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/server_test.go

@@ -0,0 +1,121 @@
+package ssh
+
+import (
+	"fmt"
+	"github.com/stretchr/testify/assert"
+	"golang.org/x/crypto/ssh"
+	"strings"
+	"testing"
+)
+
+func TestServer_AddAuthorizedKey(t *testing.T) {
+	key, err := GeneratePrivateKey(ED25519)
+	if err != nil {
+		t.Fatal(err)
+	}
+	server, err := newDefaultServer(key, "localhost:")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// add multiple keys
+	keys := map[string][]byte{}
+	for i := 0; i < 10; i++ {
+		peer := fmt.Sprintf("%s-%d", "remotePeer", i)
+		remotePrivKey, err := GeneratePrivateKey(ED25519)
+		if err != nil {
+			t.Fatal(err)
+		}
+		remotePubKey, err := GeneratePublicKey(remotePrivKey)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		err = server.AddAuthorizedKey(peer, string(remotePubKey))
+		if err != nil {
+			t.Error(err)
+		}
+		keys[peer] = remotePubKey
+	}
+
+	// make sure that all keys have been added
+	for peer, remotePubKey := range keys {
+		k, ok := server.authorizedKeys[peer]
+		assert.True(t, ok, "expecting remotePeer key to be found in authorizedKeys")
+
+		assert.Equal(t, string(remotePubKey), strings.TrimSpace(string(ssh.MarshalAuthorizedKey(k))))
+	}
+
+}
+
+func TestServer_RemoveAuthorizedKey(t *testing.T) {
+	key, err := GeneratePrivateKey(ED25519)
+	if err != nil {
+		t.Fatal(err)
+	}
+	server, err := newDefaultServer(key, "localhost:")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	remotePrivKey, err := GeneratePrivateKey(ED25519)
+	if err != nil {
+		t.Fatal(err)
+	}
+	remotePubKey, err := GeneratePublicKey(remotePrivKey)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	err = server.AddAuthorizedKey("remotePeer", string(remotePubKey))
+	if err != nil {
+		t.Error(err)
+	}
+
+	server.RemoveAuthorizedKey("remotePeer")
+
+	_, ok := server.authorizedKeys["remotePeer"]
+	assert.False(t, ok, "expecting remotePeer's SSH key to be removed")
+}
+
+func TestServer_PubKeyHandler(t *testing.T) {
+	key, err := GeneratePrivateKey(ED25519)
+	if err != nil {
+		t.Fatal(err)
+	}
+	server, err := newDefaultServer(key, "localhost:")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	var keys []ssh.PublicKey
+	for i := 0; i < 10; i++ {
+		peer := fmt.Sprintf("%s-%d", "remotePeer", i)
+		remotePrivKey, err := GeneratePrivateKey(ED25519)
+		if err != nil {
+			t.Fatal(err)
+		}
+		remotePubKey, err := GeneratePublicKey(remotePrivKey)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		remoteParsedPubKey, _, _, _, err := ssh.ParseAuthorizedKey(remotePubKey)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		err = server.AddAuthorizedKey(peer, string(remotePubKey))
+		if err != nil {
+			t.Error(err)
+		}
+		keys = append(keys, remoteParsedPubKey)
+	}
+
+	for _, key := range keys {
+		accepted := server.publicKeyHandler(nil, key)
+
+		assert.Truef(t, accepted, "expecting SSH connection to be accepted for a given SSH key %s", string(ssh.MarshalAuthorizedKey(key)))
+	}
+
+}