Simplified reverse DNS lookups.

When the user tries to query an IP address dnslookup will detect
it and convert it to a proper PTR query (if RRTYPE is not specified).

Closes #71

README.md

@@ -77,13 +77,13 @@ DNS-over-HTTPS with IP:
 dnslookup example.org https://dns.adguard.com/dns-query 94.140.14.14
 ```
 
-DNS-over-HTTPS with basic auth (supported by [AdGuard DNS](https://adguard-dns.io/)):
+DNS-over-HTTPS with basic auth (supported
+by [AdGuard DNS](https://adguard-dns.io/)):
 
 ```shell
 dnslookup example.org https://username:password@d.adguard-dns.com/dns-query
 ```
 
-
 DNSCrypt (stamp):
 
 ```shell
@@ -102,6 +102,20 @@ DNS-over-QUIC:
 dnslookup example.org quic://dns.adguard.com
 ```
 
+Sending a PTR query for an IPv4 address (IP address is recognized automatically
+when RRTYPE is not specified):
+
+```shell
+dnslookup 8.8.8.8
+```
+
+Sending a PTR query for an IPv6 address (IP address is recognized automatically
+when RRTYPE is not specified):
+
+```shell
+dnslookup 2606:4700:4700::1111
+```
+
 Machine-readable format:
 
 ```shell
@@ -154,6 +168,7 @@ EDNSOPT=65074:3132333435363738 RRTYPE=TXT dnslookup o-o.myaddr.l.google.com tls:
 ```
 
 Combine multiple options:
+
 ```shell
 RRTYPE=TXT SUBNET=1.1.1.1/24 PAD=1 dnslookup o-o.myaddr.l.google.com tls://8.8.8.8
 ```

main.go

@@ -38,11 +38,10 @@ func main() {
 	http3Enabled := os.Getenv("HTTP3") == "1"
 	verbose := os.Getenv("VERBOSE") == "1"
 	padding := os.Getenv("PAD") == "1"
-	class := getClass()
 	do := os.Getenv("DNSSEC") == "1"
 	subnetOpt := getSubnet()
 	ednsOpt := getEDNSOpt()
-	rrType := getRRType()
+	question := getQuestion()
 
 	if verbose {
 		log.SetLevel(log.DEBUG)
@@ -84,8 +83,6 @@ func main() {
 		timeout = i
 	}
 
-	domain := os.Args[1]
-
 	var server string
 	if len(os.Args) > 2 {
 		server = os.Args[2]
@@ -149,9 +146,7 @@ func main() {
 	req := &dns.Msg{}
 	req.Id = dns.Id()
 	req.RecursionDesired = true
-	req.Question = []dns.Question{
-		{Name: domain + ".", Qtype: rrType, Qclass: class},
-	}
+	req.Question = []dns.Question{question}
 
 	if subnetOpt != nil {
 		opt := getOrCreateOpt(req, do)
@@ -239,6 +234,64 @@ func getEDNSOpt() (option *dns.EDNS0_LOCAL) {
 	}
 }
 
+// getQuestion returns a DNS question for the query.
+func getQuestion() (q dns.Question) {
+	domain := os.Args[1]
+	rrType := getRRType()
+	qClass := getClass()
+
+	// If the user tries to query an IP address and does not specify any
+	// query type, convert to PTR automatically.
+	ip := net.ParseIP(domain)
+	if os.Getenv("RRTYPE") == "" && ip != nil {
+		domain = ipToPtr(ip)
+		rrType = dns.TypePTR
+	}
+
+	q.Name = dns.Fqdn(domain)
+	q.Qtype = rrType
+	q.Qclass = qClass
+
+	return q
+}
+
+func ipToPtr(ip net.IP) (ptr string) {
+	if ip.To4() != nil {
+		return ip4ToPtr(ip)
+	}
+
+	return ip6ToPtr(ip)
+}
+
+func ip4ToPtr(ip net.IP) (ptr string) {
+	parts := strings.Split(ip.String(), ".")
+	for i := range parts {
+		ptr = parts[i] + "." + ptr
+	}
+	ptr = ptr + "in-addr.arpa."
+
+	return
+}
+
+func ip6ToPtr(ip net.IP) (ptr string) {
+	addr, _ := netip.ParseAddr(ip.String())
+	str := addr.StringExpanded()
+
+	// Remove colons and reverse the order of characters.
+	str = strings.ReplaceAll(str, ":", "")
+	reversed := ""
+	for i := len(str) - 1; i >= 0; i-- {
+		reversed += string(str[i])
+		if i != 0 {
+			reversed += "."
+		}
+	}
+
+	ptr = reversed + ".ip6.arpa."
+
+	return ptr
+}
+
 func getSubnet() (option *dns.EDNS0_SUBNET) {
 	subnetStr := os.Getenv("SUBNET")
 	if subnetStr == "" {