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1SSL Certificate Verification
2============================
3
4SSL is TLS
5----------
6
7SSL is the old name. It is called TLS these days.
8
9
10Native SSL
11----------
12
13If libcurl was built with Schannel or Secure Transport support (the native SSL
14libraries included in Windows and Mac OS X), then this does not apply to
15you. Scroll down for details on how the OS-native engines handle SSL
16certificates. If you're not sure, then run "curl -V" and read the results. If
17the version string says `Schannel` in it, then it was built with Schannel
18support.
19
20It is about trust
21-----------------
22
23This system is about trust. In your local CA certificate store you have certs
24from *trusted* Certificate Authorities that you then can use to verify that the
25server certificates you see are valid. They're signed by one of the CAs you
26trust.
27
28Which CAs do you trust? You can decide to trust the same set of companies your
29operating system trusts, or the set one of the known browsers trust. That's
30basically trust via someone else you trust. You should just be aware that
31modern operating systems and browsers are setup to trust *hundreds* of
32companies and recent years several such CAs have been found untrustworthy.
33
34Certificate Verification
35------------------------
36
37libcurl performs peer SSL certificate verification by default.  This is done
38by using a CA certificate store that the SSL library can use to make sure the
39peer's server certificate is valid.
40
41If you communicate with HTTPS, FTPS or other TLS-using servers using
42certificates that are signed by CAs present in the store, you can be sure
43that the remote server really is the one it claims to be.
44
45If the remote server uses a self-signed certificate, if you don't install a CA
46cert store, if the server uses a certificate signed by a CA that isn't
47included in the store you use or if the remote host is an impostor
48impersonating your favorite site, and you want to transfer files from this
49server, do one of the following:
50
51 1. Tell libcurl to *not* verify the peer. With libcurl you disable this with
52    `curl_easy_setopt(curl, CURLOPT_SSL_VERIFYPEER, FALSE);`
53
54    With the curl command line tool, you disable this with -k/--insecure.
55
56 2. Get a CA certificate that can verify the remote server and use the proper
57    option to point out this CA cert for verification when connecting. For
58    libcurl hackers: `curl_easy_setopt(curl, CURLOPT_CAINFO, cacert);`
59
60    With the curl command line tool: --cacert [file]
61
62 3. Add the CA cert for your server to the existing default CA certificate
63    store. The default CA certificate store can be changed at compile time with
64    the following configure options:
65
66    --with-ca-bundle=FILE: use the specified file as CA certificate store. CA
67    certificates need to be concatenated in PEM format into this file.
68
69    --with-ca-path=PATH: use the specified path as CA certificate store. CA
70    certificates need to be stored as individual PEM files in this directory.
71    You may need to run c_rehash after adding files there.
72
73    If neither of the two options is specified, configure will try to auto-detect
74    a setting. It's also possible to explicitly not hardcode any default store
75    but rely on the built in default the crypto library may provide instead.
76    You can achieve that by passing both --without-ca-bundle and
77    --without-ca-path to the configure script.
78
79    If you use Internet Explorer, this is one way to get extract the CA cert
80    for a particular server:
81
82     - View the certificate by double-clicking the padlock
83     - Find out where the CA certificate is kept (Certificate>
84       Authority Information Access>URL)
85     - Get a copy of the crt file using curl
86     - Convert it from crt to PEM using the openssl tool:
87       openssl x509 -inform DES -in yourdownloaded.crt \
88       -out outcert.pem -text
89     - Add the 'outcert.pem' to the CA certificate store or use it stand-alone
90       as described below.
91
92    If you use the 'openssl' tool, this is one way to get extract the CA cert
93    for a particular server:
94
95     - `openssl s_client -showcerts -servername server -connect server:443 > cacert.pem`
96     - type "quit", followed by the "ENTER" key
97     - The certificate will have "BEGIN CERTIFICATE" and "END CERTIFICATE"
98       markers.
99     - If you want to see the data in the certificate, you can do: "openssl
100       x509 -inform PEM -in certfile -text -out certdata" where certfile is
101       the cert you extracted from logfile. Look in certdata.
102     - If you want to trust the certificate, you can add it to your CA
103       certificate store or use it stand-alone as described. Just remember that
104       the security is no better than the way you obtained the certificate.
105
106 4. If you're using the curl command line tool, you can specify your own CA
107    cert file by setting the environment variable `CURL_CA_BUNDLE` to the path
108    of your choice.
109
110    If you're using the curl command line tool on Windows, curl will search
111    for a CA cert file named "curl-ca-bundle.crt" in these directories and in
112    this order:
113      1. application's directory
114      2. current working directory
115      3. Windows System directory (e.g. C:\windows\system32)
116      4. Windows Directory (e.g. C:\windows)
117      5. all directories along %PATH%
118
119 5. Get a better/different/newer CA cert bundle! One option is to extract the
120    one a recent Firefox browser uses by running 'make ca-bundle' in the curl
121    build tree root, or possibly download a version that was generated this
122    way for you: [CA Extract](https://curl.haxx.se/docs/caextract.html)
123
124Neglecting to use one of the above methods when dealing with a server using a
125certificate that isn't signed by one of the certificates in the installed CA
126certificate store, will cause SSL to report an error ("certificate verify
127failed") during the handshake and SSL will then refuse further communication
128with that server.
129
130Certificate Verification with NSS
131---------------------------------
132
133If libcurl was built with NSS support, then depending on the OS distribution,
134it is probably required to take some additional steps to use the system-wide
135CA cert db. RedHat ships with an additional module, libnsspem.so, which
136enables NSS to read the OpenSSL PEM CA bundle. On openSUSE you can install
137p11-kit-nss-trust which makes NSS use the system wide CA certificate store. NSS
138also has a new [database format](https://wiki.mozilla.org/NSS_Shared_DB).
139
140Starting with version 7.19.7, libcurl automatically adds the 'sql:' prefix to
141the certdb directory (either the hardcoded default /etc/pki/nssdb or the
142directory configured with SSL_DIR environment variable). To check which certdb
143format your distribution provides, examine the default certdb location:
144/etc/pki/nssdb; the new certdb format can be identified by the filenames
145cert9.db, key4.db, pkcs11.txt; filenames of older versions are cert8.db,
146key3.db, secmod.db.
147
148Certificate Verification with Schannel and Secure Transport
149-----------------------------------------------------------
150
151If libcurl was built with Schannel (Microsoft's native TLS engine) or Secure
152Transport (Apple's native TLS engine) support, then libcurl will still perform
153peer certificate verification, but instead of using a CA cert bundle, it will
154use the certificates that are built into the OS. These are the same
155certificates that appear in the Internet Options control panel (under Windows)
156or Keychain Access application (under OS X). Any custom security rules for
157certificates will be honored.
158
159Schannel will run CRL checks on certificates unless peer verification is
160disabled. Secure Transport on iOS will run OCSP checks on certificates unless
161peer verification is disabled. Secure Transport on OS X will run either OCSP
162or CRL checks on certificates if those features are enabled, and this behavior
163can be adjusted in the preferences of Keychain Access.
164
165HTTPS proxy
166-----------
167
168Since version 7.52.0, curl can do HTTPS to the proxy separately from the
169connection to the server. This TLS connection is handled separately from the
170server connection so instead of `--insecure` and `--cacert` to control the
171certificate verification, you use `--proxy-insecure` and `--proxy-cacert`.
172With these options, you make sure that the TLS connection and the trust of the
173proxy can be kept totally separate from the TLS connection to the server.
174