LIBUNBOUND(3) Unbound LIBUNBOUND(3)
NAME
libunbound - Unbound DNS validating resolver 1.24.0 functions.
SYNOPSIS
#include <unbound.h>
struct ub_ctx * ub_ctx_create(void);
void ub_ctx_delete(struct ub_ctx* ctx);
int ub_ctx_set_option(struct ub_ctx* ctx, char* opt, char* val);
int ub_ctx_get_option(struct ub_ctx* ctx, char* opt, char** val);
int ub_ctx_config(struct ub_ctx* ctx, char* fname);
int ub_ctx_set_fwd(struct ub_ctx* ctx, char* addr);
int ub_ctx_set_stub(struct ub_ctx* ctx, char* zone, char* addr,
int isprime);
int ub_ctx_set_tls(struct ub_ctx* ctx, int tls);
int ub_ctx_resolvconf(struct ub_ctx* ctx, char* fname);
int ub_ctx_hosts(struct ub_ctx* ctx, char* fname);
int ub_ctx_add_ta(struct ub_ctx* ctx, char* ta);
int ub_ctx_add_ta_autr(struct ub_ctx* ctx, char* fname);
int ub_ctx_add_ta_file(struct ub_ctx* ctx, char* fname);
int ub_ctx_trustedkeys(struct ub_ctx* ctx, char* fname);
int ub_ctx_debugout(struct ub_ctx* ctx, FILE* out);
int ub_ctx_debuglevel(struct ub_ctx* ctx, int d);
int ub_ctx_async(struct ub_ctx* ctx, int dothread);
int ub_poll(struct ub_ctx* ctx);
int ub_wait(struct ub_ctx* ctx);
int ub_fd(struct ub_ctx* ctx);
int ub_process(struct ub_ctx* ctx);
int ub_resolve(struct ub_ctx* ctx, char* name,
int rrtype, int rrclass, struct ub_result** result);
int ub_resolve_async(struct ub_ctx* ctx, char* name,
int rrtype, int rrclass, void* mydata, ub_callback_type* call-
back, int* async_id);
int ub_cancel(struct ub_ctx* ctx, int async_id);
void ub_resolve_free(struct ub_result* result);
const char * ub_strerror(int err);
int ub_ctx_print_local_zones(struct ub_ctx* ctx);
int ub_ctx_zone_add(struct ub_ctx* ctx, char* zone_name, char*
zone_type);
int ub_ctx_zone_remove(struct ub_ctx* ctx, char* zone_name);
int ub_ctx_data_add(struct ub_ctx* ctx, char* data);
int ub_ctx_data_remove(struct ub_ctx* ctx, char* data);
DESCRIPTION
Unbound is an implementation of a DNS resolver, that does caching and
DNSSEC validation. This is the library API, for using the -lunbound
library. The server daemon is described in unbound(8). The library
works independent from a running unbound server, and can be used to
convert hostnames to ip addresses, and back, and obtain other informa-
tion from the DNS. The library performs public-key validation of re-
sults with DNSSEC.
The library uses a variable of type struct ub_ctx to keep context be-
tween calls. The user must maintain it, creating it with ub_ctx_create
and deleting it with ub_ctx_delete. It can be created and deleted at
any time. Creating it anew removes any previous configuration (such as
trusted keys) and clears any cached results.
The functions are thread-safe, and a context can be used in a threaded
(as well as in a non-threaded) environment. Also resolution (and vali-
dation) can be performed blocking and non-blocking (also called asyn-
chronous). The async method returns from the call immediately, so that
processing can go on, while the results become available later.
The functions are discussed in turn below.
FUNCTIONS
ub_ctx_create
Create a new context, initialised with defaults. The informa-
tion from /etc/resolv.conf and /etc/hosts is not utilised by de-
fault. Use ub_ctx_resolvconf and ub_ctx_hosts to read them.
