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Accepted
Mathias Bynens
Asked: 2016-02-11 03:32:47 +0800 CST

Testing HTTP/2 using `openssl s_client`

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CloudFlare’s blog post on tools for debugging, testing and using HTTP/2 mentions this approach to finding out which protocols are supported by a given server:

$ openssl s_client -connect www.cloudflare.com:443 -nextprotoneg ''
CONNECTED(00000003)
Protocols advertised by server: h2, spdy/3.1, http/1.1

This approach seems to work fine for many domains I’ve tested. However, the (HTTP/2-enabled) domain benchmarkjs.com produces output that looks completely different, and I don’t know why:

$ openssl s_client -connect benchmarkjs.com:443 -nextprotoneg ''
CONNECTED(00000003)
depth=0 C = US, ST = Washington, L = Seattle, O = Odin, OU = Plesk, CN = Plesk, emailAddress = [email protected]
verify error:num=18:self signed certificate
verify return:1
depth=0 C = US, ST = Washington, L = Seattle, O = Odin, OU = Plesk, CN = Plesk, emailAddress = [email protected]
verify return:1
---
Certificate chain
 0 s:/C=US/ST=Washington/L=Seattle/O=Odin/OU=Plesk/CN=Plesk/[email protected]
   i:/C=US/ST=Washington/L=Seattle/O=Odin/OU=Plesk/CN=Plesk/[email protected]
---
Server certificate
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----
subject=/C=US/ST=Washington/L=Seattle/O=Odin/OU=Plesk/CN=Plesk/[email protected]
issuer=/C=US/ST=Washington/L=Seattle/O=Odin/OU=Plesk/CN=Plesk/[email protected]
---
No client certificate CA names sent
Peer signing digest: SHA512
Server Temp Key: ECDH, P-256, 256 bits
---
SSL handshake has read 1409 bytes and written 448 bytes
---
New, TLSv1/SSLv3, Cipher is ECDHE-RSA-AES128-GCM-SHA256
Server public key is 2048 bit
Secure Renegotiation IS supported
Compression: NONE
Expansion: NONE
No ALPN negotiated
SSL-Session:
    Protocol  : TLSv1.2
    Cipher    : ECDHE-RSA-AES128-GCM-SHA256
    Session-ID: C0474D42297C21B374C558E698A69EEBBFF537B35E6365D31B4D7AA2CD4192AD
    Session-ID-ctx:
    Master-Key: 2292DF581DAE3CE4A4E8CFF5A351A3F4E99602A7DCDBD090998C38926498743B4E3A808A1C4505136D51FC847F0153A6
    Key-Arg   : None
    PSK identity: None
    PSK identity hint: None
    SRP username: None
    Start Time: 1455103745
    Timeout   : 300 (sec)
    Verify return code: 18 (self signed certificate)
---

Note that it doesn’t return to the prompt; you’re connected to the server and can now enter e.g. GET / HTTP/1.1 followed by Host: benchmarkjs.com.

So, my two questions are:

  1. What’s different about benchmarkjs.com that makes this technique not work?
  2. Is it possible to use openssl s_client to detect HTTP/2 support in a generic way that works even for this domain?
bash

1 Answers

  1. Best Answer

    My take: the openssl s_client's manual states:

    -nextprotoneg protocols

    enable Next Protocol Negotiation TLS extension and provide a list of comma- separated protocol names that the client should advertise support for. The list should contain most wanted protocols first. Protocol names are printable ASCII strings, for example "http/1.1" or "spdy/3". Empty list of protocols is treated specially and will cause the client to advertise support for the TLS extension but disconnect just after reciving ServerHello with a list of server supported protocols.

    (Emphasis mine.)

    So I'd say the SSL/TLS engine terminating your request at benchmarkjs.com merely does not support that Next Protocol Negotiation extension or has it disabled for some reason. Hence in your test the openssl s_client command advertises that is supports NPN but the server turns a blind eye onto ot. The handshake still passes OK because the extension appears to be non-essential (or at least considered to be such by openssl) and you get the connected TLS tunnel.

    As to implementing reliable checking, I'm afraid since openssl s_client apparently does not have any command-line option which would force it terminate the session immediately, you would have to go a level deeper and write your own program linking to libssl which would implement the desired behaviour. (And I would personally implement it in Go using the crypto/tls package of its standard library as it's no harder to program than JavaScript but produces a statically-linked program meaning no-brainer deploys.)

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