dns: add function to lookup PTR record by IP address

The function LookupPTR accept any IP address (either IPv4 or IPv6) and
return a single domain name on success or an error on failed.  If IP
address does not have PTR record it will return empty string without
error.

4 files changed, 200 insertions, 4 deletions

diff --git a/lib/dns/funcs.go b/lib/dns/funcs.go
index 756c3472..8687c4b7 100644
--- a/lib/dns/funcs.go
+++ b/lib/dns/funcs.go
@@ -5,7 +5,9 @@
 package dns
 
 import (
+	"fmt"
 	"net"
+	"strings"
 
 	libnet "github.com/shuLhan/share/lib/net"
 )
@@ -45,3 +47,125 @@ func ParseNameServers(nameservers []string) ([]*net.UDPAddr, error) {
 
 	return udpAddrs, nil
 }
+
+//
+// LookupPTR accept an IP address (either IPv4 or IPv6) and return a single
+// answer as domain name on sucess or an error on failed.
+// If IP address does not contains PTR record it will return an empty string
+// without error.
+//
+func LookupPTR(client Client, ip net.IP) (answer string, err error) {
+	if ip == nil {
+		return "", fmt.Errorf("empty IP address")
+	}
+
+	revIP, isIPv4 := reverseIP(ip)
+	if len(revIP) == 0 {
+		return "", fmt.Errorf("invalid IP address %q", ip)
+	}
+
+	if isIPv4 {
+		revIP = append(revIP, []byte(".in-addr.arpa")...)
+	} else {
+		revIP = append(revIP, []byte(".ip6.arpa")...)
+	}
+
+	msg, err := client.Lookup(true, QueryTypePTR, QueryClassIN, revIP)
+	if err != nil {
+		return "", err
+	}
+
+	rranswers := msg.FilterAnswers(QueryTypePTR)
+	if len(rranswers) == 0 {
+		return "", nil
+	}
+
+	banswer, ok := rranswers[0].RData().([]byte)
+	if !ok {
+		return "", fmt.Errorf("invalid PTR record data")
+	}
+
+	answer = string(banswer)
+
+	return
+}
+
+//
+// reverseIP reverse the IP address by dot.
+//
+func reverseIP(ip net.IP) (revIP []byte, isIPv4 bool) {
+	strIP := ip.String()
+
+	if strings.Count(strIP, ".") == 3 {
+		isIPv4 = true
+		revIP = reverseIPv4(strIP)
+		return
+	}
+	if strings.Count(strIP, ":") >= 2 {
+		revIP = reverseIPv6(strIP)
+		return
+	}
+
+	return nil, false
+}
+
+//
+// reverseIPv4 reverse the IPv4 address. For example, given "127.0.0.1" it
+// will return "1.0.0.127".
+//
+func reverseIPv4(ip string) (rev []byte) {
+	addrs := strings.Split(ip, ".")
+	for x := len(addrs) - 1; x >= 0; x-- {
+		if len(rev) > 0 {
+			rev = append(rev, '.')
+		}
+		rev = append(rev, addrs[x]...)
+	}
+	return
+}
+
+//
+// reverseIPv6 reverse the IPv6 address.  For example, given "2001:db8::cb01"
+// it will return "1.0.b.c.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.1.0.0.2".
+//
+func reverseIPv6(ip string) (rev []byte) {
+	addrs := strings.Split(ip, ":")
+
+	var notempty int
+	for x := 0; x < len(addrs); x++ {
+		if len(addrs[x]) != 0 {
+			notempty++
+		}
+	}
+	gap := 8 - notempty
+
+	for x := len(addrs) - 1; x >= 0; x-- {
+		addr := addrs[x]
+
+		// Fill the gap with "0.0.0.0".
+		if len(addr) == 0 {
+			for ; gap > 0; gap-- {
+				if len(rev) > 0 {
+					rev = append(rev, '.')
+				}
+				rev = append(rev, []byte("0.0.0.0")...)
+			}
+			continue
+		}
+
+		// Reverse the sub address "2001" into "1.0.0.2".
+		for y := len(addr) - 1; y >= 0; y-- {
+			if len(rev) > 0 {
+				rev = append(rev, '.')
+			}
+			rev = append(rev, addr[y])
+		}
+
+		// Fill the sub address with zero.
+		for y := len(addr); y < 4; y++ {
+			rev = append(rev, []byte(".0")...)
+		}
+	}
+
+	return
+}
diff --git a/lib/dns/funcs_test.go b/lib/dns/funcs_test.go
index fb9e2946..56699c9f 100644
--- a/lib/dns/funcs_test.go
+++ b/lib/dns/funcs_test.go
@@ -5,6 +5,7 @@
 package dns
 
 import (
+	"net"
 	"testing"
 
