NetBird SSH (#361)

This PR adds support for SSH access through the NetBird network
without managing SSH skeys.
NetBird client app has an embedded SSH server (Linux/Mac only) 
and a netbird ssh command.

client/ssh/client.go

@@ -0,0 +1,114 @@
+package ssh
+
+import (
+	"fmt"
+	"golang.org/x/crypto/ssh"
+	"golang.org/x/term"
+	"net"
+	"os"
+)
+
+// Client wraps crypto/ssh Client to simplify usage
+type Client struct {
+	client *ssh.Client
+}
+
+// Close closes the wrapped SSH Client
+func (c *Client) Close() error {
+	return c.client.Close()
+}
+
+// OpenTerminal starts an interactive terminal session with the remote SSH server
+func (c *Client) OpenTerminal() error {
+	session, err := c.client.NewSession()
+	if err != nil {
+		return fmt.Errorf("failed to open new session: %v", err)
+	}
+	defer func() {
+		err := session.Close()
+		if err != nil {
+			return
+		}
+	}()
+
+	fd := int(os.Stdout.Fd())
+	state, err := term.MakeRaw(fd)
+	if err != nil {
+		return fmt.Errorf("failed to run raw terminal: %s", err)
+	}
+	defer func() {
+		err := term.Restore(fd, state)
+		if err != nil {
+			return
+		}
+	}()
+
+	w, h, err := term.GetSize(fd)
+	if err != nil {
+		return fmt.Errorf("terminal get size: %s", err)
+	}
+
+	modes := ssh.TerminalModes{
+		ssh.ECHO:          1,
+		ssh.TTY_OP_ISPEED: 14400,
+		ssh.TTY_OP_OSPEED: 14400,
+	}
+
+	terminal := os.Getenv("TERM")
+	if terminal == "" {
+		terminal = "xterm-256color"
+	}
+	if err := session.RequestPty(terminal, h, w, modes); err != nil {
+		return fmt.Errorf("failed requesting pty session with xterm: %s", err)
+	}
+
+	session.Stdout = os.Stdout
+	session.Stderr = os.Stderr
+	session.Stdin = os.Stdin
+
+	if err := session.Shell(); err != nil {
+		return fmt.Errorf("failed to start login shell on the remote host: %s", err)
+	}
+
+	if err := session.Wait(); err != nil {
+		if e, ok := err.(*ssh.ExitError); ok {
+			switch e.ExitStatus() {
+			case 130:
+				return nil
+			}
+		}
+		return fmt.Errorf("failed running SSH session: %s", err)
+	}
+
+	return nil
+}
+
+// DialWithKey connects to the remote SSH server with a provided private key file (PEM).
+func DialWithKey(addr, user string, privateKey []byte) (*Client, error) {
+
+	signer, err := ssh.ParsePrivateKey(privateKey)
+	if err != nil {
+		return nil, err
+	}
+
+	config := &ssh.ClientConfig{
+		User: user,
+		Auth: []ssh.AuthMethod{
+			ssh.PublicKeys(signer),
+		},
+		HostKeyCallback: ssh.HostKeyCallback(func(hostname string, remote net.Addr, key ssh.PublicKey) error { return nil }),
+	}
+
+	return Dial("tcp", addr, config)
+}
+
+// Dial connects to the remote SSH server.
+func Dial(network, addr string, config *ssh.ClientConfig) (*Client, error) {
+	client, err := ssh.Dial(network, addr, config)
+	if err != nil {
+		return nil, err
+	}
+	return &Client{
+		client: client,
+	}, nil
+}