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Peer SSL Certificate Verification
=================================
(NOTE: If libcurl was built with Schannel or Secure Transport support, then
this does not apply to you. Scroll down for details on how the OS-native
engines handle SSL certificates. If you're not sure, then run "curl -V" and
read the results. If the version string says "WinSSL" in it, then it was built
with Schannel support.)
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Until 7.18.0, curl bundled a severely outdated ca bundle file that was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"
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.
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:
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.
Peer SSL Certificate Verification with NSS
==========================================
If libcurl was built 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. This library is missing in OpenSuSE, and
without it, NSS can only work with its own internal formats. NSS also has a new
database format: https://wiki.mozilla.org/NSS_Shared_DB
Starting with version 7.19.7, libcurl will check for the NSS version it runs,
and automatically add the 'sql:' prefix to the certdb directory (either the
hardcoded default /etc/pki/nssdb or the directory configured with SSL_DIR
environment variable) if 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[64]/libnssckbi.so /etc/pki/nssdb/libnssckbi.so
Peer SSL Certificate Verification with Schannel and Secure Transport
====================================================================
If libcurl was built with Schannel (Microsoft's TLS/SSL engine) or Secure
Transport (Apple's TLS/SSL engine) support, then libcurl will still perform
peer certificate verification, but instead of using a CA cert bundle, it will
use the certificates that are built into the OS. These are the same
certificates that appear in the Internet Options control panel (under Windows)
or Keychain Access application (under OS X). Any custom security rules for
certificates will be honored.
Schannel will run CRL checks on certificates unless peer verification is
disabled. Secure Transport on iOS will run OCSP checks on certificates unless
peer verification is disabled. Secure Transport on OS X will run either OCSP
or CRL checks on certificates if those features are enabled, and this behavior
can be adjusted in the preferences of Keychain Access.
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