DHE ciphers not offered

[BenjaminEHowe]

• Version: v10.16.1, v10.16.3, and v12.5.0
• Platform: Darwin REDACTED 18.7.0 Darwin Kernel Version 18.7.0: Thu Jun 20 18:42:21 PDT 2019; root:xnu-4903.270.47~4/RELEASE_X86_64 x86_64 and Linux REDACTED 4.19.66-v7l+ Switch ClosureLinter to other linter. #1253 SMP Thu Aug 15 12:02:08 BST armv7l GNU/Linux
• Subsystem: https

The https module doesn't appear to be enabling DHE ciphers. When creating a server which should offer ECDHE-RSA-AES128-GCM-SHA256 and DHE-RSA-AES128-GCM-SHA256, only the ECDHE ciphers is actually offered. I've created a sample repository.

---

Thanks to my colleague, @zamnuts, for his assistance with identifying this issue. He pointed out a smaller example of this issue:

const https = require('https');
const fs = require('fs');

https.createServer({
  key: fs.readFileSync('x509.key'),
  cert: fs.readFileSync('x509.pub')
}, (req, res) => {
  res.writeHead('200');
  res.end('hello world\n');
}).listen(8443);

According to the default cipher suite DHE-RSA-AES128-GCM-SHA256 should be offered, but isn't.

[BenjaminEHowe]

Unfortunately this was PEBCAK. The DHE ciphers weren't being used because dhparams weren't specified (see: https://nodejs.org/docs/v10.11.0/api/tls.html#tls_tls_createsecurecontext_options). I will try to create a PR to improve the docs.