$ cat /etc/os-release
PRETTY_NAME="Debian GNU/Linux 12 (bookworm)"
NAME="Debian GNU/Linux"

Raspberry Pi OSにfail2banをインストールしてみたものの以下のようなエラーが出て起動に失敗する。

$ sudo systemctl start fail2ban
$ sudo systemctl status fail2ban
fail2ban.service - Fail2Ban Service
     Loaded: loaded (/lib/systemd/system/fail2ban.service; enabled; preset>
     Active: failed (Result: exit-code) since Fri 2024-01-26 20:42:16 JST;>
   Duration: 524ms
       Docs: man:fail2ban(1)
    Process: 1287 ExecStart=/usr/bin/fail2ban-server -xf start (code=exite>
   Main PID: 1287 (code=exited, status=255/EXCEPTION)
        CPU: 410ms

 1月 26 20:42:15 cactus systemd[1]: Started fail2ban.service - Fail2Ban Se>
 1月 26 20:42:16 cactus fail2ban-server[1287]: 2024-01-26 20:42:16,134 fai>
 1月 26 20:42:16 cactus fail2ban-server[1287]: 2024-01-26 20:42:16,198 fai>
 1月 26 20:42:16 cactus fail2ban-server[1287]: 2024-01-26 20:42:16,199 fai>
 1月 26 20:42:16 cactus systemd[1]: fail2ban.service: Main process exited,>
 1月 26 20:42:16 cactus systemd[1]: fail2ban.service: Failed with result '>

'>'以降省略...。

$ sudo fail2ban-server start
2024-01-26 20:46:05,106 fail2ban.configreader   [1308]: WARNING 'allowipv6' not defined in 'Definition'. Using default one: 'auto'
2024-01-26 20:46:05,176 fail2ban                [1308]: ERROR   Failed during configuration: Have not found any log file for sshd jail
2024-01-26 20:46:05,184 fail2ban                [1308]: ERROR   Async configuration of server failed

調べた結果、OSに/var/log/auth.logなどを出力するrsyslogがインストールされていないためっだった。
rsyslogの代わりにsystemd journalを使うように設定すれば上手くいった。

$ sudo vi /etc/fail2ban/jail.conf
[DEFAULT]
backend = systemd

〜略

$ sudo systemctl start fail2ban
$ sudo systemctl status fail2ban

fail2ban.service - Fail2Ban Service
     Loaded: loaded (/lib/systemd/system/fail2ban.service; enabled; preset>
     Active: active (running) since Fri 2024-01-26 20:47:30 JST; 3s ago
       Docs: man:fail2ban(1)
   Main PID: 1318 (fail2ban-server)
      Tasks: 7 (limit: 8740)
        CPU: 554ms
     CGroup: /system.slice/fail2ban.service
             └─1318 /usr/bin/python3 /usr/bin/fail2ban-server -xf start

 1月 26 20:47:30 cactus systemd[1]: Started fail2ban.service - Fail2Ban Se>
 1月 26 20:47:30 cactus fail2ban-server[1318]: 2024-01-26 20:47:30,954 fai>
 1月 26 20:47:31 cactus fail2ban-server[1318]: Server ready

$ sudo fail2ban-server start
2024-01-26 20:50:13,722 fail2ban.configreader   [1350]: WARNING 'allowipv6' not defined in 'Definition'. Using default one: 'auto'
Server ready

（systemctlでfail2banは起動しているのでfail2ban-server startはいらない。起動メッセージ確認のため）

...そして一夜明けて

$ sudo fail2ban-client status sshd
Status for the jail: sshd
|- Filter
|  |- Currently failed: 1
|  |- Total failed:     67
|  `- Journal matches:  _SYSTEMD_UNIT=sshd.service + _COMM=sshd
`- Actions
   |- Currently banned: 20
   |- Total banned:     20
   `- Banned IP list:   〜略

動いています。