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Using PSCP to transfer files securely

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Chapter 5: Using PSCP to transfer files securely

PSCP, the PuTTY Secure Copy client, is a tool for transferring files securely between computers using an SSH connection.

If you have an SSH-2 server, you might prefer PSFTP (see chapter 6) for interactive use. PSFTP does not in general work with SSH-1 servers, however.

5.1 Starting PSCP

PSCP is a command line application. This means that you cannot just double-click on its icon to run it and instead you have to bring up a console window. With Windows 95, 98, and ME, this is called an ‘MS-DOS Prompt’ and with Windows NT, 2000, and XP, it is called a ‘Command Prompt’. It should be available from the Programs section of your Start Menu.

To start PSCP it will need either to be on your PATH or in your current directory. To add the directory containing PSCP to your PATH environment variable, type into the console window: 

set PATH=C:\path\to\putty\directory;%PATH%

This will only work for the lifetime of that particular console window. To set your PATH more permanently on Windows NT, 2000, and XP, use the Environment tab of the System Control Panel. On Windows 95, 98, and ME, you will need to edit your AUTOEXEC.BAT to include a set command like the one above.

5.2 PSCP Usage

Once you've got a console window to type into, you can just type pscp on its own to bring up a usage message. This tells you the version of PSCP you're using, and gives you a brief summary of how to use PSCP: 

C:\>pscp
PuTTY Secure Copy client
Release 0.81
Usage: pscp [options] [user@]host:source target
       pscp [options] source [source...] [user@]host:target
       pscp [options] -ls [user@]host:filespec
Options:
  -V        print version information and exit
  -pgpfp    print PGP key fingerprints and exit
  -p        preserve file attributes
  -q        quiet, don't show statistics
  -r        copy directories recursively
  -v        show verbose messages
  -load sessname  Load settings from saved session
  -P port   connect to specified port
  -l user   connect with specified username
  -pwfile file   login with password read from specified file
  -1 -2     force use of particular SSH protocol version
  -ssh -ssh-connection
            force use of particular SSH protocol variant
  -4 -6     force use of IPv4 or IPv6
  -C        enable compression
  -i key    private key file for user authentication
  -noagent  disable use of Pageant
  -agent    enable use of Pageant
  -no-trivial-auth
            disconnect if SSH authentication succeeds trivially
  -hostkey keyid
            manually specify a host key (may be repeated)
  -batch    disable all interactive prompts
  -no-sanitise-stderr  don't strip control chars from standard error
  -proxycmd command
            use 'command' as local proxy
  -unsafe   allow server-side wildcards (DANGEROUS)
  -sftp     force use of SFTP protocol
  -scp      force use of SCP protocol
  -sshlog file
  -sshrawlog file
            log protocol details to a file
  -logoverwrite
  -logappend
            control what happens when a log file already exists

(PSCP's interface is much like the Unix scp command, if you're familiar with that.)

5.2.1 The basics

To receive (a) file(s) from a remote server: 

pscp [options] [user@]host:source target

So to copy the file /etc/hosts from the server example.com as user fred to the file c:\temp\example-hosts.txt, you would type: 

pscp fred@example.com:/etc/hosts c:\temp\example-hosts.txt

To send (a) file(s) to a remote server: 

pscp [options] source [source...] [user@]host:target

So to copy the local file c:\documents\foo.txt to the server example.com as user fred to the file /tmp/foo you would type: 

pscp c:\documents\foo.txt fred@example.com:/tmp/foo

You can use wildcards to transfer multiple files in either direction, like this: 

pscp c:\documents\*.doc fred@example.com:docfiles
pscp fred@example.com:source/*.c c:\source

However, in the second case (using a wildcard for multiple remote files) you may see a warning saying something like ‘warning: remote host tried to write to a file called ‘terminal.c’ when we requested a file called ‘*.c’. If this is a wildcard, consider upgrading to SSH-2 or using the ‘-unsafe’ option. Renaming of this file has been disallowed’.

This is due to a fundamental insecurity in the old-style SCP protocol: the client sends the wildcard string (*.c) to the server, and the server sends back a sequence of file names that match the wildcard pattern. However, there is nothing to stop the server sending back a different pattern and writing over one of your other files: if you request *.c, the server might send back the file name AUTOEXEC.BAT and install a virus for you. Since the wildcard matching rules are decided by the server, the client cannot reliably verify that the filenames sent back match the pattern.

PSCP will attempt to use the newer SFTP protocol (part of SSH-2) where possible, which does not suffer from this security flaw. If you are talking to an SSH-2 server which supports SFTP, you will never see this warning. (You can force use of the SFTP protocol, if available, with -sftp - see section 5.2.2.6.)

If you really need to use a server-side wildcard with an SSH-1 server, you can use the -unsafe command line option with PSCP: 

pscp -unsafe fred@example.com:source/*.c c:\source

This will suppress the warning message and the file transfer will happen. However, you should be aware that by using this option you are giving the server the ability to write to any file in the target directory, so you should only use this option if you trust the server administrator not to be malicious (and not to let the server machine be cracked by malicious people). Alternatively, do any such download in a newly created empty directory. (Even in ‘unsafe’ mode, PSCP will still protect you against the server trying to get out of that directory using pathnames including ‘..’.)

5.2.1.1 user

The login name on the remote server. If this is omitted, and host is a PuTTY saved session, PSCP will use any username specified by that saved session. Otherwise, PSCP will attempt to use the local Windows username.

