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I have a web server with a domain on it that has a dedicated IP address. I need the shared IP address for the server to show up during a reverse DNS lookup.

The dedicated IP has an A record which works and looks like this,

example.com 14400 A 123.45.67.890 // Dedicated IP

The shared IP shows up for all other sites on the server except for this one that has the dedicated IP. I added an A record for the shared IP which does not show up when I reverse DNS lookup,

sharedip.example.com 14400 A 098.76.54.321 // Shared IP

I can't tell if I am adding the Shared IP correctly or I should be doing this in a different way.

Question

To attach the Shared IP to the domain which already has a dedicated IP is the proper way to use an A record with a sub domain 

 sharedip.example.com 14400 A 098.76.54.321

considering the dedicated is attached to the domain without a sub domain as show above? 

example.com 14400 A 123.45.67.890 // Dedicated IP
wuno
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  • What's with all of the unnecessary code blocks around things? They're only making your question more difficult to read and understand. – EEAA Apr 07 '17 at 04:12
  • Thanks for taking the time to read my question. I removed the blocks for you. If you can give it a read an help me out it would greatly be appreciated. – wuno Apr 07 '17 at 04:18
  • I see you're an AWS expert. This is actually because I need a PTR record and for them to approve it they told me to add this A record so the shared IP would also show up with a Reverse DNS Lookup. But even though the record is there and I have shown them screenshots it still does not show up for them. Basically I need to get approved to have emails sent from the server without any restrictions. – wuno Apr 07 '17 at 04:20

1 Answers1

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There is no problem with your desired state of having:

sharedip.example.com.   14400   IN      A       93.184.216.34
34.216.184.93.in-addr.arpa.     IN      PTR     sharedip.example.com.

Your service provider probably wants the A record to be working before they add the corresponding PTR record to avoid situation where the records don´t match (see informational RFC 1912, 2.1). They may not accept your screenshot because it doesn't ensure DNS is actually configured correctly.

Your question doesn't give any information on your DNS server configuration. Neither does it tell whether you maintain your own DNS server or use some kind of configuration tool for adding records, like in DNS services provided by web hosting companies or domain registrars. Therefore it's impossible to guide you on HOW you should fix this problem, but I can give some common advices.

  • If you directly modify your zone file, remember to update serial number i.e. the third part of your SOA record. This value must increment. Otherwise the DNS server (BIND) doesn't reload the zone file and secondary DNS servers won't start zone transfers. Recommended format for SN is YYYYMMDDNN (from year, month, day and version per day).

  • If using BIND, remember to rndc reload after the changes.

  • If you have DNSSEC enabled, remember to sign the zone. (SN increment might be automated.)

  • If someone else is maintaining the DNS, you can check from their manual, how often the zone is updated from the configuration environment available for the user. If that kind of information is not provided, you can dig example.com NS to get list of authoritative name servers and then test:

    • whether they all have the correct A record: dig example.com @b.iana-servers.net
    • that the SOA record has changed and SN is in correct format: dig example.com SOA

  • If your service provider didn't check the A record from your authoritative name servers but used their recursive name servers instead, notice that your TTL was set to 14400 seconds. In this case you could request for the PTR change again after the 4 hour period starting from the time you checked that the record was correctly updated on your authoritative name servers.

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Esa Jokinen
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