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res_query, res_search, res_mkquery, res_send, res_init, dn_comp, dn_expand — resolver routines

#include <sys/types.h>
#include <netinet/in.h>
#include <arpa/nameser.h>
#include <resolv.h>

int
res_query(const char *dname, int class, int type, unsigned char *answer, int anslen);

int
res_search(const char *dname, int class, int type, unsigned char *answer, int anslen);

int
res_mkquery(int op, const char *dname, int class, int type, const unsigned char *data, int datalen, const unsigned char *newrr, unsigned char *buf, int buflen);

int
res_send(const unsigned char *msg, int msglen, unsigned char *answer, int anslen);

int
res_init(void);

int
dn_comp(const char *exp_dn, unsigned char *comp_dn, int length, unsigned char **dnptrs, unsigned char **lastdnptr);

int
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 initialized (i.e. () 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_USEVC to 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 HOSTALIASES feature.
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.
Set the Checking Disabled flag on queries.

The () 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 LOCALDOMAIN. 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 the 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.

The () 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 defined in <arpa/nameser.h>.

The remaining routines are lower-level routines used by (). The () 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 update messages.

The () routine sends a pre-formatted query and returns an answer. It will call res_init() if RES_INIT is 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 errors.

The () 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 dnptrs is NULL, names are not compressed. If lastdnptr is NULL, the list of labels is not updated.

The () 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.

/etc/resolv.conf
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.

The functions res_mkquery(), res_send(), res_init(), dn_comp(), and dn_expand() appeared in 4.3BSD. The functions res_query() and res_search() appeared in 4.3BSD-Tahoe.

April 25, 2020 OpenBSD-6.8