I thought I'd write a shorter article this time. It goes hand in hand with my upcoming article series on 100% technical guide to anonymity and it's much easier to write larger articles by splitting them into smaller pieces. I can then just edit them together and produce the final article.

This article will be interesting to everyone who didn't know this awesome fact already – you can turn any Linux computer into a SOCKS5 (and SOCKS4) proxy with just one ssh command command and no extra tools.

ssh -N -D 0.0.0.0:1080 localhost

Even better – it doesn't require root privileges. The ssh command starts up dynamic -D port forwarding on port 1080 and talks to the clients via SOCSK5 or SOCKS4 protocols, just like a regular SOCKS5 proxy would. The -N option makes sure ssh stays idle and doesn't execute any commands on localhost.

If you also want the command to go into background as a daemon, then add the -f option:

ssh -f -N -D 0.0.0.0:1080 localhost

To use it, just make your software use SOCKS5 proxy on your Linux computer's IP, port 1080, and you're done – all your requests will now get proxied.

Access control can be implemented via iptables . For example, to allow only people from the ip 1.2.3.4 to use SOCKS5 proxy, add the following iptables rules:

iptables -A INPUT --src 1.2.3.4 -p tcp --dport 1080 -j ACCEPT iptables -A INPUT -p tcp --dport 1080 -j REJECT

The first rule says, allow anyone from 1.2.3.4 to connect to port 1080 , and the other rule says, deny everyone else from connecting to port 1080 .

However, executing iptables requires root privileges. If you don't have root privileges, and you don't want to leave your proxy open (and you really don't want to do that), you'll have to use some kind of a simple TCP proxy wrapper to do access control.

Here, I just wrote one in Perl. It's called tcp-proxy.pl and it uses IO::Socket::INET to abstract sockets, and IO::Select to do connection multiplexing.

#!/usr/bin/perl # use warnings; use strict; use IO::Socket::INET; use IO::Select; my @allowed_ips = ('1.2.3.4', '5.6.7.8', '127.0.0.1', '192.168.1.2'); my $ioset = IO::Select->new; my %socket_map; my $debug = 1; sub new_conn { my ($host, $port) = @_; return IO::Socket::INET->new( PeerAddr => $host, PeerPort => $port ) || die "Unable to connect to $host:$port: $!"; } sub new_server { my ($host, $port) = @_; my $server = IO::Socket::INET->new( LocalAddr => $host, LocalPort => $port, ReuseAddr => 1, Listen => 100 ) || die "Unable to listen on $host:$port: $!"; } sub new_connection { my $server = shift; my $client = $server->accept; my $client_ip = client_ip($client); unless (client_allowed($client)) { print "Connection from $client_ip denied.

" if $debug; $client->close; return; } print "Connection from $client_ip accepted.

" if $debug; my $remote = new_conn('localhost', 55555); $ioset->add($client); $ioset->add($remote); $socket_map{$client} = $remote; $socket_map{$remote} = $client; } sub close_connection { my $client = shift; my $client_ip = client_ip($client); my $remote = $socket_map{$client}; $ioset->remove($client); $ioset->remove($remote); delete $socket_map{$client}; delete $socket_map{$remote}; $client->close; $remote->close; print "Connection from $client_ip closed.

" if $debug; } sub client_ip { my $client = shift; return inet_ntoa($client->sockaddr); } sub client_allowed { my $client = shift; my $client_ip = client_ip($client); return grep { $_ eq $client_ip } @allowed_ips; } print "Starting a server on 0.0.0.0:1080

"; my $server = new_server('0.0.0.0', 1080); $ioset->add($server); while (1) { for my $socket ($ioset->can_read) { if ($socket == $server) { new_connection($server); } else { next unless exists $socket_map{$socket}; my $remote = $socket_map{$socket}; my $buffer; my $read = $socket->sysread($buffer, 4096); if ($read) { $remote->syswrite($buffer); } else { close_connection($socket); } } } }

To use it, you'll have to make a change to the previous configuration. Instead of running ssh SOCKS5 proxy on 0.0.0.0:1080 , you'll need to run it on localhost:55555 .

ssh -f -N -D 55555 localhost

After that, run the tcp-proxy.pl .

perl tcp-proxy.pl

TCP proxy will start listening on 0.0.0.0:1080 and will redirect only the allowed IPs in @allowed_ips list to localhost:55555 .

Another possibility is to use another computer instead of your own as an exit node. What I mean is you can do the following.

ssh -f -N -D 1080 other_computer.com

This will set up a SOCKS5 proxy on localhost:1080 but when you use it, ssh will automatically tunnel your requests (encrypted) via other_computer.com . This way you can hide what you're doing on the Internet from anyone who might be sniffing your link. They will see that you're communicating with other_computer.com but the traffic will be encrypted so they won't be able to tell what you're doing.

That's it. You're now the proxy king!

Download tcp-proxy.pl

Download link: catonmat.net/ftp/tcp-proxy.pl

I also pushed tcp-proxy.pl to GitHub. You can fork it and contribute. For example, you could make it redirect between any number of hosts:ports, not just two, implement onion routing, and better access control.

I'll also soon write the definitive guide to ssh port forwarding because it's an interesting but little understood topic.

See you next time!