Ubuntu 26.04 LTS Server の名前解決設定（systemd-resolved）

ls -l /etc/resolv.conf
cat /etc/resolv.conf

kazulog@sv1:~$ ls -l /etc/resolv.conf
lrwxrwxrwx 1 root root 39 Aug 24 01:24 /etc/resolv.conf -> ../run/systemd/resolve/stub-resolv.conf
kazulog@sv1:~$ cat /etc/resolv.conf
# This is /run/systemd/resolve/stub-resolv.conf managed by man:systemd-resolved(8).
# Do not edit.
#
# This file might be symlinked as /etc/resolv.conf. If you're looking at
# /etc/resolv.conf and seeing this text, you have followed the symlink.
#
# This is a dynamic resolv.conf file for connecting local clients to the
# internal DNS stub resolver of systemd-resolved. This file lists all
# configured search domains.
#
# Run "resolvectl status" to see details about the uplink DNS servers
# currently in use.
#
# Third party programs should typically not access this file directly, but only
# through the symlink at /etc/resolv.conf. To manage man:resolv.conf(5) in a
# different way, replace this symlink by a static file or a different symlink.
#
# See man:systemd-resolved.service(8) for details about the supported modes of
# operation for /etc/resolv.conf.

nameserver 127.0.0.53
options edns0 trust-ad
search kazulog.example
kazulog@sv1:~$

resolvectl status

kazulog@sv1:~$ resolvectl status
Global
         Protocols: -LLMNR -mDNS -DNSOverTLS DNSSEC=no/unsupported
  resolv.conf mode: stub

Link 2 (ens2)
    Current Scopes: DNS
         Protocols: +DefaultRoute -LLMNR -mDNS -DNSOverTLS DNSSEC=no/unsupported
Current DNS Server: 10.1.0.1
       DNS Servers: 10.1.0.1 10.1.0.2
     Default Route: yes

Link 3 (ens3)
    Current Scopes: DNS
         Protocols: +DefaultRoute -LLMNR -mDNS -DNSOverTLS DNSSEC=no/unsupported
Current DNS Server: 192.168.100.1
       DNS Servers: 192.168.100.1 2001:db8:100::1
        DNS Domain: kazulog.example
     Default Route: yes
kazulog@sv1:~$

network:
  version: 2
  ethernets:
    ens3:
      addresses:
        - 192.168.100.10/24
      nameservers:
        addresses: [192.168.100.1]
        search: [kazulog.example]

kazulog@sv1:~$ resolvectl status ens3
Link 3 (ens3)
    Current Scopes: DNS
         Protocols: +DefaultRoute -LLMNR -mDNS -DNSOverTLS DNSSEC=no/unsupported
Current DNS Server: 192.168.100.1
       DNS Servers: 192.168.100.1
        DNS Domain: kazulog.example
     Default Route: yes
kazulog@sv1:~$

resolvectl query [NAME]

kazulog@sv1:~$ resolvectl query www.kazulog.example
www.kazulog.example: 192.168.100.20            -- link: ens3

-- Information acquired via protocol DNS in 5.1987s.
-- Data is authenticated: no; Data was acquired via local or encrypted transport: no
-- Data from: network
kazulog@sv1:~$ resolvectl query www
www: 192.168.100.20                            -- link: ens3
     (www.kazulog.example)

-- Information acquired via protocol DNS in 7.0ms.
-- Data is authenticated: no; Data was acquired via local or encrypted transport: no
-- Data from: cache
kazulog@sv1:~$ resolvectl query sv1.kazulog.example
sv1.kazulog.example: 192.168.100.10            -- link: ens3

-- Information acquired via protocol DNS in 6.1378s.
-- Data is authenticated: no; Data was acquired via local or encrypted transport: no
-- Data from: network

grep '^hosts:' /etc/nsswitch.conf

kazulog@sv1:~$ grep '^hosts:' /etc/nsswitch.conf
hosts:          files dns
kazulog@sv1:~$ grep kazulog /etc/hosts
192.168.100.99 www.kazulog.example
kazulog@sv1:~$ resolvectl query www.kazulog.example
www.kazulog.example: 192.168.100.99

-- Information acquired via protocol DNS in 3.0ms.
-- Data is authenticated: yes; Data was acquired via local or encrypted transport: yes
-- Data from: synthetic
kazulog@sv1:~$ getent hosts www.kazulog.example
192.168.100.99  www.kazulog.example

sudo resolvectl dns [INTERFACE] [ADDRESS]

kazulog@sv1:~$ resolvectl dns ens3
Link 3 (ens3): 192.168.100.1
kazulog@sv1:~$ sudo resolvectl dns ens3 192.168.100.20
kazulog@sv1:~$ resolvectl dns ens3
Link 3 (ens3): 192.168.100.20
kazulog@sv1:~$ sudo netplan apply
kazulog@sv1:~$ resolvectl dns ens3
Link 3 (ens3): 192.168.100.20
kazulog@sv1:~$ sudo resolvectl revert ens3
kazulog@sv1:~$ resolvectl dns ens3
Link 3 (ens3): 192.168.100.1

sudo resolvectl statistics

kazulog@sv1:~$ sudo resolvectl statistics
Transactions
                       Current Transactions:  0
                         Total Transactions: 57
                                               
Cache                                          
                         Current Cache Size:  0
                                 Cache Hits: 27
                               Cache Misses: 33
                                               
Failure Transactions                           
                             Total Timeouts:  3
         Total Timeouts (Stale Data Served):  0
                    Total Failure Responses:  2
Total Failure Responses (Stale Data Served):  0
                                               
DNSSEC Verdicts                                
                                     Secure:  0
                                   Insecure:  0
                                      Bogus:  0
                              Indeterminate:  0

sudo resolvectl flush-caches

  sv1 (Ubuntu 26.04)              R1 (IOS XE)
   ens3 192.168.100.10/24          Gi2 192.168.100.1/24
   DNS: 192.168.100.1              ip dns server
   search: kazulog.example         www.kazulog.example  = 192.168.100.20
      |                            sv1.kazulog.example  = 192.168.100.10
      +---------- LAB-SW ----------+