The unit caddy.service has entered the 'failed' state with result 'exit-code'

修改 service（vim /usr/lib/systemd/system/caddy.service） 服务配置文件，取消注释以下三行：

CapabilityBoundingSet=CAP_NET_BIND_SERVICE
AmbientCapabilities=CAP_NET_BIND_SERVICE
NoNewPrivileges=true

完整的配置文件：

[Unit]
Description=Caddy HTTP/2 web server
Documentation=https://caddyserver.com/docs
After=network-online.target
Wants=network-online.target systemd-networkd-wait-online.service

; Do not allow the process to be restarted in a tight loop. If the
; process fails to start, something critical needs to be fixed.
StartLimitIntervalSec=14400
StartLimitBurst=10

[Service]
Restart=on-abnormal

; User and group the process will run as.
User=root
Group=root

; Letsencrypt-issued certificates will be written to this directory.
Environment=CADDYPATH=/etc/ssl/caddy

; Always set "-root" to something safe in case it gets forgotten in the Caddyfile.
ExecStart=/usr/local/bin/caddy -log stdout -log-timestamps=false -agree=true -conf=/etc/caddy/Caddyfile -root=/var/tmp
ExecReload=/bin/kill -USR1 $MAINPID

; Use graceful shutdown with a reasonable timeout
KillMode=mixed
KillSignal=SIGQUIT
TimeoutStopSec=5s

; Limit the number of file descriptors; see `man systemd.exec` for more limit settings.
LimitNOFILE=1048576
; Unmodified caddy is not expected to use more than that.
LimitNPROC=512

; Use private /tmp and /var/tmp, which are discarded after caddy stops.
PrivateTmp=true
; Use a minimal /dev (May bring additional security if switched to 'true', but it may not work on Raspberry Pi's or other devices, so it has been disabled in this dist.)
PrivateDevices=false
; Hide /home, /root, and /run/user. Nobody will steal your SSH-keys.
ProtectHome=true
; Make /usr, /boot, /etc and possibly some more folders read-only.
ProtectSystem=full
; … except /etc/ssl/caddy, because we want Letsencrypt-certificates there.
;   This merely retains r/w access rights, it does not add any new. Must still be writable on the host!
ReadWritePaths=/etc/ssl/caddy
ReadWriteDirectories=/etc/ssl/caddy

; The following additional security directives only work with systemd v229 or later.
; They further restrict privileges that can be gained by caddy. Uncomment if you like.
; Note that you may have to add capabilities required by any plugins in use.
; 取消注释下面三行
CapabilityBoundingSet=CAP_NET_BIND_SERVICE
AmbientCapabilities=CAP_NET_BIND_SERVICE
NoNewPrivileges=true

[Install]
WantedBy=multi-user.target

然后直接运行启动命令，会提示重新载入配置，重载后再次重新启动，启动成功。

[root@yisu-5fbf6b8f8c1dd caddy]# systemctl restart caddy.service
Warning: The unit file, source configuration file or drop-ins of caddy.service changed on disk. Run 'systemctl daemon-reload' to reload units.
Job for caddy.service failed because the control process exited with error code.
See "systemctl status caddy.service" and "journalctl -xe" for details.
[root@yisu-5fbf6b8f8c1dd caddy]# systemctl daemon-reload
[root@yisu-5fbf6b8f8c1dd caddy]# systemctl restart caddy.service
[root@yisu-5fbf6b8f8c1dd caddy]#