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Remove obsolete notes on name compression
These notes describe the initial compression design for BIND 9 in 1998/1999, when the IETF had some over-optimistic plans for using EDNS to change the wire format of domain names. (Another example was bitstring labels for IPv6 reverse DNS.) By the end of 2000 the EDNS name compression schemes had been abandoned, and BIND 9's compression code was rewritten to use a hash table. There is nothing left of the implementation described here, and the API functions are better described in `compress.h`, so these notes are more misleading than helpful. Those who are interested in the past can look at the version control history.
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Tony Finch
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<!--
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Copyright (C) Internet Systems Consortium, Inc. ("ISC")
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SPDX-License-Identifier: MPL-2.0
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This Source Code Form is subject to the terms of the Mozilla Public
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License, v. 2.0. If a copy of the MPL was not distributed with this
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file, you can obtain one at https://mozilla.org/MPL/2.0/.
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See the COPYRIGHT file distributed with this work for additional
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information regarding copyright ownership.
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-->
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Name Compression
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Overview.
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BIND 4.x and BIND 8.x only had one methods of compression to deal
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with 14 bit compression. BIND 9 has 3 methods of compression
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to deal with 14 bit, 16 bit and local compression (14 and 16 bit).
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In addition to this the allowed compression methods vary across
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types and across client revisions thanks to EDNS.
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To be able to compress a domain name you need to some or all of
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the following pieces of information.
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1. where the message starts.
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2. where the current rdata starts in the message (local compression).
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3. what the current owner name is (local compression).
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4. existing global 14 bit compression targets.
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5. existing global 16 bit compression targets.
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6. existing local compression targets.
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7. the current domain name.
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8. what are allowable compression methods, these are not constant
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across a message.
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BIND 4.x and BIND 8.x used a table of existing 14 bit compression
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targets.
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The implicit assumption is that we will use compression whenever
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possible and when ever there are multiple alternatives available
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we will choose the one that minimises the size of the message.
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We will need functions that determine the allowable compression
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methods, find the "best" match among the available compression
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targets, add new compression targets.
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We need to be able to back out any changes made to the compression
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targets if we are unable to add a complete RR (RRset?). This is
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only a problem for the global compression targets.
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Implementation:
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We will maintain two RBT, one for local compression targets and
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one for global compression targets. The data for these RBT will
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be the offset values. The local compression RBT only needs to
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be maintained when local compression is possible. The global
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compression RBT is maintained regardless. Unless there is a
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perfect match (or the name is ".") we will add the name to the
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compression RBTs provide the offset would not be too large for
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the valid compression methods of the RBT. All nodes of the RBT
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will have an offset excluding the root node.
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The local compression RBT will be initialised with the owner name
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and the start of the rdata will be recorded.
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We will use deepest partial match to find the potential
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compression targets.
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We only need to maintain one global RBT as 16 bit compression
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pointers are either valid or invalid for the whole message.
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dns_rdata_towire() will set the allowed methods based on the
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edns version.
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Functions:
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dns_result_t
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dns_compress_init(dns_compress_t *cctx, int edns, isc_mem_t *mctx);
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Initialises cctx to empty and sets whether 16 bit global
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compression targets are to be added to the global RBT based on the
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edns value.
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dns_result_t
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dns_compress_localinit(dns_compress_t *cctx, dns_name_t *owner,
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isc_buffer_t *target);
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Initialise a RBT for local compression, freeing and existing RBT.
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Record current offset.
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dns_compress_invalidate(dns_compress_t *cctx);
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Free any RBT's and make empty.
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dns_compress_localinvalidate(dns_compress_t *cctx);
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Free the local RBT.
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void
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dns_compress_setmethods(dns_compress_t *cctx, unsigned int allowed);
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unsigned int
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dns_compress_getmethods(dns_compress_t *cctx);
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int
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dns_compress_getedns(dns_compress_t *cctx);
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dns_result_t
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dns_name_towire(dns_name_t *name, dns_compress_t *cctx,
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isc_buffer_t *target);
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'name' contains the current name to be added to the message 'target'.
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'target' is assumed to only contain the message.
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'cctx' contains the compression context and has to hold all the
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information required that cannot be obtained from 'name' or 'target'.
