Comment 0 for bug 1785383

dnsmasq 2.79 and below omits EDNS0 OPT records when returning an empty answer for a domain it is authoritative for. systemd-resolved seems to get confused by this in certain circumstances; when using the stub resolver and requesting an address for which there are no AAAA records, there can sometimes be a five second hang in resolution.

This is fixed by upstream commit http://thekelleys.org.uk/gitweb/?p=dnsmasq.git;a=commit;h=1682d15a744880b0398af75eadf68fe66128af78

Not sure if it is worth cherry picking? I imagine the most likely trigger will be dnsmasq on routers which are not likely to be running Ubuntu, but maybe just in case.

I also think there are some logic issues in systemd-resolved, upstream bug filed:

https://github.com/systemd/systemd/issues/9785

Simple-ish test case:

---
IFACE=dummy0
SUBNET=10.0.0

ip link add $IFACE type dummy
ifconfig $IFACE ${SUBNET}.1/24
dnsmasq -h -R -d -C /dev/null -2 $IFACE -z -i $IFACE -I lo --host-record=test.test,${SUBNET}.1 &

dig -t a test.test @10.0.0.1 | grep EDNS
# should return "; EDNS ..."
dig -t aaaa test.test @10.0.0.1 | grep EDNS
# again, should return "; EDNS ..." but doesn't
---

To reproduce the systemd-resolved side of the problem

---
# as above, but
# now configure systemd-resolved to look at only 10.0.0.1, then

systemd-resolve --reset-server-features
# should exhibit five second delay then connect, assuming sshd is running :)
ssh test.test
---

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: dnsmasq-base 2.79-1
ProcVersionSignature: Ubuntu 4.15.0-23.25-generic 4.15.18
Uname: Linux 4.15.0-23-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.2
Architecture: amd64
Date: Sat Aug 4 11:33:56 2018
InstallationDate: Installed on 2018-05-31 (64 days ago)
InstallationMedia: Xubuntu 18.04 LTS "Bionic Beaver" - Release amd64 (20180426)
ProcEnviron:
 TERM=xterm
 PATH=(custom, no user)
 LANG=en_GB.UTF-8
 SHELL=/bin/bash
SourcePackage: dnsmasq
UpgradeStatus: No upgrade log present (probably fresh install)