 	"github.com/shuLhan/share/lib/test"
@@ -29,3 +30,68 @@ func TestGetSystemNameServers(t *testing.T) {
 		test.Assert(t, "NameServers", c.exp, got, true)
 	}
 }
+
+func TestReverseIP(t *testing.T) {
+	cases := []struct {
+		ip        string
+		exp       []byte
+		expIsIPv4 bool
+	}{{
+		ip: "",
+	}, {
+		ip:        "192.0.2.1",
+		exp:       []byte("1.2.0.192"),
+		expIsIPv4: true,
+	}, {
+		ip:  "2001:db8::68",
+		exp: []byte("8.6.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.8.b.d.0.1.0.0.2"),
+	}}
+
+	for _, c := range cases {
+		ip := net.ParseIP(c.ip)
+
+		gotIP, gotIsIPv4 := reverseIP(ip)
+
+		test.Assert(t, "reverseIP", c.exp, gotIP, true)
+		test.Assert(t, "isIPv4", c.expIsIPv4, gotIsIPv4, true)
+	}
+}
+
+func TestLookupPTR(t *testing.T) {
+	cl, err := NewUDPClient(testServerAddress)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	cases := []struct {
+		ip     net.IP
+		exp    string
+		expErr string
+	}{{
+		ip:     nil,
+		expErr: "empty IP address",
+	}, {
+		ip: net.ParseIP("127.0.0.2"),
+	}, {
+		ip:  net.ParseIP("127.0.0.10"),
+		exp: "kilabit.info",
+	}, {
+		ip:  net.ParseIP("::1"),
+		exp: "kilabit.info",
+	}, {
+		ip:  net.ParseIP("2001:db8::cb01"),
+		exp: "kilabit.info",
+	}}
+
+	for _, c := range cases {
+		t.Logf("ip: %s", c.ip)
+
+		got, err := LookupPTR(cl, c.ip)
+		if err != nil {
+			test.Assert(t, "error", c.expErr, err.Error(), true)
+			continue
+		}
+
+		test.Assert(t, "LookupPTR", c.exp, got, true)
+	}
+}
diff --git a/lib/dns/idpool_test.go b/lib/dns/idpool_test.go
index c5ce44fd..641f524b 100644
--- a/lib/dns/idpool_test.go
+++ b/lib/dns/idpool_test.go
@@ -11,8 +11,8 @@ import (
 )
 
 func TestIDPool(t *testing.T) {
-	test.Assert(t, "getNextID()=1", getNextID(), uint16(1), true)
-	test.Assert(t, "getNextID()=2", getNextID(), uint16(2), true)
-	test.Assert(t, "getNextID()=3", getNextID(), uint16(3), true)
-	test.Assert(t, "getNextID()=4", getNextID(), uint16(4), true)
+	test.Assert(t, "getNextID()=5", getNextID(), uint16(5), true)
+	test.Assert(t, "getNextID()=6", getNextID(), uint16(6), true)
+	test.Assert(t, "getNextID()=7", getNextID(), uint16(7), true)
+	test.Assert(t, "getNextID()=8", getNextID(), uint16(8), true)
 }
diff --git a/lib/dns/testdata/kilabit.info b/lib/dns/testdata/kilabit.info
index f6dbde12..d0165bb6 100644
--- a/lib/dns/testdata/kilabit.info
+++ b/lib/dns/testdata/kilabit.info
@@ -7,3 +7,9 @@
 
 @  IN A   127.0.0.1
       TXT "This is a test server"
+
+10.0.0.127.in-addr.arpa. PTR kilabit.info.
+
+1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.ip6.arpa. PTR kilabit.info.
+
+1.0.b.c.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.8.b.d.0.1.0.0.2.ip6.arpa. PTR kilabit.info.