Error 503 Backend fetch failed
Backend fetch failed

CentOS 6: /etc/sysconfig/varnish
CentOS 7: /etc/varnish/varnish.params
Ubuntu: /etc/default/varnish

-p http_resp_hdr_len=65536

-p http_resp_size=98304

# DAEMON_OPTS is used by the init script.
DAEMON_OPTS="-a ${VARNISH_LISTEN_ADDRESS}:${VARNISH_LISTEN_PORT}
     -f ${VARNISH_VCL_CONF}
     -T ${VARNISH_ADMIN_LISTEN_ADDRESS}:${VARNISH_ADMIN_LISTEN_PORT}
     -p thread_pool_min=${VARNISH_MIN_THREADS}
     -p thread_pool_max=${VARNISH_MAX_THREADS}
     -p http_resp_hdr_len=65536
     -p http_resp_size=98304
     -S ${VARNISH_SECRET_FILE}
     -s ${VARNISH_STORAGE}"