On my test Debian jessie 8.2 and a staging server Ubuntu trusty 14.04, I had problems being able to use cURL to fetch data from a remote HTTPs site which was secured using a free Let’s Encrypt certificate (this problem manifested itself via both PHP 7 cURL functions and curl directly).
An example of the error is:
curl --verbose https://helloworld.letsencrypt.org/
...
curl: (60) SSL certificate problem: unable to get local issuer certificate
More details here: http://curl.haxx.se/docs/sslcerts.html
To fix this, I found the following steps worked:
sudo apt-get install --reinstall ca-certificatesto ensure you have the latest certificates by your distribution – this might help you, but it didn’t assist me 🙁
So I downloaded the Let’s Encrypt root certificates and forced a rebuild of the certificate store:
sudo curl https://letsencrypt.org/certs/isrgrootx1.pem.txt -o /usr/local/share/ca-certificates/isrgrootx1.crt
sudo curl https://letsencrypt.org/certs/letsencryptauthorityx1.pem.txt -o /usr/local/share/ca-certificates/letsencryptauthorityx1.crt
sudo curl https://letsencrypt.org/certs/letsencryptauthorityx2.pem.txt -o /usr/local/share/ca-certificates/letsencryptauthorityx2.crt
sudo curl https://letsencrypt.org/certs/lets-encrypt-x1-cross-signed.pem.txt -o /usr/local/share/ca-certificates/letsencryptx1.crt
sudo curl https://letsencrypt.org/certs/lets-encrypt-x2-cross-signed.pem.txt -o /usr/local/share/ca-certificates/letsencryptx2.crt
sudo curl https://letsencrypt.org/certs/lets-encrypt-x3-cross-signed.pem.txt -o /usr/local/share/ca-certificates/letsencryptx3.crt
sudo curl https://letsencrypt.org/certs/lets-encrypt-x4-cross-signed.pem.txt -o /usr/local/share/ca-certificates/letsencryptx4.crt
sudo dpkg-reconfigure ca-certificates
and everything worked!
If you are running Java, you may need to also add the certificates to the Java Keytool:
keytool -trustcacerts -keystore $JAVA_HOME/jre/lib/security/cacerts -storepass changeit -noprompt -importcert -file /usr/local/share/ca-certificates/isrgrootx1.crt
keytool -trustcacerts -keystore $JAVA_HOME/jre/lib/security/cacerts -storepass changeit -noprompt -importcert -file /usr/local/share/ca-certificates/letsencryptauthorityx1.crt
(change the storepass password [default of “changeme”] to whatever is relevant to you)