Every line of code committed to BIND has been reviewed by ISC engineers first.
The code review process is a dialog between the original author and the reviewer. Code inspection, including documentation and tests, is part of this. Compiling and running the resulting code should be done in most cases, even for trivial changes, to ensure that it works as intended. In particular, all checks in the CI pipeline must pass run for every modification so that unexpected side-effects are identified.
When a problem or concern is found by the reviewer, these comments are placed on the merge request in GitLab so the author can respond.
First, consideration is given to whether contributed code would be useful to a significant user base (we can't take on the additional maintenance and support burden for changes that would only be useful to a tiny niche). Second, whether the approach taken is consistent with ISC's open-internet goals, BIND architecture, and DNS best practices. Third, the contribution is checked for correctness and completeness.
Obvious bottlenecks and places where performance or reliability may suffer are noted as part of the review.
New functions must be adequately commented. Public API functions are documented in the corresponding header file, static functions in the C file, above the function header. Particularly complex code should be commented throughout the function body as well.
A patch is much more likely to be accepted quickly if it includes tests providing good coverage of the new code. Tests for bugfix code should fail when run against the unmodified code; tests for new feature code should have good code coverage and address corner cases and error cases. Newly added API functions should have unit tests if possible. (See testing.)
Documentation is also reviewed. This includes all user-facing text, including log messages, manual pages, user manuals and sometimes even comments; they must be clearly written and consistent with existing style.
Every change is ultimately submitted as a GitLab merge request (MR) and reviewed there. The specifics of the workflow are documented in BIND development workflow. Take note of the section about MR title and description, which are used to generate changelog entries and release notes. These are also subject to the review process.
- Read the diff
- Read accompanying notes in the ticket
- Read the documentation, if any
- Read the tests
- Ensure the CI passes
(In some cases it may be appropriate to run tests against code from before the change to ensure that they fail as expected.) - Review the MR description and title (refer to GitLab development workflow)
- General correctness of approach
- Style errors
- Simple coding errors
- Files inadvertently omitted
- Unnecessarily complex code
- Complex code with insufficient comments
- Lack of boundary checking
- Memory and resource leaks (deallocations must match allocations)
- Places that need
REQUIRE
orINSIST
- Thread safety
- Bad function names/variable names
- Overly long functions
- Copies of code that could be unified in a helper function
- Premature optimizations
- Compiler warnings introduced
- Portability issues, such as the use of non-POSIX library calls or options
- DNS/protocol problems
- Cut/pasted code that may have been modified in one place but needs to be modified in other places as well
- No tests or inadequate tests
- Testability problems
- No documentation or inadequate documentation
- Grammar, spelling and clarity problems in documentation
- Usability problems
When a patch is contributed which is a good idea but doesn't meet our code quality requirements, we will often keep the ticket open so that we can address the issue ourselves later.
Sometimes contributed code is fine, but ISC staff still have to add documentation and/or tests -- that's okay, but it may take a long time to get to the top of our priority list. Ensuring that your patch includes tests and documentation will reduce delay.
When you submit a merge request, it triggers a CI pipeline which executes unit and system tests on various platforms. You should pay attention to any failures, as some can only occur in specific environments. Getting the CI to pass is a good start when preparing the merge request for the review.
If you want to run the system tests locally, please refer to BIND9 System Test Framework for information about running and writing system tests.
BIND uses the cmocka unit testing framework.
To build BIND with unit tests, run configure
with the --with-cmocka
option. This requires cmocka >= 1.0.0 to be installed in the system.
Unit tests are stored in /tests
subdirectories under the libraries
they test. For example, the unit tests for libisc are in lib/isc/tests
.
Particular test sets are called {module}_test.c
, where {module} is
usually the name of the module being tested; rbt_test.c
tests functions
in rbt.c
. (There are exceptions to this rule, though; for instance,
hash_test.c
tests hash functions that are implemented in several
different files in lib/isc
.)
When BIND is built with unit tests, they will be run as part of
make
check
. But if you want to run only the unit tests:
$ make unit
You can also run the unit tests for only one library:
$ cd lib/isc/tests (or lib/dns/tests)
$ make unit
Or run a particular test case (in the following example, the isc_sha512 test case in the hash unit test). This has the advantage that you can see whatever output the unit test emits, whereas in the other modes, output is redirected:
$ cd lib/isc/tests
$ ./hash_test isc_sha512
Information on writing cmocka tests can be found at the cmocka website.
New unit tests should be added whenever new API functionality is added to the libraries.
bind9/bin
: binariesbind9/bin/named
: source code for thenamed
binary; includes server configuration, interface manager, client manager, and high-level processing logic for query, update, and xfer.bind9/bin/dnssec
: DNSSEC-related tools written in C:dnssec-keygen
,dnssec-signzone
,dnssec-settime
,dnssec-revoke
,dnssec-keyfromlabel
,dnssec-dsfromkey
,dnssec-verify
(BIND 9.9+)bind9/bin/rndc
:rndc
binarybind9/bin/dig
:dig
,host
, andnslookup
bind9/bin/delv
:delv
bind9/bin/check
:named-checkconf
andnamed-checkzone
bind9/bin/confgen
:rndc-confgen
,ddns-confgen
, andtsig-keygen
(BIND 9.9+)bind9/bin/tools
: assorted useful tools:named-journalprint
,nsec3hash
, etc
bind9/lib
: librariesbind9/lib/isc
: implements basic functionality such as threads, tasks, timers, sockets, memory manager, buffers, and basic data types.bind9/lib/isc/tests
: unit tests for libisc
bind9/lib/dns
: implements higher-level DNS functionality: red-black trees, rdatasets, views, zones, ACLs, resolver, validator, etcbind9/lib/dns/tests
: unit tests for libdns
bind9/lib/isccfg
: library implementing thenamed.conf
configuration parser and checker.bind9/lib/isccc
: library implementing the control channel used byrndc
See the namespace discussion in the BIND coding style document.
