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Hamlet
Tux's lil' helper
Tux's lil' helper


Joined: 15 Apr 2005
Posts: 133
Location: Turin (Italy)

PostPosted: Thu Oct 11, 2018 6:23 am    Post subject: DNS queries timing out Reply with quote

I have a problem with the name resolution service. The problem has started suddenly on a previously working system (I update it periodically, but I would not know how to identify a suspicious update):
  • Vivaldi browser gives me a `DNS_PROBE_FINISHED_BAD_CONFIG`, a `DNS_PROBE_FINISHED_NXDOMAIN` or similar, more often than not
  • Kerberos 5 client often fails to get me a Kerberos ticket
  • SSH often can't establish a connection to remote servers
  • `emerge --sync` takes some tries to find a server
  • etc.

My connection has a cable modem connected to a router, connected to a OSX laptop and a Linux desktop (where the problem is) via local Ethernet, plus some other wireless devices.
The Linux desktop and the OSX laptop are both connected via LAN, both refer to the DNS 8.8.8.8 (secondary 8.8.4.4), referral which I believe comes from the Internet provider. Only the former manifests the problem above.
Pinging the DNS (in fact, even flooding it -- I apologise) does not lose any packet.
Wireshark shows me that when I query the DNS, quite a number of questions are submitted, and typically a few of them do not see any answer; each failure is followed by 5 seconds of timeout and then a retry.
`resolvectl` is very patient and waits, but the rest of the clients (most notably, the browser) give up pretty quickly.
Here is a spectacular 42 s query of `www.wikipedia.com`:
Code:
No.  Time           Source       Destination  Protocol Length Info
  51 65.896479279   192.168.1.2  8.8.4.4      DNS      111    Standard query 0x54f2 AAAA www.wikipedia.com OPT
  52 65.896615624   192.168.1.2  8.8.4.4      DNS      111    Standard query 0x36e0 A www.wikipedia.com OPT
  68 71.123506666   192.168.1.2  8.8.8.8      DNS      111    Standard query 0x54f2 AAAA www.wikipedia.com OPT
  69 71.123629403   192.168.1.2  8.8.4.4      DNS      111    Standard query 0x36e0 A www.wikipedia.com OPT
  72 76.373471171   192.168.1.2  8.8.8.8      DNS      111    Standard query 0x54f2 AAAA www.wikipedia.com OPT
  73 76.373595839   192.168.1.2  8.8.4.4      DNS      111    Standard query 0x36e0 A www.wikipedia.com OPT
  74 76.463031722   8.8.8.8      192.168.1.2  DNS      116    Standard query response 0x54f2 AAAA www.wikipedia.com AAAA 2620:0:863:ed1a::1 OPT
  75 76.463299593   192.168.1.2  8.8.4.4      DNS      111    Standard query 0xfbf8 SOA www.wikipedia.com OPT
  76 76.634459989   8.8.4.4      192.168.1.2  DNS      152    Standard query response 0xfbf8 SOA www.wikipedia.com SOA ns0.wikimedia.org OPT
  77 76.634706117   192.168.1.2  8.8.4.4      DNS      107    Standard query 0x8bfe DS wikipedia.com OPT
  78 76.634832638   192.168.1.2  8.8.4.4      DNS      111    Standard query 0xf28a DS www.wikipedia.com OPT
  79 76.672042886   8.8.4.4      192.168.1.2  DNS      805    Standard query response 0x8bfe DS wikipedia.com NSEC3 RRSIG SOA a.gtld-servers.net RRSIG NSEC3 RRSIG OPT
  80 76.672308732   192.168.1.2  8.8.4.4      DNS      97     Standard query 0xffa8 DNSKEY com OPT
  81 76.686307119   8.8.4.4      192.168.1.2  DNS      785    Standard query response 0xffa8 DNSKEY com DNSKEY DNSKEY RRSIG OPT
  82 76.686556909   192.168.1.2  8.8.4.4      DNS      97     Standard query 0x7be5 DS com OPT
  83 76.697220813   8.8.4.4      192.168.1.2  DNS      409    Standard query response 0x7be5 DS com DS RRSIG OPT
  84 76.697452035   192.168.1.2  8.8.4.4      DNS      93     Standard query 0xae72 DNSKEY <Root> OPT
  85 76.710338907   8.8.4.4      192.168.1.2  DNS      1456   Standard query response 0xae72 DNSKEY <Root> DNSKEY DNSKEY DNSKEY DNSKEY RRSIG OPT
  86 76.805754622   8.8.4.4      192.168.1.2  DNS      152    Standard query response 0xf28a DS www.wikipedia.com SOA ns0.wikimedia.org OPT
  87 76.806009410   192.168.1.2  8.8.4.4      DNS      107    Standard query 0xbf6d SOA wikipedia.com OPT
  88 76.879904179   8.8.4.4      192.168.1.2  DNS      148    Standard query response 0xbf6d SOA wikipedia.com SOA ns0.wikimedia.org OPT
  89 81.623492440   192.168.1.2  8.8.8.8      DNS      111    Standard query 0x36e0 A www.wikipedia.com OPT
  90 86.873321374   192.168.1.2  8.8.4.4      DNS      111    Standard query 0x36e0 A www.wikipedia.com OPT
  94 92.123468898   192.168.1.2  8.8.8.8      DNS      111    Standard query 0x36e0 A www.wikipedia.com OPT
  95 97.373317730   192.168.1.2  8.8.4.4      DNS      111    Standard query 0x36e0 A www.wikipedia.com OPT
  96 102.623480148  192.168.1.2  8.8.8.8      DNS      111    Standard query 0x36e0 A www.wikipedia.com OPT
  97 107.873317967  192.168.1.2  8.8.4.4      DNS      111    Standard query 0x36e0 A www.wikipedia.com OPT
  98 107.961115779  8.8.4.4      192.168.1.2  DNS      104    Standard query response 0x36e0 A www.wikipedia.com A 198.35.26.96 OPT

