Resolve hexadecimal notation for IPv4 addresses

e.g. `7f000001.sslip.io` → 127.0.0.1

This came about as a result of the nip.io migration to sslip.io servers:
nip.io supported hexadecimal notation; sslip.io didn't. Several nip.io
users were blindsided by the feature's lack, and raised an issue.

- The hexadecimal-notated IPv4 must be exactly 8 hexadecimal characters,
  no separators.
- Any hexadecimal notation _must_ be bookended by dots or by the
  beginning or end of the string (www.0a09091e.sslip.io or
  0a09091e.sslip.io). No dashes.
- If a normal IP notation and a hex notation are in the same hostname,
  then the normal IP notation takes precedence. This preserves existing
  behavior for sslip.io users, e.g. (0a09091e.127-0-0-1.sslip.io
  resolves to 127.0.0.1, not 10.9.9.30)

[#92]

integration_flags_test.go

@@ -136,6 +136,22 @@ var _ = Describe("flags", func() {
 			Eventually(digSession, 1).Should(Exit(0))
 			Eventually(string(serverSession.Err.Contents())).Should(MatchRegexp(`\? nil, SOA 8-8-8-8\.sslip\.io\. briancunnie\.gmail\.com\.`))
 		})
+		It("doesn't resolve public IPv4 addresses (hexadecimal)", func() {
+			digArgs := "@localhost 08080808.sslip.io -p " + strconv.Itoa(port)
+			digCmd := exec.Command("dig", strings.Split(digArgs, " ")...)
+			digSession, err := Start(digCmd, GinkgoWriter, GinkgoWriter)
+			Expect(err).ToNot(HaveOccurred())
+			Eventually(digSession, 1).Should(Exit(0))
+			Eventually(string(serverSession.Err.Contents())).Should(MatchRegexp(`\? nil, SOA 08080808\.sslip\.io\. briancunnie\.gmail\.com\.`))
+		})
+		It("resolves private IPv4 addresses (hexadecimal)", func() {
+			digArgs := "@localhost 7f000001.sslip.io -p " + strconv.Itoa(port)
+			digCmd := exec.Command("dig", strings.Split(digArgs, " ")...)
+			digSession, err := Start(digCmd, GinkgoWriter, GinkgoWriter)
+			Expect(err).ToNot(HaveOccurred())
+			Eventually(digSession, 1).Should(Exit(0))
+			Eventually(string(serverSession.Err.Contents())).Should(MatchRegexp(`TypeA 7f000001.sslip.io. \? 127.0.0.1`))
+		})
 		It("doesn't resolve public IPv6 addresses", func() {
 			digArgs := "@localhost aaaa 2600--.sslip.io -p " + strconv.Itoa(port)
 			digCmd := exec.Command("dig", strings.Split(digArgs, " ")...)

xip/xip.go

@@ -5,6 +5,7 @@ package xip
 
 import (
 	"bufio"
+	"encoding/hex"
 	"errors"
 	"fmt"
 	"io"
@@ -83,6 +84,7 @@ type DomainCustomizations map[string]DomainCustomization
 var (
 	ipv4REDots   = regexp.MustCompile(`(^|[.-])(((25[0-5]|(2[0-4]|1?\d)?\d)\.){3}(25[0-5]|(2[0-4]|1?\d)?\d))($|[.-])`)
 	ipv4REDashes = regexp.MustCompile(`(^|[.-])(((25[0-5]|(2[0-4]|1?\d)?\d)-){3}(25[0-5]|(2[0-4]|1?\d)?\d))($|[.-])`)
+	ipv4REHex    = regexp.MustCompile(`(^|\.)([[:xdigit:]]{8})($|\.)`) // no dash separators, only dots
 	// https://stackoverflow.com/questions/53497/regular-expression-that-matches-valid-ipv6-addresses
 	ipv6RE           = regexp.MustCompile(`(^|[.-])(([[:xdigit:]]{1,4}-){7}[[:xdigit:]]{1,4}|([[:xdigit:]]{1,4}-){1,7}-|([[:xdigit:]]{1,4}-){1,6}-[[:xdigit:]]{1,4}|([[:xdigit:]]{1,4}-){1,5}(-[[:xdigit:]]{1,4}){1,2}|([[:xdigit:]]{1,4}-){1,4}(-[[:xdigit:]]{1,4}){1,3}|([[:xdigit:]]{1,4}-){1,3}(-[[:xdigit:]]{1,4}){1,4}|([[:xdigit:]]{1,4}-){1,2}(-[[:xdigit:]]{1,4}){1,5}|[[:xdigit:]]{1,4}-((-[[:xdigit:]]{1,4}){1,6})|-((-[[:xdigit:]]{1,4}){1,7}|-)|fe80-(-[[:xdigit:]]{0,4}){0,4}%[\da-zA-Z]+|--(ffff(-0{1,4})?-)?((25[0-5]|(2[0-4]|1?\d)?\d)\.){3}(25[0-5]|(2[0-4]|1?\d)?\d)|([[:xdigit:]]{1,4}-){1,4}-((25[0-5]|(2[0-4]|1?\d)?\d)\.){3}(25[0-5]|(2[0-4]|1?\d)?\d))($|[.-])`)
 	ipv4ReverseRE    = regexp.MustCompile(`^(.*)\.in-addr\.arpa\.$`)
@@ -783,6 +785,28 @@ func NameToA(fqdnString string, allowPublicIPs bool) []dnsmessage.AResource {
 			}
 		}
 	}
+	if match := ipv4REHex.FindSubmatch(fqdn); match != nil {
+		hexes := match[2] // strip out leading & trailing "." by using only the 2nd capture group, e.g. "7f000001"
+		ipBytes := make([]byte, 4)
+		_, err := hex.Decode(ipBytes, []byte(hexes))
+		if err != nil || len(ipBytes) != 4 {
+			return []dnsmessage.AResource{}
+		}
+		ipv4address := net.IPv4(ipBytes[0], ipBytes[1], ipBytes[2], ipBytes[3])
+		if ipv4address == nil {
+			return []dnsmessage.AResource{}
+		}
+		ipv4address = ipv4address.To4()
+		if ipv4address == nil {
+			return []dnsmessage.AResource{}
+		}
+		if (!allowPublicIPs) && IsPublic(ipv4address) {
+			return []dnsmessage.AResource{}
+		}
+		return []dnsmessage.AResource{
+			{A: [4]byte{ipv4address[0], ipv4address[1], ipv4address[2], ipv4address[3]}},
+		}
+	}
 	return []dnsmessage.AResource{}
 }
 

