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ACCESS-FLAGS.md

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Access Control Flags

Ghostunnel uses TLS with mutual authentication for authentication and access control. This means that both the client and server present a certificate that can be verified by the other party.

Server mode

There are several flags available to restrict which clients can connect to a Ghostunnel server, based on checks on the subject of the client certificate.

Access control flags in server mode are treated as a logical disjunction (OR) when multiple flags are specified. This means that a client will be allowed to complete a connection as long as at least one flag matches.

  • --allow-all

Setting this flag allows all clients with a valid certificate, regardless of the client certificate subject. This flag is mutually exclusive with other access control flags.

  • --allow-cn

Allow clients with given common name (CN) in the subject. Can be repeated to allow multiple clients with different CNs to connect. Performs an exact string comparison on the CN field.

  • --allow-ou

Allow clients with given organizational unit (OU) field in the subject. Can be repeated to allow multiple clients with different OUs to connect. Performs an exact string comparison on the OU field.

  • --allow-dns

Allow clients with given DNS subject alternative name (DNS SAN) in the subject. Can be repeated to allow multiple clients with different DNS SANs to connect. Note that this performs the access check based on a comparison of the the DNS SAN value of the client certificate, it does not perform any DNS lookups.

  • --allow-uri

Allow clients with given URI subject alternative name (URI SAN) in the subject. Can be repeated to allow multiple clients with different URI SANs to connect. This flag may also contain * and ** wildcards that can be used to match multiple clients.

For example, setting --allow-uri=spiffe://ghostunnel/* would allow clients with spiffe://ghostunnel/client1 or spiffe://ghostunnel/client2 URI SANs (as well as other values). See documentation for the wildcard package for more information.

  • --allow-policy and --allow-query

Allow clients where a Rego policy evaluates to true with the given query. For more information, see the Open Policy Agent section below.

  • --disable-authentication

Disables client authentication entirely, no client certificate will be required from any client. This means that anyone will be able to establish a connection to the Ghostunnel server. This flag is mutually exclusive with other access control flags.

Client mode

Ghostunnel in client mode offers various flags that can be used to augment and perform additional checks on servers it connects to. Regardless of flags passed to the client, it will always perform standard hostname verification to check the hostname against the server certificate.

Access control flags in client mode are treated as a logical disjunction (OR) when multiple flags are specified. This means that a client will be allowed to complete a connection as long as at least one flag matches, assuming that hostname verification was also successful.

  • --override-server-name

If set, overrides the server name used for hostname verification to be different from the hostname that was passed in --target. This also sets the hostname passed to the backend for SNI purposes. The logic for hostname verification is implemented as part of the crypto/tls package in Go's standard library, see the ServerName field on the tls.Config struct.

  • --verify-cn

Verify the common name (CN) of the server certificate, on top of the hostname. Can be repeated to check that at least one of a set of CNs is present. This performs an exact string comparison on the CN field of the certificate.

  • --verify-ou

Verify the organizational unit (OU) of the server certificate, on top of the hostname. Can be repeated to check that at least one of a set of OUs is present. This performs an exact string comparison on the OU field of the certificate.

  • --verify-dns

Verify the presence of a DNS subject alternative name (DNS SAN) on the server certificate, on top of the hostname. This checks that the given DNS name is listed as a valid name on the certificate. Can be repeated to require that at least one of a set of hostnames is present.

  • --verify-uri

Verify the presence of a URI subject alternative name (URI SAN) on the server certificate, on top of the hostname. This checks that the given URI name is listed as a valid name on the certificate. This flag may also contain * and ** wildcards that can be used to match multiple servers.

For example, setting --verify-uri=spiffe://ghostunnel/* would allow servers with spiffe://ghostunnel/server1 or spiffe://ghostunnel/server2 URI SANs (as well as other values). See documentation for the wildcard package for more information.

  • --verify-policy and --verify-query

Verify that a Rego policy evaluates to true with the given query. For more information, see the Open Policy Agent section below.

  • --disable-authentication

Disable client authentication, no certificate will be provided to the server. This is useful if you just want to use Ghostunnel to wrap a connection in TLS but the backend doesn't require mutual authentication.

Open Policy Agent

Note: This feature is considered experimental and is subject to future breaking changes. Please report bugs if you find them!

Ghostunnel has support for Open Policy Agent (OPA), both in server and client mode. The policy file must be present on disk for Ghostunnel to use it and the use of OPA is mutually exclusive with any other allow (or verify) flags. Policy files can be reloaded at runtime much like certificates, with the --timed-reload flag or via SIGHUP on a recent release.

To use it in server mode, specify the --allow-policy and --allow-query flags.

Example:

ghostunnel server [...] --allow-policy=policy.rego --allow-query=data.policy.allow

To use it in client mode, specify the --verify-policy and --verify-query flags.

Example:

ghostunnel server [...] --verify-policy=policy.rego --allow-query=data.policy.allow

Inside your policy, you can access the reflected X.509 peer certificate using input.certificate. For example, the policy below verifies that the presented client certificate contains at least one of the allowed common names or SPIFFE IDs.

You can use the Rego Playground to test and develop policies. See the documentation for x509.Certificate for the structure of the input.certificate variable.

Example (Playground):

package policy

import input

import future.keywords.if
import future.keywords.in

default allow := false

allowed_common_names = [
	"client1",
	"client2",
]

allowed_spiffe_ids = [
	"example.com/client1",
	"example.com/client1/*",
	"example.com/client2",
	"example.com/client2/*",
]

allow if {
	# Allow if common name matches a pattern in allowed_common_names
	some common_name in allowed_common_names
	glob.match(common_name, [], input.certificate.Subject.CommonName)
}

allow if {
	# Allow if one of the URI SANs matches a pattern in allowed_spiffe_ids
	some uri in input.certificate.URIs
	some spiffe_id in allowed_spiffe_ids

	# Basic sanity checks for the URI SAN before we compare
	uri.Scheme == "spiffe"

	# User, query, fragment, etc. should not be set in the URI SAN
	not uri.User
	not uri.Opaque
	not uri.RawQuery
	not uri.Fragement
	not uri.RawFragment

	# Match host/path against the pattern
	glob.match(spiffe_id, [".", "/"], sprintf("%s%s", [uri.Host, uri.Path]))
}

The corresponding query for this policy is data.policy.allow, because we want to determine the outcome of the policy by looking at allow.

See the documentation about Golang's x509.Certificate struct for more about other properties you can match on, and the Rego documentation for more about the policy language.

Caveats

  • There is no mechanism to load a policy from a remote OPA server. The policy file has to be local, or be retrieved and stored locally out of band by a different process.
  • By standard OPA convention, we consider a policy to be "allowed" if the query is exactly one result with exactly one element that has the value true.
  • Policy evaluation timeout is the same as the connection timeout. If a policy takes more time to execute than the specified connection timeout, the connection will fail.