North American Network Operators Group
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Re: large organization nameservers sending icmp packets to dns servers.

  • From: Douglas Otis
  • Date: Thu Aug 09 15:17:20 2007


On Aug 8, 2007, at 5:35 PM, Paul Vixie wrote:


... but a TCP connection will consume a
significant amount of a name server's resources.

...wrong.

Wanting to understand this comment, ...

the resources given a nameserver to TCP connections are tightly controlled, as described in RFC 1035 4.2.2. so while TCP/53 can become unreliable during high load, the problems will be felt by initiators not targets.

The relevant entry in Section 1035 4.2.2 recommends that the server not block other activities waiting for TCP data. This is not exactly a requirement that TCP should fail before UDP.

The concern leading to a suggestion that TCP always fail was a bit different. A growing practice treats DNS as a type of web server when used to publish rather bulky script-like resource records. Due to typical sizes, it is rather common to find these records depend upon TCP fallback. This problem occurred with paypal, for example. TCP fallback is especially problematic when these records are given wildcards. Such fallback increases the amplification associated with an exploit related to the use of the script within the record.

Of course there are better ways to solve this problem, but few are as certain.

-Doug