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Using SSH tunnels

In some cases you may need to access remote hosts that have private IP addresses or are protected behind a firewall. These hosts might be accessible through an intermediate login host. The SSH tunnels (SSH port forwarding) facilitate access to multiple hosts protected behind a single host exposed to the Internet. Once the tunnels are established the hosts behind the firewall can be accessed directly. Examples were tunnels can be beneficial:

  • When using the INCD cloud service you may have multiple VMs with private IP addresses acessible through a single VM with public IP address. This enables you to directly access remote VMs while saving scarse IPv4 address space.
  • Some INCD services are not directly exposed to the Internet and may require passing through an intermediate login host for added security.

jump over SSH hosts easily from the command line

  • If you just need to connect via SSH to hosts behind the firewall by jumping over a reachable host you can use the jump host functionality of SSH whose syntax is described below and where:
    • private-remote-host is the host or IP address behind the firewall that you want to reach via SSH.
    • public-remote-host the remote host or IP address of the intermediate host that is exposed to the Internet through which you will connect to the private-remote-host.

Basic jump host syntax:

ssh -J public-remote-host   username@private-remote-host

Example of using the SSH jump host to access the SSH port (port number 22) of the remote host internal.ncg.ingrid.pt through a publicly reachable host public.ncg.ingrid.pt.

ssh -J public.ncg.ingrid.pt  username@internal.ncg.ingrid.pt

port forwarding from local to remote in detail

  • The tunnels (port forwarding) are defined upon the connection to the intermediate host that is directly acessible over the Internet.
  • Therefore we first need to start an SSH session to the intermediate host, simultaneously we define one or more port forwardings using the -L option. The basic syntax for SSH port forwarding is described below where:
    • local-port is a TCP port of your local machine (your desktop, laptop, etc). Just pick one port that is not being used above the privileged port range (above 1024) and preferrably outside of the IP ephemeral port range to avoid collisions with dynamically allocated ports (for many Linux systems outside the range of 32768-60999).
    • private-remote-host is an IP address or hostname of a remote host behind the firewall.
    • private-remote-port is the TCP port of the private-remote-host that you want to access.
    • public-remote-host the remote host or IP address of the intermediate host that is exposed to the Internet.

Basic port forwarding syntax:

ssh -L local-port:private-remote-host:private-remote-port   public-remote-host

Example of establishing a tunnel to access the SSH port (port number 22) of the remote host internal.ncg.ingrid.pt through a publicly reachable host public.ncg.ingrid.pt. We pick a random local-port of 31732 that is then mapped to the port 22 of internal.ncg.ingrid.pt.

ssh -L 31732:internal.ncg.ingrid.pt:22   public.ncg.ingrid.pt

Once the SSH connection to public.ncg.ingrid.pt is established the host internal.ncg.ingrid.pt can be directly accessed from your local host by connecting to the local-port:

ssh -p 31732 username@127.0.0.1

Another example where both port 22 (SSH) and port 80 (HTTP) are mapped to local ports. Then we can test the access to the web server on the remote port 80 using curl.

ssh -L 31732:internal.ncg.ingrid.pt:22 -L 8080:internal.ncg.ingrid.pt:80  public.ncg.ingrid.pt

curl http://127.0.0.1:8080

port forwarding from remote to local in detail

  • This enables a remote host behind the firewall to access services in your local network.
  • This can be potentially dangerous for you as it enables users in the remote machine or remote network to access your local machines.
  • The basic syntax for port forwarding from the remote side to the local side is described below where:
    • remote-port-on-public-host is a port number from the public-remote-host Just pick one port that is not being used above the privileged port range (above 1024) and preferrably outside of the IP ephemeral port range to avoid collisions with dynamically allocated ports (for many Linux systems outside the range of 32768-60999).
    • local-host a host in the local network of the user to be accessed from the public-remote-host.
    • local-port the port number of the localhost to be accessed from the public-remote-host.
    • public-remote-host the remote host or IP address of the intermediate host that is exposed to the Internet.

Basic remote port forwarding syntax:

ssh -R remote-port-on-public-host:local-host:local-port   public-remote-host

Example of forwarding the remote-port number 65532 on public.ncg.ingrid.pt to the port 22 of the local host named myotherlocalhost.

ssh -R 65532:myotherlocalhost:22   public.ncg.ingrid.pt

Then from public.ncg.ingrid.pt you can access the host in the local network with:

ssh -p 65532 127.0.0.1

jump host via config file

  • Configuring jump host in your $HOME/.ssh/config where:
    • Host is a name choosen by you to identify this mapping, this name may be different from the hostname and will be recognized as a hostname only by SSH.
    • Hostname the real hostname of the host that you want to access and is behind the firewall.
    • User the username for the host identified by Hostname.
    • ProxyJump the remote host or IP address of the intermediate host that is exposed to the Internet.

The basic configuration for remote access via a jump host

Host name-for-the-remote-host
Hostname actual-private-remote-host
User username-in-actual-private-remote-host
ProxyJump username@public-remote-host

Example of configuration:

Host ncg-internal
Hostname internal.ncg.ingrid.pt
User username-for-internal
ProxyJump username@public.ncg.ingrid.pt

Accessing the configured host:

ssh ncg-internal

using socks proxies

using sshuttle

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