165 lines
4.6 KiB
ReStructuredText
165 lines
4.6 KiB
ReStructuredText
DNS-based language dictionary
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===============================
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This example shows how to create a simple language dictionary based on **DNS**
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service within 15 minutes. The translation will be performed using TXT resource records.
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Key parts
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-----------
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Initialization
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~~~~~~~~~~~~~~~~~~~~~~~
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On **init()** module loads dictionary from a text file containing records in ``word [tab] translation`` format.
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::
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def init(id, cfg):
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log_info("pythonmod: dict init")
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f = open("examples/dict_data.txt", "r")
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...
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The suitable file can be found at http://slovnik.zcu.cz
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DNS query and word lookup
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~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Let's define the following format od DNS queries: ``word1[.]word2[.] ... wordN[.]{en,cs}[._dict_.cz.]``.
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Word lookup is done by simple ``dict`` lookup from broken DNS request.
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Query name is divided into a list of labels. This list is accesible as qname_list attribute.
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::
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aword = ' '.join(qstate.qinfo.qname_list[0:-4]) #skip last four labels
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adict = qstate.qinfo.qname_list[-4] #get 4th label from the end
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words = [] #list of words
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if (adict == "en") and (aword in en_dict):
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words = en_dict[aword]
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if (adict == "cs") and (aword in cz_dict):
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words = cz_dict[aword] # CS -> EN
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In the first step, we get a string in the form: ``word1[space]word2[space]...word[space]``.
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In the second assignment, fourth label from the end is obtained. This label should contains *"cs"* or *"en"*.
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This label determines the direction of translation.
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Forming of a DNS reply
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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DNS reply is formed only on valid match and added as TXT answer.
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::
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msg = DNSMessage(qstate.qinfo.qname_str, RR_TYPE_TXT, RR_CLASS_IN, PKT_AA)
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for w in words:
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msg.answer.append("%s 300 IN TXT \"%s\"" % (qstate.qinfo.qname_str, w.replace("\"", "\\\"")))
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if not msg.set_return_msg(qstate):
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qstate.ext_state[id] = MODULE_ERROR
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return True
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qstate.return_rcode = RCODE_NOERROR
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qstate.ext_state[id] = MODULE_FINISHED
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return True
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In the first step, a :class:`DNSMessage` instance is created for a given query *(type TXT)*.
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The fourth argument specifies the flags *(authoritative answer)*.
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In the second step, we append TXT records containing the translation *(on the right side of RR)*.
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Then, the response is finished and ``qstate.return_msg`` contains new response.
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If no error, the module sets :attr:`module_qstate.return_rcode` and :attr:`module_qstate.ext_state`.
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**Steps:**
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1. create :class:`DNSMessage` instance
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2. append TXT records containing the translation
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3. set response to ``qstate.return_msg``
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Testing
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-------
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Run the Unbound server:
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``root@localhost>unbound -dv -c ./test-dict.conf``
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In case you use own configuration file, don't forget to enable Python module::
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module-config: "validator python iterator"
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and use valid script path::
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python-script: "./examples/dict.py"
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The translation from english word *"a bar fly"* to Czech can be done by doing:
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``>>>dig TXT @127.0.0.1 a.bar.fly.en._dict_.cz``
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::
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; (1 server found)
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;; global options: printcmd
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;; Got answer:
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;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 48691
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;; flags: aa rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0
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;; QUESTION SECTION:
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;a.bar.fly.en._dict_.cz. IN TXT
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;; ANSWER SECTION:
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a.bar.fly.en._dict_.cz. 300 IN TXT "barov\253 povale\232"
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;; Query time: 5 msec
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;; SERVER: 127.0.0.1#53(127.0.0.1)
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;; WHEN: Mon Jan 01 17:44:18 2009
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;; MSG SIZE rcvd: 67
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``>>>dig TXT @127.0.0.1 nic.cs._dict_.cz``
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::
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; <<>> DiG 9.5.0-P2 <<>> TXT @127.0.0.1 nic.cs._dict_.cz
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; (1 server found)
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;; global options: printcmd
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;; Got answer:
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;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 58710
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;; flags: aa rd ra; QUERY: 1, ANSWER: 6, AUTHORITY: 0, ADDITIONAL: 0
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;; QUESTION SECTION:
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;nic.cs._dict_.cz. IN TXT
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;; ANSWER SECTION:
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nic.cs._dict_.cz. 300 IN TXT "aught"
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nic.cs._dict_.cz. 300 IN TXT "naught"
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nic.cs._dict_.cz. 300 IN TXT "nihil"
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nic.cs._dict_.cz. 300 IN TXT "nix"
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nic.cs._dict_.cz. 300 IN TXT "nothing"
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nic.cs._dict_.cz. 300 IN TXT "zilch"
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;; Query time: 0 msec
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;; SERVER: 127.0.0.1#53(127.0.0.1)
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;; WHEN: Mon Jan 01 17:45:39 2009
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;; MSG SIZE rcvd: 143
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Proof that the unbound still works as resolver.
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``>>>dig A @127.0.0.1 www.nic.cz``
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::
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; (1 server found)
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;; global options: printcmd
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;; Got answer:
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;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 19996
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;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 3, ADDITIONAL: 5
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;; QUESTION SECTION:
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;www.nic.cz. IN A
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;; ANSWER SECTION:
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www.nic.cz. 1662 IN A 217.31.205.50
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;; AUTHORITY SECTION:
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...
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Complete source code
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--------------------
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.. literalinclude:: ../../examples/dict.py
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:language: python
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