bind 9.10 fallback to tcp

[Carl Byington]

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I have a client with 9.10.2-P1-RedHat-9.10.2-2.P1.fc22 on Fedora 22, on
a machine with a pppoe link with an mtu of 1492. The routers seem to be
properly fragmenting udp - it can receive large packets such as

dig www.byington.org +dnssec +bufsiz=4000 +notcp @205.147.40.34

which says:

;; MSG SIZE  rcvd: 3790

However, a tcpdump for tcp port 53 shows a lot of traffic. In
particular,

rndc flushtree novell.com
dig www.novell.com @localhost

shows some tcp traffic to the .com servers. How does one isolate the
query or server that is causing that fallback to tcp?


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