BIND uses the "Design by Contract" pattern for most function calls.
A quick summary of the idea is that a function and its caller make a contract. If the caller meets certain preconditions, then the function promises to either fulfill its contract (i.e. guarantee a set of postconditions), or to clearly fail.
"Clearly fail" means that if the function cannot succeed, then it will not silently fail and return a value which the caller might interpret as success.
If a caller doesn't meet the preconditions, then "further execution is undefined". The function can crash, compute a garbage result, fail silently, etc. Allowing the function to define preconditions greatly simplifies many APIs, because the API need not specify a way of saying "hey caller, the values you passed in are garbage".
Typically, preconditions are specified in the functions .h file, and
encoded in its body with REQUIRE
statements. The REQUIRE
statements
cause the program to dump core if they are not true, and can be used to
identify callers that are not meeting their preconditions.
Postconditions can be encoded with ENSURE
statements. Within the body of
a function, INSIST
is used to assert that a particular expression must be
true.
Assertions must not have side effects that the function relies upon,
because assertion checking may be turned off in some environments.
(This is not recommended, however: assertion failures serve the
useful function of ensuring that named
does not continue running
in an insane state. The surfeit of assertions in BIND 9 have made
it vulnerable over the years to "packets of death" and other
denial-of-service exploits, but as of this writing - more than 14
years since the initial release - BIND 9 has never had an arbitrary
code execution vulnerability.)
A number of data structures in the ISC and DNS libraries have an
unsigned int magic
value as the first field. The purpose of the
magic number is principally to validate that a pointer that's been
passed to a subroutine really points to the type it claims to be. This
helps detect problems caused by resources being freed prematurely, that
have been corrupted, or that have not been properly initialized. It can
also be handy in debugging.
Magic numbers should always be the first field in a structure. They never require locking to access. As to the actual value to be used, something mnemonic is good:
#define TASK_MAGIC 0x5441534BU /* TASK. */
#define VALID_TASK(t) ((t) != NULL && \
(t)->magic == TASK_MAGIC)
#define TASK_MANAGER_MAGIC 0x54534B4DU /* TSKM. */
#define VALID_MANAGER(m) ((m) != NULL && \
(m)->magic ==
TASK_MANAGER_MAGIC)
Unless the memory cost is critical, most objects should have a magic number.
The magic number should be the last field set in a creation routine, so that an object will never be stamped with a magic number until it is valid.
The magic number should be set to zero immediately before the object is freed.
Magic values are usually private to the implementation of the type; i.e. they are defined in the .c file, not the .h file. There are some exceptions to this.
Validation of magic numbers is done by routines that manipulate the type, not by users of the type. (Indeed, user validation is usually not possible because the magic number is not public.)
The isc_result_t
type is provided for function result codes,
and is used throughout BIND. For example:
isc_result_t result;
FILE *fp = NULL;
result = isc_stdio_open("file", "r", &fp);
Note that an explicit result code is used, instead of mixing the error
result type with the normal result type. In contrast to the
C library routine fopen()
which returns a file pointer or NULL
on failure (setting errno
to indicate what the nature of the problem
was), BIND style always keeps indication of the function's success or
failure separate from its returned data. Similarly, the C library
function fread()
returns the number of characters read and then
depends on feof()
and ferror()
to determine whether an error occurred
or the end of file was reached, but BIND's version uses result codes:
char buffer[BUFSIZ];
size_t n;
result = isc_stdio_read(buffer, 1, sizeof(buffer), fp, &n);
if (result == ISC_R_SUCCESS) {
/* Do something with 'buffer'. */
} else if (result == ISC_R_EOF) {
/* EOF. */
result = ISC_R_SUCCESS;
} else {
/* Some other error occurred. */
}
Only functions which cannot fail (assuming the caller has provided valid
arguments) should return data directly instead of a result code. For
example, dns_name_issubdomain()
returns an bool
, because it
has no failure mode.
A result code can be converted to a human-readable error message by
calling isc_result_totext(result)
.
Many result codes have been defined and can be found in the source tree
in lib/isc/include/isc/result.h
.
ISC library result codes (many of which are generically useful elsewhere)
begin with ISC_R
: examples include ISC_R_SUCCESS
, ISC_R_FAILURE
,
ISC_R_NOMEMORY
, etc.
DNS library result codes begin with DNS_R
: DNS_R_SERVFAIL
, DNS_R_NXRRSET
,
etc). Other sets of result codes are defined for crypto functions (DST_R
and PKCS_R
).
For portability, ISC result codes are used instead of codes provided
by the operating system; for example, ISC_R_NOMEMORY
instead of
ENOMEM
. In some cases, but not all, POSIX-defined error codes can be
converted to an ISC result code by calling isc__errno2result(errno)
.
This can't be relied on; there are too many OS-specific error codes to
provide meaningful translations for all of them. Unknown errno
values
are converted to ISC_R_UNEXPECTED
.
A useful set of functions is provided for manipulating memory
buffers: the isc_buffer
API. Buffers can be used for parsing
and constructing messages in both text and binary formats.
A buffer is associated with a region of memory, which is subdivided into 'used' and 'available'. The 'used' subregion is further subdivided into 'consumed' and 'remaining'.
