4.18.2.2. Configuring policies for NNTP requests and responses

Changing the default behavior of requests is possible using the request attribute. This hash is indexed by the NNTP command name (e.g.: POST or ARTICLE). The response attribute (indexed by the command name and the response code) enables the control of NNTP responses. The possible actions are described in the following tables. See also Section 2.1, Policies for requests and responses. When looking up entries of the response attribute hash, the lookup precedence described in Section 2.1.2, Response codes is used.

ActionDescription
NNTP_REQ_ACCEPT

Allow the command.

NNTP_REQ_ACCEPT_CLIENTTEXT

Allow the command and notify the low level proxy layer that the client will send extra text (for example an article) immediately after this request.

This action is required only by certain non-standard NNTP extensions. It should be handled with great care, because its use can result in deadlocks or erroneous behavior.

NNTP_REQ_REJECT

Reject the command.

NNTP_REQ_REJECT_CLIENTTEXT

Reject the command and the extra text from the client.

NNTP_REQ_ABORT

Reject the command and terminate the NNTP session.

NNTP_REQ_POLICY

Call the function specified to make a decision about the event. The function receives three parameters: self, command, and the parameters of the command. See Section 2.1, Policies for requests and responses for details.

Table 4.51.  Action codes for NNTP requests

ActionDescription
NNTP_RSP_ACCEPT

Accept the response.

NNTP_RSP_ACCEPT_CLIENTTEXT

Accept the response and notify the low level layer that the client will send extra text (for example an article) immediately after this response.

NNTP_RSP_ACCEPT_SERVERTEXT

Accept the response and notify the low level layer of the proxy that the NNTP server will send extra text (e.g.: the list of newsgroups, or the body of an article) immediately after this response.

NNTP_RSP_REJECT

Reject the response. A response indicating a general error is sent to the client.

NNTP_RSP_REJECT_SERVERTEXT

Reject the response and notify the low level layer that although the server will send extra text following this response, it must not be forwarded to the client.

NNTP_RSP_ABORT

Reject the response and immediately terminate the current NNTP session.

NNTP_RSP_POLICY

Call the function specified to make a decision about the event. The function receives three parameters: self, response code, and the parameters of the response. See Section 2.1, Policies for requests and responses for details.

Table 4.52.  Action codes for NNTP responses

Example 4.35. Example for filtering accessible newsgroups

In this example access to certain newsgroups is disallowed: the GROUP responses are inspected by the function filterGroup (defined in the example), and responses containing the group 'disallowed.news.group' are rejected. 

class MyFilteredNntpProxy(NntpProxyStrict):
      def config(self):
              NntpProxyStrict.config(self)
              self.response["GROUP", "*"] = (NNTP_REQ_POLICY, self.filterGroup)

      def filterGroup(self, command, param):
              if param == "disallowed.news.group":
                      return NNTP_REQ_REJECT
              return NNTP_REQ_ACCEPT
Example 4.36. Example for defining policies for responses in NNTP

This example rejects all responses, except for GREETING, which is modified by the proxy. If a DATE response is received, the connection is terminated. 

class MyNntpProxy(NntpProxyStrict):
      def config(self):
              NntpProxyStrict.config(self)
              self.response["*", "*"] = (NNTP_RSP_REJECT)
              self.response["GREETING", "*"] = (NNTP_RSP_POLICY, self.changeGreeting)
              self.response["DATE", "*"] = (NNTP_RSP_ABORT)

      def changeGreeting(self, response, param):
              self.response_param = "NNTP server of Example Corporation"
              return NNTP_RSP_ACCEPT

Predefined constants are available for NNTP response codes for easier use. These are listed in Table A.2, Constants for NNTP responses .

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