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Peer SSL Certificate Verification
=================================
libcurl performs peer SSL certificate verification by default. This is done by
installing a default CA cert bundle on 'make install' (or similar), that CA
bundle package is used by default on operations against SSL servers.
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.
If the remote server uses a self-signed certificate, if you don't install
curl's CA cert bundle, if the server uses a certificate signed by a CA that
isn't included in the bundle 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 with 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 installed with the curl package is:
/usr/local/share/curl/curl-ca-bundle.crt, which 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.
(Thanks to Frankie V for this description)
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"
markers.
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.
(Thanks to Doug Kaufman for this description)
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 Mozilla 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:
http://curl.haxx.se/docs/caextract.html
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
server.
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