我正在尝试诊断 DNS 服务器的问题，在以下环境中多种的由 DHCP 设置的 DNS 服务器。
在 Ubuntu Server 20.04.2（DNS 行为没有变化）中，我需要找到哪个DNS 服务器实际上用于解析特定查询。DNS 服务器由 DHCP 设置。
本质上，我需要复制dig命令的行为或nslookup现代 Ubuntu 版本中断的行为（显示 DNS 服务器的本地主机地址，而不是“真实”服务器的地址。）
systemd-resolve/resolvectl似乎没有任何用处，因为它没有提供以下信息如何它解决了一个特定的查询。
systemd-resolve --status/resolvectl status没有用，因为它只提供某个时间点的“DNS 服务器”和“当前 DNS 服务器”（与特定的 DNS 查询无关）。
nmcli似乎不相关。
我的问题可能会通过回答“如何查看手册页的resolvectl？”这取决于那里可用的信息类型。
由于这只是流程的第一步，如果该方法能够深入了解为什么使用一个 DNS 服务器而不是其他服务器（对于特定查询），或者使用什么过程来确定这一点。
实际上并未解决我的问题的相关链接：
https://stgraber.org/2012/02/24/dns-in-ubuntu-12-04/
如何在 systemd-resolve 中列出 DNS 服务器顺序？
https://unix.stackexchange.com/questions/328131/how-to-troubleshoot-dns-with-systemd-resolved
systemd-resolve 不会将 DNS 请求转发到指定的 DNS 服务器
编辑
为了更明确地说明（我认为）原始问题中非常清楚的内容：
我知道查询是由 localhost 上的真实 DNS 服务器解析的。问题具体是关于 DHCP 分配的两个（或更多）DNS 服务器。使用本地代理会掩盖这两个服务器中的哪一个网络上的其他系统正在用于解决具体查询。
答案1
您可以设置 的日志级别systemd-resolved，使用 运行查询resolvectl query，然后检查 的日志systemd-resolved。例如：
% sudo resolvectl log-level debug
% resolvectl query example.com
example.com: 93.184.216.34 -- link: enp6s0
2606:2800:220:1:248:1893:25c8:1946 -- link: enp6s0
-- Information acquired via protocol DNS in 14.0ms.
-- Data is authenticated: no; Data was acquired via local or encrypted transport: no
-- Data from: network
并检查日志systemd-resolved：
% journalctl -u systemd-resolved
...
Apr 09 02:59:49 cthulhu systemd-resolved[193385]: Got message type=method_return sender=org.freedesktop.DBus destination=:1.1142 path=n/a interface=n/a member=n/a cookie=36 reply_cookie=62 signature=u error-name=n/a error-message=n/a
Apr 09 02:59:49 cthulhu systemd-resolved[193385]: D-Bus hostname resolution request from client PID 194160 (n/a) with UID 4294967295
Apr 09 02:59:49 cthulhu systemd-resolved[193385]: Looking up RR for example.com IN A.
Apr 09 02:59:49 cthulhu systemd-resolved[193385]: Looking up RR for example.com IN AAAA.
Apr 09 02:59:49 cthulhu systemd-resolved[193385]: Sent message type=method_call sender=n/a destination=org.freedesktop.DBus path=/org/freedesktop/DBus interface=org.freedesktop.DBus member=AddMatch cookie=63 reply_cookie=0 signature=s error-name=n/a error-message=n/a
Apr 09 02:59:49 cthulhu systemd-resolved[193385]: Sent message type=method_call sender=n/a destination=org.freedesktop.DBus path=/org/freedesktop/DBus interface=org.freedesktop.DBus member=GetNameOwner cookie=64 reply_cookie=0 signature=s error-name=n/a error-message=n/a
Apr 09 02:59:49 cthulhu systemd-resolved[193385]: Got message type=method_return sender=org.freedesktop.DBus destination=:1.1142 path=n/a interface=n/a member=n/a cookie=38 reply_cookie=64 signature=s error-name=n/a error-message=n/a
Apr 09 02:59:49 cthulhu systemd-resolved[193385]: Cache miss for example.com IN A
Apr 09 02:59:49 cthulhu systemd-resolved[193385]: Firing regular transaction 61323 for <example.com IN A> scope dns on */* (validate=yes).
Apr 09 02:59:49 cthulhu systemd-resolved[193385]: Using feature level UDP+EDNS0 for transaction 61323.
Apr 09 02:59:49 cthulhu systemd-resolved[193385]: Using DNS server 8.8.8.8 for transaction 61323.
