BIND Developer Information


  1. The code review process
  2. Testing
  3. BIND system architecture

The code review process

Every line of code comitted 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, a full regression test (make check) must be 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 RT ticket so the author can respond.

What is reviewed:

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 completness.

Obvious bottlenecks and places where performance or reliability may suffer are noted as part of the review. Tricky code needs to be commented well enough. 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. (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.

Steps in code review:

Things we look for

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.


Running system tests

To enable system tests to work, we first need to create the test loopback interfaces (as root):

    $ cd bin/tests/system
    $ sudo sh up
    $ cd ../../..

To run the tests, build BIND (be sure to use --with-atf to run unit tests), then run make check. An easy way to check the results:

    $ make check 2>&1 | tee /tmp/check.out
    $ grep '^R:' /tmp/check.out | sort | uniq -c

This will show all of the test results. One or two "R:SKIPPED" is okay; if there are a lot of them, then you probably forgot to create the loopback interfaces in the previous step. (NOTE: the summary of tests that appears at the end of make check only summarizes the system test results, not the unit tests, so you can't rely on it to catch everything.)

To run only the system tests, omitting unit tests:

    $ cd bin/tests/system
    $ sh

Or, to run an individual system test:

    $ cd bin/tests/system
    $ sh <testname>

System tests are in separate directories under bin/tests/system. For example, the "dnssec" test is in bin/tests/system/dnssec.

Writing system tests

The following standard files are found in system test directories:

All test scripts source the file bin/tests/system/ (which is generated by configure from This script provides functions and variables pointing to the binaries under test; for example, DIG contains the path to dig in the build tree being tested, RNDC points to rndc, SIGNZONE to dnssec-signzone, etc.

Building unit tests

BIND uses the Automated Testing Framework (ATF), originally from the NetBSD project, as its unit testing framework. (Note: ATF has been supplanted by a newer version called Kyua, but BIND is still using the older system.)

To build BIND with unit tests, run configure with the --with-atf option. This causes the ATF source code in the unit/atf-src subdirectory to be built.

To save time on repeated builds, you can build and install ATF in another directory, and configure BIND to use the pre-built version. (Be sure to disable shared libraries in the ATF build, and to build the ATF tools; libraries alone are not sufficient).

    $ cd atf-src
    $ configure --prefix=<prefix> --enable-tools --disable-shared
    $ make
    $ make install

After this has been done, specify the ATF prefix when building BIND:

    $ configure --with-atf=<prefix>

Running unit tests

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 ATF unit tests:

    $ sh 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

Writing unit tests

Information on writing ATF tests can be found at the NetBSD site.

Each unit test file contains at least one unit test case, and a list of test cases to execute when the test is run. These will look like the following:

    ATF_TC_HEAD(test_case, tc) {
            atf_tc_set_md_var(tc, "descr",
                              "Describe the test case here.");
    ATF_TC_BODY(serialize_align, tc) {

            result = isc_test_begin(NULL, ISC_TRUE);
            ATF_REQUIRE_EQ(result, ISC_R_SUCCESS);

            ATF_CHECK_EQ(value1, value2);
            ATF_CHECK(value1 + value2 < 100);


     * Main
    ATF_TP_ADD_TCS(tp) {
            ATF_TP_ADD_TC(tp, test_case);

            return (atf_no_error());

If the conditions specified in ATF_CHECK and ATF_CHECK_EQ directives are found to be false, then the test case will fail, but it will continue running to see if there are any more failures.

If the conditions specified in ATF_REQUIRE and ATF_REQUIRE_EQ are found to be false, the test case cannot continue running and will stop

isc_test_begin() and isc_test_end() set up necessary preconditions for checking libisc functions, such as starting a task manger and creating a memory context. Similar functions dns_test_begin() and dns_test_end() are available for testing libdns functions.

BIND system architecture

Source tree layout


See the namespace discussion in the BIND coding style document.

Design by contract

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.)

Magic numbers

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 ==

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.)

