res_query(const char *dname,
int class, int type,
unsigned char *answer, int
res_search(const char *dname,
int class, int type,
unsigned char *answer, int
const char *dname, int class,
int type, const unsigned char
*data, int datalen, const
unsigned char *newrr, unsigned char *buf,
res_send(const unsigned char
*msg, int msglen, unsigned char
*answer, int anslen);
dn_comp(const char *exp_dn,
unsigned char *comp_dn, int
length, unsigned char **dnptrs,
unsigned char **lastdnptr);
dn_expand(const unsigned char
*msg, const unsigned char *eomorig,
const unsigned char *comp_dn, char
*exp_dn, int length);
These routines are used for making, sending, and interpreting query and reply messages with Internet domain name servers.
Global configuration and state information that is used by the
resolver routines is kept in the structure _res. Most
of the values have reasonable defaults and can be ignored. Options stored in
_res.options are defined in
<resolv.h> and are as
follows. Options are stored as a simple bit mask containing the bitwise OR
of the options enabled.
- True if the initial name server address and default domain name are
res_init() has been called).
- Print debugging messages, if libc is compiled with
DEBUG. By default on OpenBSD this option does nothing.
- Accept authoritative answers only. With this option,
res_send() should continue until it finds an authoritative answer or finds an error. On OpenBSD this option does nothing.
- Use TCP connections for queries instead of UDP datagrams.
- Query the primary name server only. On OpenBSD this option does nothing.
- Ignore truncation errors, i.e. don't retry with TCP.
- Set the recursion-desired bit in queries.
res_send() does not do iterative queries and expects the name server to handle recursion.) This option is enabled by default.
- If set,
res_search() will append the default domain name to single-component names (those that do not contain a dot). This option is enabled by default.
- Used with
RES_USEVCto keep the TCP connection open between queries. This is useful only in programs that regularly do many queries. UDP should be the normal mode used.
- If this option is set,
res_search() will search for host names in the current domain and in parent domains; see hostname(7). This is used by the standard host lookup routine gethostbyname(3). This option is enabled by default.
- Do not require the IP source address on the reply packet to be equal to the server's address.
- Do not check if the query section of the reply packet is equal to that of the query packet.
- This option has no effect. In the past, it turned off the legacy
- If set, the resolver routines will set the AD flag in DNS queries and preserve the value of the AD flag in DNS replies. If not set, the resolver routines will clear the AD flag in responses. Direct use of this option to enable AD bit processing is discouraged. Instead the use of trusted name servers should be annotated with “options trust-ad” in resolv.conf(5). This option is automatically enabled if resolv.conf(5) only lists name servers on localhost.
- With this option gethostbyname(3) will return IPv6 addresses if available. This option is deprecated; software should use the getaddrinfo(3) interface instead of modifying the behavior of gethostbyname(3). On some operating systems this option also causes IPv4 addresses to be returned as IPv4-mapped IPv6 addresses. For example, 10.1.1.1 will be returned as ::ffff:10.1.1.1. IPv4-mapped IPv6 addresses are not supported on OpenBSD.
- Attach an OPT pseudo-RR for the EDNS0 extension, as specified in RFC 2671. This informs DNS servers of a client's receive buffer size, allowing them to take advantage of a non-default receive buffer size, and thus to send larger replies. DNS query packets with the EDNS0 extension are not compatible with non-EDNS0 DNS servers. OpenBSD uses 4096 bytes as input buffer size.
- Request that the resolver uses Domain Name System Security Extensions (DNSSEC), as defined in RFCs 4033, 4034, and 4035. The resolver routines will use the EDNS0 extension and set the DNSSEC DO flag in queries, asking the name server to signal validated records by setting the AD flag in the reply and to attach additional DNSSEC records. The resolver routines will clear the AD flag in replies unless the name servers are considered trusted. Also, client applications are often only interested in the value of the AD flag, making the additional DNSSEC records a waste of network bandwidth. See the description for “options trust-ad” in resolv.conf(5).
- Set the Checking Disabled flag on queries.
routine reads the configuration file (if any; see
resolv.conf(5)) to get the default domain name, search list, and the
Internet address of the local name server(s). If no server is configured,
the host running the resolver is tried. The current domain name is defined
by the hostname if not specified in the configuration file; it can be
overridden by the environment variable
This environment variable may contain several blank-separated tokens if you
wish to override the search list on a per-process basis. This is similar to
search command in the configuration file.
Another environment variable
RES_OPTIONS can be set
to override certain internal resolver options which are otherwise set by
changing fields in the _res structure or are inherited
from the configuration file's
options command. The
syntax of the
RES_OPTIONS environment variable is
explained in resolv.conf(5). Initialization normally occurs on the first
call to one of the following routines.
function provides an interface to the server query mechanism. It constructs
a query, sends it to the local server, awaits a response, and makes
preliminary checks on the reply. The query requests information of the
specified type and class for the
specified fully qualified domain name dname. The reply
message is left in the answer buffer with length
anslen supplied by the caller. Values for the
class and type fields are
routine makes a query and awaits a response like
res_query(), but in addition, it implements the
default and search rules controlled by the
options. It returns the first successful reply.
The remaining routines are lower-level routines
function constructs a standard query message and places it in
buf. It returns the size of the query, or -1 if the
query is larger than buflen. The query type
op is usually
QUERY, but can
be any of the query types defined in
<arpa/nameser.h>. The domain
name for the query is given by dname.
newrr is currently unused but is intended for making
routine sends a pre-formatted query and returns an answer. It will call
not set, send the query to the local name server, and handle timeouts and
retries. The length of the reply message is returned, or -1 if there were
function compresses the domain name exp_dn and stores
it in comp_dn. The size of the compressed name is
returned or -1 if there were errors. The size of the array pointed to by
comp_dn is given by length. The
compression uses an array of pointers dnptrs to
previously compressed names in the current message. The first pointer points
to the beginning of the message and the list ends with
NULL. The limit to the array is specified by
lastdnptr. A side effect of
dn_comp() is to update the list of pointers for
labels inserted into the message as the name is compressed. If
NULL, names are not
compressed. If lastdnptr is
NULL, the list of labels is not updated.
entry expands the compressed domain name comp_dn to a
full domain name. The compressed name is contained in a query or reply
message; msg is a pointer to the beginning of the
message. The uncompressed name is placed in the buffer indicated by
exp_dn which is of size length.
The size of compressed name is returned or -1 if there was an error.
- The configuration file.
gethostbyname(3), resolv.conf(5), hostname(7)
M. Stahl, Domain Administrators Guide, RFC 1032, November 1987.
M. Lottor, Domain Administrators Operations Guide, RFC 1033, November 1987.
P. Mockapetris, Domain Names – Concepts and Facilities, RFC 1034, November 1987.
P. Mockapetris, Domain Names – Implementation and Specification, RFC 1035, November 1987.
J. Klensin, Simple Mail Transfer Protocol, RFC 5321, October 2008.
appeared in 4.3BSD. The functions
appeared in 4.3BSD-Tahoe.