When parsing a message, the message to be parsed in in the 'used' part of the buffer. As the message is parsed, the 'consumed' subregion grows and the 'remaining' subregion shrinks.
When creating a message, data is written into the 'available' subregion, which then becomes part of 'used'.
The current sizes of these subregions can be determined by calling
isc_buffer_usedlength()
, isc_buffer_consumedlength()
,
isc_buffer_remaininglength()
, and isc_buffer_availablelength()
.
The memory associated with a buffer may be dynamically allocated
from a memory context using isc_buffer_allocate()
and freed by
isc_buffer_free()
, or it may be a static region of memory
with which we want to use buffer semantics. In that case, we
associate a new buffer object with the desired block of memory
by running isc_buffer_init()
. If the intention is to write
to the memory, nothing further is necessary; if it is to read
the memory using buffer sementaics, then we must mark the memory
as part of the 'used' subregion:
isc_buffer_t b;
char text[BUFSIZ];
unsigned int n;
result = isc_stdio_read(buf, 1, BUFSIZ, fp, &n);
if (result == ISC_R_SUCCESS && n > 0U) {
isc_buffer_init(&b, text, sizeof(text));
isc_buffer_add(&b, n);
/* now we can read the buffer */
}
Several functions are provided for both reading and writing to the buffer:
-
isc_buffer_getuint8()
: Read and return an 8-bit unsigned integer -
isc_buffer_putuint8()
: Write an 8-bit unsigned integer to a buffer -
isc_buffer_getuint16()
: Read a 16-bit unsigned integer in network byte order, convert to host byte order, and return it -
isc_buffer_putuint16()
: Convert an unsigned 16-bit integer from host to network byte order and write it to a buffer. -
isc_buffer_getuint32()
: Read a 32-bit unsigned integer in network byte order, convert to host byte order, and return it -
isc_buffer_putuint32()
: Convert an unsigned 32-bit integer from host to network byte order and write it to a buffer. -
isc_buffer_putstr()
: Copy a null-terminated string into a buffer -
isc_buffer_putmem()
: Copy a fixed-length region of memory into a buffer.
A simpler set of functions have also been provided for handling
memory regions: the isc_region
API. A region is a simple structure
that only contains a base pointer (to the beginning of the associated
memory) and a length. Buffers and buffer subregions can be converted to
regions using isc_buffer_region()
, isc_buffer_usedregion()
, etc.
Regions can be copied to buffers by using isc_buffer_copyregion()
,
or simply by running isc_buffer_init()
on the region's base pointer.
BIND tracks its memory usage internally via "memory contexts". Multiple
separate memory contexts can be created for the use of different modules or
subcomponents, and each can have its own size limits and tuning parameters
and maintain its own statistics, allocations and free lists. Memory
allocation is based on the jemalloc
library on platforms where the library
is available.
The memory system helps with diagnosis of common coding errors such as memory leaks and use after free. Newly allocated memory is populated with the repeating value 0xbe, and freed memory with 0xde. BIND tracks every memory allocation, and will complain (via an assertion failure) if any memory has not been freed when BIND shuts down.
To create a basic memory context, use:
isc_mem_t *mctx = NULL;
isc_mem_create(&mctx);
When holding a persistent reference to a memory context it is advisable to
increment its reference counter using isc_mem_attach()
. Do not just
copy an mctx
pointer; this may lead to a shutdown race in which the
memory context is freed before all references have been cleaned up.
/*
* Function to create an 'isc_foo' object.
*/
isc_result_t
isc_foo_create(isc_mem_t *mctx, isc_foo_t **foop) {
isc_foo_t *foo;
REQUIRE(mctx != NULL);
REQUIRE(foop != NULL && *foop == NULL);
foo = isc_mem_get(mctx, sizeof(isc_foo_t))
/* Attach to memory context */
isc_mem_attach(mctx, &foo->mctx);
/* Populate other isc_foo members here */
foo->magic = ISC_FOO_MAGIC;
*foop = foo;
return (ISC_R_SUCCESS);
}
When finished with a memory context, detach it with isc_mem_detach()
.
If freeing an object that contains a reference to a memory context,
you free it and detach its reference at the same time using
isc_mem_putanddetach()
.
void
isc_foo_destroy(isc_foo_t **foop) {
isc_foo_t *foo = *foop;
/* clean up various isc_foo members */
foo->magic = 0;
isc_mem_putanddetach(&foo->mctx, foo, sizeof(isc_foo_t));
*foop = NULL;
}
Two sets of allocation and deallocation functions are provided:
isc_mem_get()
and isc_mem_put()
; and isc_mem_allocate()
and
isc_mem_free()
.
The call to isc_mem_put()
must specify the number of bytes being freed,
so use isc_mem_get()
when the caller can easily keep track of the size of
the allocation.
A call to isc_mem_free()
does not need to specify the size of the
allocation, it simply frees whatever was allocated at that address, so use
isc_mem_allocate()
when use variable size blocks of memory.
The function isc_mem_strdup()
-- a version of strdup()
that uses memory
contexts -- will also return memory that can be freed with
isc_mem_free()
.
In cases where small fixed-size blocks of memory may be needed frequently,
the isc_mempool
API can be used. This creates a standing pool of blocks
of a specified size which can be passed out and returned without the need
for a new memory allocation; this can improve performance in tight inner
loops.
None of these allocation functions, including isc_mempool_get()
, can
fail. If no memory is available for allocation, the program will abort.
