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What does exactly break with double NAT?

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Jonas Stein

There are many warnings about double NAT networks.

DSL---modem with router-----second router
      configured by ISP     inner network
        first NAT            second NAT

What will exactly break in a double NAT network?

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NiKiZe
The same things as break with single NAT, just that double NAT requires double portforwards, adds latency. you now have 2 required nodes, if any of them loses their conntrack (restarts) all connections are reset.
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John Mahowald avatar Server
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John Mahowald

NAT444 makes performance worse, peer to peer applications difficult, and running an FTP server nearly impossible.

CableLabs and Time Warner Cable and others have been doing testing and a few years ago made an accessible writeup, Assessing the Impact of Carrier-Grade NAT on Network Applications. Interesting categories in the impacts section include:

  • Dropped Services
  • Performance Impacted Services
  • Loss of geolocation information
  • Lawful Intercept/Abuse Response

Of course with sufficient engineering these can be worked around. Even bigger stateful NAT gear, and logging that state. Traversal techniques, STUN, proxying, Port Control Protocol. Special applications moved to one or zero NATs where connectivity works.

Maybe would be easier to implement IPv6 end to end.

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