某个私用的子域名上了Let’s Encrypt签发的证书，上周用renew命令延长证书有效期时出现下面的错误：
Saving debug log to /var/log/letsencrypt/letsencrypt.log
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Processing /etc/letsencrypt/renewal/xxx.conf
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Cert is due for renewal, auto-renewing...
Plugins selected: Authenticator standalone, Installer None
Renewing an existing certificate
Performing the following challenges:
http-01 challenge for xxxx
Waiting for verification...
----------------------------------------
Exception happened during processing of request from ('::ffff:66.133.109.36', 45260, 0, 0)
Traceback (most recent call last):
File "/usr/lib64/python2.7/SocketServer.py", line 295, in _handle_request_noblock
self.process_request(request, client_address)
File "/usr/lib64/python2.7/SocketServer.py", line 321, in process_request
self.finish_request(request, client_address)
File "/usr/lib64/python2.7/SocketServer.py", line 334, in finish_request
self.RequestHandlerClass(request, client_address, self)
File "/usr/lib/python2.7/site-packages/acme/standalone.py", line 207, in __init__
BaseHTTPServer.BaseHTTPRequestHandler.__init__(self, *args, **kwargs)
File "/usr/lib64/python2.7/SocketServer.py", line 651, in __init__
self.finish()
File "/usr/lib64/python2.7/SocketServer.py", line 710, in finish
self.wfile.close()
File "/usr/lib64/python2.7/socket.py", line 279, in close
self.flush()
File "/usr/lib64/python2.7/socket.py", line 303, in flush
self._sock.sendall(view[write_offset:write_offset+buffer_size])
error: [Errno 32] Broken pipe
----------------------------------------
Challenge failed for domain xxxx
http-01 challenge for xxxx
Cleaning up challenges
Attempting to renew cert (xxxx) from /etc/letsencrypt/renewal/xxxx.conf produced an unexpected error: Some challenges have failed.. Skipping.
All renewal attempts failed. The following certs could not be renewed:
/etc/letsencrypt/live/xxxx/fullchain.pem (failure)
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All renewal attempts failed. The following certs could not be renewed:
/etc/letsencrypt/live/xxxx/fullchain.pem (failure)
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1 renew failure(s), 0 parse failure(s)
IMPORTANT NOTES:
- The following errors were reported by the server:
Domain: xxxx
Type: unauthorized
Detail: Invalid response from
http://xxxx/.well-known/acme-challenge/vQ5zuvoh188fvC0GE8U5KnJ5yQMf1GamNzJCW-Ho-D8
[xxx.xxx.xxx.xxx]: "<html>/n<head>/n<meta
http-equiv=/"Content-Type/" content=/"textml;charset=UTF-8/" />/n
<style>body{background-color:#FFFFFF}</style>"
To fix these errors, please make sure that your domain name was
entered correctly and the DNS A/AAAA record(s) for that domain
contain(s) the right IP address.
一直以来用的好好的，这次突然撂挑子不干了，简直莫名其妙。根据错误信息排查潜在问题，都没毛病：
- ip已经正确解析，错误信息和log中可以验证这点；
- 绑定80端口也没问题，已经停止了Nginx；
- certbot已经升级到了最新版本；
- 用Python3.6的pip重新安装certbot并尝试，问题依旧。
自行尝试加上网找方案，一番折腾无果后暂时放弃了。心想也许是服务端抽风，说不定过几天就好了。
今天再次尝试续期证书，还是同样的问题。意识到这应该不是服务端问题，不然肯定引起大面积反应了，配置或者使用上有新问题的可能性更大。
用”invalid response”等关键字搜不出什么有用的结果。过了一会后忽然想到抛出异常中的 “::ffff:66.133.109.36” 这个ip是什么玩意？复制ip上Google查询，竟然直接找到了问题原因：用于验证域名的80端口被阿里云拦截了，certbot收到的返回与预期不符合，于是提示“challenge failed for domain xxxx”的错误。至于这个ip，应该是阿里云上层防火墙的公共ip。
接着自己尝试用80端口访问域名，果然出现拦截的页面，画风如下：
知道了问题的原因，找解决办法还是挺容易的。个人能想到的解决办法有三种：
- 域名备案；这个不用说了吧，说多了也不好；
- 暂时将域名解析到境外服务器，获取到证书更新后再解析回来；
- 域名验证从http改成dns。
前两个办法比较简单，本文不再多说。这里说一下用dns方式验证域名的步骤。
dns验证有两个重要参数:
- –preferred-challenges，验证方式，填写dns或dns-01;
- –server, 验证服务器，填写https://acme-v02.api.letsencrypt.org/directory
还有一个--manual参数，但在“非交互式”环境下，必须同时指定--manual-auth-hook选项。什么是“非交互式”环境？renew就是非交互式环境，因为不需要外部输出和参与；而certonly（一般）是交互式的，因为执行过程中需要用户输入必要的信息。
本人没看auth-hook脚本的写法，所以用dns方式renew证书是不可行了。那就干脆重新再获取一次证书吧！输入命令：
certbot certonly -d 域名 --preferred-challenges dns --manual --server https://acme-v02.api.letsencrypt.org/directory
命令会询问是否同意记录当前机器的ip，必选“y”（选n会导致脚本退出）。接着命令会输出dns记录的验证信息，按照指定值在dns控制台加上 TXT 记录。dns记录添加完成后等半分钟左右让记录生效，然后按回车键，记录无误的话证书就生成了。
感谢伟大的防火墙和备案机制，又让我长见识了！