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Firefox behavior if doh is blocked by firewall

ravinder.verma
ravinder.verma
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I am working on firewall which can block Doh request to cloudfare or any custom doh service provider. I would like to know if firewall silently drops/block what is the behavior of firefox? Does it automatically fallback to premise DNS server or fallback is config driven and if fallback disabled then it continue using DOH only in which case things will continue to fail forever. Is it possible firewall can generate some end user notification to browser so it can detect block (e.g connection refused etc) connection and fallback? Yes my firewall is kind of MITM and does terminate TCP connection so its firewall/proxy.

I am working on firewall which can block Doh request to cloudfare or any custom doh service provider. I would like to know if firewall silently drops/block what is the behavior of firefox? Does it automatically fallback to premise DNS server or fallback is config driven and if fallback disabled then it continue using DOH only in which case things will continue to fail forever. Is it possible firewall can generate some end user notification to browser so it can detect block (e.g connection refused etc) connection and fallback? Yes my firewall is kind of MITM and does terminate TCP connection so its firewall/proxy.

Chosen solution

Hi,

yes, if Firefox fails to resolve a domain via DoH, it should fallback to the DNS. You can modify this behaviour using the network.trr.mode preference. Possible values of this pref are:

  • 0 - Off (default). use standard native resolving only (don't use TRR at all)
  • 1 - Reserved (used to be Race mode)
  • 2 - First. Use TRR first, and only if the name resolve fails use the native resolver as a fallback.
  • 3 - Only. Only use TRR. Never use the native (This mode also requires the bootstrapAddress pref to be set)
  • 4 - Reserved (used to be Shadow mode)
  • 5 - Off by choice. This is the same as 0 but marks it as done by choice and not done by default.


Type of the DNS connection you can check on the about:networking#dns page (it is a TRR column).

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All Replies (2)

TyDraniu
TyDraniu
  • Top 10 Contributor
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Chosen Solution

Hi,

yes, if Firefox fails to resolve a domain via DoH, it should fallback to the DNS. You can modify this behaviour using the network.trr.mode preference. Possible values of this pref are:

  • 0 - Off (default). use standard native resolving only (don't use TRR at all)
  • 1 - Reserved (used to be Race mode)
  • 2 - First. Use TRR first, and only if the name resolve fails use the native resolver as a fallback.
  • 3 - Only. Only use TRR. Never use the native (This mode also requires the bootstrapAddress pref to be set)
  • 4 - Reserved (used to be Shadow mode)
  • 5 - Off by choice. This is the same as 0 but marks it as done by choice and not done by default.


Type of the DNS connection you can check on the about:networking#dns page (it is a TRR column).

Modified by TyDraniu

ravinder.verma
ravinder.verma Question owner
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Thank you TyDraniu; this helps!