Objective-C

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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Objective-C

#import <CkoSshTunnel.h>
#import <NSString.h>

BOOL success = NO;

//  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.

CkoSshTunnel *tunnel = [[CkoSshTunnel alloc] init];

//  The fixed destination that accepted connections are forwarded to.
tunnel.DestHostname = @"db.internal.example.com";
tunnel.DestPort = [NSNumber numberWithInt: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).
tunnel.ListenBindIpAddress = @"127.0.0.1";

int sshPort = 22;
success = [tunnel Connect: @"ssh.example.com" port: [NSNumber numberWithInt: sshPort]];
if (success == NO) {
    NSLog(@"%@",tunnel.LastErrorText);
    return;
}

//  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.
NSString *password = @"mySshPassword";

success = [tunnel AuthenticatePw: @"mySshLogin" password: password];
if (success == NO) {
    NSLog(@"%@",tunnel.LastErrorText);
    return;
}

//  Pass 0 to let the operating system choose an available port.  After BeginAccepting succeeds,
//  ListenPort contains the actual port that was bound.
success = [tunnel BeginAccepting: [NSNumber numberWithInt: 0]];
if (success == NO) {
    NSLog(@"%@",tunnel.LastErrorText);
    return;
}

NSLog(@"%@%d",@"Listening on 127.0.0.1 port ",[tunnel.ListenPort intValue]);

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

BOOL waitForThreadExit = YES;
success = [tunnel CloseTunnel: waitForThreadExit];
if (success == NO) {
    NSLog(@"%@",tunnel.LastErrorText);
    return;
}