networking - Why don't more organizations use inside-to-inside NAT or similar solutions to allow NAT hairpins?

Inside-to-inside NAT aka NAT loopback solves hairpin NAT issues when accessing a web server on the external interface of an ASA or similar device from computers on the internal interface. This prevents DNS admins from having to maintain a duplicate internal DNS zone that has the corresponding RFC1918 addresses for their servers that are NATted to public addresses. I'm not a network engineer, so I might be missing something, but this seems like a no-brainer to configure and implement. Asymmetric routing can be an issue but is easily mitigated.

In my experience, network admins/engineers prefer that systems folks just run split-dns rather than configuring their firewalls to properly handle NAT hairpins. Why is this?

Answer

There are a few reasons why I wouldn't do it:

Why put extra strain on the DMZ routers and firewall if you don't need to? Most our internal services are not in the DMZ but the general corporate area, with proxying services in the DMZ for occasional remote access. Doing inside-to-inside nat adds more load to the DMZ, which in our case would be significant.

If you don't do DNAT + SNAT, you will get asymettric routing, which is notoriously tricky to get right. So you SNAT and lose source IP infomation. However, it is bloody useful to link source IPs to people for troubleshooting purposes. Or occasionally nerfshooting purposes in cases of stupidity. "Hey this IP is doing something wonky on unauthenticated service X" "Oh, let's look in the imap server logs who it is".

Comments

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