Mini Shell

# Copyright (C) Dnspython Contributors, see LICENSE for text of ISC license

# Copyright (C) 2001-2017 Nominum, Inc.
#
# Permission to use, copy, modify, and distribute this software and its
# documentation for any purpose with or without fee is hereby granted,
# provided that the above copyright notice and this permission notice
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# WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
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"""Help for building DNS wire format messages"""

import contextlib
import io
import struct
import random
import time

import dns.exception
import dns.tsig


QUESTION = 0
ANSWER = 1
AUTHORITY = 2
ADDITIONAL = 3


class Renderer:
    """Helper class for building DNS wire-format messages.

    Most applications can use the higher-level L{dns.message.Message}
    class and its to_wire() method to generate wire-format messages.
    This class is for those applications which need finer control
    over the generation of messages.

    Typical use::

        r = dns.renderer.Renderer(id=1, flags=0x80, max_size=512)
        r.add_question(qname, qtype, qclass)
        r.add_rrset(dns.renderer.ANSWER, rrset_1)
        r.add_rrset(dns.renderer.ANSWER, rrset_2)
        r.add_rrset(dns.renderer.AUTHORITY, ns_rrset)
        r.add_edns(0, 0, 4096)
        r.add_rrset(dns.renderer.ADDTIONAL, ad_rrset_1)
        r.add_rrset(dns.renderer.ADDTIONAL, ad_rrset_2)
        r.write_header()
        r.add_tsig(keyname, secret, 300, 1, 0, '', request_mac)
        wire = r.get_wire()

    output, an io.BytesIO, where rendering is written

    id: the message id

    flags: the message flags

    max_size: the maximum size of the message

    origin: the origin to use when rendering relative names

    compress: the compression table

    section: an int, the section currently being rendered

    counts: list of the number of RRs in each section

    mac: the MAC of the rendered message (if TSIG was used)
    """

    def __init__(self, id=None, flags=0, max_size=65535, origin=None):
        """Initialize a new renderer."""

        self.output = io.BytesIO()
        if id is None:
            self.id = random.randint(0, 65535)
        else:
            self.id = id
        self.flags = flags
        self.max_size = max_size
        self.origin = origin
        self.compress = {}
        self.section = QUESTION
        self.counts = [0, 0, 0, 0]
        self.output.write(b'\x00' * 12)
        self.mac = ''

    def _rollback(self, where):
        """Truncate the output buffer at offset *where*, and remove any
        compression table entries that pointed beyond the truncation
        point.
        """

        self.output.seek(where)
        self.output.truncate()
        keys_to_delete = []
        for k, v in self.compress.items():
            if v >= where:
                keys_to_delete.append(k)
        for k in keys_to_delete:
            del self.compress[k]

    def _set_section(self, section):
        """Set the renderer's current section.

        Sections must be rendered order: QUESTION, ANSWER, AUTHORITY,
        ADDITIONAL.  Sections may be empty.

        Raises dns.exception.FormError if an attempt was made to set
        a section value less than the current section.
        """

        if self.section != section:
            if self.section > section:
                raise dns.exception.FormError
            self.section = section

    @contextlib.contextmanager
    def _track_size(self):
        start = self.output.tell()
        yield start
        if self.output.tell() > self.max_size:
            self._rollback(start)
            raise dns.exception.TooBig

    def add_question(self, qname, rdtype, rdclass=dns.rdataclass.IN):
        """Add a question to the message."""

        self._set_section(QUESTION)
        with self._track_size():
            qname.to_wire(self.output, self.compress, self.origin)
            self.output.write(struct.pack("!HH", rdtype, rdclass))
        self.counts[QUESTION] += 1

    def add_rrset(self, section, rrset, **kw):
        """Add the rrset to the specified section.

        Any keyword arguments are passed on to the rdataset's to_wire()
        routine.
        """

        self._set_section(section)
        with self._track_size():
            n = rrset.to_wire(self.output, self.compress, self.origin, **kw)
        self.counts[section] += n

    def add_rdataset(self, section, name, rdataset, **kw):
        """Add the rdataset to the specified section, using the specified
        name as the owner name.

        Any keyword arguments are passed on to the rdataset's to_wire()
        routine.
        """

        self._set_section(section)
        with self._track_size():
            n = rdataset.to_wire(name, self.output, self.compress, self.origin,
                                 **kw)
        self.counts[section] += n

    def add_edns(self, edns, ednsflags, payload, options=None):
        """Add an EDNS OPT record to the message."""

        # make sure the EDNS version in ednsflags agrees with edns
        ednsflags &= 0xFF00FFFF
        ednsflags |= (edns << 16)
        opt = dns.message.Message._make_opt(ednsflags, payload, options)
        self.add_rrset(ADDITIONAL, opt)

    def add_tsig(self, keyname, secret, fudge, id, tsig_error, other_data,
                 request_mac, algorithm=dns.tsig.default_algorithm):
        """Add a TSIG signature to the message."""

        s = self.output.getvalue()

        if isinstance(secret, dns.tsig.Key):
            key = secret
        else:
            key = dns.tsig.Key(keyname, secret, algorithm)
        tsig = dns.message.Message._make_tsig(keyname, algorithm, 0, fudge,
                                              b'', id, tsig_error, other_data)
        (tsig, _) = dns.tsig.sign(s, key, tsig[0], int(time.time()),
                                  request_mac)
        self._write_tsig(tsig, keyname)

    def add_multi_tsig(self, ctx, keyname, secret, fudge, id, tsig_error,
                       other_data, request_mac,
                       algorithm=dns.tsig.default_algorithm):
        """Add a TSIG signature to the message. Unlike add_tsig(), this can be
        used for a series of consecutive DNS envelopes, e.g. for a zone
        transfer over TCP [RFC2845, 4.4].

        For the first message in the sequence, give ctx=None. For each
        subsequent message, give the ctx that was returned from the
        add_multi_tsig() call for the previous message."""

        s = self.output.getvalue()

        if isinstance(secret, dns.tsig.Key):
            key = secret
        else:
            key = dns.tsig.Key(keyname, secret, algorithm)
        tsig = dns.message.Message._make_tsig(keyname, algorithm, 0, fudge,
                                              b'', id, tsig_error, other_data)
        (tsig, ctx) = dns.tsig.sign(s, key, tsig[0], int(time.time()),
                                    request_mac, ctx, True)
        self._write_tsig(tsig, keyname)
        return ctx

    def _write_tsig(self, tsig, keyname):
        self._set_section(ADDITIONAL)
        with self._track_size():
            keyname.to_wire(self.output, self.compress, self.origin)
            self.output.write(struct.pack('!HHIH', dns.rdatatype.TSIG,
                                          dns.rdataclass.ANY, 0, 0))
            rdata_start = self.output.tell()
            tsig.to_wire(self.output)

        after = self.output.tell()
        self.output.seek(rdata_start - 2)
        self.output.write(struct.pack('!H', after - rdata_start))
        self.counts[ADDITIONAL] += 1
        self.output.seek(10)
        self.output.write(struct.pack('!H', self.counts[ADDITIONAL]))
        self.output.seek(0, io.SEEK_END)

    def write_header(self):
        """Write the DNS message header.

        Writing the DNS message header is done after all sections
        have been rendered, but before the optional TSIG signature
        is added.
        """

        self.output.seek(0)
        self.output.write(struct.pack('!HHHHHH', self.id, self.flags,
                                      self.counts[0], self.counts[1],
                                      self.counts[2], self.counts[3]))
        self.output.seek(0, io.SEEK_END)

    def get_wire(self):
        """Return the wire format message."""

        return self.output.getvalue()