SPF for sub server

I have a virtual server domain.com and a sub server sub.domain.com
Though user@domain.com can send mail that passes SPF, mail from user@sub.domain.com does not use SPF at all, and I’m not sure how to enable it since sub servers don’t seem to have a DNS Options section where one would configure SPF. Is there something I need to be configuring in the parent server’s SPF settings or DNS records? Is this a fundamental limitation of a sub server, or am I missing something?

In your main server, add the following DNS record:
subdomain.domain.com. IN TXT “v=spf1 ip4: -all”

There should also be a:
subdomain.domain.com. IN A

Where is your IP

I ultimately ended up just creating the TXT record by hand and putting it in the DNS records for sub.domain.com. Seems to work ok, it was just confusing that there was no DNS Options section. Kind of implied to me that the function wasn’t there, but that could be my lack of understanding how SPF works.