dgram: skip dns.lookup() for literal IP addresses

Every unconnected send(), and the implicit bind on first send, resolved the
destination through dns.lookup() even when it was already a literal IP. The
address resolves to itself, so the call is redundant, and tools that
instrument dns.lookup() record a lookup for every datagram sent to an IP.

Skip the resolver for a literal IP of the socket's family and report it on the
next tick, keeping dns.lookup()'s asynchronous contract. A custom lookup
function is still consulted for every address.

Refs: DataDog/dd-trace-js#2984
Signed-off-by: Ruben Bridgewater <ruben@bridgewater.de>

Author: BridgeAR

benchmark/dgram/send-to-ip.js

@@ -0,0 +1,43 @@
+// Measure the send rate to a literal IP destination. The destination needs no
+// name resolution, so this isolates the per-send overhead the default lookup
+// pays before the packet reaches the socket.
+'use strict';
+
+const common = require('../common.js');
+const dgram = require('dgram');
+const PORT = common.PORT;
+
+// `n` is the number of send requests queued each round. Keep it high (>10) so
+// the measurement reflects send overhead rather than event loop cycles.
+const bench = common.createBenchmark(main, {
+  n: [100],
+  dur: [5],
+});
+
+function main({ dur, n }) {
+  const chunk = Buffer.allocUnsafe(1);
+  let sent = 0;
+  const socket = dgram.createSocket('udp4');
+
+  function onsend() {
+    if (sent++ % n === 0) {
+      setImmediate(() => {
+        for (let i = 0; i < n; i++) {
+          socket.send(chunk, PORT, '127.0.0.1', onsend);
+        }
+      });
+    }
+  }
+
+  socket.on('listening', () => {
+    bench.start();
+    onsend();
+
+    setTimeout(() => {
+      bench.end(sent);
+      process.exit(0);
+    }, dur * 1000);
+  });
+
+  socket.bind(PORT);
+}

doc/api/dgram.md

@@ -1039,6 +1039,8 @@ changes:
   * `recvBufferSize` {number} Sets the `SO_RCVBUF` socket value.
   * `sendBufferSize` {number} Sets the `SO_SNDBUF` socket value.
   * `lookup` {Function} Custom lookup function. **Default:** [`dns.lookup()`][].
+    When the default is used, a literal IP address of the socket's family
+    resolves to itself without calling [`dns.lookup()`][].
   * `signal` {AbortSignal} An AbortSignal that may be used to close a socket.
   * `receiveBlockList` {net.BlockList} `receiveBlockList` can be used for discarding
     inbound datagram to specific IP addresses, IP ranges, or IP subnets. This does not

lib/internal/dgram.js

@@ -8,6 +8,7 @@ const {
 const { codes: {
   ERR_SOCKET_BAD_TYPE,
 } } = require('internal/errors');
+const { isIP } = require('internal/net');
 const { UDP } = internalBinding('udp_wrap');
 const { guessHandleType } = require('internal/util');
 const {
@@ -28,13 +29,22 @@ function lookup6(lookup, address, callback) {
   return lookup(address || '::1', 6, callback);
 }
 
+// A literal IP of the socket's family resolves to itself, so skip dns.lookup().
+// Defer with nextTick to keep the callback async (e.g. bind()'s 'listening').
+function defaultLookup(address, family, callback) {
+  if (isIP(address) === family) {
+    process.nextTick(callback, null, address, family);
+    return;
+  }
+  if (dns === undefined) {
+    dns = require('dns');
+  }
+  return dns.lookup(address, family, callback);
+}
+
 function newHandle(type, lookup) {
   if (lookup === undefined) {
-    if (dns === undefined) {
-      dns = require('dns');
-    }
-
-    lookup = dns.lookup;
+    lookup = defaultLookup;
   } else {
     validateFunction(lookup, 'lookup');
   }

test/parallel/test-dgram-custom-lookup.js

@@ -4,10 +4,12 @@ const assert = require('assert');
 const dgram = require('dgram');
 const dns = require('dns');
 
+const originalLookup = dns.lookup;
+
 {
   // Verify that the provided lookup function is called.
   const lookup = common.mustCall((host, family, callback) => {
-    dns.lookup(host, family, callback);
+    originalLookup(host, family, callback);
   });
 
   const socket = dgram.createSocket({ type: 'udp4', lookup });
@@ -18,17 +20,17 @@ const dns = require('dns');
 }
 
 {
-  // Verify that lookup defaults to dns.lookup().
-  const originalLookup = dns.lookup;
-
+  // Verify that the default lookup forwards host names to dns.lookup().
   dns.lookup = common.mustCall((host, family, callback) => {
     dns.lookup = originalLookup;
-    originalLookup(host, family, callback);
+    assert.strictEqual(host, 'example.invalid');
+    assert.strictEqual(family, 4);
+    callback(null, '127.0.0.1', 4);
   });
 
   const socket = dgram.createSocket({ type: 'udp4' });
 
-  socket.bind(common.mustCall(() => {
+  socket.bind(0, 'example.invalid', common.mustCall(() => {
     socket.close();
   }));
 }

test/parallel/test-dgram-default-lookup-ip.js

@@ -0,0 +1,80 @@
+'use strict';
+
+// The default dgram lookup resolves a literal IP address of the socket's own
+// family to itself, without calling dns.lookup(). Each case below stubs the
+// process-global dns.lookup(), so they run sequentially to keep one case from
+// observing another's stub.
+
+const common = require('../common');
+const assert = require('assert');
+const dgram = require('dgram');
+const dns = require('dns');
+
+const originalLookup = dns.lookup;
+
+function ipv4SendSkipsLookup(next) {
+  dns.lookup = common.mustNotCall('dns.lookup() ran for an IPv4 literal');
+
+  const receiver = dgram.createSocket('udp4');
+  const sender = dgram.createSocket('udp4');
+
+  receiver.on('message', common.mustCall((msg) => {
+    assert.strictEqual(msg.toString(), 'payload');
+    dns.lookup = originalLookup;
+    receiver.close();
+    sender.close();
+    next();
+  }));
+
+  receiver.bind(0, '127.0.0.1', common.mustCall(() => {
+    sender.send('payload', receiver.address().port, '127.0.0.1', common.mustCall());
+  }));
+}
+
+function ipv6BindSkipsLookup(next) {
+  if (!common.hasIPv6) {
+    next();
+    return;
+  }
+
+  dns.lookup = common.mustNotCall('dns.lookup() ran for an IPv6 literal');
+
+  const socket = dgram.createSocket('udp6');
+
+  socket.bind(0, '::1', common.mustCall(() => {
+    dns.lookup = originalLookup;
+    socket.close();
+    next();
+  }));
+}
+
+function mismatchedFamilyFallsThrough(next) {
+  // '::1' is not an IPv4 literal, so a udp4 socket still resolves it via
+  // dns.lookup() rather than short-circuiting.
+  dns.lookup = common.mustCall((host, family, callback) => {
+    dns.lookup = originalLookup;
+    assert.strictEqual(host, '::1');
+    assert.strictEqual(family, 4);
+    callback(null, '127.0.0.1', 4);
+  });
+
+  const socket = dgram.createSocket('udp4');
+
+  socket.bind(0, '::1', common.mustCall(() => {
+    socket.close();
+    next();
+  }));
+}
+
+const cases = [
+  ipv4SendSkipsLookup,
+  ipv6BindSkipsLookup,
+  mismatchedFamilyFallsThrough,
+];
+
+(function runNext() {
+  const testCase = cases.shift();
+  if (testCase !== undefined) {
+    testCase(runNext);
+  }
+})();