DHCP_UPDATE is defined via /etc/named.key


------/etc/named.conf------
include "/etc/named.key"

zone "dyn" in {
        type master;
        file "dyn/dyn.zon";
        allow-transfer { 10.10.10.200; };
        allow-update { key DHCP_UPDATE; };
        notify yes;
};

zone "10.10.10.in-addr.arpa" in {
        type master;
        file "dyn/10.10.10.zon";
        allow-transfer { 10.10.10.200; };
        allow-update { key DHCP_UPDATE; };
        notify yes;
};

---------------------------

-------/etc/dhcpd.conf-----
authorative;
ddns-update-style interim;
ddns-updates on;
#ignore client-updates;
include "/etc/named.key";

subnet 10.10.10.0 netmask 255.255.255.0 {

        range 10.10.10.50 10.10.10.55;
        option routers 10.10.10.200;
        option domain-name-servers 10.10.10.201;

        zone dyn. { 
                primary 127.0.0.1; 
                key DHCP_UPDATE; 
        }

        zone 10.10.10.in-addr.arpa. { 
                primary 127.0.0.1; 
                key DHCP_UPDATE; 
        }

--------------------------

(CLIENT PART)
-----/etc/dhclient.conf---

send fqdn.fqdn "suse-dell.dyn.";
send fqdn.encoded on;
send fqdn.server-update off;

key DHCP_UPDATE {
        algorithm HMAC-MD5;
        secret "V3WAtyWFX5To2SSPJBNo8DQQE5zQdgOPXZSAzBG3ogPDG//CYdzMxzbE4HJFAnmPk4iBOvggfsTAmzKyXgjVzw==";
}

zone dyn {
        key DHCP_UPDATE;
}
----------------------------

ZONE-FILES
----10.10.10.in-addr.arpa---
10.10.10.in-addr.arpa.  IN SOA  dyn. admin.dyn. (
                                2007120408 ; serial
                                10800      ; refresh (3 hours)
                                3600       ; retry (1 hour)
                                604800     ; expire (1 week)
                                36400      ; minimum (10 hours 6 minutes 40 seconds)
                                )
                        NS      ns1.dyn.
                        NS      ns2.dyn.10.10.10.in-addr.arpa.

100                     PTR     suse1.dyn.
200                     PTR     suse2.dyn.
201                     PTR     suse3.dyn.
------------------------------

----dyn.zon-------------------
dyn.            IN SOA  dyn. admin.dyn. (
                                2007120409
                                10800
                                3600
                                604800
                                38400
                                )
                        NS      ns1.dyn.
                        NS      ns2.dyn.
                        A       10.10.10.100
                        MX      10 mail.dyn.

mail                    CNAME   dyn.
ns1                     A       10.10.10.201
ns2                     A       10.10.10.200
suse1                   CNAME   dyn.
suse2                   A       10.10.10.200
suse3                   A       10.10.10.201
-----------------------------