requests-gssapi 1.3.0

A GSSAPI authentication handler for python-requests

Requests is an HTTP library, written in Python, for human beings. This library adds optional GSSAPI authentication support and supports mutual authentication.

It provides a fully backward-compatible shim for the old python-requests-kerberos library: simply replace import requests_kerberos with import requests_gssapi. A more powerful interface is provided by the HTTPSPNEGOAuth component, but this is of course not guaranteed to be compatible. Documentation below is written toward the new interface.

Basic GET usage:

>>> import requests
>>> from requests_gssapi import HTTPSPNEGOAuth
>>> r = requests.get("http://example.org", auth=HTTPSPNEGOAuth())
...

The entire requests.api should be supported.

Setup

In order to use this library, there must already be a Kerberos Ticket-Granting Ticket (TGT) in a credential cache (ccache). Whether a TGT is available can be easily determined by running the klist command. If no TGT is available, then it first must be obtained (for instance, by running the kinit command, or pointing the $KRB5CCNAME to a credential cache with a valid TGT).

In short, the library will handle the “negotiations” of Kerberos authentication, but ensuring that a credentials are available and valid is the responsibility of the user.

Authentication Failures

Client authentication failures will be communicated to the caller by returning a 401 response. A 401 response may also be the result of expired credentials (including the TGT).

Mutual Authentication

Mutual authentication is a poorly-named feature of the GSSAPI which doesn’t provide any additional security benefit to most possible uses of requests_gssapi. Practically speaking, in most mechanism implementations (including krb5), it requires another round-trip between the client and server during the authentication handshake. Many clients and servers do not properly handle the authentication handshake taking more than one round-trip. If you encounter a MutualAuthenticationError, this is probably why.

So long as you’re running over a TLS link whose security guarantees you trust, there’s no benefit to mutual authentication. If you don’t trust the link at all, mutual authentication won’t help (since it’s not tamper-proof, and GSSAPI isn’t being used post-authentication. There’s some middle ground between the two where it helps a small amount (e.g., passive adversary over encrypted-but-unverified channel), but for Negotiate (what we’re doing here), it’s not generally helpful.

For a more technical explanation of what mutual authentication actually guarantees, I refer you to rfc2743 (GSSAPIv2), rfc4120 (krb5 in GSSAPI), rfc4178 (SPNEGO), and rfc4559 (HTTP Negotiate).

DISABLED

By default, there’s no need to explicitly disable mutual authentication. However, for compatability with older versions of request_gssapi or requests_kerberos, you can explicitly request it not be attempted:

>>> import requests
>>> from requests_gssapi import HTTPSPNEGOAuth, DISABLED
>>> gssapi_auth = HTTPSPNEGOAuth(mutual_authentication=DISABLED)
>>> r = requests.get("https://example.org", auth=gssapi_auth)
...

REQUIRED

This was historically the default, but no longer is. If requested, HTTPSPNEGOAuth will require mutual authentication from the server, and if a server emits a non-error response which cannot be authenticated, a requests_gssapi.errors.MutualAuthenticationError will be raised. (See above for what this means.) If a server emits an error which cannot be authenticated, it will be returned to the user but with its contents and headers stripped. If the response content is more important than the need for mutual auth on errors, (eg, for certain WinRM calls) the stripping behavior can be suppressed by setting sanitize_mutual_error_response=False:

>>> import requests
>>> from requests_gssapi import HTTPSPNEGOAuth, REQUIRED
>>> gssapi_auth = HTTPSPNEGOAuth(mutual_authentication=REQUIRED, sanitize_mutual_error_response=False)
>>> r = requests.get("https://windows.example.org/wsman", auth=gssapi_auth)
...

OPTIONAL

This will cause requests_gssapi to attempt mutual authentication if the server advertises that it supports it, and cause a failure if authentication fails, but not if the server does not support it at all. This is probably not what you want: link tampering will either cause hard failures, or silently cause it to not happen at all. It is retained for compatability.

