feat(protonvpn): port forwarding support with NAT-PMP (#1543)

Co-authored-by: Nicholas Xavier <nicho@nicho.dev>

internal/natpmp/externaladdress.go

@@ -0,0 +1,28 @@
+package natpmp
+
+import (
+	"context"
+	"encoding/binary"
+	"fmt"
+	"net/netip"
+	"time"
+)
+
+// ExternalAddress fetches the duration since the start of epoch and the external
+// IPv4 address of the gateway.
+// See https://www.ietf.org/rfc/rfc6886.html#section-3.2
+func (c *Client) ExternalAddress(ctx context.Context, gateway netip.Addr) (
+	durationSinceStartOfEpoch time.Duration,
+	externalIPv4Address netip.Addr, err error) {
+	request := []byte{0, 0} // version 0, operationCode 0
+	const responseSize = 12
+	response, err := c.rpc(ctx, gateway, request, responseSize)
+	if err != nil {
+		return 0, externalIPv4Address, fmt.Errorf("executing remote procedure call: %w", err)
+	}
+
+	secondsSinceStartOfEpoch := binary.BigEndian.Uint32(response[4:8])
+	durationSinceStartOfEpoch = time.Duration(secondsSinceStartOfEpoch) * time.Second
+	externalIPv4Address = netip.AddrFrom4([4]byte{response[8], response[9], response[10], response[11]})
+	return durationSinceStartOfEpoch, externalIPv4Address, nil
+}