The _res Data Structure

Global information used by these resolver calls is kept in the _res data structure. This structure is defined in the <RESOLV.H> file and contains the following members:

┌─────────────────────────────────────────────────────────────────┐
│Type                  Member                Contents             │
├─────────────────────────────────────────────────────────────────┤
│int                   retrans               Retransmission time  │
│                                            interval             │
├─────────────────────────────────────────────────────────────────┤
│int                   retry                 Number of times to   │
│                                            retransmit           │
├─────────────────────────────────────────────────────────────────┤
│long                  options               Option flags         │
├─────────────────────────────────────────────────────────────────┤
│int                   nscount               Number of name       │
│                                            servers              │
├─────────────────────────────────────────────────────────────────┤
│struct                nsaddr_list           Address of name      │
│sockaddr_in[MAXNS]                          server               │
├─────────────────────────────────────────────────────────────────┤
│unsigned short        id                    Current packet id    │
├─────────────────────────────────────────────────────────────────┤
│char[MAXDNAME]        defdname              Default domain       │
├─────────────────────────────────────────────────────────────────┤
│char*[MAXDNSRCH+1]    dnsrch                Components of domain │
│                                            to search            │
└─────────────────────────────────────────────────────────────────┘

The options field of the _res data structure is constructed by logically ORing the following values:

RES_INIT

Indicates whether the initial name server and default domain name have been initialized (that is, whether the res_init() call has been called).

Prints debugging messages.

Uses Transmission Control Protocol/Internet Protocol (TCP/IP) connections for queries instead of User Datagram Protocol/Internet Protocol (UDP/IP).

Used with the RES_USEVC value, keeps the TCP/IP connection open between queries. While UDP/IP is the mode normally used, TCP/IP mode and this option are useful for programs that regularly perform many queries.

Sets the Recursion Desired bit for queries. This is the default.

Appends the default domain name to single-label queries. This is the default.

These environment variables affect values related to the _res data structure:

LOCALDOMAIN

Overrides the default local domain, which is read from the ETC\RESOLV.conf file and stored in the defdname field of the _res data structure.

Overrides the default value of the retrans field of the _res data structure, which is the value of the RES_TIMEOUT constant defined in the <RESOLV.H> file. This value is the base timeout period in seconds between queries to the name servers. After each failed attempt, the timeout period is doubled. The timeout period is divided by the number of name servers defined. The minimum timeout period is 1 second.

Overrides the default value for the retry field of the _res data structure, which is 4. This value is the number of times the resolver tries to query the name servers before giving up. Setting RES_RETRY to 0 prevents the resolver from querying the name servers.

The res_send() call does not perform interactive queries and expects the name server to handle recursion.