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I am not in charge of the network, but I have responsibility for the Email Server at my workplace. We outsource networking, and how it was set is like this:

Mail Server (192.168.0.2)

Internet Firewall (155.x.y.z)

Internal DNS Server (mail.example.com -> 192.168.0.2)

Public DNS (World) (mail.example.com -> 155.x.y.z)

If i ping 155.x.y.z from within my LAN 192.168.0.0/24, I do not get a reply. When I consulted my network team, they said that I was not able to access 155.x.y.z from within the 192.168.0.0/24.

Problem My users use laptops, when they come into the LAN, for a while they still have (mail.example.com -> 155.x.y.z) in their DNS cache, and thus cannot get any reply from the mail server.

The options I have used are using a shorter TTL, but it has not worked because some of it is being overridden.

How can I fix the above?

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  • 1
    Windows? Flush the DNS cache ipconfig /flushdns
    – DavidPostill
    Jul 7, 2016 at 14:24
  • @DavidPostill many executive users use this setup, day in day out. Hard to tell them to run a cmd daily. Jul 7, 2016 at 14:28
  • 1
    You can probably automate it ...
    – DavidPostill
    Jul 7, 2016 at 14:30
  • Smart, maybe a windows service which they run when they come into the network...or even detects network change and runs itself, smart...make it an answer :D Jul 7, 2016 at 14:34
  • Not enough meat for an answer. When you figure it out you can answer your own question :)
    – DavidPostill
    Jul 7, 2016 at 14:36

1 Answer 1

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What you need is equipment capable of providing NAT loopback or hairpin NAT which allows the public IP to be accessible from within your LAN.

This is usually only available on business-grade network equipment so you'll have two options - buy a better router or try and flash your router with a firmware such as DD-WRT etc.

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  • Oh I see, so that is in the network place of things, can I ask, is this what enterprise-grade networks employ? Jul 7, 2016 at 13:09
  • "is this what enterprise-grade networks employ?" Yes. Jul 7, 2016 at 14:49
  • @Ƭᴇcʜιᴇ007 I see thanks. Will read this up to give me some info, probably might not implement for reason of the size of department. But Thanks! Jul 7, 2016 at 14:51
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    A couple hundred pounds could get you an entry - level solution.
    – Kinnectus
    Jul 7, 2016 at 14:53

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