On Wed, Aug 14, 2019 at 06:51:52PM +0100, Dominic Preston wrote: > On Wed, 14 Aug 2019 at 18:43, Simon Kelley <si...@thekelleys.org.uk> wrote: > > On 11/08/2019 21:01, Dominic Preston wrote: > > > Hi, > > > > > > I have a fresh installation of Ubuntu 18.04 on Google Cloud Platform. > > > > > > dig @127.0.0.1 pir.org > > > > > > After that there's a long pause, and the result comes back with the > > > following line at the top of dig: > > > > > > ;; Truncated, retrying in TCP mode. > > > > > > dnsmasq log says: > > > > > > dnsmasq: reducing DNS packet size for nameserver 22.214.171.124 to 1280 > > > > > > If I run this, dig comes back immediately with no pause and no TCP mode: > > > > > > dig @126.96.36.199 pir.org > > > > > > Any ideas why the first dig command has problems and the second dig > > > command is fine? > > > > > > > It's likely that the MTU for the path from 188.8.131.52 to you it limited, > > and a reply for one of the queries needed to verify the query is > > getting dropped. Hence dnsmasq reduces the packet size to the more > > conservative 1280, and the query has to be done over TCP. > > > > It works fast the second time because the information you're asking for > > is cached by dnsmasq. > > > > Cheers, > > > > Simon. > > > > Thanks Simon, that makes sense. > > Is there a straightforward (non dnsmasq) network command I can run so > I can demonstrate this MTU issue to the Google Cloud team?
https://duckduckgo.com/?q=test+MTU+size does hint `ping` I don't know how well it goes in this situation. Road that I would explore is how to mimic the dnsmasq request to upstream DNS with `dig`. My attempt with `dig @184.108.40.206 pir.org ANY` returns "MSG SIZE rcvd: 2509", but no sign of ";; Truncated, retrying in TCP mode." Interresting problem. > Regards, > Dom. Groeten Geert Stappers -- Leven en laten leven _______________________________________________ Dnsmasq-discuss mailing list Dnsmasqemail@example.com http://lists.thekelleys.org.uk/mailman/listinfo/dnsmasq-discuss