SSSD 无法执行动态 DNS 更新：Could not resolve address for this machine, error:Timeout while contacting DNS servers

Issue

• 加入 AD 域的 RHEL 主机无法执行动态 DNS 更新。

• SSSD 调试日志返回 Timeout while contacting DNS servers，表示 DNS 服务器无法访问：

  (2025-08-06 10:54:46): [be[ad.example.com]] [ad_dyndns_update_send] (0x0400): Performing update
  
  <...>
  
  (2025-08-06 10:54:46): [be[ad.example.com]] [resolv_is_unix] (0x4000): [rhel.ad.example.com] does not look like a unix domain socket
  (2025-08-06 10:54:46): [be[ad.example.com]] [resolv_is_address] (0x4000): [rhel.ad.example.com] does not look like an IP address
  (2025-08-06 10:54:46): [be[ad.example.com]] [resolv_gethostbyname_step] (0x2000): Querying DNS
  (2025-08-06 10:54:46): [be[ad.example.com]] [resolv_gethostbyname_dns_query] (0x0100): Trying to resolve A record of 'rhel.ad.example.com' in DNS
  
  <...>
  
  (2025-08-06 10:54:47): [be[ad.example.com]] [resolv_gethostbyname_done] (0x0040): querying hosts database failed [5]: Input/output error
  (2025-08-06 10:54:47): [be[ad.example.com]] [nsupdate_get_addrs_done] (0x0040): Could not resolve address for this machine, error [5]: Input/output error, resolver returned: [12]: Timeout while contacting DNS servers
  (2025-08-06 10:54:47): [be[ad.example.com]] [nsupdate_get_addrs_done] (0x0040): nsupdate_get_addrs_done failed: [5]: [Input/output error]
  (2025-08-06 10:54:47): [be[ad.example.com]] [sdap_dyndns_dns_addrs_done] (0x0040): Could not receive list of current addresses [5]: Input/output error
  (2025-08-06 10:54:47): [be[ad.example.com]] [ad_dyndns_sdap_update_done] (0x0040): Dynamic DNS update failed [5]: Input/output error
  (2025-08-06 10:54:47): [be[ad.example.com]] [sdap_id_op_destroy] (0x4000): releasing operation connection
  (2025-08-06 10:54:47): [be[ad.example.com]] [sdap_id_conn_data_idle] (0x4000): Marking connection as idle
  (2025-08-06 10:54:47): [be[ad.example.com]] [be_ptask_done] (0x0040): Task [Dyndns update]: failed with [5]: Input/output error
  

• 需要在 DNS 服务器中设置的主机名为 rhel.ad.example.com。

• 系统主机名被设置为一个短主机名：

  # hostname
  rhel
  

• 完全限定域名 (FQDN) 在 sssd.conf 中设置

  # cat /etc/sssd/sssd.conf | grep hostname
  ad_hostname = rhel.ad.example.com
  

• 主机名在 /etc/hosts 中设置

  # cat /etc/hosts
  127.0.0.1      localhost localhost.localdomain localhost4 localhost4.localdomain4
  ::1            localhost localhost.localdomain localhost6 localhost6.localdomain6
  192.168.1.200  rhel.ad.example.com rhel
  

• 另外，dig 显示 DNS 服务器可以被访问，主机名也可以被解析。

  # dig rhel.ad.example.com
  
  <...>
  
  ;; ANSWER SECTION:
  rhel.ad.example.com.    3600    IN  A   192.168.1.200
  
  ;; Query time: 1 msec
  ;; SERVER: 192.168.1.100#53(192.168.1.100) (UDP)
  ;; WHEN: Wed Aug 06 11:03:12 AEST 2025
  ;; MSG SIZE  rcvd: 66