From a74450a5452846a62e5d96785f7f9ed6097baeae Mon Sep 17 00:00:00 2001
From: Kevin Wallace <doof@doof.net>
Date: Wed, 20 Feb 2019 01:06:49 -0800
Subject: add -txt flag to also look up TXT records

---
 main.go | 52 +++++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 51 insertions(+), 1 deletion(-)

diff --git a/main.go b/main.go
index 2096965..7f9f8b8 100644
--- a/main.go
+++ b/main.go
@@ -8,6 +8,10 @@ import (
 	"strings"
 )
 
+var (
+	txt = flag.Bool("txt", false, "also look up TXT records")
+)
+
 func main() {
 	flag.Parse()
 	for _, arg := range flag.Args() {
@@ -19,10 +23,20 @@ func main() {
 				ip := ipnet.IP
 				for ipnet.Contains(ip) {
 					if names, err := net.LookupAddr(ip.String()); err != nil {
-						fmt.Fprintf(os.Stderr, "%s\t# error: %s\n", ip, err)
+						fmt.Fprintf(os.Stderr, "%s\t# PTR error: %s\n", ip, err)
 					} else {
 						fmt.Printf("%s\t%s\n", ip, strings.Join(names, " "))
 					}
+					if *txt {
+						recs, err := net.LookupTXT(arpa(ip))
+						if err != nil {
+							fmt.Fprintf(os.Stderr, "%s\t# TXT error: %s\n", ip, err)
+						} else {
+							for _, rec := range recs {
+								fmt.Printf("%s\tTXT %q\n", ip, rec)
+							}
+						}
+					}
 					ip = next(ip)
 				}
 			}
@@ -30,6 +44,42 @@ func main() {
 	}
 }
 
+// arpa returns the .in-addr.arpa or .ip6.arpa name corresponding with the given IP.
+func arpa(ip net.IP) string {
+	if ip4 := ip.To4(); ip4 != nil {
+		return fmt.Sprintf(
+			"%d.%d.%d.%d.in-addr.arpa",
+			ip4[3], ip4[2], ip4[1], ip4[0],
+		)
+	} else if ip6 := ip.To16(); ip6 != nil {
+		return fmt.Sprintf(
+			"%x.%x.%x.%x.%x.%x.%x.%x."+
+				"%x.%x.%x.%x.%x.%x.%x.%x."+
+				"%x.%x.%x.%x.%x.%x.%x.%x."+
+				"%x.%x.%x.%x.%x.%x.%x.%x."+
+				"ip6.arpa",
+			ip6[15]&0xf, ip6[15]&0xf0>>4,
+			ip6[14]&0xf, ip6[14]&0xf0>>4,
+			ip6[13]&0xf, ip6[13]&0xf0>>4,
+			ip6[12]&0xf, ip6[12]&0xf0>>4,
+			ip6[11]&0xf, ip6[11]&0xf0>>4,
+			ip6[10]&0xf, ip6[10]&0xf0>>4,
+			ip6[9]&0xf, ip6[9]&0xf0>>4,
+			ip6[8]&0xf, ip6[8]&0xf0>>4,
+			ip6[7]&0xf, ip6[7]&0xf0>>4,
+			ip6[6]&0xf, ip6[6]&0xf0>>4,
+			ip6[5]&0xf, ip6[5]&0xf0>>4,
+			ip6[4]&0xf, ip6[4]&0xf0>>4,
+			ip6[3]&0xf, ip6[3]&0xf0>>4,
+			ip6[2]&0xf, ip6[2]&0xf0>>4,
+			ip6[1]&0xf, ip6[1]&0xf0>>4,
+			ip6[0]&0xf, ip6[0]&0xf0>>4,
+		)
+	} else {
+		panic("non-ip4 non-ip6 ip? " + ip.String())
+	}
+}
+
 // 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))
-- 
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