| Peer SSL Certificate Verification
|libcurl performs peer SSL certificate verification by default. This is done
|by using CA cert bundle that the SSL library can use to make sure the peer's
|server certificate is valid.
|If you communicate with HTTPS or FTPS servers using certificates that are
|signed by CAs present in the bundle, you can be sure that the remote server
|really is the one it claims to be.
|Until 7.18.0, curl bundled a severely outdated ca bundle file that was
|installed by default. These days, the curl archives include no ca certs at
|all. You need to get them elsewhere. See below for example.
|If the remote server uses a self-signed certificate, if you don't install a CA
|cert bundle, if the server uses a certificate signed by a CA that isn't
|included in the bundle you use or if the remote host is an impostor
|impersonating your favorite site, and you want to transfer files from this
|server, do one of the following:
| 1. Tell libcurl to *not* verify the peer. With libcurl you disable this with
| curl_easy_setopt(curl, CURLOPT_SSL_VERIFYPEER, FALSE);
| With the curl command line tool, you disable this with -k/--insecure.
| 2. Get a CA certificate that can verify the remote server and use the proper
| option to point out this CA cert for verification when connecting. For
| libcurl hackers: curl_easy_setopt(curl, CURLOPT_CAPATH, capath);
| With the curl command line tool: --cacert [file]
| 3. Add the CA cert for your server to the existing default CA cert bundle.
| The default path of the CA bundle used can be changed by running configure
| with the --with-ca-bundle option pointing out the path of your choice.
| To do this, you need to get the CA cert for your server in PEM format and
| then append that to your CA cert bundle.
| If you use Internet Explorer, this is one way to get extract the CA cert
| for a particular server:
| o View the certificate by double-clicking the padlock
| o Find out where the CA certificate is kept (Certificate>
| Authority Information Access>URL)
| o Get a copy of the crt file using curl
| o Convert it from crt to PEM using the openssl tool:
| openssl x509 -inform DES -in yourdownloaded.crt \
| -out outcert.pem -text
| o Append the 'outcert.pem' to the CA cert bundle or use it stand-alone
| as described below.
| If you use the 'openssl' tool, this is one way to get extract the CA cert
| for a particular server:
| o openssl s_client -connect xxxxx.com:443 |tee logfile
| o type "QUIT", followed by the "ENTER" key
| o The certificate will have "BEGIN CERTIFICATE" and "END CERTIFICATE"
| o If you want to see the data in the certificate, you can do: "openssl
| x509 -inform PEM -in certfile -text -out certdata" where certfile is
| the cert you extracted from logfile. Look in certdata.
| o If you want to trust the certificate, you can append it to your
| cert_bundle or use it stand-alone as described. Just remember that the
| security is no better than the way you obtained the certificate.
| 4. If you're using the curl command line tool, you can specify your own CA
| cert path by setting the environment variable CURL_CA_BUNDLE to the path
| of your choice.
| If you're using the curl command line tool on Windows, curl will search
| for a CA cert file named "curl-ca-bundle.crt" in these directories and in
| this order:
| 1. application's directory
| 2. current working directory
| 3. Windows System directory (e.g. C:\windows\system32)
| 4. Windows Directory (e.g. C:\windows)
| 5. all directories along %PATH%
| 5. Get a better/different/newer CA cert bundle! One option is to extract the
| one a recent Firefox browser uses by running 'make ca-bundle' in the curl
| build tree root, or possibly download a version that was generated this
| way for you:
|Neglecting to use one of the above methods when dealing with a server using a
|certificate that isn't signed by one of the certificates in the installed CA
|cert bundle, will cause SSL to report an error ("certificate verify failed")
|during the handshake and SSL will then refuse further communication with that
| Peer SSL Certificate Verification with NSS
|If libcurl is build with NSS support then depending on the OS distribution it
|is probably required to take some additional steps to use the system-wide CA
|cert db. RedHat ships with an additional module libnsspem.so which enables NSS
|to read the OpenSSL PEM CA bundle. With OpenSuSE this lib is missing, and NSS
|can only work with its own internal formats. Also NSS got a new database
|Starting with version 7.19.7 libcurl will check for the NSS version it runs,
|and add automatically the 'sql:' prefix to the certdb directory (either the
|hardcoded default /etc/pki/nssdb or the directory configured with SSL_DIR
|environment variable) if a version 3.12.0 or later is detected.
|To check which certdb format your distribution provides examine the default
|certdb location /etc/pki/nssdb; the new certdb format can be identified by
|the filenames cert9.db, key4.db, pkcs11.txt; filenames of older versions are
|cert8.db, key3.db, modsec.db.
|Usually these cert databases are empty; but NSS also has built-in CAs which are
|provided through a shared library libnssckbi.so; if you want to use these
|built-in CAs then create a symlink to libnssckbi.so in /etc/pki/nssdb:
|ln -s /usr/lib/libnssckbi.so /etc/pki/nssdb/libnssckbi.so