firewall: add option to send REJECT replies (#738)

* firewall: add option to send REJECT replies

This change allows you to configure the firewall to send REJECT packets
when a packet is denied.

    firewall:
      # Action to take when a packet is not allowed by the firewall rules.
      # Can be one of:
      #   `drop` (default): silently drop the packet.
      #   `reject`: send a reject reply.
      #     - For TCP, this will be a RST "Connection Reset" packet.
      #     - For other protocols, this will be an ICMP port unreachable packet.
      outbound_action: drop
      inbound_action: drop

These packets are only sent to established tunnels, and only on the
overlay network (currently IPv4 only).

    $ ping -c1 192.168.100.3
    PING 192.168.100.3 (192.168.100.3) 56(84) bytes of data.
    From 192.168.100.3 icmp_seq=2 Destination Port Unreachable

    --- 192.168.100.3 ping statistics ---
    2 packets transmitted, 0 received, +1 errors, 100% packet loss, time 31ms

    $ nc -nzv 192.168.100.3 22
    (UNKNOWN) [192.168.100.3] 22 (?) : Connection refused

This change also modifies the smoke test to capture tcpdump pcaps from
both the inside and outside to inspect what is going on over the wire.
It also now does TCP and UDP packet tests using the Nmap version of
ncat.

* calculate seq and ack the same was as the kernel

The logic a bit confusing, so we copy it straight from how the kernel
does iptables `--reject-with tcp-reset`:

- https://github.com/torvalds/linux/blob/v5.19/net/ipv4/netfilter/nf_reject_ipv4.c#L193-L221

* cleanup

overlay/tun_disabled.go

@@ -1,7 +1,6 @@
 package overlay
 
 import (
-	"encoding/binary"
 	"fmt"
 	"io"
 	"net"
@@ -75,38 +74,15 @@ func (t *disabledTun) Read(b []byte) (int, error) {
 }
 
 func (t *disabledTun) handleICMPEchoRequest(b []byte) bool {
-	// Return early if this is not a simple ICMP Echo Request
-	//TODO: make constants out of these
-	if !(len(b) >= 28 && len(b) <= 9001 && b[0] == 0x45 && b[9] == 0x01 && b[20] == 0x08) {
+	out := make([]byte, len(b))
+	out = iputil.CreateICMPEchoResponse(b, out)
+	if out == nil {
 		return false
 	}
 
-	// We don't support fragmented packets
-	if b[7] != 0 || (b[6]&0x2F != 0) {
-		return false
-	}
-
-	buf := make([]byte, len(b))
-	copy(buf, b)
-
-	// Swap dest / src IPs and recalculate checksum
-	ipv4 := buf[0:20]
-	copy(ipv4[12:16], b[16:20])
-	copy(ipv4[16:20], b[12:16])
-	ipv4[10] = 0
-	ipv4[11] = 0
-	binary.BigEndian.PutUint16(ipv4[10:], ipChecksum(ipv4))
-
-	// Change type to ICMP Echo Reply and recalculate checksum
-	icmp := buf[20:]
-	icmp[0] = 0
-	icmp[2] = 0
-	icmp[3] = 0
-	binary.BigEndian.PutUint16(icmp[2:], ipChecksum(icmp))
-
 	// attempt to write it, but don't block
 	select {
-	case t.read <- buf:
+	case t.read <- out:
 	default:
 		t.l.Debugf("tun_disabled: dropped ICMP Echo Reply response")
 	}
@@ -154,22 +130,3 @@ func (p prettyPacket) String() string {
 
 	return s.String()
 }
-
-func ipChecksum(b []byte) uint16 {
-	var c uint32
-	sz := len(b) - 1
-
-	for i := 0; i < sz; i += 2 {
-		c += uint32(b[i]) << 8
-		c += uint32(b[i+1])
-	}
-	if sz%2 == 0 {
-		c += uint32(b[sz]) << 8
-	}
-
-	for (c >> 16) > 0 {
-		c = (c & 0xffff) + (c >> 16)
-	}
-
-	return ^uint16(c)
-}