June 4, 1993 RESOLVER 3

### NAME

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

### LIBRARY

Standard C Library (libc, -lc)

### SYNOPSIS

res_query(`` ```const char *dname` ```int class` ```int type` ```u_char *answer` ```int anslen` ) res_search(`` ```const char *dname` ```int class` ```int type` ```u_char *answer` ```int anslen` ) res_mkquery(`` ```int op` ```const char *dname` ```int class` ```int type` ```const char *data` ```int datalen` ```struct rrec *newrr` ```char *buf` ```int buflen` ) res_send(`` ```const u_char *msg` ```int msglen` ```u_char *answer` ```int anslen` ) res_init(``) dn_comp(`` ```const char *exp_dn` ```u_char *comp_dn` ```int length` ```u_char **dnptrs` ```u_char **lastdnptr` ) dn_expand(`` ```const u_char *msg` ```const u_char *eomorig` ```const u_char *comp_dn` ```u_char *exp_dn` ```int length` )

### DESCRIPTION

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.

```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_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`
  Unused currently (ignore truncation errors, i.e., don't retry with TCP).

```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(``) will append 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(``) 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.

```RES_USE_INET6`
  Enables support for IPv6-only applications. This causes IPv4 addresses to be returned as an IPv4 mapped address. For example, 10.1.1.1 will be returned as ::ffff:10.1.1.1. The option is meaningful with certain kernel configuration only.

```RES_USE_EDNS0`
  Enables support for OPT pseudo-RR for EDNS0 extension. With the option, resolver code will attach OPT pseudo-RR into DNS queries, to inform of our receive buffer size. The option will allow DNS servers to take advantage of non-default receive buffer size, and to send larger replies. DNS query packets with EDNS0 extension is not compatible with non-EDNS0 DNS servers.

The res_init(``) 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 res_query(``) 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.

The res_search(``) routine makes a query and awaits a response 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.

The remaining routines are lower-level routines used by res_query(``). The res_mkquery(``) 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 res_send(``) 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 dn_comp(``) 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 dnptr is ```NULL`, names are not compressed. If ```lastdnptr` is ```NULL`, the list of labels is not updated.

The dn_expand(``) 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.

### FILES

```/etc/resolv.conf`
  The configuration file, see resolv.conf(5).

### SEE ALSO

gethostbyname(3), resolv.conf(5), hostname(7), named(8)

### HISTORY

The res_query function appeared in 4.3BSD.