Comment 8 for bug 1805183

@juliank
I *believe* this is impacting our AWS ubuntu EC2 machines and causing wild DNS errors impacting our users, I tried to apply the patch in this thread but it doesn't work, it complains like this

    /etc/dhcp/dhclient-enter-hooks.d/resolved: Syntax error: "(" unexpected

about this line

    if ! cmp --quiet $oldstate <(md5sum $statedir/isc-dhcp-v4-$interface.conf $statedir/isc-dhcp-v6-$interface.conf 2>&1); then

Also for me this line

+ md5sum $statedir/isc-dhcp-v4-$interface.conf $statedir/isc-dhcp-v6-$interface.conf &> $oldstate

outputs to stdout

   $ sudo dhclient
   RTNETLINK answers: File exists
   d41d8cd98f00b204e9800998ecf8427e /run/systemd/resolved.conf.d/isc-dhcp-v4-ens5.conf
   md5sum: /run/systemd/resolved.conf.d/isc-dhcp-v6-ens5.conf: No such file or directory

and the resulting temp file is empty.

Dunno why your patch misbehaves like this but I really have no time to investigate further.

I've attached a patch that seems to be working on our servers in case others are experiencing the same issue and need a quick fix.

----
Adding few server details in case you need 'em:

$ uname -a
Linux ip-172-31-14-255 4.15.0-42-generic #45-Ubuntu SMP Thu Nov 15 19:32:57 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux
$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 18.04.1 LTS
Release: 18.04
Codename: bionic
$ dhclient --version
isc-dhclient-4.3.5
$ systemd --version
systemd 237
+PAM +AUDIT +SELINUX +IMA +APPARMOR +SMACK +SYSVINIT +UTMP +LIBCRYPTSETUP +GCRYPT +GNUTLS +ACL +XZ +LZ4 +SECCOMP +BLKID +ELFUTILS +KMOD -IDN2 +IDN -PCRE2 default-hierarchy=hybrid
$ bash --version
GNU bash, version 4.4.19(1)-release (x86_64-pc-linux-gnu)