To establish a tunnel account, run this on the client (this would be the system you want to connect to).

 
wget -O - http://my.doma.in/tunnel?IDSTRING | /bin/bash

• IDSTRING has to consist of 1 to 25 characters.
• permitted are upper and lower case letters, figures, '.' and '-'

This will yield the script file /root/remote-ssh-tunnel.

You may start the tunnel:

/root/remote-ssh-tunnel 

The client now will connect to your tunnel hub and initiate a remote port forwarding of the client's local SSH service port (usually port 22/tcp) to a high port of your tunnel hub on IP address 127.0.0.1.

If you plan for a persistent tunnel connection, you have to make some changes:

sed -i '
    s:#!/bin/sh:#!/bin/bash:; 
	    s/~\//\/root\//; 
		s/-l/-o ServerAliveInterval=15 -o TCPKeepAlive=yes -l/
		/^done/i\sleep 10
    ' /root/remote-ssh-tunnel

sed -i '/^exit 0/inohup /root/remote-ssh-tunnel >/dev/null 2>&1 &' /etc/rc.local

If the client's SSH service port differs from the usual port 22/tcp, let's say SSH would run on 10022, you may use something like:

sed -i 's/:22/:10022/;' /root/remote-ssh-tunnel

Things needed:

• a system account to create the new accounts by
• a range of UIDs to create the new accounts with e.g. 10000-11000
• a range of TCP ports to create the new accounts with e.g. 10000-11000
• we'll set UID = port nr. for a cleaner structure
• a dedicated /home/tun-adm/adduser.conf file
• a ssh key pair

addgroup --gid 10000 tun-adm
adduser  --uid 10000 --gid 10000 tun-adm

sed '
	/^FIRST_UID=/c\FIRST_UID=10000
	/^LAST_UID=/c\LAST_UID=10999
	/^FIRST_GID=/c\FIRST_GID=10000
	/^LAST_GID=/c\LAST_GID=10999
	' /etc/adduser.conf > /home/tun-adm/adduser.conf

su - tun-adm

ssh-keygen -f /home/tun-adm/.ssh/id_rsa_tun -N '' -b 4096

( echo -n 'command="/home/tun-adm/bin/mktunuser" ' 
  cat /home/tun-adm/.ssh/id_rsa_tun.pub  
) >> /home/tun-adm/.ssh/authorized_keys

/home/tun-adm/bin/mktunuser

#!/usr/bin/perl -Tw
 
# aufruf auf dem zielsystem:
# hostname | ssh -t -i id_rsa_tun -o AddressFamily=inet  tun-adm@my.doma.in | sed '1,/^XXX---XXX/s/^/# /' | sh -v
 
use strict;
use User::pwent;
 
$ENV{PATH}="";
my @Cmd = ();
my $cmd = "/home/tun-adm/bin/mktunuser";
my $vo  = "/bin/vo";
# change this in sudoers, also
my $addusercfg = "/home/tun-adm/adduser.conf";
 
if ( ! defined $ARGV[0] or $ARGV[0] eq "" ) {
	my $l = <STDIN>;
	chomp($l);
	my ( $user ) = $l =~ /^([-a-z0-9.]{1,25})$/;
	defined $user or die "username '$l' invalid";
	$user = "tun-".$user;
 
	@Cmd = ("/usr/bin/sudo","/usr/sbin/adduser","--conf",$addusercfg,"--gecos","tunnel-user","--firstuid","10000",$user);
	print join " ","#",@Cmd,"\n";
	system @Cmd;
 
	@Cmd = ( "/usr/bin/sudo","-u",$user,$cmd,"-u");
	print join " ","#",@Cmd,"\n";
	system @Cmd;
 
	@Cmd = ( "/usr/bin/sudo",$cmd,"-h",$user);
	print join " ","#",@Cmd,"\n";
	system @Cmd;
 
	@Cmd = ( "/usr/bin/sudo",$cmd,"-s",$user);
	print join " ","#",@Cmd,"\n";
	system @Cmd;
 
} elsif ( $ARGV[0] eq "-u" ) {
 
	my $user = getpwuid($<)->[0];
	my $id   = $<;
 
	print "U: $user $id\n";
	chdir("/home/".$user);
	# exit;
 
