you can't nslookup a URL, only the hostname "epo.phantom.us".
Now if you want to test the URL, which in this case is "https://repo.phantom.us/phantom/4.5/base/7/x86_64/phantom_repo-4.5.7532-1.x86_64.rpm" you'll need a different tool. Right now it sounds like you can get an IP address so Splunk knowns who to call. but splunk needs to talk to that server over HTTPS. nslookup doesn't understand HTTPS or any other protocol other than DNS, which is used to retrieve an IP using a host name. If your not too familiar with DNS I would highly recommend read up on a simple introduction. it will help you in future.
Okay enough explanation, the next step is to see if we can actually talk to that server over HTTPS. Their are a number of tools that can be used to test this, but my personal favorite is nmap. see if you have nmap install using the command "nmap -V". if you get a "command not found" you'll need to install it. you can install it using this command as root "yum install nmap -y"
okay, with nmap we can test ports. The protocol HTTPS run over 443 (usually). so the command "nmap -p 443 repo.phantom.us" will tell us if that port is opened. give that a try and let me know if the port state says "open".
sorry for the long reply
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