here is how we have configured dynamic sub domain, where application
determine the resource based on subdomain name.
for example -
test.mydomain.com, here "test" is subdomain

write this to your dns configuration -
*.mydomain.com. 14400 IN A 123.123.123.123

now any host lookups (eg. blah.mydomain.com, foo.mydomain.com) will
resolve to 123.123.123.123

best wishes,
--
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nhm tanveer hossain khan (hasan)
http://hasan.we4tech.com
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mobile: +880 1713 090 511
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"work for fun"


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