From 9e3656fae97f7d2f826479c460852419f978250a Mon Sep 17 00:00:00 2001
From: Karan Sharma <hello@mrkaran.dev>
Date: Fri, 25 Jun 2021 17:25:04 +0530
Subject: [PATCH] feat: Add support for reverse DNS lookups

Closes https://github.com/mr-karan/doggo/issues/25
---
 README.md                 |  1 +
 cmd/doggo/cli/cli.go      | 10 ++++++++++
 cmd/doggo/cli/help.go     |  1 +
 internal/app/questions.go | 20 ++++++++++++++++++++
 pkg/models/models.go      |  7 ++++---
 5 files changed, 36 insertions(+), 3 deletions(-)

diff --git a/README.md b/README.md
index 500a407..6f828fb 100644
--- a/README.md
+++ b/README.md
@@ -32,6 +32,7 @@ to experiment with writing a DNS Client from scratch in `Go` myself. Hence the n
 - Supports IPv4 **and** IPv6 _both_.
 - Available as a web tool as well: [https://doggo.mrkaran.dev](https://doggo.mrkaran.dev).
 - Shell completions for `zsh` and `fish`.
+- Reverse DNS Lookups.
 
 ## Installation
 
diff --git a/cmd/doggo/cli/cli.go b/cmd/doggo/cli/cli.go
index ff8a9f6..a413f18 100644
--- a/cmd/doggo/cli/cli.go
+++ b/cmd/doggo/cli/cli.go
@@ -37,6 +37,7 @@ func main() {
 	f.StringSliceP("type", "t", []string{}, "Type of DNS record to be queried (A, AAAA, MX etc)")
 	f.StringSliceP("class", "c", []string{}, "Network class of the DNS record to be queried (IN, CH, HS etc)")
 	f.StringSliceP("nameservers", "n", []string{}, "Address of the nameserver to send packets to")
+	f.BoolP("reverse", "x", false, "Performs a DNS Lookup for an IPv4 or IPv6 address. Sets the query type and class to PTR and IN respectively.")
 
 	// Resolver Options
 	f.Int("timeout", 5, "Sets the timeout for a query to T seconds. The default timeout is 5 seconds.")
@@ -94,6 +95,15 @@ func main() {
 	app.QueryFlags.QClasses = append(app.QueryFlags.QClasses, qc...)
 	app.QueryFlags.QNames = append(app.QueryFlags.QNames, qn...)
 
+	// Check if reverse flag is passed. If it is, then set
+	// query type as PTR and query class as IN.
+	// Modify query name like 94.2.0.192.in-addr.arpa if it's an IPv4 address.
+	// Use IP6.ARPA nibble format otherwise.
+
+	if app.QueryFlags.ReverseLookup {
+		app.ReverseLookup()
+	}
+
 	// Load fallbacks.
 	app.LoadFallbacks()
 
diff --git a/cmd/doggo/cli/help.go b/cmd/doggo/cli/help.go
index c27ccf8..a391dbc 100644
--- a/cmd/doggo/cli/help.go
+++ b/cmd/doggo/cli/help.go
@@ -43,6 +43,7 @@ var appHelpTextTemplate = `{{ "NAME" | color "" "heading" }}:
   {{"-t, --type=TYPE" | color "yellow" ""}}             Type of the DNS Record ({{"A, MX, NS" | color "cyan" ""}} etc).
   {{"-n, --nameserver=ADDR" | color "yellow" ""}}       Address of a specific nameserver to send queries to ({{"9.9.9.9, 8.8.8.8" | color "cyan" ""}} etc).
   {{"-c, --class=CLASS" | color "yellow" ""}}           Network class of the DNS record ({{"IN, CH, HS" | color "cyan" ""}} etc).
+  {{"-x, --reverse" | color "yellow" ""}}               Performs a DNS Lookup for an IPv4 or IPv6 address. Sets the query type and class to PTR and IN respectively.
 
 {{ "Resolver Options" | color "" "heading" }}:
   {{"--ndots=INT" | color "yellow" ""}}        Specify ndots parameter. Takes value from /etc/resolv.conf if using the system namesever or 1 otherwise.
diff --git a/internal/app/questions.go b/internal/app/questions.go
index d95de6d..b31a1e3 100644
--- a/internal/app/questions.go
+++ b/internal/app/questions.go
@@ -33,3 +33,23 @@ func (app *App) PrepareQuestions() {
 		}
 	}
 }
+
+// ReverseLookup is used to perform a reverse DNS Lookup
+// using an IPv4 or IPv6 address.
+// Query Type is set to PTR, Query Class is set to IN.
+// Query Names must be formatted in in-addr.arpa. or ip6.arpa format.
+func (app *App) ReverseLookup() {
+	app.QueryFlags.QTypes = []string{"PTR"}
+	app.QueryFlags.QClasses = []string{"IN"}
+	formattedNames := make([]string, 0, len(app.QueryFlags.QNames))
+
+	for _, n := range app.QueryFlags.QNames {
+		addr, err := dns.ReverseAddr(n)
+		if err != nil {
+			app.Logger.WithError(err).Error("error formatting address")
+			app.Logger.Exit(2)
+		}
+		formattedNames = append(formattedNames, addr)
+	}
+	app.QueryFlags.QNames = formattedNames
+}
diff --git a/pkg/models/models.go b/pkg/models/models.go
index a3c0852..e6cca57 100644
--- a/pkg/models/models.go
+++ b/pkg/models/models.go
@@ -3,11 +3,11 @@ package models
 import "time"
 
 const (
-	// DefaultTLSPort specifies the default port for a DNS server connecting over TCP over TLS
+	// DefaultTLSPort specifies the default port for a DNS server connecting over TCP over TLS.
 	DefaultTLSPort = "853"
-	// DefaultUDPPort specifies the default port for a DNS server connecting over UDP
+	// DefaultUDPPort specifies the default port for a DNS server connecting over UDP.
 	DefaultUDPPort = "53"
-	// DefaultTCPPort specifies the default port for a DNS server connecting over TCP
+	// DefaultTCPPort specifies the default port for a DNS server connecting over TCP.
 	DefaultTCPPort   = "53"
 	UDPResolver      = "udp"
 	DOHResolver      = "doh"
@@ -31,6 +31,7 @@ type QueryFlags struct {
 	DisplayTimeTaken bool          `koanf:"time" json:"-"`
 	ShowJSON         bool          `koanf:"json" json:"-"`
 	UseSearchList    bool          `koanf:"search" json:"-"`
+	ReverseLookup    bool          `koanf:"reverse" reverse:"-"`
 }
 
 // Nameserver represents the type of Nameserver