OSX issues, in comparison, a single query ("Standard query 0x0426 A www.wikipedia.com OPT").
The local DNS cache (`resolvectl statistics`) clears quick and misses often, even after repeated `resolvectl query` queries for the same address.

Is this failure rate normal? is it my timeout unreasonably high? (a successful query takes a few tenth of a second) If so, where is that timeout controlled in my systemd/dhcpcd platform?
Any other suggestion?
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turtles
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Joined: 31 Dec 2004
Posts: 1340

PostPosted: Fri Oct 19, 2018 5:45 am    Post subject: Reply with quote

Those AAAA requests are ipv6
Do you really have an ipv6 nameserver?

My guess is your ipv6 DNS requests have top priority then requests are timing out and your system falls back to ipv4, which is all most networks have.

Another handy one is
Code:
/usr/sbin/tcpdump -vvv -s 0 -l port 53


Check out /etc/gai.conf

ipv6 is rarely used but once was thought to be replacing ipv4 'soon' that was 20 years ago. It probably slows down more Gentoo systems than anybody realizes.
Unless your running a Gentoo router or know your system needs ipv6 I would get rid of it.
I now disable the USE flag
Code:
-ipv6
in my make.conf these days .
A few packages "need it" like net-dialup/ppp and I enable it on a per package basis.
Some other packages need it in the kernel (but not enabled) So I keep compiling it in as modules.
/etc/default/grub
Code:
GRUB_CMDLINE_LINUX="ipv6.disable=1"


Hope that helps

Cheers
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Hamlet
Tux's lil' helper
Tux's lil' helper


Joined: 15 Apr 2005
Posts: 133
Location: Turin (Italy)

PostPosted: Mon Nov 05, 2018 7:00 am    Post subject: Reply with quote

I tried with "-ipv6" and killing IPv6 support from the kernel, but it did not seem to help.
Thank you for the hint, though, and for your thoughts on IPv6. It seems now to be more of a presence than it used to be, but I might be better off without it for the time being anyway.

I had this problem for two weeks, making navigation almost impossible. A few days ago, it went back to normal. Why? no clue.
Probably I'll whine and reopen the case in another two weeks, when it will be come back...
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