net/packet, wgengine/filter: support SCTP

Add proto to flowtrack.Tuple.

Add types/ipproto leaf package to break a cycle.

Server-side ACL work remains.

Updates #1516

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

types/ipproto/ipproto.go

@@ -0,0 +1,74 @@
+// Copyright (c) 2021 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package ipproto contains IP Protocol constants.
+package ipproto
+
+import "fmt"
+
+// Proto is an IP subprotocol as defined by the IANA protocol
+// numbers list
+// (https://www.iana.org/assignments/protocol-numbers/protocol-numbers.xhtml),
+// or the special values Unknown or Fragment.
+type Proto uint8
+
+const (
+	// Unknown represents an unknown or unsupported protocol; it's
+	// deliberately the zero value. Strictly speaking the zero
+	// value is IPv6 hop-by-hop extensions, but we don't support
+	// those, so this is still technically correct.
+	Unknown Proto = 0x00
+
+	// Values from the IANA registry.
+	ICMPv4 Proto = 0x01
+	IGMP   Proto = 0x02
+	ICMPv6 Proto = 0x3a
+	TCP    Proto = 0x06
+	UDP    Proto = 0x11
+	SCTP   Proto = 0x84
+
+	// TSMP is the Tailscale Message Protocol (our ICMP-ish
+	// thing), an IP protocol used only between Tailscale nodes
+	// (still encrypted by WireGuard) that communicates why things
+	// failed, etc.
+	//
+	// Proto number 99 is reserved for "any private encryption
+	// scheme". We never accept these from the host OS stack nor
+	// send them to the host network stack. It's only used between
+	// nodes.
+	TSMP Proto = 99
+
+	// Fragment represents any non-first IP fragment, for which we
+	// don't have the sub-protocol header (and therefore can't
+	// figure out what the sub-protocol is).
+	//
+	// 0xFF is reserved in the IANA registry, so we steal it for
+	// internal use.
+	Fragment Proto = 0xFF
+)
+
+func (p Proto) String() string {
+	switch p {
+	case Unknown:
+		return "Unknown"
+	case Fragment:
+		return "Frag"
+	case ICMPv4:
+		return "ICMPv4"
+	case IGMP:
+		return "IGMP"
+	case ICMPv6:
+		return "ICMPv6"
+	case UDP:
+		return "UDP"
+	case TCP:
+		return "TCP"
+	case SCTP:
+		return "SCTP"
+	case TSMP:
+		return "TSMP"
+	default:
+		return fmt.Sprintf("IPProto-%d", int(p))
+	}
+}