SOA record for hosts?

[Ecol]

I want to set some hosts' records to expire very quickly- with TTL=0 (or
30 minutes for example). So I set in my files:
$ORIGIN com.
mydomain          IN      SOA     nms.mydomain.com. root.mydomain.com. (

                1999090204 10800 3600 604800 86400 )
                IN      NS      nms.mydomain.com.
                IN      NS      nms2.mydomain.com.
$INCLUDE mydomain.com.dyn

and in file mydomain.com.dyn:
ftp.mydomain.com.          IN      SOA   nms.mydomain.com.
root.mydomain.com. (
                1999090207 10800 3600 604800 1800 )
                IN      CNAME   host1.mydomain.com.

and after named restart I can see in log file:
nms named[6268]: mydomain.com.dyn:1: SOA for "ftp.mydomain.com" not at
zone top "mydomain.com"
nms named[6268]: Zone "mydomain.com" (file mydomain.com): multiple SOA
RRs found
nms named[6268]: master zone "mydomain.com" (IN) rejected due to errors
(serial 1999090207)
nms named[6269]: Ready to answer queries.

but when I ask :

nslookup -type=any ftp.mydomain.com
Server:  nms.mydomain.com
Address:  x.x.x.x

Non-authoritative answer:
ftp.mydomain.com
        origin = nms.mydomain.com
       (..)
        minimum ttl = 1800 (30M)
ftp.mydomain.com canonical name = host1.mydomain.com

How to make it right (look at "Non-authoritative answer......")?
Any help will be appreciated.

E.Colanski
ecol at ibud.vr9.com