root@sunday:~# curl https://letsencrypt.org/
curl: (60) server certificate verification failed. CAfile: /etc/ssl/certs/ca-certificates.crt CRLfile: none
More details here: http://curl.haxx.se/docs/sslcerts.html
curl performs SSL certificate verification by default, using a "bundle"
of Certificate Authority (CA) public keys (CA certs). If the default
bundle file isn't adequate, you can specify an alternate file
using the --cacert option.
If this HTTPS server uses a certificate signed by a CA represented in
the bundle, the certificate verification probably failed due to a
problem with the certificate (it might be expired, or the name might
not match the domain name in the URL).
If you'd like to turn off curl's verification of the certificate, use
the -k (or --insecure) option.
ubuntu 16.04 已安装过openssl 1.0.2g-1ubuntu4.20依旧是证书过期解决
在证书配置文件中，删除“DST_Root_CA_X3”证书，并重载根证书配置
ubuntu
sudo sed -i '/^mozilla\/DST_Root_CA_X3/s/^/!/' /etc/ca-certificates.conf && sudo update-ca-certificates -f
centos
sudo yum install ca-certificates -y
sudo update-ca-trust force-enable