gistfile1.pl

#!/usr/bin/env perl
# -*- perl -*-

# location of the server-side scp we want to run
$scp_server = "/usr/bin/scp";

sub fail {
    my ($msg) = @_;
    print STDERR $msg, "\n";
    exit 1;
}

# This just makes me feel better.

$TRUE  = (0 == 0);
$FALSE = (0 == 1);

# Since this script is called as a forced command, need to get the
# original scp command given by the client.

($command = $ENV{SSH_ORIGINAL_COMMAND})
    || fail;

# Split the command string to make an argument list, and remove the first
# element (the command name; we'll supply our own);

@scp_argv = split /[ \t]+/, $command;

# Complain if the command is not "scp".

fail "account restricted: only scp is allowed"
    unless $scp_argv[0] eq "scp";

# Wipe the environment as a security precaution.  This might conceivably
# break something, but if it does you can filter the environment more
# selectively here.

%ENV = ();

# Ensure that either -t or -f is on the command line, to enforce running
# scp in server mode.

$ok = $FALSE;
foreach $arg (@scp_argv) {
    if ($arg eq '-t' || $arg eq '-f') {
	$ok = $TRUE;
	last;
    }
}

fail "Restricted; only server mode allowed."
    unless $ok;

# if we're OK, run our desired "scp" with arguments.

shift(@scp_argv);
exec($scp_server, @scp_argv);