18.04服务器-systemd-解析返回缓存的cname NODATA进行查找

Question

摘要:由于缓存的CNAME NODATA查找，DNS记录查找无法解决。

详细信息:使用DNS查找报告错误的邮件日志：

 Host or domain name not found. Name service error for name=google.com type=A: Host found but no data record of requested type

启用“解决调试”之后，我看到dns查找首先查询域的CNAME记录。这通常是根域，它没有CNAME，查找正确地返回NODATA。

但是，当执行后续的A查找时，CNAME查找的NODATA结果将从缓存返回，而不是执行A查找。

通过发出以下命令，我可以始终如一地重新创建该命令：

~$ dig google.com CNAME
~$ dig google.com A

以下是调试日志的结果：

Aug 08 11:09:04 leopard systemd-resolved[555]: Transaction 17304 for  scope dns on eth0/*.
Aug 08 11:09:04 leopard systemd-resolved[555]: Using feature level UDP+EDNS0 for transaction 17304.
Aug 08 11:09:04 leopard systemd-resolved[555]: Using DNS server 8.8.8.8 for transaction 17304.
Aug 08 11:09:04 leopard systemd-resolved[555]: Sending query packet with id 17304.
Aug 08 11:09:04 leopard systemd-resolved[555]: Processing query...
Aug 08 11:09:04 leopard systemd-resolved[555]: Processing incoming packet on transaction 17304. (rcode=SUCCESS)
Aug 08 11:09:04 leopard systemd-resolved[555]: Added NODATA cache entry for google.com IN CNAME 1799s
Aug 08 11:09:04 leopard systemd-resolved[555]: Transaction 17304 for  on scope dns on eth0/* now complete with  from network (unsigned).
Aug 08 11:09:04 leopard systemd-resolved[555]: Sending response packet with id 22860 on interface 1/AF_INET.
Aug 08 11:09:04 leopard systemd-resolved[555]: Freeing transaction 17304.

A纪录查询结果：

Aug 08 11:09:37 leopard systemd-resolved[555]: Processing query...
Aug 08 11:09:51 leopard systemd-resolved[555]: Got DNS stub UDP query packet for id 3119
Aug 08 11:09:51 leopard systemd-resolved[555]: Looking up RR for google.com IN A.
Aug 08 11:09:51 leopard systemd-resolved[555]: NODATA cache hit for google.com IN A
Aug 08 11:09:51 leopard systemd-resolved[555]: Transaction 45189 for  on scope dns on eth0/* now complete with  from cache (unsigned).
Aug 08 11:09:51 leopard systemd-resolved[555]: Freeing transaction 45189.
Aug 08 11:09:51 leopard systemd-resolved[555]: Sending response packet with id 3119 on interface 1/AF_INET.

进一步的信息:服务器正在运行一个LEMP堆栈。看起来，nginx在每个请求之前都会进行DNS查找，它首先是CNAME查找，然后是A查找，然后是AAAA查找。这将导致缓存CNAME NODATA。随后，当邮件服务器试图发送邮件时，它将从resolve获得缓存的NODATA记录，从而导致上述错误。

问题:这是预期的行为(CNAME返回进行查找)吗？是否有一些配置，我可以更改，以防止缓存的CNAME查找返回A查找？

诊断信息：

~$ ip route
default via 85.159.215.1 dev eth0 proto static 
85.159.215.0/24 dev eth0 proto kernel scope link src 85.159.215.159 


~$ sudo systemd-resolve --status
Global
          DNSSEC NTA: 10.in-addr.arpa
                      16.172.in-addr.arpa
                      168.192.in-addr.arpa
                      17.172.in-addr.arpa
                      18.172.in-addr.arpa
                      19.172.in-addr.arpa
                      20.172.in-addr.arpa
                      21.172.in-addr.arpa
                      22.172.in-addr.arpa
                      23.172.in-addr.arpa
                      24.172.in-addr.arpa
                      25.172.in-addr.arpa
                      26.172.in-addr.arpa
                      27.172.in-addr.arpa
                      28.172.in-addr.arpa
                      29.172.in-addr.arpa
                      30.172.in-addr.arpa
                      31.172.in-addr.arpa
                      corp
                      d.f.ip6.arpa
                      home
                      internal
                      intranet
                      lan
                      local
                      private
                      test

Link 2 (eth0)
      Current Scopes: DNS
       LLMNR setting: yes
MulticastDNS setting: no
      DNSSEC setting: no
    DNSSEC supported: no
         DNS Servers: 8.8.8.8
                      8.8.4.4


~$ cat /etc/resolv.conf
nameserver 127.0.0.53

Answer 1

在目前使用的systemd Ubuntu 18.04版本上，这似乎是一个真正的问题。https://github.com/systemd/systemd/issues/9833和https://bugs.launchpad.net/ubuntu/+source/systemd/+bug/1818527发射台

但不确定Ubuntu是否会升级systemd版本。