resolver(3N)

NAME

resolver: dn_comp(), dn_expand(), get_resfield(), herror(), res_init(), res_mkquery(), res_query(), res_search(), res_send(), set_resfield() — resolver routines

SYNOPSIS

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

ssize_t res_query( 
     char *dname, 
     int class, 
     int type, 
     u_char *answer, 
     int anslen 
); 

ssize_t res_search( 
     char *dname, 
     int class, 
     int type, 
     u_char *answer, 
     int anslen 
); 

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

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

int res_init(); 

ssize_t dn_comp( 
     const char *exp_dn, 
     u_char *comp_dn, 
     ssize_t length, 
     u_char **dnptrs, 
     u_char **lastdnptr 
); 

ssize_t dn_expand( 
     const u_char *msg, 
     const u_char *eomorig, 
     const u_char *comp_dn, 
     u_char *exp_dn, 
     int  length 
); 

int set_resfield( 
    int field, 
    void *value 
); 

int get_resfield( 
    int field, 
    void *value, 
    sizeof value 
); 

OBSOLESCENT INTERFACE

void herror(const char *s);

DESCRIPTION

These routines are used for making, sending, and interpreting query and reply messages with Internet domain name servers.

Global configuration and state information used by the resolver routines are kept in the structure _res and are defined in <resolv.h>. Most of the fields have reasonable defaults and can be ignored. The resolver options are stored in the _res.options field and are listed below. The options are stored as a simple bit mask containing the bitwise OR of the options enabled.

In a multithreaded environment, a thread specific _res structure is allocated for each thread.

RES_INIT: True if the initial name server address and default domain name are initialized (i.e., res_init() has been called).
RES_DEBUG: Print debugging messages.
RES_AAONLY: Accept authoritative answers only. With this option, res_send() should continue until it finds an authoritative answer or finds an error. Currently this is not implemented.
RES_PRIMARY: Query the primary server only. Currently this is not implemented.
RES_USEVC: Use TCP connections for queries instead of UDP datagrams.
RES_STAYOPEN: 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.
RES_IGNTC: The name server will set the truncation bit if all of the data does not fit into the response datagram packet. If RES_IGNTC is set, res_send() will not retry the query with TCP (i.e., ignore truncation errors).
RES_RECURSE: Set the recursion-desired bit in queries. This is the default. (res_send() does not do iterative queries and expects the name server to handle recursion.)
RES_DEFNAMES: If set, res_search() appends the default domain name to single-component names (those that do not contain a dot). This option is enabled by default.
RES_DNSRCH: If this option is set, res_search() searches for host names in the current domain and in parent domains; see hostname(5). This is used by the standard host lookup routine gethostbyname() (see gethostent(3N)). This option is enabled by default.

Initialization of the resolver structure normally occurs on the first call to one of the resolver routines below. If there are errors in the configuration file, they are silently ignored.

The values for retransmission timeout and number of retries to be attempted can be configured. These correspond to the retrans and retry fields in the _res structure. The following three options, listed in the order of precedence, have been provided for configuring the retransmission timeout and retry values.

1.: Environment Variables,
2.: Configuration file /etc/resolv.conf,
3.: Through calls to API set_resfield().

retrans and retry can be configured through the Environmental Variables RES_RETRANS and RES_RETRY as follows:

RES_RETRANS=values in milliseconds 
RES_RETRY=number of retries 

Alternatively in resolv.conf the following name-value pairs can be added :

retrans value in milliseconds
retry number of retries

While the Environmental Variables and entries in the resolv.conf file are interpreted when the res_init() API is called, the API set_resfield() has to be explicitly called from within the code. Setting the retrans and retry values through a lower precedence option will be ignored if these values have been configured through higher precedence option. A message is flagged in syslog when an invalid value is specified in either resolv.conf or Environmental variables. The retrans is to be specified in milliseconds, and its default value is 5000 milliseconds. The default value for retry is 4.