The memory context can be set to check if all memory allocated via the said
memory context was freed before the memory context was destroyed by calling
isc_mem_checkdestroyed()
. This could lead to false positives on abnormal
shutdowns, so the checking is only enabled in dig
and named
applications on
normal shutdown.
The memory context are normally used only for internal allocations, but several
external libraries allow replacing their allocators (namely libxml2, libuv and
OpenSSL). As there has been known memory leak in the OpenSSL when
engine_pkcs11
is loaded, memory checking at destroy is disabled by default in
the memory contexts used for external libraries and it needs to be enabled with
a --enable-leak-detection
autoconf option.
A set of macros are provided for creating, modifying and iterating
doubly-linked lists. These are defined in <isc/list.h>
.
To create a structure that will be part of a linked list, specify
an ISC_LINK
as one of its members:
typedef struct isc_foo isc_foo_t;
struct isc_foo {
unsigned int magic;
/* other contents */
ISC_LINK(isc_foo_t) link;
};
(Note the typedef
of isc_foo_t
prior to the structure declaration.)
When creating an instance of this structure, initialize the link:
isc_result_t
isc_foo_create(isc_mem_t mctx, isc_foo_t **foop) {
isc_foo_t *foo;
REQUIRE(foop != NULL && *foop == NULL);
foo = isc_mem_get(mctx, sizeof(isc_foo_t));
ISC_LINK_INIT(foo, link);
/* initialize other members */
foo->magic = ISC_FOO_MAGIC;
*foop = foo;
return (ISC_R_SUCCESS);
}
To make a list of these elements, first create a list variable
by declaring it using the ISC_LIST
macro, then initialize it
with ISC_LIST_INIT
:
ISC_LIST(isc_foo_t) foolist;
ISC_LIST_INIT(foolist);
The list can then be modified:
ISC_LIST_APPEND(foolist, foo1, link);
Several macros are provided for this purpose, including ISC_LIST_PREPEND
,
ISC_LIST_INSERTBEFORE
, and ISC_LIST_INSERTAFTER
.
More macros are provided for iterating the list:
isc_foo_t *foo;
for (foo = ISC_LIST_HEAD(foolist);
foo != NULL;
foo = ISC_LIST_NEXT(foo, link))
{
/* do things */
}
There are also ISC_LIST_TAIL
and ISC_LIST_PREV
macros for walking the
list in reverse order.
Items can be removed from the list using ISC_LIST_UNLINK
:
ISC_LIST_UNLINK(foolist, foo, link);
The dns_name
API has facilities for processing DNS names and labels,
both dynamically and statically allocated, relative and absolute,
compressed and not, with straightforward conversions from text to
wire format and vice versa.
When a name object is initialized, a pointer to an "offset table"
(dns_offsets_t
) may optionally be supplied; this will improve
performance of most name operations if the name is used more than
once.
dns_name_t name1, name2;
dns_offsets_t offsets1;
dns_name_init(&name1, &offsets1);
dns_name_init(&name2, NULL);
There are three methods for copying name objects:
-
dns_name_clone()
makes a target refer to the same data as the source, but does not copy the data. The source must not be changed or freed while the target is still in use. -
dns_name_copy()
copies the source data from one name object into another, which must already have a dedicated buffer associated with it to receive the data. The target name can have a buffer assigned to it usingdns_name_setbuffer()
:dns_name_t target; unsigned char namedata[DNS_NAME_MAXWIRE]; isc_buffer_t buffer; isc_buffer_init(&buffer, namedata, sizeof(namedata)); dns_name_init(&target, NULL); dns_name_setbuffer(target, &buffer); dns_name_copy(source, &target);
Using a fixed name (see below) for the target ensures that it has sufficient buffer space without needing to set a buffer.
-
dns_name_dup()
copies a name into a new name object, dynamically allocating buffer space as needed.dns_name_dupwithoffsets()
does the same, but also dynamically allocates space for the copied offset table. Targets created by these functions must be freed by callingdns_name_free()
.
To create a name object from a wire format message such as a DNS
query or response, use dns_name_fromwire()
. Generally this is
done with names in a DNS message object (dns_message_t
), and some
names may be compressed; the ongoing decompression state for a message
is maintained in a "decompression context" object (dns_decompress_t
)
which must be initialized before the first call to dns_name_fromwire()
for a given message, and passed to each additional call until all
the names have been extracted.
Similarly, dns_name_towire()
converts name objects into DNS wire
format, using an ongoing "compression context" object (dns_compress_t
).
Converting text representations of names to name objects is
usually done by calling dns_name_fromtext()
, which converts a name
found in a source buffer object
When using dns_name_fromtext()
, the target name must have a buffer
associated with it, or else a buffer must be passed in separately which
will be used to store name data. An origin
parameter indicates a zone origin
name, which is appended to the converted name; for absolute names, the root
zone name, dns_rootname
, should be used as origin. If the
DNS_NAME_DOWNCASE
flag is set in the options
parameter, then the target
name will be converted to lower case, regardless of the case of the source
name.
char *text = "foo.com";
unsigned char namedata[DNS_NAME_MAXWIRE];
isc_buffer_t buf;
dns_name_t name;
dns_name_init(&name, NULL);
isc_buffer_init(&buf, namedata, sizeof(namedata));
isc_buffer_add(&buf, strlen(text));
result = dns_name_fromtext(&name, &buf, dns_rootname, 0, NULL);
if (result != ISC_R_SUCCESS) {
/* something went wrong */
}
An alternate mechanism dns_name_fromstring()
converts a standard
null-terminated string to a name object. When using this function,
if the target name has a buffer associated with it, then that buffer
is used for the resulting name data; otherwise, memory is allocated for
the purpose and the name will need to be freed with dns_name_free()
later.
