我正在使用 Fedora 33 和 systemd 246。为了提供一些背景信息，我在通过多播 DNS 解析时遇到了问题，nss-mdns并确定了单播 SOA 启发式是一个问题。
然后我注意到，即使 systemd-resolved 向当前 DNS 服务器发送的查询导致 NXDOMAIN，它也会以 NOERROR 响应代码进行响应。
$ resolvectl status
Global
LLMNR setting: resolve
MulticastDNS setting: no
DNSOverTLS setting: no
DNSSEC setting: no
DNSSEC supported: no
Fallback DNS Servers: 1.1.1.1
8.8.8.8
1.0.0.1
8.8.4.4
2606:4700:4700::1111
2001:4860:4860::8888
2606:4700:4700::1001
2001:4860:4860::8844
Link 3 (wlo1)
Current Scopes: DNS LLMNR/IPv4 LLMNR/IPv6
DefaultRoute setting: yes
LLMNR setting: yes
MulticastDNS setting: no
DNSOverTLS setting: no
DNSSEC setting: no
DNSSEC supported: no
Current DNS Server: 192.168.1.1
DNS Servers: 192.168.1.1
DNS Domain: ~.
lan
向 DNS 服务器查询：
$ dig @192.168.1.1 -t SOA example.local
; <<>> DiG 9.11.24-RedHat-9.11.24-2.fc33 <<>> @192.168.1.1 -t SOA example.local
; (1 server found)
;; global options: +cmd
;; Got answer:
;; WARNING: .local is reserved for Multicast DNS
;; You are currently testing what happens when an mDNS query is leaked to DNS
;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id: 4136
;; flags: qr aa rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 1
;; QUESTION SECTION:
;example.local. IN SOA
;; AUTHORITY SECTION:
example.local. 10800 IN SOA example.local. nobody.invalid. 1 3600 1200 604800 10800
;; ADDITIONAL SECTION:
explanation.invalid. 10800 IN TXT "Blocking is mandated by standards, see references on https://www.iana.org/assignments/special-use-domain-names/special-use-domain-names.xhtml"
;; Query time: 2 msec
;; SERVER: 192.168.1.1#53(192.168.1.1)
;; WHEN: St pro 02 16:42:24 CET 2020
;; MSG SIZE rcvd: 254
向 systemd 解析的本地 DNS 存根侦听器进行查询：
$ dig @127.0.0.53 -t SOA example.local
; <<>> DiG 9.11.24-RedHat-9.11.24-2.fc33 <<>> @127.0.0.53 -t SOA example.local
; (1 server found)
;; global options: +cmd
;; Got answer:
;; WARNING: .local is reserved for Multicast DNS
;; You are currently testing what happens when an mDNS query is leaked to DNS
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 5771
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 65494
;; QUESTION SECTION:
;example.local. IN SOA
;; ANSWER SECTION:
example.local. 10800 IN SOA example.local. nobody.invalid. 1 3600 1200 604800 10800
;; Query time: 3 msec
;; SERVER: 127.0.0.53#53(127.0.0.53)
;; WHEN: St pro 02 16:44:21 CET 2020
;; MSG SIZE rcvd: 92
使用 resolvectl 查询：
$ resolvectl query -t SOA example.local
example.local IN SOA example.local nobody.invalid 1 3600 1200 604800 10800 -- link: wlo1
-- Information acquired via protocol DNS in 5.0ms.
-- Data is authenticated: no
tcpdump捕获的相应数据包：
$ tcpdump -n -i wlo1 port domain
listening on wlo1, link-type EN10MB (Ethernet), capture size 262144 bytes
16:49:57.660945 IP 192.168.1.187.37304 > 192.168.1.1.domain: 32798+ SOA? example.local. (31)
16:49:57.664257 IP 192.168.1.1.domain > 192.168.1.187.37304: 32798 NXDomain* 0/1/1 (254)
对应的systemd-resolved日志：
systemd-resolved[186024]: Got message type=method_call sender=:1.3283 destination=org.freedesktop.resolve1 path=/org/freedesktop/resolve1 interface=org.freedesktop.resolve1.Manager member=ResolveRecord cookie=2 reply_cookie=0 signature=isqqt error-name=n/a error-message=n/a
systemd-resolved[186024]: Looking up RR for example.local IN SOA.
systemd-resolved[186024]: Sent message type=method_call sender=n/a destination=org.freedesktop.DBus path=/org/freedesktop/DBus interface=org.freedesktop.DBus member=AddMatch cookie=89 reply_cookie=0 signature=s error-name=n/a error-message=n/a
systemd-resolved[186024]: Sent message type=method_call sender=n/a destination=org.freedesktop.DBus path=/org/freedesktop/DBus interface=org.freedesktop.DBus member=GetNameOwner cookie=90 reply_cookie=0 signature=s error-name=n/a error-message=n/a
systemd-resolved[186024]: Got message type=method_return sender=org.freedesktop.DBus destination=:1.3260 path=n/a interface=n/a member=n/a cookie=4294967295 reply_cookie=90 signature=s error-name=n/a error-message=n/a
systemd-resolved[186024]: Cache miss for example.local IN SOA
systemd-resolved[186024]: Transaction 13951 for <example.local IN SOA> scope dns on wlo1/*.
systemd-resolved[186024]: Using feature level UDP+EDNS0 for transaction 13951.
systemd-resolved[186024]: Using DNS server 192.168.1.1 for transaction 13951.
systemd-resolved[186024]: Sending query packet with id 13951.
systemd-resolved[186024]: Got message type=method_return sender=org.freedesktop.DBus destination=:1.3260 path=n/a interface=n/a member=n/a cookie=4294967295 reply_cookie=89 signature= error-name=n/a error-message=n/a
systemd-resolved[186024]: Match type='signal',sender='org.freedesktop.DBus',path='/org/freedesktop/DBus',interface='org.freedesktop.DBus',member='NameOwnerChanged',arg0=':1.3283' successfully installed.
systemd-resolved[186024]: Processing incoming packet on transaction 13951 (rcode=NXDOMAIN).
systemd-resolved[186024]: Server doesn't support EDNS(0) properly, downgrading feature level...
systemd-resolved[186024]: Using degraded feature set UDP instead of UDP+EDNS0 for DNS server 192.168.1.1.
systemd-resolved[186024]: Server feature level is now lower than when we began our transaction. Restarting with new ID.
systemd-resolved[186024]: Transaction 13951 is now 31028.
systemd-resolved[186024]: Cache miss for example.local IN SOA
systemd-resolved[186024]: Transaction 31028 for <example.local IN SOA> scope dns on wlo1/*.
systemd-resolved[186024]: Using feature level UDP for transaction 31028.
systemd-resolved[186024]: Sending query packet with id 31028.
systemd-resolved[186024]: Processing incoming packet on transaction 31028 (rcode=NXDOMAIN).
systemd-resolved[186024]: Transaction 31028 for <example.local IN SOA> on scope dns on wlo1/* now complete with <success> from network (unsigned).
systemd-resolved[186024]: Sent message type=method_return sender=n/a destination=:1.3283 path=n/a interface=n/a member=n/a cookie=91 reply_cookie=2 signature=a(iqqay)t error-name=n/a error-message=n/a
systemd-resolved[186024]: Sent message type=method_call sender=n/a destination=org.freedesktop.DBus path=/org/freedesktop/DBus interface=org.freedesktop.DBus member=RemoveMatch cookie=92 reply_cookie=0 signature=s error-name=n/a error-message=n/a
systemd-resolved[186024]: Freeing transaction 31028.
为什么会发生这种情况？