常规方法安装完Caddy，写好Caddyfile和systemd服务配置文件后sudo systemctl start caddy，结果发现在获取证书时失败，导致整个服务启动失败：
failed to obtain certificate: acme: Error -> One or more domains had a problem:
Mar 05 09:55:48 instance-2 caddy[1846]: [ mydomain.example.com ] [ mydomain.example.com ] acme: error presenting token: presenting with standard provider server: could not start HTTPS server for challenge -> listen tcp :443: bind: permission denied
开始以为是 setcap 的问题，但是检查过后，发现安装时的设置没问题：
getcap /usr/local/bin/caddy
/usr/local/bin/caddy = cap_net_bind_service+ep
而且直接命令行启动也没有问题，那么看来应该是systemd服务配置中的问题，一番搜索后发现了这个：Caddy won’t start – Could not start HTTP server for challenge -> listen tcp :80: bind: permission denied ，参考问题中的解决方法，修改caddy.service配置为：
[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
[Service]
Restart=on-abnormal
; User and group the process will run as.
User=caddy
Group=caddy
; 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 -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
PrivateDevices=true
; 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!
ReadWriteDirectories=/etc/ssl/caddy
; The following additional security directives only work with systemd v229 or later.
; They further retrict 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.
CapabilityBoundingSet=CAP_NET_BIND_SERVICE
AmbientCapabilities=CAP_NET_BIND_SERVICE
NoNewPrivileges=true
[Install]
WantedBy=multi-user.target
也就是打开这三行原来的注释：
CapabilityBoundingSet=CAP_NET_BIND_SERVICE
AmbientCapabilities=CAP_NET_BIND_SERVICE
NoNewPrivileges=true
然后，sudo systemctl daemon-reload，再次启动服务，一切正常！
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