Ironport has its own caching DNS server on board. If you have to choose between using the root servers and the DNS of your ISP I suggest you use the root servers. Why? Because it's more reliable... if your ISP has more than 3 DNS servers you are lucky and even those can all be unreachable, if you walk the DNS tree yourself there is not much risk that you will be “out of DNS servers“.
Please be aware that Ironport is a HUGE DNS consumer. The Senderbase stuff is fully DNS based. For each incoming connection your device will at least launch 3 DNS queries. For the connections that have an acceptable Senderbase score it will be even more. Especially when you have a busy Ironport infrastructure, your ISP might not be happy with the load you put on their DNS systems.
We use our own (non-Ironport) caching DNS servers because we want to use DNS also for routing to the internal systems. These DNS servers are used by Ironport and the web proxy systems and the load on those is 90% Ironport, 10% web proxy.
Table of ContentsIntroductionVersion HistoryPossible Future
UpdatesDocuments PurposeNAT Operation in ASA 8.3+ SectionsRule Types
Network Object NATTwice NAT / Manual NATRule Types used per SectionNAT
Types used with Twice NAT / Manual NAT and Network Obje...
Table of Contents Introduction:This document describes details on how
NAT-T works. Background: ESP encrypts all critical information,
encapsulating the entire inner TCP/UDP datagram within an ESP header.
ESP is an IP protocol in the same sense that TCP an...