1                      Peer SSL Certificate Verification
2                      =================================
3
4(NOTE: If libcurl was built with Schannel or Secure Transport support, then
5this does not apply to you. Scroll down for details on how the OS-native
6engines handle SSL certificates. If you're not sure, then run "curl -V" and
7read the results. If the version string says "WinSSL" in it, then it was built
8with Schannel support.)
9
10libcurl performs peer SSL certificate verification by default.  This is done
11by using CA cert bundle that the SSL library can use to make sure the peer's
12server certificate is valid.
13
14If you communicate with HTTPS or FTPS servers using certificates that are
15signed by CAs present in the bundle, you can be sure that the remote server
16really is the one it claims to be.
17
18Until 7.18.0, curl bundled a severely outdated ca bundle file that was
19installed by default. These days, the curl archives include no ca certs at
20all. You need to get them elsewhere. See below for example.
21
22If the remote server uses a self-signed certificate, if you don't install a CA
23cert bundle, if the server uses a certificate signed by a CA that isn't
24included in the bundle you use or if the remote host is an impostor
25impersonating your favorite site, and you want to transfer files from this
26server, do one of the following:
27
28 1. Tell libcurl to *not* verify the peer. With libcurl you disable this with
29    curl_easy_setopt(curl, CURLOPT_SSL_VERIFYPEER, FALSE);
30
31    With the curl command line tool, you disable this with -k/--insecure.
32
33 2. Get a CA certificate that can verify the remote server and use the proper
34    option to point out this CA cert for verification when connecting. For
35    libcurl hackers: curl_easy_setopt(curl, CURLOPT_CAPATH, capath);
36
37    With the curl command line tool: --cacert [file]
38
39 3. Add the CA cert for your server to the existing default CA cert bundle.
40    The default path of the CA bundle used can be changed by running configure
41    with the --with-ca-bundle option pointing out the path of your choice.
42
43    To do this, you need to get the CA cert for your server in PEM format and
44    then append that to your CA cert bundle.
45
46    If you use Internet Explorer, this is one way to get extract the CA cert
47    for a particular server:
48
49     o View the certificate by double-clicking the padlock
50     o Find out where the CA certificate is kept (Certificate>
51       Authority Information Access>URL)
52     o Get a copy of the crt file using curl
53     o Convert it from crt to PEM using the openssl tool:
54       openssl x509 -inform DES -in yourdownloaded.crt \
55       -out outcert.pem -text
56     o Append the 'outcert.pem' to the CA cert bundle or use it stand-alone
57       as described below.
58
59    If you use the 'openssl' tool, this is one way to get extract the CA cert
60    for a particular server:
61
62     o openssl s_client -connect xxxxx.com:443 |tee logfile
63     o type "QUIT", followed by the "ENTER" key
64     o The certificate will have "BEGIN CERTIFICATE" and "END CERTIFICATE"
65       markers.
66     o If you want to see the data in the certificate, you can do: "openssl
67       x509 -inform PEM -in certfile -text -out certdata" where certfile is
68       the cert you extracted from logfile. Look in certdata.
69     o If you want to trust the certificate, you can append it to your
70       cert_bundle or use it stand-alone as described. Just remember that the
71       security is no better than the way you obtained the certificate.
72
73 4. If you're using the curl command line tool, you can specify your own CA
74    cert path by setting the environment variable CURL_CA_BUNDLE to the path
75    of your choice.
76
77    If you're using the curl command line tool on Windows, curl will search
78    for a CA cert file named "curl-ca-bundle.crt" in these directories and in
79    this order:
80      1. application's directory
81      2. current working directory
82      3. Windows System directory (e.g. C:\windows\system32)
83      4. Windows Directory (e.g. C:\windows)
84      5. all directories along %PATH%
85
86 5. Get a better/different/newer CA cert bundle! One option is to extract the
87    one a recent Firefox browser uses by running 'make ca-bundle' in the curl
88    build tree root, or possibly download a version that was generated this
89    way for you:
90
91        http://curl.haxx.se/docs/caextract.html
92
93Neglecting to use one of the above methods when dealing with a server using a
94certificate that isn't signed by one of the certificates in the installed CA
95cert bundle, will cause SSL to report an error ("certificate verify failed")
96during the handshake and SSL will then refuse further communication with that
97server.
98
99                      Peer SSL Certificate Verification with NSS
100                      ==========================================
101
102If libcurl was built with NSS support, then depending on the OS distribution,
103it is probably required to take some additional steps to use the system-wide CA
104cert db. RedHat ships with an additional module, libnsspem.so, which enables
105NSS to read the OpenSSL PEM CA bundle. This library is missing in OpenSuSE, and
106without it, NSS can only work with its own internal formats. NSS also has a new
107database format: https://wiki.mozilla.org/NSS_Shared_DB
108
109Starting with version 7.19.7, libcurl will check for the NSS version it runs,
110and automatically add the 'sql:' prefix to the certdb directory (either the
111hardcoded default /etc/pki/nssdb or the directory configured with SSL_DIR
112environment variable) if version 3.12.0 or later is detected. To check which
113certdb format your distribution provides, examine the default
114certdb location: /etc/pki/nssdb; the new certdb format can be identified by
115the filenames cert9.db, key4.db, pkcs11.txt; filenames of older versions are
116cert8.db, key3.db, modsec.db.
117
118Usually these cert databases are empty, but NSS also has built-in CAs which are
119provided through a shared library, libnssckbi.so; if you want to use these
120built-in CAs, then create a symlink to libnssckbi.so in /etc/pki/nssdb:
121ln -s /usr/lib[64]/libnssckbi.so /etc/pki/nssdb/libnssckbi.so
122
123     Peer SSL Certificate Verification with Schannel and Secure Transport
124     ====================================================================
125
126If libcurl was built with Schannel (Microsoft's TLS/SSL engine) or Secure
127Transport (Apple's TLS/SSL engine) support, then libcurl will still perform
128peer certificate verification, but instead of using a CA cert bundle, it will
129use the certificates that are built into the OS. These are the same
130certificates that appear in the Internet Options control panel (under Windows)
131or Keychain Access application (under OS X). Any custom security rules for
132certificates will be honored.
133
134Schannel will run CRL checks on certificates unless peer verification is
135disabled. Secure Transport on iOS will run OCSP checks on certificates unless
136peer verification is disabled. Secure Transport on OS X will run either OCSP
137or CRL checks on certificates if those features are enabled, and this behavior
138can be adjusted in the preferences of Keychain Access.
139