Before you call this, use the openssl functions
CRYPTO_set_id_callback and CRYPTO_set_locking_callback to set up
asynchronous operation if you use lib openssl (the application
calls these functions once for initialisation). Openssl 1.0.0
or later uses the CRYPTO_THREADID_set_callback function.
ub_ctx_delete
Delete validation context and free associated resources. Out-
standing async queries are killed and callbacks are not called
for them.
ub_ctx_set_option
A power-user interface that lets you specify one of the options
from the config file format, see unbound.conf(5). Not all op-
tions are relevant. For some specific options, such as adding
trust anchors, special routines exist. Pass the option name
with the trailing ':'.
ub_ctx_get_option
A power-user interface that gets an option value. Some options
cannot be gotten, and others return a newline separated list.
Pass the option name without trailing ':'. The returned value
must be free(2)d by the caller.
ub_ctx_config
A power-user interface that lets you specify an unbound config
file, see unbound.conf(5), which is read for configuration. Not
all options are relevant. For some specific options, such as
adding trust anchors, special routines exist. This function is
thread-safe only if a single instance of ub_ctx* exists in the
application. If several instances exist the application has to
ensure that ub_ctx_config is not called in parallel by the dif-
ferent instances.
ub_ctx_set_fwd
Set machine to forward DNS queries to, the caching resolver to
use. IP4 or IP6 address. Forwards all DNS requests to that ma-
chine, which is expected to run a recursive resolver. If the
proxy is not DNSSEC capable, validation may fail. Can be called
several times, in that case the addresses are used as backup
servers. At this time it is only possible to set configuration
before the first resolve is done.
ub_ctx_set_stub
Set a stub zone, authoritative dns servers to use for a particu-
lar zone. IP4 or IP6 address. If the address is NULL the stub
entry is removed. Set isprime true if you configure root hints
with it. Otherwise similar to the stub zone item from unbound's
config file. Can be called several times, for different zones,
or to add multiple addresses for a particular zone. At this
time it is only possible to set configuration before the first
resolve is done.
ub_ctx_set_tls
Enable DNS over TLS (DoT) for machines set with ub_ctx_set_fwd.
At this time it is only possible to set configuration before the
first resolve is done.
ub_ctx_resolvconf
By default the root servers are queried and full resolver mode
is used, but you can use this call to read the list of name-
servers to use from the filename given. Usually "/etc/re-
solv.conf". Uses those nameservers as caching proxies. If they
do not support DNSSEC, validation may fail. Only nameservers
are picked up, the searchdomain, ndots and other settings from
resolv.conf(5) are ignored. If fname NULL is passed, "/etc/re-
solv.conf" is used (if on Windows, the system-wide configured
nameserver is picked instead). At this time it is only possible
to set configuration before the first resolve is done.
ub_ctx_hosts
Read list of hosts from the filename given. Usually
"/etc/hosts". When queried for, these addresses are not marked
DNSSEC secure. If fname NULL is passed, "/etc/hosts" is used
(if on Windows, etc/hosts from WINDIR is picked instead). At
this time it is only possible to set configuration before the
first resolve is done.
ub_ctx_add_ta
Add a trust anchor to the given context. At this time it is
only possible to add trusted keys before the first resolve is
done. The format is a string, similar to the zone-file format,
[domainname] [type] [rdata contents]. Both DS and DNSKEY
records are accepted.
ub_ctx_add_ta_autr
Add filename with automatically tracked trust anchor to the
given context. Pass name of a file with the managed trust an-
chor. You can create this file with unbound-anchor(8) for the
root anchor. You can also create it with an initial file with
one line with a DNSKEY or DS record. If the file is writable,
it is updated when the trust anchor changes. At this time it is
only possible to add trusted keys before the first resolve is
done.
ub_ctx_add_ta_file
Add trust anchors to the given context. Pass name of a file
with DS and DNSKEY records in zone file format. At this time it
is only possible to add trusted keys before the first resolve is
done.
ub_ctx_trustedkeys
Add trust anchors to the given context. Pass the name of a
bind-style config file with trusted-keys{}. At this time it is
only possible to add trusted keys before the first resolve is
done.
ub_ctx_debugout
Set debug and error log output to the given stream. Pass NULL
to disable output. Default is stderr. File-names or using sys-
log can be enabled using config options, this routine is for us-
ing your own stream.
ub_ctx_debuglevel
Set debug verbosity for the context. Output is directed to
stderr. Higher debug level gives more output.
