test/dgram: Add support for IPv6 link local scope IDs in received UDP datagrams

doc/api/dgram.md

@@ -94,6 +94,12 @@ The event handler function is passed two arguments: `msg` and `rinfo`.
   * `port` {number} The sender port.
   * `size` {number} The message size.
 
+If the source address of the incoming packet is an IPv6 link local
+address, the interface name is added to the `address`.  For
+example, a packet received on the `en0` interface might have the
+address field set to `'fe80::2618:1234:ab11:3b9c%en0'`, where `'%en0'`
+is the interface name as a zone id suffix.
+
 ### `socket.addMembership(multicastAddress[, multicastInterface])`
 <!-- YAML
 added: v0.6.9

src/tcp_wrap.cc

@@ -337,9 +337,10 @@ Local<Object> AddressToJS(Environment* env,
                           const sockaddr* addr,
                           Local<Object> info) {
   EscapableHandleScope scope(env->isolate());
-  char ip[INET6_ADDRSTRLEN];
+  char ip[INET6_ADDRSTRLEN + UV_IF_NAMESIZE];
   const sockaddr_in* a4;
   const sockaddr_in6* a6;
+
   int port;
 
   if (info.IsEmpty())
@@ -349,6 +350,18 @@ Local<Object> AddressToJS(Environment* env,
   case AF_INET6:
     a6 = reinterpret_cast<const sockaddr_in6*>(addr);
     uv_inet_ntop(AF_INET6, &a6->sin6_addr, ip, sizeof ip);
+    // Add an interface identifier to a link local address.
+    if (IN6_IS_ADDR_LINKLOCAL(&a6->sin6_addr)) {
+        const size_t addrlen = strlen(ip);
+        CHECK_LT(addrlen, sizeof(ip));
+        ip[addrlen] = '%';
+        size_t scopeidlen = sizeof(ip) - addrlen - 1;
+        CHECK_GE(scopeidlen, UV_IF_NAMESIZE);
+        const int r = uv_if_indextoiid(a6->sin6_scope_id,
+                                       ip + addrlen + 1,
+                                       &scopeidlen);
+        CHECK_EQ(r, 0);
+    }
     port = ntohs(a6->sin6_port);
     info->Set(env->context(),
               env->address_string(),

test/parallel/test-dgram-udp6-link-local-address.js

@@ -0,0 +1,51 @@
+'use strict';
+const common = require('../common');
+if (!common.hasIPv6)
+  common.skip('no IPv6 support');
+
+const assert = require('assert');
+const dgram = require('dgram');
+const os = require('os');
+
+function linklocal() {
+  for (const [ifname, entries] of Object.entries(os.networkInterfaces())) {
+    for (const { address, family, scopeid } of entries) {
+      if (family === 'IPv6' && address.startsWith('fe80:')) {
+        return { address, ifname, scopeid };
+      }
+    }
+  }
+}
+const iface = linklocal();
+
+if (!iface)
+  common.skip('cannot find any IPv6 interfaces with a link local address');
+
+const address = `${iface.address}%${iface.ifname}`;
+const message = 'Hello, local world!';
+
+// Create a client socket for sending to the link-local address.
+const client = dgram.createSocket('udp6');
+
+// Create the server socket listening on the link-local address.
+const server = dgram.createSocket('udp6');
+
+server.on('listening', common.mustCall(() => {
+  const port = server.address().port;
+  client.send(message, 0, message.length, port, address);
+}));
+
+server.on('message', common.mustCall((buf, info) => {
+  const received = buf.toString();
+  assert.strictEqual(received, message);
+  // Check that the sender address is the one bound,
+  // including the link local scope identifier.
+  assert.strictEqual(
+    info.address,
+    common.isWindows ? `${iface.address}%${iface.scopeid}` : address
+  );
+  server.close();
+  client.close();
+}, 1));
+
+server.bind({ address });