default.vcl

# ...

include "dynamic_backends.vcl";

# ...

update-backends.sh

#!/bin/bash

set -e

elb_hostname=${elb_hostname:-myapp.herokuapp.com}
backend_vcl=${backend_vcl:-/etc/varnish/dynamic_backends.vcl}
ips=$(ruby -rresolv -e "puts Resolv::DNS.new.getaddresses('$elb_hostname').map(&:to_s)")

# clear out existing backends
cat <<EOF > $backend_vcl
director my_backend round-robin {
EOF

# create a new backends.vcl
for ip in $ips; do
  cat <<EOF >> $backend_vcl
  {
    .backend = {
      .host = "$ip";
      .port = "http";
    }
  }
EOF
done

cat <<EOF >> $backend_vcl
}
EOF

# restart varnish
/etc/init.d/varnish reload