net: fallback to IPv6-only when dual-stack bind fails

lib/net.js

@@ -63,6 +63,7 @@ const {
 const assert = require('internal/assert');
 const {
   UV_EADDRINUSE,
+  UV_EAFNOSUPPORT,
   UV_EBADF,
   UV_EINVAL,
   UV_ENOTCONN,
@@ -2264,6 +2265,19 @@ function createServerHandle(address, port, addressType, fd, flags) {
       }
     } else if (addressType === 6) {
       err = handle.bind6(address, port, flags);
+      // On IPv6-only systems, setting IPV6_V6ONLY to false can fail with
+      // EAFNOSUPPORT because the IPv4 address family is unavailable. In that
+      // case, retry with an IPv6-only socket before falling back to IPv4.
+      if (err === UV_EAFNOSUPPORT &&
+          !(flags & TCPConstants.UV_TCP_IPV6ONLY)) {
+        handle.close();
+        handle = new TCP(TCPConstants.SERVER);
+        err = handle.bind6(
+          address,
+          port,
+          flags | TCPConstants.UV_TCP_IPV6ONLY,
+        );
+      }
     } else {
       err = handle.bind(address, port, flags);
     }

test/sequential/test-net-server-listen-ipv6-only-system.js

@@ -0,0 +1,38 @@
+// Flags: --expose-internals
+'use strict';
+
+const common = require('../common');
+const assert = require('assert');
+const net = require('net');
+const { internalBinding } = require('internal/test/binding');
+const { TCP, constants: TCPConstants } = internalBinding('tcp_wrap');
+const { UV_EAFNOSUPPORT } = internalBinding('uv');
+
+const probe = new TCP(TCPConstants.SOCKET);
+if (probe.bind6('::1', 0, TCPConstants.UV_TCP_IPV6ONLY) !== 0) {
+  probe.close();
+  common.skip('no IPv6 support');
+}
+probe.close();
+
+const bind6 = TCP.prototype.bind6;
+let firstBind = true;
+
+TCP.prototype.bind6 = function(...args) {
+  if (firstBind && args[2] === 0) {
+    firstBind = false;
+    return UV_EAFNOSUPPORT;
+  }
+  return bind6.apply(this, args);
+};
+
+const server = net.createServer();
+server.on('error', common.mustNotCall());
+
+server.listen({ port: 0 }, common.mustCall(() => {
+  assert.strictEqual(server.address().family, 'IPv6');
+
+  server.close(common.mustCall(() => {
+    TCP.prototype.bind6 = bind6;
+  }));
+}));