5.2.1.2 host

The name of the remote server, or the name of an existing PuTTY saved session. In the latter case, the session's settings for hostname, port number, cipher type and username will be used.

5.2.1.3 source

One or more source files. Wildcards are allowed. The syntax of wildcards depends on the system to which they apply, so if you are copying from a Windows system to a UNIX system, you should use Windows wildcard syntax (e.g. *.*), but if you are copying from a UNIX system to a Windows system, you would use the wildcard syntax allowed by your UNIX shell (e.g. *).

If the source is a remote server and you do not specify a full pathname (in UNIX, a pathname beginning with a / (slash) character), what you specify as a source will be interpreted relative to your home directory on the remote server.

5.2.1.4 target

The filename or directory to put the file(s). When copying from a remote server to a local host, you may wish simply to place the file(s) in the current directory. To do this, you should specify a target of .. For example: 

pscp fred@example.com:/home/tom/.emacs .

...would copy /home/tom/.emacs on the remote server to the current directory.

As with the source parameter, if the target is on a remote server and is not a full path name, it is interpreted relative to your home directory on the remote server.

5.2.2 Options

PSCP accepts all the general command line options supported by the PuTTY tools, except the ones which make no sense in a file transfer utility. See section 3.11.3 for a description of these options. (The ones not supported by PSCP are clearly marked.)

PSCP also supports some of its own options. The following sections describe PSCP's specific command-line options.

5.2.2.1 -ls list remote files

If the -ls option is given, no files are transferred; instead, remote files are listed. Only a hostname specification and optional remote file specification need be given. For example: 

pscp -ls fred@example.com:dir1

The SCP protocol does not contain within itself a means of listing files. If SCP is in use, this option therefore assumes that the server responds appropriately to the command ls -la; this may not work with all servers.

If SFTP is in use, this option should work with all servers.

5.2.2.2 -p preserve file attributes

By default, files copied with PSCP are timestamped with the date and time they were copied. The -p option preserves the original timestamp on copied files.

5.2.2.3 -q quiet, don't show statistics

By default, PSCP displays a meter displaying the progress of the current transfer: 

mibs.tar          |   168 kB |  84.0 kB/s | ETA: 00:00:13 |  13%

The fields in this display are (from left to right), filename, size (in kilobytes) of file transferred so far, estimate of how fast the file is being transferred (in kilobytes per second), estimated time that the transfer will be complete, and percentage of the file so far transferred. The -q option to PSCP suppresses the printing of these statistics.

5.2.2.4 -r copies directories recursively

By default, PSCP will only copy files. Any directories you specify to copy will be skipped, as will their contents. The -r option tells PSCP to descend into any directories you specify, and to copy them and their contents. This allows you to use PSCP to transfer whole directory structures between machines.

5.2.2.5 -batch avoid interactive prompts

If you use the -batch option, PSCP will never give an interactive prompt while establishing the connection. If the server's host key is invalid, for example (see section 2.2), then the connection will simply be abandoned instead of asking you what to do next.

This may help PSCP's behaviour when it is used in automated scripts: using -batch, if something goes wrong at connection time, the batch job will fail rather than hang.

5.2.2.6 -sftp, -scp force use of particular file transfer protocol

As mentioned in section 5.2.1, there are two different file transfer protocols in use with SSH. Despite its name, PSCP (like many other ostensible scp clients) can use either of these protocols.

The older SCP protocol does not have a written specification and leaves a lot of detail to the server platform. Wildcards are expanded on the server. The simple design means that any wildcard specification supported by the server platform (such as brace expansion) can be used, but also leads to interoperability issues such as with filename quoting (for instance, where filenames contain spaces), and also the security issue described in section 5.2.1.

The newer SFTP protocol, which is usually associated with SSH-2 servers, is specified in a more platform independent way, and leaves issues such as wildcard syntax up to the client. (PuTTY's SFTP wildcard syntax is described in section 6.2.2.) This makes it more consistent across platforms, more suitable for scripting and automation, and avoids security issues with wildcard matching.

Normally PSCP will attempt to use the SFTP protocol, and only fall back to the SCP protocol if SFTP is not available on the server.

The -scp option forces PSCP to use the SCP protocol or quit.

The -sftp option forces PSCP to use the SFTP protocol or quit. When this option is specified, PSCP looks harder for an SFTP server, which may allow use of SFTP with SSH-1 depending on server setup.

5.2.2.7 -no-sanitise-stderr: control error message sanitisation

The -no-sanitise-stderr option will cause PSCP to pass through the server's standard-error stream literally, without stripping control characters from it first. This might be useful if the server were sending coloured error messages, but it also gives the server the ability to have unexpected effects on your terminal display. For more discussion, see section 7.2.3.5.

5.2.3 Return value

PSCP returns an ERRORLEVEL of zero (success) only if the files were correctly transferred. You can test for this in a batch file, using code such as this: 

pscp file*.* user@hostname:
if errorlevel 1 echo There was an error

5.2.4 Using public key authentication with PSCP

Like PuTTY, PSCP can authenticate using a public key instead of a password. There are three ways you can do this.

Firstly, PSCP can use PuTTY saved sessions in place of hostnames (see section 5.2.1.2). So you would do this:

Secondly, you can supply the name of a private key file on the command line, with the -i option. See section 3.11.3.18 for more information.

Thirdly, PSCP will attempt to authenticate using Pageant if Pageant is running (see chapter 9). So you would do this:

For more general information on public-key authentication, see chapter 8.

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[PuTTY release 0.81]
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