util/linuxfw: initial implementation of package

This package is an initial implementation of something that can read
netfilter and iptables rules from the Linux kernel without needing to
shell out to an external utility; it speaks directly to the kernel using
syscalls and parses the data returned.

Currently this is read-only since it only knows how to parse a subset of
the available data.

Signed-off-by: Andrew Dunham <andrew@tailscale.com>
Change-Id: Iccadf5dcc081b73268d8ccf8884c24eb6a6f1ff5

util/linuxfw/helpers.go

@@ -0,0 +1,35 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package linuxfw
+
+import (
+	"encoding/hex"
+	"fmt"
+	"strings"
+	"unicode"
+)
+
+func formatMaybePrintable(b []byte) string {
+	// Remove a single trailing null, if any
+	if len(b) > 0 && b[len(b)-1] == 0 {
+		b = b[:len(b)-1]
+	}
+
+	nonprintable := strings.IndexFunc(string(b), func(r rune) bool {
+		return r > unicode.MaxASCII || !unicode.IsPrint(r)
+	})
+	if nonprintable >= 0 {
+		return "<hex>" + hex.EncodeToString(b)
+	}
+	return string(b)
+}
+
+func formatPortRange(r [2]uint16) string {
+	if r == [2]uint16{0, 65535} {
+		return fmt.Sprintf(`any`)
+	} else if r[0] == r[1] {
+		return fmt.Sprintf(`%d`, r[0])
+	}
+	return fmt.Sprintf(`%d-%d`, r[0], r[1])
+}