ub_ctx_async
Set a context behaviour for asynchronous action. if set to
true, enables threading and a call to ub_resolve_async creates a
thread to handle work in the background. If false, a process is
forked to handle work in the background. Changes to this set-
ting after ub_resolve_async calls have been made have no effect
(delete and re-create the context to change).
ub_poll
Poll a context to see if it has any new results. Do not poll in
a loop, instead extract the fd below to poll for readiness, and
then check, or wait using the wait routine. Returns 0 if noth-
ing to read, or nonzero if a result is available. If nonzero,
call ub_process to do callbacks.
ub_wait
Wait for a context to finish with results. Calls ub_process af-
ter the wait for you. After the wait, there are no more out-
standing asynchronous queries.
ub_fd Get file descriptor. Wait for it to become readable, at this
point answers are returned from the asynchronous validating re-
solver. Then call the ub_process to continue processing.
ub_process
Call this routine to continue processing results from the vali-
dating resolver (when the fd becomes readable). Will perform
necessary callbacks.
ub_resolve
Perform resolution and validation of the target name. The name
is a domain name in a zero terminated text string. The rrtype
and rrclass are DNS type and class codes. The result structure
is newly allocated with the resulting data.
ub_resolve_async
Perform asynchronous resolution and validation of the target
name. Arguments mean the same as for ub_resolve except no data
is returned immediately, instead a callback is called later.
The callback receives a copy of the mydata pointer, that you can
use to pass information to the callback. The callback type is a
function pointer to a function declared as:
void my_callback_function(void* my_arg, int err,
struct ub_result* result);
The async_id is returned so you can (at your option) decide to
track it and cancel the request if needed. If you pass a NULL
pointer the async_id is not returned.
ub_cancel
Cancel an async query in progress. This may return an error if
the query does not exist, or the query is already being deliv-
ered, in that case you may still get a callback for the query.
ub_resolve_free
Free struct ub_result contents after use.
ub_strerror
Convert error value from one of the unbound library functions to
a human readable string.
ub_ctx_print_local_zones
Debug printout the local authority information to debug output.
ub_ctx_zone_add
Add new zone to local authority info, like local-zone
unbound.conf(5) statement.
ub_ctx_zone_remove
Delete zone from local authority info.
ub_ctx_data_add
Add resource record data to local authority info, like lo-
cal-data unbound.conf(5) statement.
ub_ctx_data_remove
Delete local authority data from the name given.
RESULT DATA STRUCTURE
The result of the DNS resolution and validation is returned as struct
ub_result. The result structure contains the following entries:
struct ub_result {
char* qname; /* text string, original question */
int qtype; /* type code asked for */
int qclass; /* class code asked for */
char** data; /* array of rdata items, NULL terminated*/
int* len; /* array with lengths of rdata items */
char* canonname; /* canonical name of result */
int rcode; /* additional error code in case of no data */
void* answer_packet; /* full network format answer packet */
int answer_len; /* length of packet in octets */
int havedata; /* true if there is data */
int nxdomain; /* true if nodata because name does not exist */
int secure; /* true if result is secure */
int bogus; /* true if a security failure happened */
char* why_bogus; /* string with error if bogus */
int was_ratelimited; /* true if the query was ratelimited (SERVFAIL) by unbound */
int ttl; /* number of seconds the result is valid */
};
If both secure and bogus are false, security was not enabled for the
domain of the query. Else, they are not both true, one of them is
true.
RETURN VALUES
Many routines return an error code. The value 0 (zero) denotes no er-
ror happened. Other values can be passed to ub_strerror to obtain a
readable error string. ub_strerror returns a zero terminated string.
ub_ctx_create returns NULL on an error (a malloc failure). ub_poll re-
turns true if some information may be available, false otherwise.
ub_fd returns a file descriptor or -1 on error. ub_ctx_config and
ub_ctx_resolvconf attempt to leave errno informative on a function re-
turn with file read failure.
SEE ALSO
unbound.conf(5), unbound(8).
AUTHOR
Unbound developers are mentioned in the CREDITS file in the distribu-
tion.
COPYRIGHT
1999-2025, NLnet Labs
1.24.0 Sep 18, 2025 LIBUNBOUND(3)
