
>>> stderr closed

It turns out its important to wait for those zeros by the way. So, here’s my implementation of the protocol to send more than one file. Turning this into paramiko code is left as an exercise for the reader.

#!/usr/bin/python

import fcntl
import os
import select
import string
import subprocess
import sys
import traceback

def printable(s):
  out = ''

  for c in s:
    if c == '\n':
      out += '\\n'
    elif c in string.printable:
      out += c
    else:
      out += '\\%d' % ord(c)

  return out

try:
  dialog = ['C0644 21 c\n',
            'file c file c file c\n\0',
            'C0644 21 d\n',
            'file d file d file d\n\0']

  proc = subprocess.Popen(['scp', '-v', '-d', '-t', '/tmp'],
                          stdin=subprocess.PIPE,
                          stdout=subprocess.PIPE,
                          stderr=subprocess.PIPE)

  r = [proc.stdout, proc.stderr]
  w = []
  e = [proc.stdout, proc.stderr]

  fl = fcntl.fcntl(proc.stdout, fcntl.F_GETFL)
  fcntl.fcntl(proc.stdout, fcntl.F_SETFL, fl | os.O_NONBLOCK)
  fl = fcntl.fcntl(proc.stderr, fcntl.F_GETFL)
  fcntl.fcntl(proc.stderr, fcntl.F_SETFL, fl | os.O_NONBLOCK)

  stdin_closed = False
  while proc.returncode is None:
    (readable, _, errorable) = select.select(r, w, e)

    for flo in readable:
      if flo == proc.stdout:
        d = os.read(proc.stdout.fileno(), 1024)
        if len(d) > 0:
          sys.stdout.write('O: %s\n' % printable(d))

          if len(dialog) > 0:
            sys.stdout.write('I: %s\n' % printable(dialog[0]))
            os.write(proc.stdin.fileno(), dialog[0])
            dialog = dialog[1:]

          if len(dialog) == 0 and not stdin_closed:
            sys.stdout.write('>>> stdin closed\n')
            proc.stdin.close()
            stdin_closed = True

        else:
          sys.stdout.write('>>> stdout closed\n')
          r.remove(proc.stdout)
          e.remove(proc.stdout)

      elif flo == proc.stderr:
        d = os.read(proc.stderr.fileno(), 1024)
        if len(d) > 0:
          sys.stdout.write('E: %s\n' % printable(d))
        else:
          sys.stdout.write('>>> stderr closed\n')
          r.remove(proc.stderr)
          e.remove(proc.stderr)

      else:
        sys.stdout.write('>>> Unknown readable: %s: %s\n'
                         %(repr(flo), flo.read()))

    for flo in errorable:
      sys.stdout.write('>>> Error on %s\n' % repr(flo))
      r.remove(flo)
      e.remove(flo)

    proc.poll()

  print '#: %s' % proc.returncode

except:
  exc = sys.exc_info()
  for tb in traceback.format_exception(exc[0], exc[1], exc[2]):
    print tb
    del tb

Regular readers will note that I’ve been interested in how scp works and paramiko for the last couple of days. There are previous examples of how to do scp with paramiko out there, but the code isn’t all on one page, you have to read through the mail thread and work it out from there. I figured I might save someone some time (possibly me!) and note a complete example of scp with paramiko…

#!/usr/bin/python

# A simple scp example for Paramiko.
# Args:
#   1: hostname
#   2: username
#   3: local filename
#   4: remote filename

import getpass
import os
import paramiko
import socket
import sys

# Socket connection to remote host
sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
sock.connect((sys.argv[1], 22))

# Build a SSH transport
t = paramiko.Transport(sock)
t.start_client()
t.auth_password(sys.argv[2], getpass.getpass('Password: '))

# Start a scp channel
scp_channel = t.open_session()

f = file(sys.argv[3], 'rb')
scp_channel.exec_command('scp -v -t %s\n'
                         % '/'.join(sys.argv[4].split('/')[:-1]))
scp_channel.send('C%s %d %s\n'
                 %(oct(os.stat(sys.argv[3]).st_mode)[-4:],
                   os.stat(sys.argv[3])[6],
                   sys.argv[4].split('/')[-1]))
scp_channel.sendall(f.read())

# Cleanup
f.close()
scp_channel.close()
t.close()
sock.close()

Tag: scp

Multiple file support with scp

Implementing SCP with paramiko