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<TITLE>RE: DNS DOS increasing?</TITLE>
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<P><FONT SIZE=2>-----Original Message-----</FONT>
<BR><FONT SIZE=2>From: E.B. Dreger [<A HREF="mailto:eddy+public+spam@noc.everquick.net">mailto:eddy+public+spam@noc.everquick.net</A>]</FONT>
<BR><FONT SIZE=2>Sent: Tuesday, January 22, 2002 12:51 AM</FONT>
<BR><FONT SIZE=2>To: just me</FONT>
<BR><FONT SIZE=2>Cc: Miquel van Smoorenburg; nanog@merit.edu</FONT>
<BR><FONT SIZE=2>Subject: Re: DNS DOS increasing?</FONT>
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<P><FONT SIZE=2>That's not the problem. It's ill-behaved clients that ignore TTL</FONT>
<BR><FONT SIZE=2>and query every 10s no matter what. See some of James Smith's</FONT>
<BR><FONT SIZE=2>posts...</FONT>
<BR><FONT SIZE=2>-----</FONT>
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<P><FONT SIZE=2>Methinks I have been misunderstood or I have obfuscated my own point... The dns server is set to give a 10 second TTL to the dns client. The entry ages out in 10 seconds, so the client (following expected practice) ages the entry out. 15 seconds later, when they click on the next button on the web page (for example), they have to go get the IP again. This the DOS (DDOS?) like behavior.</FONT></P>
<P><FONT SIZE=2>Sure the dns client is hammering the dns server, but the server is telling it to by giving out an absurdly short TTL... The server is ASKING FOR IT by setting it's TTL to 10 seconds. The client can't help it, it is just doing what it has been told.</FONT></P>
<P><FONT SIZE=2>Why It Does It This Way</FONT>
<BR><FONT SIZE=2>The mechanism this dns server uses for selecting which IP to respond with is a ping to check upstream connectivity. This box pings constantly, looking for a fail. When link failure is detected, the box starts feeding DNS queries with responses from the other links subnet. The short ttl ensures that dns clients age out the info fast enough to make a near seamless failover to the other link. </FONT></P>
<P><FONT SIZE=2>Since the box is authoritative for the zone, and has interfaces in more than one subnet or provider, the failure of one link means that the normal dns mechanism of going to the next responsive dns server points users to the remaining good link, and the box obliges by serving out responses that point the client back down the good link.</FONT></P>
<P><FONT SIZE=2>James H. Smith II NNCDS NNCSE</FONT>
<BR><FONT SIZE=2>Systems Engineer</FONT>
<BR><FONT SIZE=2>The Presidio Corporation</FONT>
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<P><FONT SIZE=2>I speak for myself, and that gets me into enough trouble.</FONT>
<BR><FONT SIZE=2>(back to lurk mode...)</FONT>
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