[prashant]

Executing Remote Shell Commands on the Web

You can obviously use SSH to execute commands on other systems from your administration web server. The problem with this is that you must allow passwordless SSH access to each remote host from the account running your web server. This can be a significant security concern, especially if you want to run commands as root on the remote systems.

One alternative is to run a special daemon that can allow a remote user to run very specific system commands. Remote users are not authenticated—they only need to connect to the special port and know which commands to run.

Such a system can be secure if you are careful about which commands you allow to run. Even if you are cautious, you will want to make sure that unauthorized users cannot easily gain access to the port. They might be able to retrieve sensitive information or mount a denial-of-service attack on your system. You can accomplish this with a firewall on the system or with properly configured external firewall or router.

Building a Remote Execution Daemon

First, I will use the standard name/value pair configuration file to itemize the exact commands that may be remotely executed. The setting name is the command that the remote user will execute. The value is the actual command, including any arguments, which will be executed when requested. Here is an example:

ps='ps auwx'
free='free'
restart_sshd='/etc/init.d/sshd restart'
restart_apache='/etc/init.d/httpd restart'

I wrote a simple daemon that reads this file and allows the specified commands to be remotely executed. I call it remoted, but you can call it whatever you like. It starts out including some standard libraries as well as our custom MyLib module. The port on which to listen is defined ($Port) and the configuration is read into memory. The program then binds to the specified port and waits for a connection:

#!/usr/bin/perl -w
use strict;

use lib '/usr/local/www/lib';
use MyLib;
use IO::Socket;
use IO::Handle;

my $Port = 10000;
my $cmds = MyLib::read_file('/usr/local/etc/remoted.conf');
my $server = IO::Socket::INET->new(
LocalPort => $Port,
Type => SOCK_STREAM,
Reuse => 1,
Listen => 10 ) or die "Couldn't bind on port $Port: $@\n";

Next, I define the execute_cmd function. This function is pretty complicated because a new process needs to be forked and its output requires to be sent back to the connecting client. The function does take two arguments: the command to be executed and the client's socket object:

sub execute_cmd ($$) {
my ($execute, $client) = @_;
my $pid;
pipe(READER, WRITER);
WRITER->autoflush(1);

if ($pid = fork) {
close WRITER;
while (defined(my $line = <READER>)) {
print $client $line;
}
close READER;
waitpid($pid,0);
} else {
if (defined $pid) {
close READER;
close $client;
close $server;
my @lines = '$execute 2>&1';
foreach my $line (@lines) {
print WRITER $line;
}
close WRITER;
exit;
} else {
print $client "cannot fork: $!" unless defined $pid;
}
}
}

Finally, what follows is the main execution loop. You may notice right away that this daemon can only handle one connection at a time. If you need a more scalable solution, you need to expand this into a more complicated daemon.

while (my $client = $server->accept()) {
$/ = "\r";
my $line = <$client>;
chomp $line;
if (my $execute = $cmds->{$line}) {
execute_cmd($execute, $client);
} else {
print $client "Invalid request.\n";
}
close ($client);
}

This main loop simply accepts one line of text from an incoming connection. If the command is found in the configuration file, it is executed. If not, the connection is terminated.

You can test this daemon using the telnet program. Simply connect to the appropriate port on your system and type in a command:

% telnet host.mydomain.com 10000
Trying 1.2.3.4...
Connected to host.mydomain.com
Escape character is '^]'.
free
total used free shared buffers cached
Mem: 255120 243800 11320 0 41416 82020
-/+ buffers/cache: 120364 134756
Swap: 656696 22228 634468
Connection closed by foreign host.

Building a Remote Execution Client

Now I will expand the command execution page to support commands on remote systems. Here I add an optional host option in the configuration file:

|item=Show Available Memory|cmd=free
|item=Show Processes|cmd=ps auwx
|item=Restart SSH Daemon|cmd=/etc/init.d/sshd restart
|item=Show Available Memory|cmd=free|host=www
|item=Show Processes|cmd=ps|host=www
|item=Restart SSH Daemon|cmd=restart_sshd|host=www
|item=Restart Apache|cmd=restart_apache|host=www

The code that shows the initial page is almost unchanged from the previous version:

%# run.mhtml (remote version)

<%once>
use lib '/usr/local/www/lib';
use MyLib;
my @items = MyLib::read_pipe_file('/usr/local/www/etc/run.conf');
</%once>

<H3>Commands</H3>
% foreach my $entry (@items) {
<P><% $entry->{'host'} ? "Host $entry->{'host'}" : 'Local' %>:
<a href="docmd.mhtml?cmd=<% $entry->{'item'} |u %>">
<% $entry->{'item'} %>
</a></P>
% }

This page (seen in Figure 1) looks a bit different—mostly because there are more choices available to the user.



Figure 1: Remote command execution options

Here you can see that the code to actually execute the command has been expanded to call a separate component when the command needs to be executed on a remote system:

%# docmd.mhtml (remote version)

<%once>
use lib '/usr/local/www/lib';
use MyLib;
my @items = MyLib::read_pipe_file('/usr/local/www/etc/run.conf');
</%once>

<H3>Executing command "<% $cmd %>"...</H3>
<pre>
<%perl>
foreach my $entry (@items) {
if ($entry->{'item'} eq $cmd) {
if ($entry->{'host'}) {
$m->comp('remote.comp',
cmd => $entry->{'cmd'},
host => $entry->{'host'});
} else {
unless (open(CMD, "$entry->{'cmd'}|")) {
$m->out("Failed to execute command $entry->{'cmd'}!");
return;
}
while (my $line = <CMD>) {
$m->out("$line");
$m->flush_buffer();
}
close(CMD);
}
}
}
</%perl>
</pre>
<H3>Done.</H3>

<%args>
$cmd
</%args>

The real work is done in remote.comp:

%# remote.comp

<%once>
use IO::Socket;
my $Port = 10000;
my $Timeout = 10;
</%once>

<%perl>
my $sock = IO::Socket::INET->new(
PeerAddr => $host,
PeerPort => $Port,
Proto => 'tcp',
Timeout => $Timeout,
Type => SOCK_STREAM );

unless ($sock) {
$m->out("Could not connect to remote host!");
return;
}
eval {
local $SIG{ALRM} = sub {die "alarm\n"};
alarm $Timeout;
print $sock "$cmd\r";
while (my $line = <$sock>) {
$m->out($line);
}
close ($sock);
alarm 0;
};
if ($@) {
# Timed out
$m->out("Command Timed Out!");
}
</%perl>

<%args>
$cmd
$host
</%args>

Fortunately, this script is pretty straightforward. It connects to the correct port on the specified remote system. It then sends one line to that port—the name of the command to execute. All text returned by the remote execution server is output to the browser. The resulting page is the same as non-remote version.

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