Apr 09 02:59:49 cthulhu systemd-resolved[193385]: Announcing packet size 1472 in egress EDNS(0) packet.
Apr 09 02:59:49 cthulhu systemd-resolved[193385]: Emitting UDP, link MTU is 1500, socket MTU is 1500, minimal MTU is 40
Apr 09 02:59:49 cthulhu systemd-resolved[193385]: Sending query packet with id 61323 of size 40.
Apr 09 02:59:49 cthulhu systemd-resolved[193385]: Cache miss for example.com IN AAAA
Apr 09 02:59:49 cthulhu systemd-resolved[193385]: Firing regular transaction 17323 for <example.com IN AAAA> scope dns on */* (validate=yes).
Apr 09 02:59:49 cthulhu systemd-resolved[193385]: Using feature level UDP+EDNS0 for transaction 17323.
Apr 09 02:59:49 cthulhu systemd-resolved[193385]: Using DNS server 8.8.8.8 for transaction 17323.
Apr 09 02:59:49 cthulhu systemd-resolved[193385]: Announcing packet size 1472 in egress EDNS(0) packet.
Apr 09 02:59:49 cthulhu systemd-resolved[193385]: Emitting UDP, link MTU is 1500, socket MTU is 1500, minimal MTU is 40
Apr 09 02:59:49 cthulhu systemd-resolved[193385]: Sending query packet with id 17323 of size 40.
Apr 09 02:59:49 cthulhu systemd-resolved[193385]: Cache miss for example.com IN AAAA
Apr 09 02:59:49 cthulhu systemd-resolved[193385]: Firing regular transaction 7013 for <example.com IN AAAA> scope dns on enp6s0/* (validate=yes).
Apr 09 02:59:49 cthulhu systemd-resolved[193385]: Using feature level UDP+EDNS0 for transaction 7013.
Apr 09 02:59:49 cthulhu systemd-resolved[193385]: Using DNS server 8.8.8.8 for transaction 7013.
Apr 09 02:59:49 cthulhu systemd-resolved[193385]: Announcing packet size 1472 in egress EDNS(0) packet.
Apr 09 02:59:49 cthulhu systemd-resolved[193385]: Emitting UDP, link MTU is 1500, socket MTU is 0, minimal MTU is 40
Apr 09 02:59:49 cthulhu systemd-resolved[193385]: Sending query packet with id 7013 of size 40.
Apr 09 02:59:49 cthulhu systemd-resolved[193385]: Cache miss for example.com IN A
Apr 09 02:59:49 cthulhu systemd-resolved[193385]: Firing regular transaction 32950 for <example.com IN A> scope dns on enp6s0/* (validate=yes).
Apr 09 02:59:49 cthulhu systemd-resolved[193385]: Using feature level UDP+EDNS0 for transaction 32950.
Apr 09 02:59:49 cthulhu systemd-resolved[193385]: Using DNS server 8.8.8.8 for transaction 32950.
Apr 09 02:59:49 cthulhu systemd-resolved[193385]: Announcing packet size 1472 in egress EDNS(0) packet.
Apr 09 02:59:49 cthulhu systemd-resolved[193385]: Emitting UDP, link MTU is 1500, socket MTU is 0, minimal MTU is 40
Apr 09 02:59:49 cthulhu systemd-resolved[193385]: Sending query packet with id 32950 of size 40.
Apr 09 02:59:49 cthulhu systemd-resolved[193385]: Got message type=method_return sender=org.freedesktop.DBus destination=:1.1142 path=n/a interface=n/a member=n/a cookie=37 reply_cookie=63 signature=n/a error-name=n/a error-message=n/a
Apr 09 02:59:49 cthulhu systemd-resolved[193385]: Match type='signal',sender='org.freedesktop.DBus',path='/org/freedesktop/DBus',interface='org.freedesktop.DBus',member='NameOwnerChanged',arg0=':1.1170' successfully installed.
Apr 09 02:59:49 cthulhu systemd-resolved[193385]: Received dns UDP packet of size 56, ifindex=2, ttl=0, fragsize=0, sender=8.8.8.8, destination=192.168.0.2
Apr 09 02:59:49 cthulhu systemd-resolved[193385]: Processing incoming packet of size 56 on transaction 61323 (rcode=SUCCESS).
Apr 09 02:59:49 cthulhu systemd-resolved[193385]: Added positive unauthenticated non-confidential cache entry for example.com IN A 7200s on enp6s0/INET/8.8.8.8
Apr 09 02:59:49 cthulhu systemd-resolved[193385]: Regular transaction 61323 for <example.com IN A> on scope dns on */* now complete with <success> from network (unsigned; non-confidential).
