使用Caddy在签发Let’s Encrypt 免费证书时出现了问题，报错误如下：

service caddy status
● caddy.service - Caddy HTTP/2 web server
   Loaded: loaded (/lib/systemd/system/caddy.service; enabled; vendor preset: enabled)
   Active: failed (Result: exit-code) since Tue 2019-12-24 13:12:24 CST; 39s ago
     Docs: https://caddyserver.com/docs
  Process: 4573 ExecStart=/usr/local/bin/caddy -log stdout -log-timestamps=false -agree=true -conf=/etc/caddy/Caddyfile -root=/var/tmp (code=exited, status=1/FAILURE)
 Main PID: 4573 (code=exited, status=1/FAILURE)
 Dec 24 13:12:14 oranme6 caddy[4573]: 2019/12/24 13:12:14 [INFO] [oran.wzfou.me] acme: Trying to solve TLS-ALPN-01
Dec 24 13:12:22 oranme6 caddy[4573]: 2019/12/24 13:12:22 [INFO] Deactivating auth: https://acme-v02.api.letsencrypt.org/acme/authz-v3/1898165386
Dec 24 13:12:22 oranme6 caddy[4573]: 2019/12/24 13:12:22 [INFO] Unable to deactivate the authorization: https://acme-v02.api.letsencrypt.org/acme/authz-v3/1898165386
Dec 24 13:12:22 oranme6 caddy[4573]: [ERROR][oran.wzfou.me] failed to obtain certificate: acme: Error -> One or more domains had a problem:
Dec 24 13:12:22 oranme6 caddy[4573]: [oran.wzfou.me] acme: error: 403 :: urn:ietf:params:acme:error:unauthorized :: Cannot negotiate ALPN protocol "acme-tls/1" for tls-alpn-01 challenge
Dec 24 13:12:23 oranme6 caddy[4573]: 2019/12/24 13:12:23 [INFO] [oran.wzfou.me] acme: Obtaining bundled SAN certificate
Dec 24 13:12:23 oranme6 caddy[4573]: [ERROR][oran.wzfou.me] failed to obtain certificate: acme: error: 429 :: POST :: https://acme-v02.api.letsencrypt.org/acme/new-order :: urn:ietf:par
Dec 24 13:12:24 oranme6 systemd[1]: caddy.service: Main process exited, code=exited, status=1/FAILURE
Dec 24 13:12:24 oranme6 systemd[1]: caddy.service: Unit entered failed state.
Dec 24 13:12:24 oranme6 systemd[1]: caddy.service: Failed with result 'exit-code'.

解决办法就是尝试使用 DNS Challenge 获取证书。参考：https://caddyserver.com/v1/docs/automatic-https#dns-challenge