How to SSH into a Private Instance via a Bastion Host Using Go's x/crypto/ssh?

Establish SSH Connection to Private Instance via Bastion Node using Go x/crypto/ssh

Overview:

This guide demonstrates how to establish an SSH connection to a private instance over a bastion node using Go's x/crypto/ssh library.

Scenario:

Consider an AWS VPC with a public and a private subnet. A "bastion" instance is deployed in the public subnet, while the "service instance" runs in the private subnet.

Objective:

Connect to the "service instance" from a local laptop over the "bastion", run commands, and upload files.

Solution:

1. Establishing the Bastion Connection:

Use the ssh.Dial function to connect to the bastion host:

bClient, err := ssh.Dial("tcp", bastionAddr, config)
if err != nil {
    log.Fatal(err)
}

2. Dialing the Service Connection:

Use the Dial method of the bastion client to establish a connection to the service host:

conn, err := bClient.Dial("tcp", serviceAddr)
if err != nil {
    log.Fatal(err)
}

3. Creating the Service Client:

Create a new ssh.ClientConn and ssh.Client using the established connection:

ncc, chans, reqs, err := ssh.NewClientConn(conn, serviceAddr, config)
if err != nil {
    log.Fatal(err)
}

sClient := ssh.NewClient(ncc, chans, reqs)

4. Using the Service Client:

The created sClient can now be used to execute commands and transfer files:

// Run a command on the service instance
cmd := sClient.Run("ls -l")
output, err := cmd.Output()
if err != nil {
    log.Fatal(err)
}
fmt.Printf("Output: %s", output)

// Upload a file to the service instance
f, err := os.Open("./local_file.txt")
if err != nil {
    log.Fatal(err)
}
defer f.Close()

w, err := sClient.NewWriter("service_file.txt")
if err != nil {
    log.Fatal(err)
}
defer w.Close()

if _, err := io.Copy(w, f); err != nil {
    log.Fatal(err)
}

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