There are also multiple functions for converting name objects to text.
dns_name_tostring()
writes the name into a buffer object, which must
have at least DNS_NAME_MAXTEXT
bytes of available space.
dns_name_format()
writes the name into a null-terminated
string, which must have space for at least DNS_NAME_FORMATSIZE
bytes. dns_name_tostring()
allocates memory for the text, which
must later be freed with isc_mem_free()
.
Several functions are provided for inspecting and modifying name objects. These include:
dns_name_countlabels()
returns the number of labels in a name.dns_name_getlabel()
locates a specified label in a name and references it in a region object. In the name "www.example.com", label 0 is "www", label 1 is "example", label 2 is "com", and label 3 is the root zone.dns_name_getlabelsequence
copies a specified label and a specified number of labels after it into a new name object.dns_name_split()
separates a name into prefix and a suffix on a specified label boundary. For example, "www.example.com" can be split into "www" and "example.com".dns_name_concatenate()
concatenates a prefix and a suffix into a single name.
DNS name comparisons are more complex than simple string comparisons. When
sorting names, labels at the end of the name are more significant than
labels at the beginning ("zzz.com" is less than "aaa.zzz.com").
Furthermore, it's necessary to determine relationships between names other
than simple ordering: Whether one name is the ancestor of another, or
whether they share a common ancestor, and if so how many labels they
have in common. The dns_name_fullcompare()
function determines these
things. Its return value is the relationship between two names:
dns_namereln_t rel;
unsigned int common;
int order;
/*
* Get relationship between two names; store the sort
* order in 'order' and the number of common labels in
* 'common'
*/
rel = dns_name_fullcompare(name1, name2, &order, &common);
The return value may be:
dns_namereln_contains
: name1 contains name2dns_namereln_subdomain
: name2 contains name1dns_name_commonancestor
: name1 and name2 share some labelsdns_name_equal
: name1 and name2 are the same
Some simpler comparison functions are provided for convenience when not all of this information is required:
dns_name_compare()
: returns the sort order of two names but not their relationshipdns_name_equal()
: returnstrue
when names are equivalentdns_name_caseequal()
: same asdns_name_equal()
, but case-sensitivedns_name_issubdomain()
: returnstrue
if one name contains another
dns_fixedname_t
is a convenience type containing a name, an offsets
table, and a dedicated buffer big enough for the longest possible DNS
name. This allows names to be stack-allocated with minimal initialization:
dns_fixedname_t fn;
dns_name_t *name;
name = dns_fixedname_initname(&fn);
name
is now a pointer to a dns_name
object in which a name can be
stored for the duration of this function; there is no need to initialize,
allocate, or free memory.
An rdataset (dns_rdataset_t
) is BIND's representation of a DNS RRset,
excluding the owner name but including the type, TTL, and the contents of
each RR. The rdataset object does not hold the data itself: it is a view
that refers to data held elsewhere -- for example, in a DNS message, or in
an rbtdb (for cached or authoritative data).
It is a vaguely object-oriented polymorphic data structure, with different implementations depending on the backing data structure that actually holds the records. The rdataset is explicitly associated/disassociated with the backing data structure so that it can maintain reference counts.
One important rdataset implementation is part of the red-black tree
database, implemented in rdata.c
.
Another backing data structure for an rdataset is the rdatalist
(dns_rdatalist_t
) -- a linked list of rdata structures. An rdatalist is
used to record the locations of records in a DNS message. It does not
maintain reference counts. An rdatalist can be converted to or from an
rdataset using dns_rdatalist_tordataset()
and
dns_rdatalist_fromrdataset()
.
See the RRATA Types document for details on type-specific rdata conversions.
Retrieving data from BIND databases involves the use of iterator functions to walk from entry to entry. Several iterator function sets have been defined:
dns_dbiterator
: Walks the nodes in a databasedns_rdatasetiter
: Walks the RRsets in a nodedns_rdataset
: Walks the resource records in an RRsetdns_rriterator
: A combination of the previous three; walks all the RRs or RRsets in a databasedns_rbtnodechain
: Walks the nodes in a red-black tree
Each of these has a first()
, next()
and current()
function; for
example, dns_rdataset_first()
, dns_rdataset_next()
, and
dns_rdataset_current()
.
The first()
and next()
functions move the iterator's cursor and so that
the data at a new location can be retrieved. (Most of these can only step
by one item at a time, but dns_rriterator
provides both next()
and
nextrrset()
, enabling it to step by RR or RRset.) These functions return
isc_result_t
, with ISC_R_SUCCESS
indicating that there is data to
retrieve and ISC_R_NOMORE
indicating that the iterator is finished.
The current()
function has no return value; it simply retrieves the
data at the current cursor location.
To use an iterator, call the first()
function, then the current()
function, then loop over the next()
function until it no longer returns
success:
for (result = dns_rdataset_first(rdataset);
result == ISC_R_SUCCESS;
result = dns_rdataset_next(rdataset))
{
dns_rdata_t rdata = DNS_RDATA_INIT;
dns_rdataset_current(rdataset, &rdata);
/* rdata is now populated with an RR */
}
In some cases, calling an iterator function causes the acquisition of
database and/or node locks. Rather than reacquire these locks every time
one of these functions is called, they are often simply held until the
iterator is destroyed. If a caller wishes to hold an iterator open but not
use it for a while, it should call the iterator's pause()
function (such
as dns_dbiterator_pause()
); this will release all the locks that are
currently held by the iterator so that other threads may proceed.
