pexpect.pxssh — Pexpect 4.1.0 documentation

# Source code for pexpect.pxssh

    '''This class extends pexpect.spawn to specialize setting up SSH connections.
    This adds methods for login, logout, and expecting the shell prompt.

    PEXPECT LICENSE

        This license is approved by the OSI and FSF as GPL-compatible.
            http://opensource.org/licenses/isc-license.txt

        Copyright (c) 2012, Noah Spurrier <noah@noah.org>
        PERMISSION TO USE, COPY, MODIFY, AND/OR DISTRIBUTE THIS SOFTWARE FOR ANY
        PURPOSE WITH OR WITHOUT FEE IS HEREBY GRANTED, PROVIDED THAT THE ABOVE
        COPYRIGHT NOTICE AND THIS PERMISSION NOTICE APPEAR IN ALL COPIES.
        THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
        WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
        MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
        ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
        WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
        ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
        OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.

    '''

    from pexpect import ExceptionPexpect, TIMEOUT, EOF, spawn
    import time
    import os

    __all__ = ['ExceptionPxssh', 'pxssh']

    # Exception classes used by this module.
    class ExceptionPxssh(ExceptionPexpect):
        '''Raised for pxssh exceptions.
        '''

    class pxssh (spawn):
        '''This class extends pexpect.spawn to specialize setting up SSH
        connections. This adds methods for login, logout, and expecting the shell
        prompt. It does various tricky things to handle many situations in the SSH
        login process. For example, if the session is your first login, then pxssh
        automatically accepts the remote certificate; or if you have public key
        authentication setup then pxssh won't wait for the password prompt.

        pxssh uses the shell prompt to synchronize output from the remote host. In
        order to make this more robust it sets the shell prompt to something more
        unique than just $ or #. This should work on most Borne/Bash or Csh style
        shells.

        Example that runs a few commands on a remote server and prints the result::

            from pexpect import pxssh
            import getpass
            try:
                s = pxssh.pxssh()
                hostname = raw_input('hostname: ')
                username = raw_input('username: ')
                password = getpass.getpass('password: ')
                s.login(hostname, username, password)
                s.sendline('uptime')   # run a command
                s.prompt()             # match the prompt
                print(s.before)        # print everything before the prompt.
                s.sendline('ls -l')
                s.prompt()
                print(s.before)
                s.sendline('df')
                s.prompt()
                print(s.before)
                s.logout()
            except pxssh.ExceptionPxssh as e:
                print("pxssh failed on login.")
                print(e)

        Example showing how to specify SSH options::

            from pexpect import pxssh
            s = pxssh.pxssh(options={
                                "StrictHostKeyChecking": "no",
                                "UserKnownHostsFile": "/dev/null"})
            ...

        Note that if you have ssh-agent running while doing development with pxssh
        then this can lead to a lot of confusion. Many X display managers (xdm,
        gdm, kdm, etc.) will automatically start a GUI agent. You may see a GUI
        dialog box popup asking for a password during development. You should turn
        off any key agents during testing. The 'force_password' attribute will turn
        off public key authentication. This will only work if the remote SSH server
        is configured to allow password logins. Example of using 'force_password'
        attribute::

                s = pxssh.pxssh()
                s.force_password = True
                hostname = raw_input('hostname: ')
                username = raw_input('username: ')
                password = getpass.getpass('password: ')
                s.login (hostname, username, password)
        '''

        def __init__ (self, timeout=30, maxread=2000, searchwindowsize=None,
                        logfile=None, cwd=None, env=None, ignore_sighup=True, echo=True,
                        options={}, encoding=None, codec_errors='strict'):

            spawn.__init__(self, None, timeout=timeout, maxread=maxread,
                           searchwindowsize=searchwindowsize, logfile=logfile,
                           cwd=cwd, env=env, ignore_sighup=ignore_sighup, echo=echo,
                           encoding=encoding, codec_errors=codec_errors)

            self.name = '<pxssh>'

            #SUBTLE HACK ALERT! Note that the command that SETS the prompt uses a
            #slightly different string than the regular expression to match it. This
            #is because when you set the prompt the command will echo back, but we
            #don't want to match the echoed command. So if we make the set command
            #slightly different than the regex we eliminate the problem. To make the
            #set command different we add a backslash in front of $. The $ doesn't
            #need to be escaped, but it doesn't hurt and serves to make the set
            #prompt command different than the regex.

