modify Retry-after http response

[dwi kristianto]

hi all,

i came from http://www.varnish-cache.org/trac/wiki/VCL#vcl_error and
dont see any documentation about it.

for now, i'm suffering from some bots that abusing my web server.
using varnish and iptables it may cut down the abuse. despite of that,
for other good bots and SEO purposes, it's good to return
"Retry-After" header when error happened.

how can i do that?
should i use vcl_error sub?

regards,
dwi.