The ISC logging system is designed to provide a flexible, extensible method of writing messages, either to the system's logging facility, directly to a file, or into the bitbucket -- usually configured per the desires of the user of the program.
Each log message is associated with a particular category (eg, "security" or "database") that reflects its nature, and a particular module (such as the library's source file) that reflects its origin. Messages are also assigned a priority level which states how remarkable the message is; the program's user may use this to decide how much detail is desired.
Libraries which use the ISC logging system can be linked against each other without fear of conflict. A program is able to select which, if any, libraries will write log messages.
Log messages are associated with three pieces of information that are used to determine their disposition: a category, a module, and a level (aka "priority").
A category describes the conceptual nature of the message, that is,
what general aspect of the code it is concerned with. For example,
the DNS library defines categories that include the workings of the
database as well security issues. Macros for naming categories are
typically provided in the library's log header file, such as
DNS_LOGCATEGORY_DATABASE
and DNS_LOGCATEGORY_SECURITY
in <dns/log.h>
.
The special category ISC_LOGCATEGORY_DEFAULT
is associated with
any message that does not match a particular category (or matches a
category but not a module, as seen in the next paragraph).
A module is loosely the origin of a message. There may not be a
one-to-one correspondence of source files with modules, but it is typical
that a module's name reflect the source file in which it is used. So, for
example, the module identifier DNS_LOGMODULE_RBT
would be used by
messages coming from within the lib/dns/rbt.c
source file.
The specification of the combination of a category and a module for a message are called the message's "category/module pair".
The level of a message is an indication of its severity. There are six standard logging levels, in order here from most to least severe (least to most common):
ISC_LOG_CRITICAL
: An error so severe it causes the program to exit.ISC_LOG_ERROR
: A very notable error, but the program can go on.ISC_LOG_WARNING
: Something is probably not as it should be.ISC_LOG_NOTICE
: Notable events that occur while the program runs.ISC_LOG_INFO
: Statistics and routine announcements.ISC_LOG_DEBUG(unsigned int level)
: Detailed debugging messages.
ISC_LOG_DEBUG
is not quite like the others in that it takes an
argument the defines roughly how detailed the message is; a higher
level means more copious detail, so that values near 0 would be used
at places like the entry to major sections of code, while greater
numbers would be used inside loops.
The next building block of the logging system is a channel. A channel specifies where a message of a particular priority level should go, as well as any special options for that destination. There are four basic destinations, as follows:
ISC_LOG_TOSYSLOG
: Send it to syslog.ISC_LOG_TOFILE
: Write to a file.ISC_LOG_TOFILEDESC
: Write to a (previously opened) file descriptor.ISC_LOG_TONULL
: Do not write the message when selected.
A file destination names a path to a log file. It also specifies the
maximum allowable byte size of the file before it is closed (where 0
means no limit) and the number of versions of a file to keep (where
ISC_LOG_ROLLNEVER
means the logging system never renames the log file,
and ISC_LOG_ROLLINFINITE
means no cap, other than integer size, on the
number of versions). Version control is done just before a file is opened,
so a program that used it would start with a fresh log file (unless using
ISC_LOG_ROLLNEVER
) each time it ran. If you want to use an external
rolling method, use ISC_LOG_ROLLNEVER
and ensure that your program has a
mechanism for calling isc_log_closefilelogs()
.
A file descriptor destination is simply associated with a previously opened stdio file descriptor. This is mostly used for associating stdout or stderr with log messages, but could also be used, for example, to send logging messages down a pipe that has been opened by the program. File descriptor destinations are never closed, have no maximum size limit, and do not do version control.
Syslog destinations are associated with the standard syslog facilities
available on your system: generally syslogd
on UNIX and Linux
systems. They too have no maximum size limit and do no version
control.
Since null channels go nowhere, no additional destination specification is necessary.
Channels have string names that are their primary external reference. There are four predefined logging channels (five, as of BIND 9.11):
"default_stderr"
: Descriptor channel to stderr at priorityISC_LOG_INFO
"default_logfile"
: File channel created if the user specifies a logfile usingnamed -L
at priorityISC_LOG_DYNAMIC
(9.11 and higher only)"default_debug"
: Descriptor channel to stderr at priorityISC_LOG_DYNAMIC
"default_syslog"
-- Syslog channel toLOG_DAEMON
at priorityISC_LOG_INFO
"null"
-- Null channel
Other channels may be configured by the user via named.conf
.
ISC_LOG_DYNAMIC
indicates to the logging system that
debugging messages are desired, but only at the current debugging level
of the program. The debugging level can be modified dynamically at
runtime; in named
this can be done by the "rndc trace"
command.
When the debugging level is 0 (turned off), then no debugging messages are
written to the channel. If the debugging level is raised, only debugging
messages up to the current debugging level are written to the channel.
These objects -- the category, module, and channel -- direct hessages
to desired destinations. Each category/module pair can be associated
with a specific channel, and the correct destination will be used
when a message is logged by isc_log_write()
.
In isc_log_write()
, the logging system first looks up a list that
consists of all of the channels associated with a particular category.
It walks down the list looking for each channel that also has the
indicated module associated with it, and writes the message to each
channel it encounters. If no match is found in the list for the
module, the default channel (associated with ISC_LOGCATEGORY_DEFAULT
)
is used. The default is also used if no channels have been specified
for the category at all.
The type used by programs for configuring log message destinations is
isc_logconfig_t
. It is used to store the configurable specification of
message destinations, which can be changed during the course of the program.