            # used to match the command-line prompt
            self.UNIQUE_PROMPT = "\[PEXPECT\][\$\#] "
            self.PROMPT = self.UNIQUE_PROMPT

            # used to set shell command-line prompt to UNIQUE_PROMPT.
            self.PROMPT_SET_SH = "PS1='[PEXPECT]\$ '"
            self.PROMPT_SET_CSH = "set prompt='[PEXPECT]\$ '"
            self.SSH_OPTS = ("-o'RSAAuthentication=no'"
                    + " -o 'PubkeyAuthentication=no'")
    # Disabling host key checking, makes you vulnerable to MITM attacks.
    #                + " -o 'StrictHostKeyChecking=no'"
    #                + " -o 'UserKnownHostsFile /dev/null' ")
            # Disabling X11 forwarding gets rid of the annoying SSH_ASKPASS from
            # displaying a GUI password dialog. I have not figured out how to
            # disable only SSH_ASKPASS without also disabling X11 forwarding.
            # Unsetting SSH_ASKPASS on the remote side doesn't disable it! Annoying!
            #self.SSH_OPTS = "-x -o'RSAAuthentication=no' -o 'PubkeyAuthentication=no'"
            self.force_password = False

            # User defined SSH options, eg,
            # ssh.otions = dict(StrictHostKeyChecking="no",UserKnownHostsFile="/dev/null")
            self.options = options

        def levenshtein_distance(self, a, b):
            '''This calculates the Levenshtein distance between a and b.
            '''

            n, m = len(a), len(b)
            if n > m:
                a,b = b,a
                n,m = m,n
            current = range(n+1)
            for i in range(1,m+1):
                previous, current = current, [i]+[0]*n
                for j in range(1,n+1):
                    add, delete = previous[j]+1, current[j-1]+1
                    change = previous[j-1]
                    if a[j-1] != b[i-1]:
                        change = change + 1
                    current[j] = min(add, delete, change)
            return current[n]

        def try_read_prompt(self, timeout_multiplier):
            '''This facilitates using communication timeouts to perform
            synchronization as quickly as possible, while supporting high latency
            connections with a tunable worst case performance. Fast connections
            should be read almost immediately. Worst case performance for this
            method is timeout_multiplier * 3 seconds.
            '''

            # maximum time allowed to read the first response
            first_char_timeout = timeout_multiplier * 0.5

            # maximum time allowed between subsequent characters
            inter_char_timeout = timeout_multiplier * 0.1

            # maximum time for reading the entire prompt
            total_timeout = timeout_multiplier * 3.0

            prompt = self.string_type()
            begin = time.time()
            expired = 0.0
            timeout = first_char_timeout

            while expired < total_timeout:
                try:
                    prompt += self.read_nonblocking(size=1, timeout=timeout)
                    expired = time.time() - begin # updated total time expired
                    timeout = inter_char_timeout
                except TIMEOUT:
                    break

            return prompt

        def sync_original_prompt (self, sync_multiplier=1.0):
            '''This attempts to find the prompt. Basically, press enter and record
            the response; press enter again and record the response; if the two
            responses are similar then assume we are at the original prompt.
            This can be a slow function. Worst case with the default sync_multiplier
            can take 12 seconds. Low latency connections are more likely to fail
            with a low sync_multiplier. Best case sync time gets worse with a
            high sync multiplier (500 ms with default). '''

            # All of these timing pace values are magic.
            # I came up with these based on what seemed reliable for
            # connecting to a heavily loaded machine I have.
            self.sendline()
            time.sleep(0.1)

            try:
                # Clear the buffer before getting the prompt.
                self.try_read_prompt(sync_multiplier)
            except TIMEOUT:
                pass

            self.sendline()
            x = self.try_read_prompt(sync_multiplier)

            self.sendline()
            a = self.try_read_prompt(sync_multiplier)

            self.sendline()
            b = self.try_read_prompt(sync_multiplier)

            ld = self.levenshtein_distance(a,b)
            len_a = len(a)
            if len_a == 0:
                return False
            if float(ld)/len_a < 0.4:
                return True
            return False

