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176 lines
8.3 KiB
Markdown
176 lines
8.3 KiB
Markdown
SSL Certificate Verification
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============================
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SSL is TLS
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----------
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SSL is the old name. It is called TLS these days.
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Native SSL
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----------
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If libcurl was built with Schannel or Secure Transport support (the native SSL
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libraries included in Windows and Mac OS X), then this does not apply to
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you. Scroll down for details on how the OS-native engines handle SSL
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certificates. If you are not sure, then run "curl -V" and read the results. If
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the version string says `Schannel` in it, then it was built with Schannel
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support.
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It is about trust
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-----------------
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This system is about trust. In your local CA certificate store you have certs
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from *trusted* Certificate Authorities that you then can use to verify that
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the server certificates you see are valid. They are signed by one of the
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certificate authorities you trust.
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Which certificate authorities do you trust? You can decide to trust the same
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set of companies your operating system trusts, or the set one of the known
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browsers trust. That is basically trust via someone else you trust. You should
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just be aware that modern operating systems and browsers are setup to trust
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*hundreds* of companies and in recent years several certificate authorities
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have been found untrustworthy.
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Certificate Verification
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------------------------
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libcurl performs peer SSL certificate verification by default. This is done
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by using a CA certificate store that the SSL library can use to make sure the
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peer's server certificate is valid.
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If you communicate with HTTPS, FTPS or other TLS-using servers using
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certificates in the CA store, you can be sure that the remote server really is
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the one it claims to be.
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If the remote server uses a self-signed certificate, if you do not install a CA
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cert store, if the server uses a certificate signed by a CA that is not
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included in the store you use or if the remote host is an impostor
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impersonating your favorite site, and you want to transfer files from this
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server, do one of the following:
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1. Tell libcurl to *not* verify the peer. With libcurl you disable this with
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`curl_easy_setopt(curl, CURLOPT_SSL_VERIFYPEER, FALSE);`
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With the curl command line tool, you disable this with `-k`/`--insecure`.
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2. Get a CA certificate that can verify the remote server and use the proper
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option to point out this CA cert for verification when connecting. For
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libcurl hackers: `curl_easy_setopt(curl, CURLOPT_CAINFO, cacert);`
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With the curl command line tool: `--cacert [file]`
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3. Add the CA cert for your server to the existing default CA certificate
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store. The default CA certificate store can be changed at compile time with
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the following configure options:
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`--with-ca-bundle=FILE`: use the specified file as the CA certificate
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store. CA certificates need to be concatenated in PEM format into this
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file.
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`--with-ca-path=PATH`: use the specified path as CA certificate store. CA
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certificates need to be stored as individual PEM files in this directory.
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You may need to run c_rehash after adding files there.
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If neither of the two options is specified, configure will try to
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auto-detect a setting. It's also possible to explicitly not set any
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default store but rely on the built in default the crypto library may
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provide instead. You can achieve that by passing both
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`--without-ca-bundle` and `--without-ca-path` to the configure script.
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If you use Internet Explorer, this is one way to get extract the CA cert
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for a particular server:
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- View the certificate by double-clicking the padlock
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- Find out where the CA certificate is kept (Certificate>
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Authority Information Access>URL)
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- Get a copy of the crt file using curl
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- Convert it from crt to PEM using the OpenSSL tool:
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`openssl x509 -inform DES -in yourdownloaded.crt -out outcert.pem -text`
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- Add the `outcert.pem` to the CA certificate store or use it stand-alone
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as described below.
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If you use the `openssl` tool, this is one way to get extract the CA cert
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for a particular server:
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- `openssl s_client -showcerts -servername server -connect server:443 > cacert.pem`
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- type "quit", followed by the "ENTER" key
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- The certificate will have "BEGIN CERTIFICATE" and "END CERTIFICATE"
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markers.
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- If you want to see the data in the certificate, you can do: `openssl
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x509 -inform PEM -in certfile -text -out certdata` where `certfile` is
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the cert you extracted from logfile. Look in `certdata`.
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- If you want to trust the certificate, you can add it to your CA
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certificate store or use it stand-alone as described. Just remember that
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the security is no better than the way you obtained the certificate.
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4. If you are using the curl command line tool, you can specify your own CA
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cert file by setting the environment variable `CURL_CA_BUNDLE` to the path
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of your choice.
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If you are using the curl command line tool on Windows, curl will search
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for a CA cert file named "curl-ca-bundle.crt" in these directories and in
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this order:
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1. application's directory
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2. current working directory
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3. Windows System directory (e.g. C:\windows\system32)
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4. Windows Directory (e.g. C:\windows)
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5. all directories along %PATH%
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5. Get a better/different/newer CA cert bundle! One option is to extract the
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one a recent Firefox browser uses by running 'make ca-bundle' in the curl
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build tree root, or possibly download a version that was generated this
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way for you: [CA Extract](https://curl.se/docs/caextract.html)
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Neglecting to use one of the above methods when dealing with a server using a
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certificate that is not signed by one of the certificates in the installed CA
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certificate store, will cause SSL to report an error ("certificate verify
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failed") during the handshake and SSL will then refuse further communication
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with that server.
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Certificate Verification with NSS
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---------------------------------
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If libcurl was built with NSS support, then depending on the OS distribution,
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it is probably required to take some additional steps to use the system-wide
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CA cert db. Red Hat ships with an additional module, libnsspem.so, which
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enables NSS to read the OpenSSL PEM CA bundle. On openSUSE you can install
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p11-kit-nss-trust which makes NSS use the system wide CA certificate
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store. NSS also has a new [database
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format](https://wiki.mozilla.org/NSS_Shared_DB).
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Starting with version 7.19.7, libcurl automatically adds the `sql:` prefix to
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the certificate database directory (either the set default `/etc/pki/nssdb` or
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the directory configured with the `SSL_DIR` environment variable). To check
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which certificate database format your distribution provides, examine the
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default certificate database location: `/etc/pki/nssdb`; the new certificate
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database format can be identified by the filenames `cert9.db`, `key4.db`,
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`pkcs11.txt`; filenames of older versions are `cert8.db`, `key3.db`,
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`secmod.db`.
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Certificate Verification with Schannel and Secure Transport
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-----------------------------------------------------------
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If libcurl was built with Schannel (Microsoft's native TLS engine) or Secure
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Transport (Apple's native TLS engine) support, then libcurl will still perform
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peer certificate verification, but instead of using a CA cert bundle, it will
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use the certificates that are built into the OS. These are the same
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certificates that appear in the Internet Options control panel (under Windows)
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or Keychain Access application (under OS X). Any custom security rules for
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certificates will be honored.
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Schannel will run CRL checks on certificates unless peer verification is
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disabled. Secure Transport on iOS will run OCSP checks on certificates unless
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peer verification is disabled. Secure Transport on OS X will run either OCSP
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or CRL checks on certificates if those features are enabled, and this behavior
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can be adjusted in the preferences of Keychain Access.
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HTTPS proxy
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-----------
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Since version 7.52.0, curl can do HTTPS to the proxy separately from the
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connection to the server. This TLS connection is handled separately from the
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server connection so instead of `--insecure` and `--cacert` to control the
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certificate verification, you use `--proxy-insecure` and `--proxy-cacert`.
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With these options, you make sure that the TLS connection and the trust of the
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proxy can be kept totally separate from the TLS connection to the server.
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