Actions are used with the return(<action>) keyword, which returns control from subroutines back to varnish. The action determines how processing in varnish continues as shown in Varnish Processing States.

Common actions are documented here, while additional actions specific to only one or some subroutines are documented in Built-in subroutines as well as which action can be used from which built in subroutine.

common actions for the client and backend side


Transition to vcl_synth on the client side as for return(synth(503, "VCL Failed")), but with any request state changes undone as if std.rollback() was called and forcing a connection close.

Intended for fatal errors, for which only minimal error handling is possible.

common actions for the client side

synth(status code, reason)

Transition to vcl_synth with resp.status and resp.reason being preset to the arguments of synth().


Switch to pass mode, making the current request not use the cache and not putting its response into it. Control will eventually pass to vcl_pass.


Switch to pipe mode. Control will eventually pass to vcl_pipe.


Restart the transaction. Increases the req.restarts counter.

If the number of restarts is higher than the max_restarts parameter, control is passed to vcl_synth as for return(synth(503, "Too many restarts"))

For a restart, all modifications to req attributes are preserved except for req.restarts and req.xid, which need to change by design.

common actions for the backend side


Abandon the backend request. Unless the backend request was a background fetch, control is passed to vcl_synth on the client side with resp.status preset to 503.