Apr 09 02:59:49 cthulhu systemd-resolved[193385]: Received dns UDP packet of size 68, ifindex=2, ttl=0, fragsize=0, sender=8.8.8.8, destination=192.168.0.2
Apr 09 02:59:49 cthulhu systemd-resolved[193385]: Processing incoming packet of size 68 on transaction 7013 (rcode=SUCCESS).
Apr 09 02:59:49 cthulhu systemd-resolved[193385]: Added positive unauthenticated non-confidential cache entry for example.com IN AAAA 1277s on enp6s0/INET/8.8.8.8
Apr 09 02:59:49 cthulhu systemd-resolved[193385]: Regular transaction 7013 for <example.com IN AAAA> on scope dns on enp6s0/* now complete with <success> from network (unsigned; non-confidential).
Apr 09 02:59:49 cthulhu systemd-resolved[193385]: Received dns UDP packet of size 68, ifindex=2, ttl=0, fragsize=0, sender=8.8.8.8, destination=192.168.0.2
Apr 09 02:59:49 cthulhu systemd-resolved[193385]: Processing incoming packet of size 68 on transaction 17323 (rcode=SUCCESS).
Apr 09 02:59:49 cthulhu systemd-resolved[193385]: Added positive unauthenticated non-confidential cache entry for example.com IN AAAA 7200s on enp6s0/INET/8.8.8.8
Apr 09 02:59:49 cthulhu systemd-resolved[193385]: Regular transaction 17323 for <example.com IN AAAA> on scope dns on */* now complete with <success> from network (unsigned; non-confidential).
Apr 09 02:59:49 cthulhu systemd-resolved[193385]: Freeing transaction 61323.
Apr 09 02:59:49 cthulhu systemd-resolved[193385]: Freeing transaction 7013.
Apr 09 02:59:49 cthulhu systemd-resolved[193385]: Freeing transaction 32950.
Apr 09 02:59:49 cthulhu systemd-resolved[193385]: Added socket 26 to graveyard
Apr 09 02:59:49 cthulhu systemd-resolved[193385]: Sent message type=method_return sender=n/a destination=:1.1170 path=n/a interface=n/a member=n/a cookie=65 reply_cookie=2 signature=a(iiay)st error-name=n/a error-message=n/a
Apr 09 02:59:49 cthulhu systemd-resolved[193385]: Sent message type=method_call sender=n/a destination=org.freedesktop.DBus path=/org/freedesktop/DBus interface=org.freedesktop.DBus member=RemoveMatch cookie=66 reply_cookie=0 signature=s error-name=n/a error-message=n/a
Apr 09 02:59:49 cthulhu systemd-resolved[193385]: Freeing transaction 17323.
...
我无法解释这种行为，但你可以详细地看到这种行为。
另请检查man systemd-resolved.service，在我看来，它对所使用的算法有相当详细的解释：
• Queries for multi-label names are routed via unicast DNS on
local interfaces that have a DNS server configured, plus the
globally configured DNS servers if there are any. Which
interfaces are used is determined by the routing logic based
on search and route-only domains, described below. [...]
If lookups are routed to multiple interfaces, the first
successful response is returned (thus effectively merging the
lookup zones on all matching interfaces). If the lookup failed on
all interfaces, the last failing response is returned.
[...]
The following query routing logic applies for unicast DNS lookups
initiated by systemd-resolved.service:
• If a name to look up matches (that is: is equal to or has as
suffix) any of the configured routing domains (search or
route-only) of any link, or the globally configured DNS
settings, "best matching" routing domain is determined: the
matching one with the most labels. The query is then sent to
all DNS servers of any links or the globally configured DNS
servers associated with this "best matching" routing domain.
(Note that more than one link might have this same "best
matching" routing domain configured, in which case the query
is sent to all of them in parallel).
In case of single-label names, when search domains are
defined, the same logic applies, except that the name is
first suffixed by each of the search domains in turn. Note
that this search logic doesn't apply to any names with at
least one dot. Also see the discussion about compatibility
with the traditional glibc resolver below.
• If a query does not match any configured routing domain
(either per-link or global), it is sent to all DNS servers
that are configured on links with the DefaultRoute= option
set, as well as the globally configured DNS server.
• If there is no link configured as DefaultRoute= and no global
DNS server configured, one of the compiled-in fallback DNS
servers is used.
• Otherwise the unicast DNS query fails, as no suitable DNS
servers can be determined.