一、Let's Encrypt
Let's Encrypt SSL证书是一个免费的公益项目，由Mozilla、Cisco、Akamai、IdenTrust、EFF等组织人员发起，主要的目是为了推进网站从HTTP向HTTPS过度的进程，目前已经有越来越多的商家加入和赞助支持。
使用Let's Encrypt生成域名证书的前置条件：
- 拥有域名，能自主配置DNS记录。或者提供web服务器验证，需要在网站目录下放一个文件，推荐第一种方式。
- 获取证书的环境要能访问到DNS服务器，因为中途需要校验DNS解析。
- 拥有主机的超级权限，中途需要更新和安装组件。
二、申请流程
第一步：拉代码，代码开源于Github。
> git clone https://github.com/letsencrypt/letsencrypt
Cloning into 'letsencrypt'...
remote: Enumerating objects: 29, done.
remote: Counting objects: 100% (29/29), done.
remote: Compressing objects: 100% (27/27), done.
remote: Total 61424 (delta 7), reused 4 (delta 2), pack-reused 61395
Receiving objects: 100% (61424/61424), 20.12 MiB | 5.01 MiB/s, done.
Resolving deltas: 100% (44625/44625), done.完成后执行命令生成证书：
> ./certbot-auto certonly -d *.maqian.art --manual \
> --preferred-challenges dns \
> --server https://acme-v02.api.letsencrypt.org/directory解释一下各个参数的含义：
certonly: 表示当前为安装模式--manual: 表示手动安装插件，不要自动安装了--preferred-challenges dns: 校验方式为dns验证-d *.maqian.art: 要生成的域名列表，可以为多个，如果是多个分别以-d加上即可--server: Let's Encrypt ACME v2版本使用的服务器
接下来的步骤一直接受即可，直到出现添加DNS记录为止：
Requesting to rerun ./certbot-auto with root privileges...
[sudo] password for ma:
Saving debug log to /var/log/letsencrypt/letsencrypt.log
Plugins selected: Authenticator manual, Installer None
Enter email address (used for urgent renewal and security notices) (Enter 'c' to
cancel): maqian@dyxmq.cn
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Please read the Terms of Service at
https://letsencrypt.org/documents/LE-SA-v1.2-November-15-2017.pdf. You must
agree in order to register with the ACME server at
https://acme-v02.api.letsencrypt.org/directory
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(A)gree/(C)ancel: A
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Would you be willing to share your email address with the Electronic Frontier
Foundation, a founding partner of the Let's Encrypt project and the non-profit
organization that develops Certbot? We'd like to send you email about our work
encrypting the web, EFF news, campaigns, and ways to support digital freedom.
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(Y)es/(N)o: Y
Obtaining a new certificate
Performing the following challenges:
dns-01 challenge for sinfor.maqian.io
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NOTE: The IP of this machine will be publicly logged as having requested this
certificate. If you're running certbot in manual mode on a machine that is not
your server, please ensure you're okay with that.
Are you OK with your IP being logged?
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(Y)es/(N)o: Y
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Please deploy a DNS TXT record under the name
_acme-challenge.maqian.art with the following value:
zw1MeEkmGZOqSqiySp9Ke8S5a9BXC3O4tYzlbjwU-CU
Before continuing, verify the record is deployed.
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Press Enter to Continue此时需要在DNS服务商处添加dns解析，记录类型为TXT，记录为_acme-challenge，值为zw1MeEkmGZOqSqiySp9Ke8S5a9BXC3O4tYzlbjwU-CU。当DNS记录设置好后，新开一个终端查询解析是否生效：
> nslookup -type=txt _acme-challenge.maqian.art
Server: 100.100.2.136
Address: 100.100.2.136#53
Non-authoritative answer:
_acme-challenge.maqian.art text = "zw1MeEkmGZOqSqiySp9Ke8S5a9BXC3O4tYzlbjwU-CU"
Authoritative answers can be found from:
当查询到的记录和给定的都一致之后按下任意键执行下一步，如果DNS验证成功就会出现以下信息：
Waiting for verification...
Cleaning up challenges
IMPORTANT NOTES:
- Congratulations! Your certificate and chain have been saved at:
/etc/letsencrypt/live/maqian.art-0001/fullchain.pem
Your key file has been saved at:
/etc/letsencrypt/live/maqian.art-0001/privkey.pem
Your cert will expire on 2019-03-22. To obtain a new or tweaked
version of this certificate in the future, simply run certbot-auto
again. To non-interactively renew *all* of your certificates, run
"certbot-auto renew"
- If you like Certbot, please consider supporting our work by:
Donating to ISRG / Let's Encrypt: https://letsencrypt.org/donate
Donating to EFF: https://eff.org/donate-le
此时就表示证书已经申请完成了，存放的路径为：/etc/letsencrypt/live/maqian.art-0001。
> sudo ls /etc/letsencrypt/live/maqian.art-0001/ -l
total 4
lrwxrwxrwx 1 root root 39 Dec 22 23:46 cert.pem -> ../../archive/maqian.art-0001/cert1.pem
lrwxrwxrwx 1 root root 40 Dec 22 23:46 chain.pem -> ../../archive/maqian.art-0001/chain1.pem
lrwxrwxrwx 1 root root 44 Dec 22 23:46 fullchain.pem -> ../../archive/maqian.art-0001/fullchain1.pem
lrwxrwxrwx 1 root root 42 Dec 22 23:46 privkey.pem -> ../../archive/maqian.art-0001/privkey1.pem
-rw-r--r-- 1 root root 692 Dec 22 23:46 README三、安装到nginx
上面一共生成了四个文件，各自的用途为：
cert.pem: Apache服务器端证书chain.pem: Apache根证书和中继证书fullchain.pem: Nginx所需要ssl_certificate文件privkey.pem: 安全证书KEY文件
部署到nginx只需要添加一下指令即可：
ssl on;
ssl_certificate /etc/letsencrypt/live/maqian.art-0001/fullchain.pem;
ssl_certificate_key /etc/letsencrypt/live/maqian.art-0001/privkey.pem;打开网站，点开左上角地址栏的https，查看证书：