        ### TODO: This is getting messy and I'm pretty sure this isn't perfect.
        ### TODO: I need to draw a flow chart for this.
        def login (self, server, username, password='', terminal_type='ansi',
                    original_prompt=r"[#$]", login_timeout=10, port=None,
                    auto_prompt_reset=True, ssh_key=None, quiet=True,
                    sync_multiplier=1, check_local_ip=True):
            '''This logs the user into the given server.

            It uses
            'original_prompt' to try to find the prompt right after login. When it
            finds the prompt it immediately tries to reset the prompt to something
            more easily matched. The default 'original_prompt' is very optimistic
            and is easily fooled. It's more reliable to try to match the original
            prompt as exactly as possible to prevent false matches by server
            strings such as the "Message Of The Day". On many systems you can
            disable the MOTD on the remote server by creating a zero-length file
            called :file:`~/.hushlogin` on the remote server. If a prompt cannot be found
            then this will not necessarily cause the login to fail. In the case of
            a timeout when looking for the prompt we assume that the original
            prompt was so weird that we could not match it, so we use a few tricks
            to guess when we have reached the prompt. Then we hope for the best and
            blindly try to reset the prompt to something more unique. If that fails
            then login() raises an :class:`ExceptionPxssh` exception.

            In some situations it is not possible or desirable to reset the
            original prompt. In this case, pass ``auto_prompt_reset=False`` to
            inhibit setting the prompt to the UNIQUE_PROMPT. Remember that pxssh
            uses a unique prompt in the :meth:`prompt` method. If the original prompt is
            not reset then this will disable the :meth:`prompt` method unless you
            manually set the :attr:`PROMPT` attribute.
            '''

            ssh_options = ''.join([" -o '%s=%s'" % (o, v) for (o, v) in self.options.items()])
            if quiet:
                ssh_options = ssh_options + ' -q'
            if not check_local_ip:
                ssh_options = ssh_options + " -o'NoHostAuthenticationForLocalhost=yes'"
            if self.force_password:
                ssh_options = ssh_options + ' ' + self.SSH_OPTS
            if port is not None:
                ssh_options = ssh_options + ' -p %s'%(str(port))
            if ssh_key is not None:
                try:
                    os.path.isfile(ssh_key)
                except:
                    raise ExceptionPxssh('private ssh key does not exist')
                ssh_options = ssh_options + ' -i %s' % (ssh_key)
            cmd = "ssh %s -l %s %s" % (ssh_options, username, server)

            # This does not distinguish between a remote server 'password' prompt
            # and a local ssh 'passphrase' prompt (for unlocking a private key).
            spawn._spawn(self, cmd)
            i = self.expect(["(?i)are you sure you want to continue connecting", original_prompt, "(?i)(?:password)|(?:passphrase for key)", "(?i)permission denied", "(?i)terminal type", TIMEOUT, "(?i)connection closed by remote host"], timeout=login_timeout)

            # First phase
            if i==0:
                # New certificate -- always accept it.
                # This is what you get if SSH does not have the remote host's
                # public key stored in the 'known_hosts' cache.
                self.sendline("yes")
                i = self.expect(["(?i)are you sure you want to continue connecting", original_prompt, "(?i)(?:password)|(?:passphrase for key)", "(?i)permission denied", "(?i)terminal type", TIMEOUT])
            if i==2: # password or passphrase
                self.sendline(password)
                i = self.expect(["(?i)are you sure you want to continue connecting", original_prompt, "(?i)(?:password)|(?:passphrase for key)", "(?i)permission denied", "(?i)terminal type", TIMEOUT])
            if i==4:
                self.sendline(terminal_type)
                i = self.expect(["(?i)are you sure you want to continue connecting", original_prompt, "(?i)(?:password)|(?:passphrase for key)", "(?i)permission denied", "(?i)terminal type", TIMEOUT])

