I just need someone to follow the logic and see if what I am thinking is right..

I have my home internet through Virgin Media. They do DHCP and thus I have a dynamic IP address on the external interface on my router. It changed yesterday for the first time in about.. 18 months, which has led me to think on this stuff.

I have a network device on my home LAN that updates DynDNS.org, so if you go to blahblah.dyndns.org, you get sent to my home router (NOTE: MY dynamic host name is not blahblah.dyndns.org, please don't haxx0r some innocent person), On my Mac, I have the no-ip client running so if you go to blahblah.no-ip.net you also get directed to my home router (NOTE: MY dynamic host name is not blahblah.no-ip.net, please don't haxx0r some innocent person). You may ask why I'm doing this twice - it's historical, but it's mainly because if you don't keep them up to date every so often you lose the hostname and I want to keep them.

Obviously the IP address also has the Virgin Media host name that they allocate to it in their DNS, which is something like blahblah.network.virginmedia.net (NOTE: My ISP provided host name is not blahblah.network.virginmedia.net, please don't haxx0r some innocent person).

The question then is this - if someone does a reverse lookup on my router's external IP address, what will they get? My theory is this : if they're also on Virgin Media, they'll get 'blahblah.network.virginmedia.net'. If they're not on Virgin Media, will they get one of the other three at random depending on how their local DNS resolution works, or do reverse lookups work the same way as forward lookups in that there is an 'authoritative master' for a given segment of addresses which will always be preferred? I can't remember if the latter is true or not.

The 'why' of this is to do with email routing - a lot of email servers do a reverse lookup on the sending host IP address and if it doesn't match the 'from domain' in the email envelope or doesn't have the correct spf record, they massively up the spam score of the message. If I set up my own home MTA, I don't want everything coming from it to be marked as spam.

Jon

the reverse DNS will always be that of Virgin. There is no way for anyone to deduce your other domain names from your IP address. The IP belongs to Virgin, and it is to Virgin that the reverse DNS request will go.

You may be able to request a static IP and then a rDNS from your ISP.

Figured as much. Thanks for the confirmation.


As far as I can tell, Virgin don't offer static IPs unless you get a business account .

Jon