** Description changed: [Impact] * If a DNS lookup returns SERVFAIL, systemd-resolved seems to cache the result for very long (infinity?). I have to restart systemd-resolved to have the negative caching purged. * After SERVFAIL DNS server issue has been resolved, chromium/firefox still returns DNS error despite host can correctly resolve the name. [Test Case] * If a lookup returns SERVFAIL systemd-resolved will cache the result for 30s (See 201d995), however, there are several use cases on which this condition is not acceptable (See #5552 comments) and the only workaround would be to disable cache entirely or flush it , which isn't optimal. * Configure /etc/systemd/resolved.conf as follows: Cache=yes (default) * Restart systemd-resolved (systemctl restart systemd-resolved.service) * Run a host/getent command against a entry that will return SERVFAIL and check the journalctl output to see that the reply gets served from cache. - root@systemd-disco:/home/ubuntu# host www.no-record.cl + root@systemd-disco:/home/ubuntu# host www.montemar.cl Host www.montemar.cl not found: 2(SERVFAIL) root@systemd-disco:/home/ubuntu# journalctl -u systemd-resolved -n -- Logs begin at Fri 2019-07-12 18:09:42 UTC, end at Tue 2019-07-23 15:10:17 UTC. -- Jul 23 15:10:10 systemd-disco systemd-resolved[1282]: Transaction 6222 for <ntp.ubuntu.com IN AAAA> on scope dns on ens3/* now complete with <success> Jul 23 15:10:10 systemd-disco systemd-resolved[1282]: Sending response packet with id 61042 on interface 1/AF_INET. Jul 23 15:10:10 systemd-disco systemd-resolved[1282]: Freeing transaction 6222. Jul 23 15:10:17 systemd-disco systemd-resolved[1282]: Got DNS stub UDP query packet for id 53580 Jul 23 15:10:17 systemd-disco systemd-resolved[1282]: Looking up RR for www.no-record.cl IN A. Jul 23 15:10:17 systemd-disco systemd-resolved[1282]: RCODE SERVFAIL cache hit for www.no-record.cl IN A Jul 23 15:10:17 systemd-disco systemd-resolved[1282]: Transaction 58570 for < www.no-record.cl IN A> on scope dns on ens3/* now complete with <rcode-fai Jul 23 15:10:17 systemd-disco systemd-resolved[1282]: Freeing transaction 58570. Jul 23 15:10:17 systemd-disco systemd-resolved[1282]: Sending response packet with id 53580 on interface 1/AF_INET. Jul 23 15:10:17 systemd-disco systemd-resolved[1282]: Processing query... [Regression Potential] * The default options (Yes/No) will remain as default Yes, behaving in the same original way, by setting it to no-negative any negative answer will be skipped from being cached. * No regression potential has been detected as this just introduces a new possible option for the Cache configuration directive. [Fix] With the cache option set to 'no-negative', negative DNS answers are entirely avoided to being cached. root@systemd-disco:/home/ubuntu# host www.metaklass.org Host www.metaklass.org not found: 2(SERVFAIL) * Look at the systemd-resolved entries root@systemd-disco:/home/ubuntu# journalctl -u systemd-resolved -n -- Logs begin at Fri 2019-07-12 18:09:42 UTC, end at Fri 2019-07-12 18:48:31 UTC. -- Jul 12 18:48:31 systemd-disco systemd-resolved[2635]: Cache miss for www.metaklass.org IN A Jul 12 18:48:31 systemd-disco systemd-resolved[2635]: Transaction 22382 for <www.metaklass.org IN A> scope dns on ens3/. Jul 12 18:48:31 systemd-disco systemd-resolved[2635]: Using feature level UDP for transaction 22382. Jul 12 18:48:31 systemd-disco systemd-resolved[2635]: Sending query packet with id 22382. Jul 12 18:48:31 systemd-disco systemd-resolved[2635]: Processing incoming packet on transaction 22382 (rcode=SERVFAIL). Jul 12 18:48:31 systemd-disco systemd-resolved[2635]: Server returned error: SERVFAIL Jul 12 18:48:31 systemd-disco systemd-resolved[2635]: Not caching negative entry for: www.metaklass.org IN A, cache mode set to no-negative Jul 12 18:48:31 systemd-disco systemd-resolved[2635]: Transaction 22382 for <www.metaklass.org IN A> on scope dns on ens3/ now complete with from network (unsigned). Jul 12 18:48:31 systemd-disco systemd-resolved[2635]: Sending response packet with id 31060 on interface 1/AF_INET. The following patch https://github.com/systemd/systemd/pull/13047 implements the required changes. [Other Info] Note that systemd in Eoan is being upgraded to upstream 242, so I am not adding this to Eoan now, as I don't want to disturb the merge. If needed after the merge, I'll add to Eoan.
-- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1668771 Title: [SRU] systemd-resolved negative caching for extended period of time To manage notifications about this bug go to: https://bugs.launchpad.net/systemd/+bug/1668771/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