            # Second phase
            if i==0:
                # This is weird. This should not happen twice in a row.
                self.close()
                raise ExceptionPxssh('Weird error. Got "are you sure" prompt twice.')
            elif i==1: # can occur if you have a public key pair set to authenticate.
                ### TODO: May NOT be OK if expect() got tricked and matched a false prompt.
                pass
            elif i==2: # password prompt again
                # For incorrect passwords, some ssh servers will
                # ask for the password again, others return 'denied' right away.
                # If we get the password prompt again then this means
                # we didn't get the password right the first time.
                self.close()
                raise ExceptionPxssh('password refused')
            elif i==3: # permission denied -- password was bad.
                self.close()
                raise ExceptionPxssh('permission denied')
            elif i==4: # terminal type again? WTF?
                self.close()
                raise ExceptionPxssh('Weird error. Got "terminal type" prompt twice.')
            elif i==5: # Timeout
                #This is tricky... I presume that we are at the command-line prompt.
                #It may be that the shell prompt was so weird that we couldn't match
                #it. Or it may be that we couldn't log in for some other reason. I
                #can't be sure, but it's safe to guess that we did login because if
                #I presume wrong and we are not logged in then this should be caught
                #later when I try to set the shell prompt.
                pass
            elif i==6: # Connection closed by remote host
                self.close()
                raise ExceptionPxssh('connection closed')
            else: # Unexpected
                self.close()
                raise ExceptionPxssh('unexpected login response')
            if not self.sync_original_prompt(sync_multiplier):
                self.close()
                raise ExceptionPxssh('could not synchronize with original prompt')
            # We appear to be in.
            # set shell prompt to something unique.
            if auto_prompt_reset:
                if not self.set_unique_prompt():
                    self.close()
                    raise ExceptionPxssh('could not set shell prompt '
                                         '(received: %r, expected: %r).' % (
                                             self.before, self.PROMPT,))
            return True

        def logout (self):
            '''Sends exit to the remote shell.

            If there are stopped jobs then this automatically sends exit twice.
            '''
            self.sendline("exit")
            index = self.expect([EOF, "(?i)there are stopped jobs"])
            if index==1:
                self.sendline("exit")
                self.expect(EOF)
            self.close()

        def prompt(self, timeout=-1):
            '''Match the next shell prompt.

            This is little more than a short-cut to the :meth:`~pexpect.spawn.expect`
            method. Note that if you called :meth:`login` with
            ``auto_prompt_reset=False``, then before calling :meth:`prompt` you must
            set the :attr:`PROMPT` attribute to a regex that it will use for
            matching the prompt.

            Calling :meth:`prompt` will erase the contents of the :attr:`before`
            attribute even if no prompt is ever matched. If timeout is not given or
            it is set to -1 then self.timeout is used.

            :return: True if the shell prompt was matched, False if the timeout was
                     reached.
            '''

            if timeout == -1:
                timeout = self.timeout
            i = self.expect([self.PROMPT, TIMEOUT], timeout=timeout)
            if i==1:
                return False
            return True

        def set_unique_prompt(self):
            '''This sets the remote prompt to something more unique than ``#`` or ``$``.
            This makes it easier for the :meth:`prompt` method to match the shell prompt
            unambiguously. This method is called automatically by the :meth:`login`
            method, but you may want to call it manually if you somehow reset the
            shell prompt. For example, if you 'su' to a different user then you
            will need to manually reset the prompt. This sends shell commands to
            the remote host to set the prompt, so this assumes the remote host is
            ready to receive commands.

            Alternatively, you may use your own prompt pattern. In this case you
            should call :meth:`login` with ``auto_prompt_reset=False``; then set the
            :attr:`PROMPT` attribute to a regular expression. After that, the
            :meth:`prompt` method will try to match your prompt pattern.
            '''

            self.sendline("unset PROMPT_COMMAND")
            self.sendline(self.PROMPT_SET_SH) # sh-style
            i = self.expect ([TIMEOUT, self.PROMPT], timeout=10)
            if i == 0: # csh-style
                self.sendline(self.PROMPT_SET_CSH)
                i = self.expect([TIMEOUT, self.PROMPT], timeout=10)
                if i == 0:
                    return False
            return True

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