IPAddressV(46) methods to convert IPs to their inverse in-addr.arpa / ip6.arpa representation

Summary: As per the title, this adds methods to convert IPv(46) to their inverse representation, this is useful for applications that needs to calculate the inverse arpa representation, typically used when making PTR DNS requests.

Reviewed By: Orvid

Differential Revision: D4867502

fbshipit-source-id: 190e5c309b17a633e1c97b077f212ab38725860f

folly/IPAddressV4.cpp

@@ -224,6 +224,16 @@ string IPAddressV4::str() const {
   return detail::fastIpv4ToString(addr_.inAddr_);
 }
 
+// public
+string IPAddressV4::toInverseArpaName() const {
+  return sformat(
+      "{}.{}.{}.{}.in-addr.arpa",
+      addr_.bytes_[3],
+      addr_.bytes_[2],
+      addr_.bytes_[1],
+      addr_.bytes_[0]);
+}
+
 // public
 uint8_t IPAddressV4::getNthMSByte(size_t byteIndex) const {
   const auto highestIndex = byteCount() - 1;

folly/IPAddressV4.h

@@ -183,6 +183,8 @@ class IPAddressV4 {
   // @see IPAddress#str
   std::string str() const;
 
+  std::string toInverseArpaName() const;
+
   // return underlying in_addr structure
   in_addr toAddr() const { return addr_.inAddr_; }
 

folly/IPAddressV6.cpp

@@ -397,6 +397,19 @@ string IPAddressV6::toFullyQualified() const {
   return detail::fastIpv6ToString(addr_.in6Addr_);
 }
 
+// public
+string IPAddressV6::toInverseArpaName() const {
+  constexpr folly::StringPiece lut = "0123456789abcdef";
+  std::array<char, 32> a;
+  int j = 0;
+  for (int i = 15; i >= 0; i--) {
+    a[j] = (lut[bytes()[i] & 0xf]);
+    a[j + 1] = (lut[bytes()[i] >> 4]);
+    j += 2;
+  }
+  return sformat("{}.ip6.arpa", join(".", a));
+}
+
 // public
 uint8_t IPAddressV6::getNthMSByte(size_t byteIndex) const {
   const auto highestIndex = byteCount() - 1;

folly/IPAddressV6.h

@@ -274,6 +274,8 @@ class IPAddressV6 {
   // @see IPAddress#toFullyQualified
   std::string toFullyQualified() const;
 
+  std::string toInverseArpaName() const;
+
   // @see IPAddress#str
   std::string str() const;
 

folly/test/IPAddressTest.cpp

@@ -405,6 +405,17 @@ TEST(IPAddress, ToString) {
                               " - ", addr_1, " - ", addr_10_1_2_3));
 }
 
+TEST(IPaddress, toInverseArpaName) {
+  IPAddressV4 addr_ipv4("10.0.0.1");
+  EXPECT_EQ("1.0.0.10.in-addr.arpa", addr_ipv4.toInverseArpaName());
+  IPAddressV6 addr_ipv6("2620:0000:1cfe:face:b00c:0000:0000:0003");
+  EXPECT_EQ(
+      sformat(
+          "{}.ip6.arpa",
+          "3.0.0.0.0.0.0.0.0.0.0.0.c.0.0.b.e.c.a.f.e.f.c.1.0.0.0.0.0.2.6.2"),
+      addr_ipv6.toInverseArpaName());
+}
+
 // Test that invalid string values are killed
 TEST_P(IPAddressCtorTest, InvalidCreation) {
   string addr = GetParam();