Default value of a session resumption timeout (300 seconds vs 7200 seconds)

[Matt Caswell]

On 26/01/2021 18:13, Harish Kulkarni wrote:
> Thank you both for bringing this to my attention, your points are
> invaluable.
> 
> If this is something which gets set from server on client side. can
> client override this?. Can i change this to something less and try?. Has
> anyone tried?.
> 
> Whats the option in openssl.conf or some other place?.

The session timeout is something entirely controlled by the server. The
client has no influence on this*. If the server is using session tickets
for its sessions then it provides a lifetime hint to the client to say
how long the client can expect the session to be good for. The client
can query this using SSL_SESSION_get_ticket_lifetime_hint().

On the server the timeout can be configured using SSL_CTX_set_timeout().
I don't think this is possible to change via openssl.conf.

Matt


* Note that the client may be managing a cache of sessions provided by
servers. That's not something that happens by default in OpenSSL but can
be configured using SSL_CTX_set_session_cache_mode(). In that case there
will be separate timeouts associated with the life of the session in the
client cache. Those timeouts may not be the same as the server's timeouts.

> 
> -thanks
> harish
> 
> 
> On Mon, Jan 25, 2021 at 11:08 PM Matt Caswell <matt at openssl.org
> <mailto:matt at openssl.org>> wrote:
> 
> 
> 
>     On 23/01/2021 15:22, John Thoe wrote:
>     > Hi list,
>     >
>     > The session reuse question posted on the mailing list earlier
>     >
>     (https://mta.openssl.org/pipermail/openssl-users/2021-January/013360.html)
>     > reminded of a somewhat similar question I have.
>     >
>     > As per the docs,
>     >
>     https://www.openssl.org/docs/man1.0.2/man3/SSL_get_default_timeout.html,
>     > it says the default value is 300 seconds for which a session resuse
>     > will be accepted. The docs say that it is the same for all
>     > protocols.
>     >
>     > However I tried it with my setup where I didn't explicitly set the
>     > timeout and I am getting 7200 seconds as the default value. s_client
>     > output: TLS session ticket lifetime hint: 7200 (seconds). My client
>     > openssl.conf has no setting override (not that it should matter
>     > because this is a server preference). No OpenSSL settings on the
>     > server have been modified as well.
> 
>     Looks to me like the docs are wrong. They probably should say 7200.
> 
> 
>     >
>     > In ssl/ssl_sess.c#L80, the code matches the document: ss->timeout =
>     > 60 * 5 + 4;   /* 5 minute timeout by default */ ... (with additional
>     > four seconds?)
> 
> 
>     This gets set during construction and then later overwritten when we
>     actually get a new session via "ssl_get_new_session":
> 
>         /* If the context has a default timeout, use it */
>         if (s->session_ctx->session_timeout == 0)
>             ss->timeout = SSL_get_default_timeout(s);
>         else
>             ss->timeout = s->session_ctx->session_timeout;
> 
>     In most cases SSL_get_default_timeout() calls tls1_default_timeout() (it
>     can end up somewhere different for certain protocol versions - but all
>     the different variants are the same!):
> 
>     long tls1_default_timeout(void)
>     {
>         /*
>          * 2 hours, the 24 hours mentioned in the TLSv1 spec is way too
>     long for
>          * http, the cache would over fill
>          */
>         return (60 * 60 * 2);
>     }
> 
>     60 * 60 * 2 = 7200
> 
> 
>     Matt
>