xip/xip_test.go

@@ -198,8 +198,8 @@ var _ = Describe("Xip", func() {
 		DescribeTable("when it succeeds",
 			func(fqdn string, expectedA dnsmessage.AResource) {
 				ipv4Answers := xip.NameToA(fqdn, true)
-				Expect(len(ipv4Answers)).To(Equal(1))
 				Expect(ipv4Answers[0]).To(Equal(expectedA))
+				Expect(len(ipv4Answers)).To(Equal(1))
 			},
 			Entry("custom record", "CusTom.RecOrd.", dnsmessage.AResource{A: [4]byte{78, 46, 204, 247}}),
 			// dots
@@ -213,6 +213,18 @@ var _ = Describe("Xip", func() {
 			Entry("IETF protocol assignments with domain and www", "www-192-0-0-1-com", dnsmessage.AResource{A: [4]byte{192, 0, 0, 1}}),
 			// dots-and-dashes, mix-and-matches
 			Entry("Pandaxin's paradox", "minio-01.192-168-1-100.sslip.io", dnsmessage.AResource{A: [4]byte{192, 168, 1, 100}}),
+			Entry("Hexadecimal #0", "filer.7f000001.sslip.io", dnsmessage.AResource{A: [4]byte{127, 0, 0, 1}}),
+			Entry("Hexadecimal #1, TLD", "0A09091E", dnsmessage.AResource{A: [4]byte{10, 9, 9, 30}}),
+			Entry("Hexadecimal #1, TLD #2", "0A09091E.", dnsmessage.AResource{A: [4]byte{10, 9, 9, 30}}),
+			Entry("Hexadecimal #1, TLD #3", ".0A09091E.", dnsmessage.AResource{A: [4]byte{10, 9, 9, 30}}),
+			Entry("Hexadecimal #1, TLD #4", "www.0A09091E.", dnsmessage.AResource{A: [4]byte{10, 9, 9, 30}}),
+			Entry("Hexadecimal #2, mixed case", "ffffFFFF.nip.io", dnsmessage.AResource{A: [4]byte{255, 255, 255, 255}}),
+			Entry("Hexadecimal #3, different numbers", "www.fedcba98.nip.io", dnsmessage.AResource{A: [4]byte{254, 220, 186, 152}}),
+			Entry("Hexadecimal #3, different numbers #2", "www.76543210.nip.io", dnsmessage.AResource{A: [4]byte{118, 84, 50, 16}}),
+			Entry("Hexadecimal #4, dashes trump hex", "www.127-0-0-53.76543210.nip.io", dnsmessage.AResource{A: [4]byte{127, 0, 0, 53}}),
+			Entry("Hexadecimal #4, dashes trump hex #2", "www.76543210.127-0-0-53.nip.io", dnsmessage.AResource{A: [4]byte{127, 0, 0, 53}}),
+			Entry("Hexadecimal #4, dots trump hex", "www.127.0.0.53.76543210.nip.io", dnsmessage.AResource{A: [4]byte{127, 0, 0, 53}}),
+			Entry("Hexadecimal #4, dots trump hex #2", "www.76543210.127.0.0.53.nip.io", dnsmessage.AResource{A: [4]byte{127, 0, 0, 53}}),
 		)
 		DescribeTable("when it does NOT match an IP address",
 			func(fqdn string) {
@@ -228,6 +240,10 @@ var _ = Describe("Xip", func() {
 			Entry("NS but no dot", "ns-hetzner.sslip.io"),
 			Entry("NS + cruft at beginning", "p-ns-hetzner.sslip.io"),
 			Entry("test-net address with dots-and-dashes mixed", "www-192.0-2.3.example-me.com"),
+			Entry("Hexadecimal with too many digits (9 instead of 8)", "www.0A09091E0.com"),
+			Entry("Hexadecimal with too few  digits (7 instead of 8)", "www.0A09091.com"),
+			Entry("Hexadecimal with a dash instead of a .", "www-0A09091E.com"),
+			Entry("Hexadecimal with a dash instead of a . #2", "www.0A09091E-com"),
 		)
 		When("There is more than one A record", func() {
 			It("returns them all", func() {