When using curl, you may receive the following error message if you are missing the CA certificate in the directory tree /usr/local/share/ca-certificates/ for the site you are trying to connect to:

user@hardy:/tmp$ curl https://example.com
curl: (60) SSL certificate problem, verify that the CA cert is OK. Details:
error:14090086:SSL routines:SSL3_GET_SERVER_CERTIFICATE:certificate verify failed
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). The default
 bundle is named curl-ca-bundle.crt; 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).

The easiest way around this is to turn off curl’s verification of the certificate, using the -k (or –insecure) option. However, the best way is to add the associated CA certificate to your system by following these directions: Adding Additional SSL CA certificates.
On Ubuntu Hardy, curl is compiled to use the file /etc/ssl/certs/ca-certificates.crt. You will see the following when executing curl against a site using HTTPS:

* successfully set certificate verify locations:
*   CAfile: /etc/ssl/certs/ca-certificates.crt
  CApath: none

On Ubuntu Lucid, curl is compiled to use the CA certificate directory /etc/ssl/certs/. You will see the following when executing curl against a site using HTTPS:

* successfully set certificate verify locations:
*   CAfile: none
  CApath: /etc/ssl/certs

You can override this with the –ca-cert or –capath options.

On Ubuntu, SSL CA certificates are stored in subdirectories of /usr/local/share/ca-certificates. In order to add additional CA certificates, first create a new subdirectory to store your CAs:

sudo mkdir /usr/local/share/ca-certificates/added

Then add your CA certificates to this directory. These files should have a .crt extension (e.g., my_ca.crt). Then append a line for each certificate you add to the configuration file /etc/ca-certificates.conf (e.g., “added/my_ca.crt“). Finally, run

sudo update-ca-certificates --fresh

This command reads the file /etc/ca-certificates.conf, updates the directory /etc/ssl/certs to hold SSL certificates and generates the file /etc/ssl/certs/certificates.crt. The script update-ca-certificates will use the command c_rehash take a hash value of each .crt file. It then creates symbolic links in the directory /etc/ssl/certs for each of the files named by the hash value. This is useful as many programs require directories to be set up like this in order to find the certificates they require. /etc/ssl/certs/certificates.crt is a concatenated single-file version of CA certificates. It contains all CA certificates that were activated in /etc/ca-certificates.conf.
Use the file command to verify that the .crt files you add use Unix newline characters, otherwise the /etc/ssl/certs/certificates.crt file may not be generated properly.

References

• http://manpages.ubuntu.com/manpages/lucid/man8/update-ca-certificates.8.html
• http://manpages.ubuntu.com/manpages/lucid/en/man1/c_rehash.1ssl.html
• http://manpages.ubuntu.com/manpages/lucid/en/man1/openssl.1ssl.html
• http://manpages.ubuntu.com/manpages/lucid/man1/fromdos.1.html