+++ This bug was initially created as a clone of Bug #2228188 +++ LLMNR is multicast resolution protocol. It is enabled in systemd-resolved since it has become default in f33 and provides Windows compatible resolution of names. It works over multicast on local networks and is potentially insecure. Any machine on the same network link can impersonate any hostname without dot present. But even Microsoft, which is behind its RFC has announced its deprecation [MS-LLMNR]. What is not caused by the protocol itself, but the way systemd-resolved implements it, it directs single label names to LLMNR resolution *only*. So it also answers for queries not really intended for LLMNR resolution [stub-llmnr]. Reproducible: Always Steps to Reproduce: 1. resolvectl llmnr 2. dig -t ns org. Actual Results: # resolvectl llmnr Global: resolve Link 2 (eth0): resolve # dig -t ns org. ; <<>> DiG 9.18.17 <<>> -t ns org. ;; global options: +cmd ;; Got answer: ;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id: 45940 ;; flags: qr aa rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 1 ;; OPT PSEUDOSECTION: ; EDNS: version: 0, flags:; udp: 65494 ;; QUESTION SECTION: ;org. IN NS ;; Query time: 3580 msec ;; SERVER: 127.0.0.53#53(127.0.0.53) (UDP) ;; WHEN: Tue Aug 01 12:58:27 EDT 2023 ;; MSG SIZE rcvd: 32 Expected Results: # resolvectl llmnr Global: no Link 2 (eth0): no # dig -t ns org. ; <<>> DiG 9.18.17 <<>> -t ns org. ;; global options: +cmd ;; Got answer: ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 18927 ;; flags: qr rd ra; QUERY: 1, ANSWER: 6, AUTHORITY: 0, ADDITIONAL: 1 ;; OPT PSEUDOSECTION: ; EDNS: version: 0, flags:; udp: 65494 ;; QUESTION SECTION: ;org. IN NS ;; ANSWER SECTION: org. 3593 IN NS b2.org.afilias-nst.org. org. 3593 IN NS b0.org.afilias-nst.org. org. 3593 IN NS c0.org.afilias-nst.info. org. 3593 IN NS d0.org.afilias-nst.org. org. 3593 IN NS a0.org.afilias-nst.info. org. 3593 IN NS a2.org.afilias-nst.info. ;; Query time: 0 msec ;; SERVER: 127.0.0.53#53(127.0.0.53) (UDP) ;; WHEN: Tue Aug 01 12:59:53 EDT 2023 ;; MSG SIZE rcvd: 170 Unlike Server variant, it may still make sense on networks with Windows 7, which do not have enabled mdns responder enabled and LLMNR is the only suitable protocol supported. But Windows 7 are already unsupported, so I doubt that should be reason for enabling the protocol in default installations on Fedora 39+. [MS-LLMNR] https://techcommunity.microsoft.com/t5/networking-blog/aligning-on-mdns-ramping-down-netbios-name-resolution-and-llmnr/ba-p/3290816 [stub-llmnr] https://github.com/systemd/systemd/issues/23622
Alternative change might be changing build default to ResolveUnicastSingleLabel=yes That would still use LLMNR, but would try ALSO unicast DNS. Which it should do especially in cases when the query were received over port domain (53).
I'm unsetting POST because there's no patch or clear decision yet.
This bug appears to have been reported against 'rawhide' during the Fedora Linux 39 development cycle. Changing version to 39.
I think the change can be used from upstream PR #28263. Isn't that usable for disabling it? Is there different change under consideration?
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