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struct dns_compress {
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unsigned int allowed; /* Allowed methods. */
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unsigned int rdata; /* Start of local rdata */
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bool global16; /* 16 bit offsets allowed */
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dns_rbt_t *local; /* Local RBT */
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dns_rbt_t *global; /* Global RBT */
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isc_mem_t *mctx; /* Required by RBT */
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};
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sets allowed based on the value of edns.
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bool
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dns_compress_findglobal(dns_compress_t *cctx, dns_name_t *name,
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dns_name_t *prefix, dns_name_t *suffix,
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uint16_t *offset, isc_buffer_t *workspace);
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bool
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dns_compress_findlocal(dns_compress_t *cctx, dns_name_t *name,
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dns_name_t *prefix, dns_name_t *suffix,
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uint16_t *offset, isc_buffer_t *workspace);
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Find the best best match in the global / local RBT. Returns prefix,
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suffix and offset of the bestmatch. Findglobal(), findlocal()
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requires as workspace as it may be necessary to spit a bit stream
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label. The result prefix will be such that it can be added to the
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wire format followed by a compression pointer pointing to offset.
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Suffix is returned so that it is possible to add the compression
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pointers via dns_compress_add().
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void
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dns_compress_add(dns_compress_t *cctx, dns_name_t *prefix,
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dns_name_t *suffix, uint16_t offset);
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Add compression pointers pointing to lebels (if any) in prefix.
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The offset to the first label is passed in offset.
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Dependency:
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Requires RBT deepest match.
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Requires the ability to walk the RBT and remove any node which
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meets the removal condition.
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@ -1,107 +0,0 @@
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<!--
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Copyright (C) Internet Systems Consortium, Inc. ("ISC")
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SPDX-License-Identifier: MPL-2.0
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This Source Code Form is subject to the terms of the Mozilla Public
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License, v. 2.0. If a copy of the MPL was not distributed with this
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file, you can obtain one at https://mozilla.org/MPL/2.0/.
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See the COPYRIGHT file distributed with this work for additional
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information regarding copyright ownership.
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-->
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Name Decompression
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Overview.
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There are 4 type of compression: global 14 bit, global 16 bit,
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local 14 bit and local 16 bit.
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In general the resolver / nameserver should accept any compression
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method at any time regardless of whether it was legal to
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send it. This fits with the principle of being liberal with
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what you accept and strict with what you send.
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There are a few cases where it does not make sense to accept
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compression pointers of a given type. i.e. the first domain name
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in a message, local compression pointers in the ownername of a RR
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or in a question.
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When performing regression testing however we should be as strict
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as possible. Hence we need to be able modify the behaviour of the
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decompression routines.
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To be able to decompress a domain name we need some or all of the
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following pieces of information.
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1. where the message starts.
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2. where the current rdata starts in the message (local compression).
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3. what the current owner name is (local compression).
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4. where the domainname we are decompressing starts.
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5. what are allowable decompression method. These vary across type
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and edn version.
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Implementation:
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dns_rdata_fromwire will set the allowed decompression methods allowed
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by looking at edns, strict and the type values.
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Types:
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struct dns_decompress {
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unsigned int magic;
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unsigned int allowed;
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int edns;
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dns_name_t owner_name;
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unsigned int rdata;
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bool strict;
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}
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Functions:
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void
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dns_decompress_init(dns_decompress_t *dctx, int edns,
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bool strict);
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initialise dctx
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dctx->ownername is invalidated
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void
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dns_decompress_localinit(dns_decompress_t *dctx, dns_name_t *name,
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isc_buffer_t *source);
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initialise dctx->ownername
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record source->current to dctx->rdata
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void
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dns_decompress_invalidate(dns_decompress_t *dctx);
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invalidate dctx
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void
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dns_decompress_localinvalidate(dns_decompress_t *dctx);
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invalidate dctx->ownername
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void
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dns_decompress_setmethods(dns_decompress_t *dctx, unsigned int allowed);
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sets dctx->allowed
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unsigned int
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dns_decompress_getmethods(dns_decompress_t *dctx);
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returns dctx->allowed
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int
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dns_decompress_edns(dns_decompress_t *dctx);
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returns dctx->edns
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bool
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dns_decompress_strict(dns_decompress_t *dctx);
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returns dctx->strict
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dns_result_t
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dns_name_fromwire(dns_name_t *name, isc_buffer_t *source,
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dns_decompress_t *dctx, bool downcase,
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isc_buffer_t *target)
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