Variables substitution in jail.conf

[Niklaas Baudet von Gersdorff]

I am experimenting with jail.conf, trying to automate everything as much as
I can. I would like to execute pfctl commands automatically once a jail is
started or stopped; that is, adding the IP of the jail to a table that passes
connection and deleting it again once it's no longer needed. This is my
jail.conf:

    host.hostname = "$name.box-fra-01.klaas";
    path          = "/usr/local/jails/$name";
    ip4.addr      = "lo1|10.15.$network.$id";
    ip6.addr      = "vtnet0|2a00:XXX:XXXX:XXXX:X::$network:$id";
    mount         = "/usr/local/jails/templates/base-10.2-RELEASE /usr/local/jails/$name/     nullfs ro 0 0";
    mount        += "/usr/local/jails/thinjails/$name             /usr/local/jails/$name/jail nullfs rw 0 0";
    mount.devfs;

    exec.start    = "/bin/sh /etc/rc";
    exec.stop     = "/bin/sh /etc/rc.shutdown";

    exec.clean;

    www {
        $id            = 1;
        $network       = 1;
        exec.poststart = "pfctl -t www -T add ${ip4.addr} {$ip6.addr}";
        exec.poststop  = "pfctl -t www -T delete {$ip4.addr} {$ip6.addr}";
    }

However, I get an error that ip6 is not defined. I have already realised that
pfctl will give an error (because ip{4,6}.addr includes {lo1,vtnet0}) but what
I do not understand is why the parameter is not recognised.

I also tried setting things up with additional variables my_ip4 and my_ip6 but
that didn't work either. After reading jail.conf(5) I thought about putting
everything in hierarchical jails but I am not sure whether that will help to
make substitution work the way I want it to.

I am happy for any advise.