Primary Routines

res_init(): Reads the configuration file, /etc/resolv.conf, to get the default domain name, search list, the Internet address of the local name server(s), the values for retrans and retry. 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 and overrides the search list on a per process basis. This is similar to the search command in the configuration file. Another environment variable such as, RES_OPTIONS can be set to override certain internal resolver options which are set by calling some of the configuration routines above, or by using the configuration file's options command. The syntax of the RES_OPTIONS environment variable is explained in resolver(4). The resolv.conf entries for retrans and retry can be overridden by RES_RETRANS and RES_RETRY Environmental Variables respectively.
res_query(): 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.
res_search(): Makes a query and awaits a response much like res_query(), but in addition, it implements the default and search rules controlled by the RES_DEFNAMES and RES_DNSRCH options. It returns the first successful reply.
set_resfield(): Sets the value for the retry and retrans fields in the _res structure. The value for the retrans option must be specified in milliseconds. This routine also validates the values which the user tries to set for the retry and retrans options. set_resfield() returns 0 on success, and -1 on failure. Calls to set_resfield() fails when values for the field passed as argument is already set by any higher precedence option like entering name-value pairs in the resolv.conf file or setting the Environmental Variables RES_RETRANS and RES_RETRY.
get_resfield(): Get the value for the retry and retrans fields in the _res structure. get_resfield() returns the value of the field requested on success and -1 if on failure. It fails when the arguments do not refer to retrans or retry.

Other Routines

Routines described here are lower-level routines used by res_query().

res_mkquery(): 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. class can be any of the query classes defined in <arpa/nameser.h>. type can be any of the query types defined in <arpa/nameser.h>. data is the data for an inverse query (IQUERY). newrr is currently unused but is intended for making update messages.
res_send(): Sends a pre-formatted query and returns an answer. It calls res_init() if RES_INIT is not set, sends the query to the local name server, and handles timeouts and retries. res_send() returns the length of the reply message, or -1 if there were errors.
dn_comp(): 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. length is the size of the array pointed to by comp_dn. The compression uses an array of pointers dnptrs to previously compressed names in the current message. The first pointer points to 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 dnptr is NULL, names are not compressed. If lastdnptr is NULL, the list of labels is not updated.
dn_expand(): 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.

Obsolescent Routine

herror()

herror() supports existing applications to print an error message describing a failure. ANSI applications must specify the following definition for herror():

void herror(const char *);

The argument string s is printed first, followed by a colon, a blank, the message, and a new-line. herror() may be removed in future releases.

RETURN VALUE

Error return status from res_search() is indicated by a return value of -1. The external integer h_errno can then be checked to see whether this is a temporary failure or an invalid or unknown host.

In a multithreaded application using kernel thread, a thread specific h_errno is allocated for each thread.

ERRORS

h_errno can have the following values:

HOST_NOT_FOUND: No such host is known.
TRY_AGAIN: This is usually a temporary error and means that the local server did not receive a response from an authoritative server. A retry at some later time may succeed.
NO_RECOVERY: Some unexpected server failure was encountered. This is a non-recoverable error.
NO_DATA: The name is known to the name server, but there is no data of the requested type associated with this name; this is not a temporary error. Another type of request to the name server using this domain name will result in an answer.

WARNINGS

The _res.options field should be modified only by using set_resfield() call, and should not be manipulated directly. h_errno is referenced as an extern int for non-threaded applications and is defined as function call macro for multithreaded applications in file /usr/include/netdb.h. Applications which reference h_errno should include /usr/include/netdb.h.

_res is referenced as an extern struct _res_state for non-threaded applications and is defined as function call macro for multithreaded application in file /usr/include/resolv.h. Applications which reference _res should include /usr/include/resolv.h.

AUTHOR

These resolver routines were developed by the University of California, Berkeley.

FILES

/etc/resolv.conf: Resolver configuration file.