A starting configuration (isc_logconfig_t
) is created implicitly. The pointer
to this configuration is returned via isc_logconfig_get()
so that it can then
be configured. A new log configuration can be established by creating it with
isc_logconfig_create()
, configuring it, then installing it as the active
configuration with isc_logconfig_set()
.
The entire logging context is thread-locked for most of the duration of the
isc_log_write()
. However, isc_log_write()
avoids the delays caused by
locking when it is clear that there are no possible outputs for a message
based on its debugging level --- this is so that a program can have
debugging messages sprinkled liberally throughout it but not incur any
locking penalty when debugging is not enabled.
To enable the messages from a library that uses the logging system, the following steps need to be taken to initialize it.
-
Include the main logging header file as well as the logging header file for any additional library you are using. For example, when using the DNS library, include the following:
#include <isc/log.h> log.h>/log.h>
-
Initialize a logging context. A logging context needs a valid memory context in order to work, so the following code snippet shows a rudimentary initialization of both.
isc_mem_t *mctx; isc_logconfig_t *lcfg; lcfg = isc_logconfig_get();
-
Initialize any additional libraries. The convention for the name of the initialization function is
{library}_log_init()
, with a pointer to the logging context as an argument. The function can only be called once in a program or it will generate an assertion.`dns_log_init();`
If you do not want a library to write any log messages, simply do not call its the initialization function.
-
Create any channels you want in addition to the internal channels of
default_syslog
,default_stderr
,default_debug
and null. A destination structure needs to be filled for any destination other than null. The following examples show use of a file log, a file descriptor log, and syslog.isc_logdestination_t destination; destination.file.name = "/var/log/example"; destination.file.maximum_size = 0; /* No byte limit. */ destination.file.versions = ISC_LOG_ROLLNEVER; /* External rolling. */ isc_log_createchannel(lcfg, "sample1", ISC_LOG_TOFILE, ISC_LOG_DYNAMIC, &destination, ISC_LOG_PRINTTIME); destination.file.stream = stdout; isc_log_createchannel(lcfg, "sample2", ISC_LOG_TOFILEDESC, ISC_LOG_INFO, &destination, ISC_LOG_PRINTTIME); destination.facility = LOG_ERR; isc_log_createchannel(lcfg, "sample3", ISC_LOG_SYSLOG, ISC_LOG_ERROR, &destination, 0);
ISC_LOG_DYNAMIC
is used to define a channel that wants any of the messages up to the current debugging level of the program.ISC_LOG_DEBUG(level)
can define a channel that always gets messages up to the debug level specified, regardless of the debugging state of the server. -
Direct the various log categories and modules to the desired destination. This step is not necessary if the normal behavior of sending all messages to
default_stderr
is acceptable. The following examples sends DNS security messages to stderr, DNS database messages to null, and all other messages to syslog.result = isc_log_usechannel(lcfg, "default_stderr", DNS_LOGCATEGORY_SECURITY, ISC_LOGMODULE_DEFAULT); if (result != ISC_R_SUCCESS) oops_it_didnt_work(); result = isc_log_usechannel(lcfg, "null", DNS_LOGCATEGORY_DATABASE, ISC_LOGMODULE_DEFAULT); if (result != ISC_R_SUCCESS) oops_it_didnt_work(); result = isc_log_usechannel(lcfg, "default_syslog", ISC_LOGCATEGORY_DEFAULT, ISC_LOGMODULE_DEFAULT); if (result != ISC_R_SUCCESS) oops_it_didnt_work();
Providing a NULL argument for the category means "associate the channel with the indicated module in all known categories":
ISC_CATEGORY_DEFAULT
.Providing a NULL argument for the module means "associate the channel with all modules that use this category."
There are three additional functions you might find useful in your program to control logging behavior, two to work with the debugging level and one to control the closing of log files.
void isc_log_setdebuglevel(unsigned int level);
unsigned int isc_log_getdebuglevel();
These set and retrieve the current debugging level of the program.
isc_log_getdebuglevel()
can be used so that you need not keep track of
the level yourself in another variable.
void isc_log_closefilelogs();
This function closes any open log files. This is useful for programs that
do not want to do file rotation as with the internal rolling mechanism.
For example, a program that wanted to keep daily logs would define a
channel which used ISC_LOG_ROLLNEVER
, then once a day would rename the
log file and call isc_log_closefilelogs()
. The next time a message needs
to be written a file that has been closed, it is reopened.
BIND 9 was designed to make it relatively easy for anyone with sufficient knowledge of C to add user defined resource record (RR) types.
The descriptions of all the record types known to BIND are in a directory
structure under lib/dns/rdata in the source tree. This directory is
structured at the first level by the DNS CLASS the record type belongs to.
The name of the directory is the {class}_{code}
(for example, IN is
in_1
).
The currently existing classes are in_1
, ch_3
, hs_4
, any_255
and
generic
-- the first four hold RR types that are specific to a particular
class, and "generic" holds RR types that are the same across all classes.
Within each of these directories there are pairs of files which describe
the actual types. These files are named {type}_{code}.c
and
{type}_{code}.h
: for examle, the description of the MX record, which has
the RR type code 15, is in mx_15.c
and mx_15.h
.
Within each of these files there are method functions for various
operations that apply to types, such as how to print out a type, how to
read a type from a text file, how to read a record from a DNS message in
wire format, etc. These methods have names constructed from the type,
class (if the record is class specific) and operation to be performed.
These methods are called from the dns_rdata_{method}
functions which are
declared in <dns/rdata.h>
.
