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Cisco Bug: CSCvu63661 - Umbrella: DNS resolution hangs globally after sending multiple TCP DNS requests with large response

Last Modified

Aug 17, 2020

Products (1)

  • Cisco AnyConnect Secure Mobility Client

Known Affected Releases

4.8(1090) 4.9(1045)

Description (partial)

DNS resolution stops working (hangs) regardless of DNS query or app issuing it.

- Platform: maCOS, Windows
- AnyConnect Umbrella DNS protection active. 
- Send multiple TCP DNS requests that produce a large (>512 bytes) response, e.g. via command
for /l %i in (1,1,400) do nslookup -type=txt -vc
on Windows.
- Note that it may take a few hundred requests to reproduce.
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  • Full Description (including symptoms, conditions and workarounds)
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