Result codes

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 occured 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(bufer), 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 isc_boolean_t, 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 header files called result.h (for example, the result codes defined for the ISC library are in lib/isc/include/isc/result.h.

ISC library result codes (many of which are generically useful elsewhere) begin with ISC_R: examples inclue 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.

Buffers and regions

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:

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.

Memory management

BIND manages its own memory 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.

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;
    result = isc_mem_create(0, 0, &mctx);

(The zeroes are tuning parameters, max_size and target_size: Any allocations smaller than max_size will be satisfied by getting blocks of size target_size from the operating system's memory allocator and breaking them up into pieces, while larger allocations will call the system allocator directly. These parameters are rarely used.)

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_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))
            if (foo == NULL)
                    return (ISC_R_NOMEMORY);

            /* 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().

    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().

Every allocation and deallocation requires a memory context lock to be acquired. This will cause performance problems if you write code that allocates and deallocates memory frequently. Whenever possible, inner loop functions should be passed static buffers rather than allocating memory.

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 locking the entire memory context.


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_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));
            if (foo == NULL)
                    return (ISC_R_NOMEMORY);

            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;

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);

A similar but smaller set of ISC_QUEUE macros, including ISC_QUEUE_PUSH and ISC_QUEUE_POP, are provided to implement strict FIFO lists, with built-in fine-grained locking.


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 two methods for copying name objects: dns_name_clone() makes a target refer to the same data as the source without copying the data, so the source must not be changed while the target is still in use. dns_name_dup() and dns_name_dupwithoffsets() create a true copy of the name, dynamically allocating memory as needed; targets created by these must be freed by calling dns_name_free().

Wire format

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).

Text format

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 = "";
    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().

Manipulating names

Several functions are provided for inspecting and modifying name objects. These include:


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 ("" is less than ""). 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:

Some simpler comparison functions are provided for convenience when not all of this information is required:

Fixed names

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_name(&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.


Retrieving data from BIND databases involves the use of iterator functions to walk from entry to entry. Several iterator function sets have been defined:

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 reaquire 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_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:

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 and the Application log in the Event Viewer on Windows 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):

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 modifid 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.

Externally visible structure

The types used by programs for configuring log message destinations are isc_log_t and isc_logconfig_t. The isc_log_t type is normally created only once by a program, to hold static information about what categories and modules exist in the program and some other housekeeping information. isc_logconfig_t 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 when the context (isc_log_t) is created. The pointer to this configuration is returned via a parameter to isc_log_create() 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_use().

Logging in multithreaded programs

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.

Using libraries that use the logging system

To enable the messages from a library that uses the logging system, the following steps need to be taken to initialize it.

  1. 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>
    #include <dns/log.h>
  2. 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_log_t *lctx;
    isc_logconfig_t *lcfg;
    if (isc_mem_create(0, 0, &mctx) != ISC_R_SUCCESS) ||
        isc_log_create(mctx, &lctx, &lcfg) != ISC_R_SUCCESS))
  3. Initalize 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.


    If you do not want a library to write any log messages, simply do not call its the initialization function.

  4. 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;
     = "/var/log/example";
    destination.file.maximum_size = 0;              /* No byte limit. */
    destination.file.versions = ISC_LOG_ROLLNEVER;  /* External rolling. */
    result = isc_log_createchannel(lcfg, "sample1", ISC_LOG_TOFILE,
                                   ISC_LOG_DYNAMIC, &destination,
    if (result != ISC_R_SUCCESS)
     = stdout;
    result = isc_log_createchannel(lcfg, "sample2", ISC_LOG_TOFILEDESC,
                                   ISC_LOG_INFO, &destination,
    if (result != ISC_R_SUCCESS)
    destination.facility = LOG_ERR;
    result = isc_log_createchannel(lcfg, "sample3", ISC_LOG_SYSLOG,
                                   ISC_LOG_ERROR, &destination, 0);
    if (result != ISC_R_SUCCESS)

    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.