Once the two files containing the method and type definitions for the
structures have been written you need to run "make clean"
then "make"
to
incorporate the new record type. This will cause the lib/dns/rdata
directory structure to be scanned and header files to be rebuilt which will
include the new files. All the tools that are part of BIND will know about
the new type.
You can also define auxiliary functions to help walk the structure returned
by dns_rdata_tostruct()
, such as dns_rdata_txt_first()
and
dns_rdata_txt_next()
, which are used to walk the text strings in a TXT
record. The code goes into the .c file and the function prototype into the
.h file the contents of which are included in <dns/rdatastruct.h>
.
lib/dns/rdata/generic/proforma.c
and lib/dns/rdata/generic/proforma.h
can be copied and used as starting points when defining a adding a new type.
Please also look as the existing record types for examples of how to
implement a method.
Type values range from 0 to 65536. These have been further divided into reserved values, values that have global definition and values that have local definition as defined in RFC 6895. Please use an appropriate value. You can use a private value (65280 - 65534) while waiting for a type assignment to be made, then rename the file and update the type values when the assignment has been made.
"fromtext" reads a series of tokens from lexer
and constructs
a DNS record in wire format, which it stores in target
. It performs
sanity checks on the entered content, rejecting any invalid records.
static isc_result_t
fromtext[_<class>]_<type>(int rdclass, dns_rdatatype_t type,
isc_lex_t *lexer, dns_name_t *origin,
unsigned int options, isc_buffer_t *target,
dns_rdatacallbacks_t *callbacks);
static isc_result_t
totext[_<class>]_<type>(dns_rdata_t *rdata,
dns_rdata_textctx_t *tctx,
isc_buffer_t *target);
"totext" takes a record in wire format, converts it to presentation format, and stores it in a buffer for later printing.
static isc_result_t
fromwire[_<class>]_<type>(int rdclass, dns_rdatatype_t type,
isc_buffer_t *source,
dns_decompress_t *dctx,
unsigned int options,
isc_buffer_t *target_t);
"fromwire" copies in a record received in a DNS message. It performs sanity checks to ensure that the record conforms to the specification for the RR type. It expands any compressed domain names, and copies out the expanded record to a buffer. (NOTE: It is critical to the security of the name server that only valid records are accepted by this function, as other parts of the name server do not verify the contents of incoming records.)
static isc_result_t
towire[_<class>]_<type>(dns_rdata_t *rdata, dns_compress_t *cctx,
isc_buffer_t *target);
"towire" takes a record in wire format and adds it to a DNS
message, optionally compressing domain names if that is allowed by the
type's definition. (NOTE: Compression is no longer allowed in new
RR types, so this is effectively a wrapper around memmove()
.)
static int
compare[_<class>]_<type>(const dns_rdata_t *rdata1,
const dns_rdata_t *rdata2);
"compare" takes two records and compares them according to the
DNSSEC ordering rules. For all new record types, this is effectively a
wrapper around memcmp()
.
static isc_result_t
fromstruct[_<class>]_<type>(int rdclass, dns_rdatatype_t type,
void *source, isc_buffer_t *target);
"fromstruct" takes a C structure (as described in
tostruct()
, below) and turns it into a record in wire format.
static isc_result_t
tostruct[_<class>]_<type>(dns_rdata_t *rdata, void *target,
isc_mem_t *mctx);
"tostruct" take a record in wire format and breaks it down into
a type-specific C structure defined in the header file. The name of this
structure is dns_rdata_<type>[_<class>]_t
; the first element of
the structure must be "dns_rdatacommon_t common;"
.
If no memory context is passed in, then the caller will preserve the
contents of the record in wire form until the structure is freed or no
longer in use. If a memory context is passed, in then memory should
be allocated for anything not directly part of the structure.
static void
freestruct[_<class>]_<type>(void *source);
"freestruct" frees any memory allocated by tostruct()
.
static isc_result_t
additional[_<class>]_<type>(dns_rdata_t *rdata,
dns_additionaldatafunc_t add,
void *arg);
"additional" provides the ability to add related records to the additional section of a message when this record is added to a message. An empty method is usual here.
static isc_result_t
digest[_<class>]_<type>(dns_rdata_t *rdata,
dns_digestfunc_t digest,
void *arg);
"digest" passes the record contents to the digest
function,
performing any needed DNSSEC canonicalisation. For all new record types,
this simply involves adding the entire record to a region and passing that
to digest
, because new record types are treated as opaque blobs of data
by DNSSEC.
static bool
checkowner[_<class>]_<type>(dns_name_t *name,
dns_rdataclass_t rdclass,
dns_rdatatype_t type,
bool wildcard);
"checkowner" takes the owner name of the record and checks
that it meets appropriate rules that are defined external to the DNS.
In most cases this can just be a function that returns true
.
static bool
checknames[_<class>]_<type>(dns_rdata_t *rdata,
dns_name_t *owner,
dns_name_t *bad);
"checknames" checks the contents of the rdata with the given
owner name to ensure that it meets externally defined syntax rules.
If false
is returned, then bad
will point to the name that
caused the problem.
static int
casecompare[_<class>]_<type>(const dns_rdata_t *rdata1,
const dns_rdata_t *rdata2);
"casecompare" compares two rdatas case-insensitively.
In nearly all cases, this is simply a wrapper around the compare()
function, except where DNSSEC comparisons are specified as
case-sensitive. Unknown RR types are always compared case-sensitively.
Asynchronous operations are processed using the event loop manager; see the Loop Manager document for details.
Further architectural details on BIND to be added here in the future.