From 5603a64f542f454bcb22523c9d84049abd001748 Mon Sep 17 00:00:00 2001
From: Kevin Wallace <kevin@pentabarf.net>
Date: Thu, 9 Jun 2016 22:42:27 -0700
Subject: initial commit

---
 main.go | 99 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 99 insertions(+)
 create mode 100644 main.go

diff --git a/main.go b/main.go
new file mode 100644
index 0000000..241a6b5
--- /dev/null
+++ b/main.go
@@ -0,0 +1,99 @@
+package main
+
+import (
+	"flag"
+	"fmt"
+	"net"
+	"os"
+	"strings"
+)
+
+func main() {
+	flag.Parse()
+	for _, arg := range flag.Args() {
+		if ipnets, err := resolveArg(arg); err != nil {
+			fmt.Fprintf(os.Stderr, "# couldn't resolve %s: %s\n", arg, err)
+		} else {
+			fmt.Printf("# %s\n", arg)
+			for _, ipnet := range ipnets {
+				ip := ipnet.IP
+				for ipnet.Contains(ip) {
+					line := ip.String() + "\t"
+					names, err := net.LookupAddr(ip.String())
+					line += strings.Join(names, " ")
+					if err != nil {
+						line += fmt.Sprintf(" # %s", err)
+					}
+					fmt.Println(line)
+					ip = next(ip)
+				}
+			}
+		}
+	}
+}
+
+// next returns the next IP address after ip, or nil if there is no such address.
+func next(ip net.IP) net.IP {
+	next := make(net.IP, len(ip))
+	copy(next, ip)
+	for i := len(next) - 1; i >= 0; i-- {
+		next[i]++
+		if next[i] != 0 {
+			return next
+		}
+	}
+	return nil
+}
+
+// resolveArg parses an argument of one of the following forms:
+// * 1.2.3.4
+// * 1.2.3.4/24
+// * 2001::
+// * 2001::/16
+// * hostname.example.com
+// If the given address is a hostname, it is resolved to all of its A and AAAA records.
+// It returns the specified networks as a slice of net.IPNet.
+func resolveArg(arg string) ([]net.IPNet, error) {
+	parts := strings.SplitN(arg, "/", 2)
+	ips, err := resolveHost(parts[0], len(parts) == 1)
+	if err != nil {
+		return nil, err
+	}
+
+	ipnets := make([]net.IPNet, len(ips))
+	for i, ip := range ips {
+		if len(parts) > 1 {
+			if _, ipnet, err := net.ParseCIDR(ip.String() + "/" + parts[1]); err != nil {
+				return nil, err
+			} else {
+				ipnets[i] = *ipnet
+			}
+		} else {
+			ipnets[i].IP = ip
+			ipnets[i].Mask = net.CIDRMask(len(ip)*8, len(ip)*8)
+		}
+	}
+	return ipnets, nil
+}
+
+func resolveHost(host string, canLookup bool) ([]net.IP, error) {
+	ip := parseIP(host)
+	if ip != nil {
+		return []net.IP{ip}, nil
+	}
+	if canLookup {
+		return net.LookupIP(host)
+	}
+	return nil, fmt.Errorf("can't parse IP")
+}
+
+func parseIP(s string) net.IP {
+	ip := net.ParseIP(s)
+	// net.ParseIP provides no way to distinguish "::ffff:1.2.3.4" from "1.2.3.4";
+	// this attempts to do so by assuming any IPv6 address contains a ':'.
+	// I think this is an accurate heuristic, but I am not sure of it.
+	if !strings.Contains(s, ":") {
+		ip = ip.To4()
+	}
+	return ip
+}
-- 
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