How to simulate "TLS 1.3 Session Resumption" through OpenSSL tools?

Matt Caswell matt at
Fri Jul 2 09:45:59 UTC 2021

On 02/07/2021 10:09, Nan Xiao wrote:
> Hi OpenSSL users,
> Greetings from me! From this article
> ( and pcap file
> (, I know we can use
> s_server and s_client to simulate "TLS 1.3 Session Resumption". I
> tried following command:
> echo | openssl s_client -tls1_3  -connect -reconnect

That looks like you've stumbled across an s_client bug. This should 
work, but it doesn't appear to. I just raised an issue for it:

> But it seems not to work since there is no "pre_shared_key" extension,
> and every time s_client just initiated a new connection instead of
> resumption.
> Could anybody advise how to simulate "TLS 1.3 Session Resumption"
> through OpenSSL tools? Thanks very much in advance!

You can do this another way. Create an initial connection (add 
"-connect" option as appropriate):

openssl s_client -tls1_3 -sess_out sess.pem

And then resume like this:

openssl s_client -tls1_3 -sess_in sess.pem

However, note that with TLSv1.3 the session data doesn't arrive until 
post-handshake. In the case of the cloudflare server it doesn't send any 
session tickets until it has received some application data from the 
client. So in order to get a valid resumable session you will have to 
type some HTTP command into s_client once it has created its initial 
connection. This should cause the cloudflare server to respond with a 
session ticket, which will get saved to the sess.pem file. You can then 
use that in the subsequent resumption attempt.


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