Tcl

SSH Tunnel Static (Fixed-Destination) Port Forwarding

Demonstrates fixed-destination port forwarding with the DestHostname, DestPort, ListenBindIpAddress, and ListenPort properties. Every local connection accepted by the listener is forwarded to the same fixed destination through the SSH server.

Background: This is the classic ssh -L scenario: expose a single remote service — a database, an internal web app — at a local port. Binding the listener to 127.0.0.1 keeps the tunnel private to the local machine, whereas 0.0.0.0 would let other hosts route through it. Passing 0 to BeginAccepting lets the OS pick a free port, which ListenPort then reports — handy when a fixed port might already be in use.

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Tcl


load ./chilkat.dll

set success 0

#  Demonstrates fixed-destination (static) port forwarding with the SshTunnel properties
#  DestHostname, DestPort, ListenBindIpAddress, and ListenPort.
#  
#  Every local connection accepted by the listener is forwarded to the same fixed destination
#  through the SSH server.

set tunnel [new_CkSshTunnel]

#  The fixed destination that accepted connections are forwarded to.
CkSshTunnel_put_DestHostname $tunnel "db.internal.example.com"
CkSshTunnel_put_DestPort $tunnel 5432

#  Bind the listener to loopback so only local processes can use the tunnel.  Use "0.0.0.0" to
#  accept connections from other machines (less secure).
CkSshTunnel_put_ListenBindIpAddress $tunnel "127.0.0.1"

set sshPort 22
set success [CkSshTunnel_Connect $tunnel "ssh.example.com" $sshPort]
if {$success == 0} then {
    puts [CkSshTunnel_lastErrorText $tunnel]
    delete_CkSshTunnel $tunnel
    exit
}

#  Normally you would not hard-code the password in source.  You should instead obtain it
#  from an interactive prompt, environment variable, or a secrets vault.
set password "mySshPassword"

set success [CkSshTunnel_AuthenticatePw $tunnel "mySshLogin" $password]
if {$success == 0} then {
    puts [CkSshTunnel_lastErrorText $tunnel]
    delete_CkSshTunnel $tunnel
    exit
}

#  Pass 0 to let the operating system choose an available port.  After BeginAccepting succeeds,
#  ListenPort contains the actual port that was bound.
set success [CkSshTunnel_BeginAccepting $tunnel 0]
if {$success == 0} then {
    puts [CkSshTunnel_lastErrorText $tunnel]
    delete_CkSshTunnel $tunnel
    exit
}

puts "Listening on 127.0.0.1 port [CkSshTunnel_get_ListenPort $tunnel]"

#  Applications now connect to 127.0.0.1:ListenPort as if it were the database server.

set waitForThreadExit 1
set success [CkSshTunnel_CloseTunnel $tunnel $waitForThreadExit]
if {$success == 0} then {
    puts [CkSshTunnel_lastErrorText $tunnel]
    delete_CkSshTunnel $tunnel
    exit
}


delete_CkSshTunnel $tunnel