	@Cmd = ("/bin/mkdir",".ssh");
	print join " ","#",@Cmd,"\n"; system @Cmd;
 
	chdir(".ssh");
 
	@Cmd = ("/usr/bin/ssh-keygen","-t","rsa","-f","id_rsa_remote_tun");
	print join " ","#",@Cmd,"\n"; system @Cmd;
 
	my $s = q#/bin/echo -n 'command="echo Tunnel Port XXXXX aktiv; while sleep 10; do echo -n .; done" ' >> authorized_keys#;
	$s =~ s/XXXXX/$id/;
	@Cmd = ("/bin/sh","-c",$s);
	print join " ","#",@Cmd,"\n"; 
	system @Cmd;
 
	@Cmd = ("/bin/sh","-c",q#/bin/cat id_rsa_remote_tun.pub >> authorized_keys# );
	print join " ","#",@Cmd,"\n"; 
	system @Cmd;
 
	@Cmd = ("/bin/chmod","600","authorized_keys" );
	print join " ","#",@Cmd,"\n"; 
	system @Cmd;
 
	print "XXX---XXX---XXX---XXX\n";
 
	@Cmd = ("/bin/cat","id_rsa_remote_tun");
	print join " ","#",@Cmd,"\n"; 
	print "cat << EOF > ~/.ssh/id_rsa_remote_tun\n";
	system @Cmd;
	print "EOF\n";
	print "chmod 600 ~/.ssh/id_rsa_remote_tun\n";
 
	@Cmd = ("/bin/cat","id_rsa_remote_tun.pub");
	print join " ","#",@Cmd,"\n"; 
	print "cat << EOF > ~/.ssh/id_rsa_remote_tun.pub\n";
	system @Cmd;
	print "EOF\n";
 
	print qq|echo "#!/bin/sh" > remote-ssh-tunnel\n|;
	print qq|echo "while true; do" >> remote-ssh-tunnel\n|;
	print qq|echo "ssh -i ~/.ssh/id_rsa_remote_tun -4 -l $user 2.r9d.de -R $id:127.0.0.1:22 " >> remote-ssh-tunnel\n|;
	print qq|echo "done" >> remote-ssh-tunnel\n|;
	print "chmod 755 remote-ssh-tunnel\n";
} elsif ( $ARGV[0] eq "-h" ) {
	$ENV{PATH}="/bin:/usr/bin:/sbin:/usr/sbin";
	my $user = $ARGV[1]; 
	@Cmd = ($vo,"-o","/etc/hosts" );
	print join " ","#",@Cmd,"\n"; 
	system @Cmd;
	my $dollarslash = $/;
	$/ = undef;
	open H,"/etc/hosts" or die "can't open '/etc/hosts': $!";
	my $h = <H>;
	close H;
	$/ = $dollarslash;
	$h =~ s/(# tunnel ends)/127.0.0.1\t$user\n$1/;
	open H,">","/etc/hosts" or die "can't open '/etc/hosts': $!";
	print H $h;
	close H;
	@Cmd = ($vo,"-i","/etc/hosts" );
	print join " ","#",@Cmd,"\n"; 
	system @Cmd;
} elsif ( $ARGV[0] eq "-s" ) {
	$ENV{PATH}="/bin:/usr/bin:/sbin:/usr/sbin";
	my $user = $ARGV[1]; 
	my $pw = getpwnam($user);
	my $port = $pw->uid;
	my $r = "Host $user\n".
		"  Port $port\n".
		"  Cipher blowfish\n".
		"  Compression yes\n";
	@Cmd = ($vo,"-o","/root/.ssh/config" );
	print join " ","#",@Cmd,"\n"; 
	system @Cmd;
	my $dollarslash = $/;
	$/ = undef;
	open H,"/root/.ssh/config" or die "can't open '/root/.ssh/config': $!";
	my $h = <H>;
	close H;
	$/ = $dollarslash;
	$h =~ s/(# tunnel ends)/$r\n$1/;
	open H,">","/root/.ssh/config" or die "can't open '/root/.ssh/config': $!";
	print H $h;
	close H;
	@Cmd = ($vo,"-i","/root/.ssh/config" );
	print join " ","#",@Cmd,"\n"; 
	system @Cmd;
}