>>> import requests
>>> from requests_gssapi import HTTPSPNEGOAuth, OPTIONAL
>>> gssapi_auth = HTTPSPNEGOAuth(mutual_authentication=OPTIONAL)
>>> r = requests.get("https://example.org", auth=gssapi_auth)
...

Opportunistic Authentication

HTTPSPNEGOAuth can be forced to preemptively initiate the GSSAPI exchange and present a token on the initial request (and all subsequent). By default, authentication only occurs after a 401 Unauthorized response containing a Negotiate challenge is received from the origin server. This can cause mutual authentication failures for hosts that use a persistent connection (eg, Windows/WinRM), as no GSSAPI challenges are sent after the initial auth handshake. This behavior can be altered by setting opportunistic_auth=True:

>>> import requests
>>> from requests_gssapi import HTTPSPNEGOAuth
>>> gssapi_auth = HTTPSPNEGOAuth(opportunistic_auth=True)
>>> r = requests.get("https://windows.example.org/wsman", auth=gssapi_auth)
...

Expect-Continue

Since httplib does not support the Expect-Continue header, a request with a body will fail with 401 Unauthorized and must be repeated with a GSSAPI exchange. This causes several issues:

• Additional overhead for request retransmission

• Requests with non-repeatable bodies will fail

• Some servers will already send the approriate error response while your client is still streaming the request. Not all reverse proxies can handle that properly and will rather fail.

Therefore, in such cases you must enable opportunistic authentication.

Hostname Override

If communicating with a host whose DNS name doesn’t match its hostname (eg, behind a content switch or load balancer), the hostname used for the GSSAPI exchange can be overridden by passing in a custom name (string or gssapi.Name):

>>> import requests
>>> from requests_gssapi import HTTPSPNEGOAuth
>>> gssapi_auth = HTTPSPNEGOAuth(target_name="internalhost.local")
>>> r = requests.get("https://externalhost.example.org/", auth=gssapi_auth)
...

Explicit Principal

HTTPSPNEGOAuth normally uses the default principal (ie, the user for whom you last ran kinit or kswitch, or an SSO credential if applicable). However, an explicit credential can be in instead, if desired.

>>> import gssapi
>>> import requests
>>> from requests_gssapi import HTTPSPNEGOAuth
>>> name = gssapi.Name("user@REALM", gssapi.NameType.user)
>>> creds = gssapi.Credentials(name=name, usage="initiate")
>>> gssapi_auth = HTTPSPNEGOAuth(creds=creds)
>>> r = requests.get("http://example.org", auth=gssapi_auth)
...

Explicit Mechanism

HTTPSPNEGOAuth normally lets SPNEGO decide which negotiation mechanism to use. However, an explicit mechanism can be used instead if desired. The mech parameter will be passed straight through to gssapi without interference. It is expected to be an instance of gssapi.mechs.Mechanism.

>>> import gssapi
>>> import requests
>>> from requests_gssapi import HTTPSPNEGOAuth
>>> try:
...   krb5 = gssapi.mechs.Mechanism.from_sasl_name("GS2-KRB5")
... except AttributeError:
...   krb5 = gssapi.OID.from_int_seq("1.2.840.113554.1.2.2")
>>> gssapi_auth = HTTPSPNEGOAuth(mech=krb5)
>>> r = requests.get("http://example.org", auth=gssapi_auth)
...

Delegation

requests_gssapi supports credential delegation (GSS_C_DELEG_FLAG). To enable delegation of credentials to a server that requests delegation, pass delegate=True to HTTPSPNEGOAuth:

>>> import requests
>>> from requests_gssapi import HTTPSPNEGOAuth
>>> r = requests.get("http://example.org", auth=HTTPSPNEGOAuth(delegate=True))
...

Be careful to only allow delegation to servers you trust as they will be able to impersonate you using the delegated credentials.

Logging

This library makes extensive use of Python’s logging facilities.

Log messages are logged to the requests_gssapi and requests_gssapi.gssapi named loggers.

If you are having difficulty we suggest you configure logging. Issues with the underlying GSSAPI libraries will be made apparent. Additionally, copious debug information is made available which may assist in troubleshooting if you increase your log level all the way up to debug.