Upgrading to Varnish 6.4.0

Upgrading to Varnish 6.4 from 6.3 should not require any changes to VCL.

This document contains information about other relevant aspects which should be considered when upgrading.


  • The hash algorithm of the hash director was changed, so backend selection will change once only when upgrading.

    Users of the hash director are advised to consider using the shard director instead, which, amongst other advantages, offers more stable backend selection through consistent hashing. See VMOD directors - Varnish Directors Module for details.

  • We fixed a case where send_timeout had no effect on HTTP/1 connections when streaming from a backend fetch, in other words, a connection might not have got closed despite the send_timeout having been reached. HTTP/2 was not affected.

    When send_timeout is reached on HTTP/1, the MAIN.sc_tx_error is increased. Users with long running backend fetches and HTTP/1 clients should watch out for an increase of that counter compared to before the deployment and consider increasing send_timeout appropriately.

    The timeout can also be set per connection from VCL as sess.send_timeout.


  • The MAIN.sess_drop counter is gone. It should be removed from any statistics gathering tools, if present

  • sess.timeout_idle / timeout_idle being reached on HTTP/1 used to be accounted to the MAIN.rx_timeout statistic. We have now added the MAIN.rx_close_idle counter for this case specifically.

  • sess.send_timeout / send_timeout being reached on HTTP/1 used to be accounted to MAIN.sc_rem_close. Such timeout events are now accounted towards MAIN.sc_tx_error.

See varnish-counters for details.


  • The Process timestamp for vcl_synth {} was wrongly issued before the VCL subroutine was called, now it gets emitted after VCL returns for consistency with vcl_deliver {}.

    Users of this timestamp should be aware that it now includes vcl_synth {} processing time and appears at a different position in the log.

  • A Notice VSL tag has been added.