  5. 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, NULL);
    if (result != ISC_R_SUCCESS)
    result = isc_log_usechannel(lcfg, "null",
                                DNS_LOGCATEGORY_DATABASE, NULL);
    if (result != ISC_R_SUCCESS)
    result = isc_log_usechannel(lcfg, "default_syslog",
                                ISC_LOGCATEGORY_DEFAULT, NULL);
    if (result != ISC_R_SUCCESS)

    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(isc_log_t *lctx, unsigned int level);
    unsigned int isc_log_getdebuglevel(isc_log_t *lctx);

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(isc_log_t *lcxt);

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.

Adding a new RR type


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 value selection

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 isc_boolean_t
    checkowner[_<class>]_<type>(dns_name_t *name,
                                dns_rdataclass_t rdclass,
                                dns_rdatatype_t type,
                                isc_boolean_t 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 ISC_TRUE.

    static isc_boolean_t
    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 ISC_FALSE is returned, then bad will point to the name that caused the probelm.

    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.

Task and timer model

The BIND task/timer management system can be thought of as comparable to a simple non-preemptive multitasking operating system.

In this model, what BIND calls a "task" (or an isc_task_t object) is the equivalent of what is usually called a "process" in an operating system: a persistent execution context in which functions run when action is required. When the action is complete, the task goes to sleep, and wakes up again when more work arrives. A "worker thread" is comparable to an operating system's CPU; when a task is ready to run, a worker thread will run it. By default, BIND creates one worker thread for each system CPU.

An "event" object can be associated with a task, and triggered when a a specific condition occurs. Each event object contains a pointer to a specific function and arguments to be passed to it. When the event triggers, the task is placed onto a "ready queue" in the task manager. As each running task finishes, the worker threads pull new tasks off the ready queue. When the task associated with a given event reaches the head of the queue, the specified function will be called.


isc_socket_recv() calls the recv() system call asynchronously: rather than waiting for data, it returns immediately, but it sets up an event to be triggered when the recv() call completes; BIND can now do other work instead of waiting for I/O. Once the recv() is finished, the associated event is triggered.

     * Function to handle a completed recv()
    static void
    recvdone(isc_task_t *task, isc_event_t *event) {
            /* Arguments are in event->ev_arg. */


     * Call recv() on socket 'sock', put results into 'region',
     * minimum read size 1, and call recvdone() with NULL as
     * argument.  (Note: 'sock' is already associated with a
     * particular task, so that doesn't need to be specified
     * here.)
    isc_socket_recv(sock, &region, 1, recvdone, NULL);

A timer is set for a specifed time in the future, and the event will be triggered at that time.

     * Function to handle a timeout
    static void
    timeout(isc_task_t *task, isc_event_t *event) {
           /* do things */


      * Set up a timer in timer manager 'timermgr', to run
      * once, with a NULL expiration time, after 'interval'
      * has passed; it will run the function 'timeout' with
      * 'arg' as its argument in task 'task'.
     isc_timer_t *timer = NULL;
     result = isc_timer_create(timermgr, isc_timertype_once, NULL,
                               interval, task, timeout, arg, &timer);

An event can also be explicitly triggered via isc_task_send().

    static void
    do_things(isc_task_t *task, isc_event_t *event) {
            /* this function does things */


     * Allocate an event that calls 'do_things' with a 
     * NULL argument, using 'myself' as ev_sender.
     * DNS_EVENT_DOTHINGS must be defined in <dns/events.h>.
     * (Note that 'size' must be specified because there are
     * event objects that inherit from isc_event_t, incorporating
     * common members via the ISC_EVENT_COMMON member and then
     * following them with other members.)
    isc_event_t *event;
    event = isc_event_allocate(mctx, myself, DNS_EVENT_DOTHINGS,
                               do_things, NULL, sizeof(isc_event_t));
    if (event == NULL)
            return (ISC_R_NOMEMORY);


     * Send the allocated event to task 'task'
    isc_task_send(task, event);


Further architectural details on BIND to be added here in the future.