gip: expand the README to inlcude explanations

Signed-off-by: Nagy Károly Gábriel <k@jpi.io>

README.md

@@ -1,3 +1,8 @@
 # gip
 
 Find your public IP
+gip will print your public IPv4 followed by a tab and your public IPv6.
+If either is missing they will be replaced by "". Please note that due 
+to their way usage the IPv4 will most probably be the public IP of your
+gateway while the IPv6 will be the public IP of the computer gip is run 
+on.

gip.go

@@ -2,8 +2,8 @@ package main
 
 import (
 	"fmt"
-
 	"github.com/miekg/dns"
+	"os"
 )
 
 /*
@@ -11,58 +11,102 @@ dig {-4,-6} +short myip.opendns.com @resolver1.opendns.com
 dig {-4,-6} TXT +short o-o.myaddr.l.google.com @ns1.google.com
 */
 
-func get4(c *dns.Client, m *dns.Msg, nameserver string) (*dns.Msg, error) {
-	c.Net = "udp4"
-	m.SetQuestion("myip.opendns.com.", dns.TypeA)
-
-	r, _, err := c.Exchange(m, nameserver)
-	return r, err
-}
-
-func get6(c *dns.Client, m *dns.Msg, nameserver string) (*dns.Msg, error) {
-	c.Net = "udp6"
-	m.SetQuestion("myip.opendns.com.", dns.TypeAAAA)
-
-	r, _, err := c.Exchange(m, nameserver)
-	return r, err
-
-}
-
-func printIP(ra []dns.RR) []string {
-	var ips []string
-	for _, ansa := range ra {
-		switch ansb := ansa.(type) {
-		case *dns.A:
-			ips = append(ips, ansb.A.String())
-		case *dns.AAAA:
-			ips = append(ips, ansb.AAAA.String())
-		}
-	}
-	return ips
-}
-
-func main() {
+func getO() (ipv4 string, ipv6 string) {
 	nameserver := dns.Fqdn("resolver1.opendns.com") + ":53"
 
 	c := new(dns.Client)
-	c.Net = "udp4"
 	m := &dns.Msg{
 		MsgHdr: dns.MsgHdr{
 			RecursionDesired: true,
 		},
 		Question: make([]dns.Question, 1),
 	}
-	r4, err := get4(c, m, nameserver)
-	if err != nil {
-		fmt.Println(err)
+	// get IPv4 first
+	c.Net = "udp4"
+	m.SetQuestion("myip.opendns.com.", dns.TypeA)
+	r4, _, e4 := c.Exchange(m, nameserver)
+	if e4 != nil {
+		ipv4 = ""
 	} else {
-		fmt.Println(printIP(r4.Answer))
+		ipv4 = getIP(r4.Answer)
 	}
-	r6, e6 := get6(c, m, nameserver)
-	if err != nil {
-		fmt.Println(e6)
+	// now IPv6
+	c.Net = "udp6"
+	m.SetQuestion("myip.opendns.com.", dns.TypeAAAA)
+	r6, _, e6 := c.Exchange(m, nameserver)
+	if e6 != nil {
+		ipv6 = ""
 	} else {
-		fmt.Println(printIP(r6.Answer))
+		ipv6 = getIP(r6.Answer)
 	}
-
+	return ipv4, ipv6
+}
+
+func getG() (ipv4 string, ipv6 string) {
+	nameserver := dns.Fqdn("ns1.google.com") + ":53"
+
+	c := new(dns.Client)
+	m := &dns.Msg{
+		MsgHdr: dns.MsgHdr{
+			RecursionDesired: true,
+		},
+		Question: make([]dns.Question, 1),
+	}
+	// get IPv4 first
+	c.Net = "udp4"
+	m.SetQuestion("o-o.myaddr.l.google.com.", dns.TypeTXT)
+	r4, _, e4 := c.Exchange(m, nameserver)
+	if e4 != nil {
+		ipv4 = ""
+	} else {
+		ipv4 = getIP(r4.Answer)
+	}
+	// now IPv6
+	c.Net = "udp6"
+	m.SetQuestion("o-o.myaddr.l.google.com.", dns.TypeTXT)
+	r6, _, e6 := c.Exchange(m, nameserver)
+	if e6 != nil {
+		ipv6 = ""
+	} else {
+		ipv6 = getIP(r6.Answer)
+	}
+	return ipv4, ipv6
+}
+
+func getIP(ra []dns.RR) string {
+	//the answer will always be one IP or none
+	var ip string
+	for _, ansa := range ra {
+		switch ansb := ansa.(type) {
+		case *dns.A:
+			ip = ansb.A.String()
+		case *dns.AAAA:
+			ip = ansb.AAAA.String()
+		case *dns.TXT:
+			// we always expect only one IP in the field
+			ip = ansb.Txt[0]
+		}
+	}
+	return ip
+}
+
+func main() {
+	var ipv4, ipv6 string
+	exit := 0
+	o4, o6 := getO()
+	g4, g6 := getG()
+	if o4 == g4 {
+		ipv4 = o4
+	} else {
+		ipv4 = ""
+		exit = 111
+	}
+	if o6 == g6 {
+		ipv6 = o6
+	} else {
+		ipv6 = ""
+		exit = 111
+	}
+	fmt.Printf("%s\t%s\n", ipv4, ipv6)
+	os.Exit(exit)
 }