[dns-operations] Force TCP for external quereis to Open Resolvers?

Jim Reid jim at rfc1035.com
Sun Mar 31 14:22:09 UTC 2013

On 31 Mar 2013, at 14:36, "Patrick W. Gilmore" <patrick at ianai.net> wrote:

> CloudFlare, CacheFly, and a few other CDNs who anycast web server addresses would probably disagree.

Yeah. We both know we have had those discussions before Patrick and (hopefully) agreed to disgagree. :-)

>> Keeping state for bazillions of DNS TCP connections to a resolving server will present further challenges. [Maybe TCPCT could help.] That could lead to a new DoS attack vector: overwhelming a resolving server with too much TCP traffic. Though that could be done already I suppose.
> Shouldn't be difficult to keep TCP in a different thread or process, so UDP is unaffected.

Isolating TCP and UDP traffic at the DNS server is not the issue I think. Keeping bazillions of protocol control blocks (or equivalent) in the kernel, one for each TCP connection, is. Though I'd welcome getting told I am wrong about that. Those PCBs have to stick around for twice the maximum segment life time, typically a minute or more. DNS over TCP could easily mean resolvers handling orders of magnitude more connections (ie PCBs) than the busiest of web servers. A DNS server getting ~10Kqps over TCP would have around 1 million "active" PCBs in the kernel: nasty.

>> Another problem is lots of crapware -- CPE, hotel networks, coffee shop wi-fi, etc -- assume DNS is only ever done over UDP. Anyone stuck behind that already loses whenever they get a truncated response. They'll have much bigger problems if resolving servers default to truncation and TCP retries for everything. I suppose more use of DNS over TCP could provide an incentive to get those broken middleboxes fixed. Wouldn't hold my breath though....
> Maybe it would be an incentive to fix those broken clients?

It's not the users' clients that are broken. [Though they may be bust too.] It's the middleware crap that these clients sit behind: the DSL or cable box that the typical Internet user has or the hotel/coffee-shop network that mangles DNS packets. I already said forcing DNS over TCP could provide an incentive to get those middleware devices fixed but doubted this would ever happen. Good luck getting a Wal-mart or Verizon (say) to beat up their Chinese suppliers for shipping DSL boxes that constrain DNS to UDP packets of less than 512 bytes.

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