Magento – Error 503 Backend fetch failed

In Magento 2, this error is caused because the length of cache tags is more than 8192 characters. You can fix this error by increasing http_resp_hdr_len value in varnish configuration file.

I followed this tutorial https://magentip.com/magento-2-error-503-backend-fetch-failed-with-varnish-cache/ and successfully fixed that error.

Step 1: Depend on your OS, navigate to:

Ubuntu: /etc/default/varnish

CentOS 6.x: /etc/sysconfig/varnish

CentOS 7.x: /etc/varnish/varnish.params

Step 2:

Edit config file and add the following line:

-p http_resp_hdr_len=42000

For example:

DAEMON_OPTS="-a ${VARNISH_LISTEN_ADDRESS}:${VARNISH_LISTEN_PORT} \
         -f ${VARNISH_VCL_CONF} \
         -T ${VARNISH_ADMIN_LISTEN_ADDRESS}:${VARNISH_ADMIN_LISTEN_PORT} \
         -t ${VARNISH_TTL} \
         -p thread_pool_min=${VARNISH_MIN_THREADS} \
         -p thread_pool_max=${VARNISH_MAX_THREADS} \
         -p http_resp_hdr_len=42000 \

Save file and restart your server. The error should be gone.