25

I just installed a new Ubuntu Server 18.04. I set my hostname hostnamectl set-hostname ****.openbayou.biz and I set /etc/hosts:

127.0.0.1 localhost
[ip address] ****.openbayou.biz hostname
# The following lines are desirable for IPv6 capable hosts
[ip6 address] *****.openbayou.biz hostname
::1     localhost ip6-localhost ip6-loopback
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters

I also installed OSSEC to monitor for new files, errors and changes to my server and I'm now getting these alerts:

Server returned error NXDOMAIN, mitigating potential DNS violation DVE-2018- 
0001, retrying transaction with reduced feature level UDP.`

It's now repeating itself:

systemd-resolved[3195]: message repeated 4 times: [ Server returned error 
NXDOMAIN, mitigating potential DNS violation DVE-2018-0001, retrying transaction 
with reduced feature level UDP.]

I've looked online for a solution and nobody is reporting this issue.

  • Are you behind a captive portal? – dobey Jul 23 '18 at 18:56
  • No, this is a Linode 4GB server – Gregory Schultz Jul 23 '18 at 19:27
  • If you comment out the two lines you added, does it make a difference? I don't think the errors are about your /etc/hosts. They are happening because of the infrastructure the server is behind is likely doing something wrong. github.com/systemd/systemd/pull/8608 seems to be the issue you're having, and was the first search result for "DVE-2018-0001." I don't think you're going to get a satisfactory answer until the upstream issue is fixed and released. – dobey Jul 23 '18 at 19:27
21

Server returned error NXDOMAIN, mitigating potential DNS violation DVE-2018- 0001, retrying transaction with reduced feature level UDP.

The same error happened to my desktop machine, I don't know if it applies to server too.

It seems that my system had the old config in the place, resulting in a conflict between two services: resolvconf and systemd-resolved.

The symlink /etc/resolv.conf pointed to ../run/resolvconf/resolv.conf

Changing it to point to /run/systemd/resolve/resolv.conf which is managed by systemd, fixed it for me.

Read more here.

Hope that helped.

  • 3
    Mine is pointing to /run/systemd/resolve/stub-resolv.conf on an Ubuntu 18.10 instance. – datashaman Nov 11 '18 at 6:14
  • Forgot to mention my system. Latest KDE Neon, (Ubuntu based), 18.04.1, 4.15.0-39-generic. – Panagiotis Tabakis Nov 18 '18 at 12:38
  • This fixed the problem for me, too. Thx! – Witek Nov 26 '18 at 10:03
  • 3
    @datashaman It was the same case for me but changing the symlink to point /run/systemd/resolve/resolv.conf from /run/systemd/resolve/stub-resolv.conf fixed the issue for me. I no longer see that error. – Karthic Raghupathi Dec 1 '18 at 22:21
  • Same worked for me. I'm on 18.10, but migrated from 18.04. Changing the /etc/resolv.conf -> /run/systemd/resolve/resolv.conf did the trick. – Igor Kupczyński Jan 9 at 12:44
8

I asked on the OSSEC GitHub about this error and they recommended writing a rule to ignore NXDOMAIN errors. Add to /var/ossec/rules/local_rules.xml

<rule id="234567" level="0">
 <program_name>systemd-resolved</program_name>
 <match>Server returned error NXDOMAIN</match>
 <description>Usless systemd-resolvd log message</description>
</rule>
5

I noticed the same thing on an Ubuntu 18.04 server which was recently updated to 18.04.1.

It would appear that systemd-resolve logs that message whenever it gets any NXDOMAIN response. In my case I have postfix running. So I get a lot of NXDOMAINS when random servers connect that don't have PTR record set.

You can test it with

systemd-resolve securelogin.example.com

Then you should see the log message appear.

With this in mind it would appear to be a relatively innocuous error and you can ignore it.

  • Added PTR record and haven't gotten a notice (so far). Thanks! – Gregory Schultz Jul 26 '18 at 1:09
  • Nope. Still getting them. Think next stage is to get OSSEC to ignore them. Would it be something related to Cloudflare as it's going through their systems and not being bypassed? Also, I see that OSSEC has an update (on 2.9.4, update to 3.0.0). Will update and see what happens. – Gregory Schultz Jul 26 '18 at 21:29
  • It's just part of how systemd works. If systemd-resolve tries to resolve a domain that doesn't resolve it logs that message. – Rwky Jul 27 '18 at 22:07
3

This warning is logged by systemd-resolved, whenever a name can not be resolved by the DNS system (e.g. nslookup www.kjfoiqaefah34876asdf.com). This can be tolerated and is no reason to be alarmed. This is no error and nothing needs to be fixed.

Redirecting /etc/resolv.conf to /run/systemd/resolve/resolv.conf is wrong, because this way systemd-resolved is skipped and the application with the faulty DNS request talks directly to the name server and not to the systemd-resolved stub anymore. This way systemd-resolved does not notice the NXDOMAIN events any more and therefore cannot log it any more.

The NXDOMAIN events are caused by packages, which try to access non-existing servers during system startup.

  • Is there any way to discover what the unresolved names are? – OrangeDog Jan 2 at 10:53
2

Summary:

NXDOMAIN error message means that a domain does not exist.

Some ISPs started DNS hijacking or DNS redirection for NXDOMAIN error messages. It is the practice of redirecting the resolution of Domain Name System (DNS) names to other DNS servers or web servers.

Commonly used for displaying advertisements or collecting statistics.

This practice violates the RFC standard for DNS (NXDOMAIN) responses.

Phishing: Cross-site scripting attacks can occur due to malicious hijacking.

Censorship: DNS service providers to block access to selected domains.

Shown up here: https://www.dnsknowledge.com/whatis/nxdomain-non-existent-domain-2/

0

I was able to get rid of the message, and by the way I was also able to finally connect to my samba server, by changing the server name to server.domain instead of only server.

0

My understanding after having read the previous answers and other web pages such as Ubuntu 18.04 systemd-resolved error NXDOMAIN is that this is more a warning than an error and there is nothing I can do on my side about it.

Therefore, I agree with those who say that we should not try to do something on our side so that these messages are not produced anymore. If we succeed, it is likely that we have altered the normal way the system resolve DNS requests.

However, since I have thousands of them (I am also in a desktop - it's not a server), I don't want them in my syslog file. Therefore, following https://www.rsyslog.com/doc/v8-stable/configuration/filters.html and Number pair prefix to config files, I added a file named 10-resolv.conf with a single line :msg, contains, "Server returned error NXDOMAIN, mitigating potential DNS violation DVE-2018-0001, retrying transaction with reduced feature level UDP" ~ in the directory /etc/rsyslog.d .

The name 10-resolv.conf is not important, but it must precede all other file names in the directory in alphabetic order. The command :msg, contains, <message-part> ~ says that all messages that contains <message-part> must be ignored: the tilde ~ in the command says to drop the message.

Note added: Since I wrote this answer, I installed some packages (for other reasons) and the error message is not produced anymore as checked with journalctl -u systemd-resolved -f. One installed package that might explain the disappearance of this message is libnss-resolve.

0

This appears related to EDNS. The difference between using stub-resolv.conf and resolv.conf is options edns0.

Extension mechanisms for DNS (EDNS) is a specification for expanding the size of several parameters of the Domain Name System (DNS) protocol which had size restrictions that the Internet engineering community deemed too limited for increasing functionality of the protocol.

https://en.wikipedia.org/wiki/Extension_mechanisms_for_DNS

More details under this issue: https://bugs.launchpad.net/ubuntu/+source/systemd/+bug/1766969

It sounds like, you can simply turn off that "option".

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