gethostbyname()--Get Host Information for Host Name

BSD 4.3 Syntax

  #include <netdb.h>

 struct hostent *gethostbyname(char *host_name)

  Service Program Name: QSOSRV2

  Default Public Authority: *USE

  Threadsafe: No; see Usage Notes.

UNIX^® 98 Compatible Syntax

  #define _XOPEN_SOURCE 520
  #include <netdb.h>

 struct hostent *gethostbyname(const char *host_name)

  Service Program Name: QSOSRV2

  Default Public Authority: *USE

  Threadsafe: No; see Usage Notes.

The gethostbyname() function is used to retrieve information about a host.

There are two versions of the API, as shown above. The base IBM^® i API uses BSD 4.3 structures and syntax. The other uses syntax and structures compatible with the UNIX 98 programming interface specifications. You can select the UNIX 98 compatible interface with the _XOPEN_SOURCE macro.

Parameters

host_name: (Input) The pointer to the character string that contains the name of the host for which information is to be retrieved.

Authorities

Authorization of *R (allow access to the object) to the host aliases file specified by the HOSTALIASES environment variable.

You also need *X authority to each directory in the path of the host aliases file.

Return Value

gethostbyname() returns a pointer. Possible values are:

NULL (unsuccessful)
p (successful), where p is a pointer to struct hostent.

The structure struct hostent is defined in <netdb.h>.

      struct hostent {
        char   *h_name;
        char   **h_aliases;
        int    h_addrtype;
        int    h_length;
        char   **h_addr_list;
      };

      #define h_addr  h_addr_list[0]

h_name points to the character string that contains the name of the host. h_aliases is a pointer to a NULL-terminated list of pointers, each of which points to a character string that represents an alternative name for the host. h_addrtype contains the address type of the host (for example, AF_INET). h_length contains the address length. h_addr_list is a pointer to a NULL-terminated list of pointers, each of which points to a network address for the host, in network byte order. Note that the array of address pointers points to structures of type in_addr defined in <netinet/in.h>.

Error Conditions

When gethostbyname() fails, h_errno (defined in <netdb.h>) can be set to one of the following:

[HOST_NOT_FOUND]: The host name specified by the host_name parameter was not found.
[NO_DATA]: The host name is a valid name, but there is no corresponding IP address.
[NO_RECOVERY]: An unrecoverable error has occurred.
[TRY_AGAIN]: The local server did not receive a response from an authoritative server. An attempt at a later time may succeed.

When the gethostbyname() function fails, errno can be set to:

[EACCES]: Permission denied. The process does not have the appropriate privileges to the host aliases file specified by the HOSTALIASES environment variable.

Usage Notes

System i^® Navigator or the following CL commands can be used to access the local host table:
- ADDTCPHTE (Add TCP/IP Host Table Entry)
- RMVTCPHTE (Remove TCP/IP Host Table Entry)
- CHGTCPHTE (Change TCP/IP Host Table Entry)
- RNMTCPHTE (Rename TCP/IP Host Table Entry)
- MRGTCPHT (Merge TCP/IP Host Tables)
- CFGTCP (Configure TCP/IP), option "10. Work with TCP/IP host table entries"
There are limits to both the number of entries and the size of those entries returned in the hostent structure. The limits are defined in <netdb.h> and entries may be truncated. The string and pointer arrays should be traversed by looking for null terminators rather than relying on hardcoded limits.
The pointer returned by gethostbyname() points to static storage that is overwritten on subsequent calls to the gethostbyname(), gethostbyaddr(), or gethostent() functions.
There are two sources from which host information can be obtained: the domain name server, and the local host table. The path taken depends on whether an IP address is configured for a name server with System i Navigator or with option 12, Change TCP/IP domain information, on the Configure TCP/IP (CFGTCP) menu.
Note: A person with a UNIX background would expect this information to exist in a file known as /etc/resolv.conf.

If the IP address is found (indicating that the local network is a domain network), the gethostbyaddr() function attempts to query the domain name server for information about a host. If the query fails, the information is obtained from the local host table. If the name server IP address is not found (indicating that local network is a flat network), the local host table is used to obtain the address.
If the host_name parameter does specify a domain qualified name, the gethostbyaddr() function appends a domain name to the specified host name, if possible. The domain name that is appended is configured with System i Navigator or with the CFGTCP menu option 12, Change TCP/IP domain information.
When the host information is obtained from the local host table, the table is opened and the host information is retrieved (if it exists) from the table. The table is then closed only if a sethostent() with a nonzero parameter value was not previously done.
If a sethostent() with a nonzero parameter value was previously done, the gethostbyname() routine, when obtaining host information from the domain name server, communicates with the domain name server over a connection-oriented transport service (for example, TCP). Otherwise, gethostbyname() uses a connectionless transport service (for example, UDP).
A job has a coded character set identifier (CCSID) and a default CCSID. The default CCSID is the same as the job CCSID unless the job CCSID specifies 65535, which requests that no translation be performed. In this case, the default CCSID is set by the system based on the language ID in effect for the job.
If the host information is retrieved from the domain name server, sockets converts the host name specified by the host_name parameter from the default (CCSID) to ASCII before communicating with the domain name server. If the host information is retrieved from the local host table, no conversion is done on the host name specified by the host_name parameter unless the CCSID of the job is something other than 65535. In addition, the host names returned in the hostent structure will be returned in the default CCSID of the job if they are obtained from the domain name server. For translation to occur for the host names returned in the hostent structure when they are obtained from the local host table, you must use a job CCSID of something other than 65535.
Address families are defined in <sys/socket.h>, and the in_addr structure is defined in <netinet/in.h>.
Do not use the gethostbyname() function in a multithreaded environment. See the multithread alternative gethostbyname_r() function.
gethostbyname() will resolve local host aliases to a domain name which are then resolved with a query using DNS. See res_hostalias() for more information about aliases.
When you develop in C-based languages and an application is compiled with the _XOPEN_SOURCE macro defined to the value 520 or greater, the gethostbyname() API is mapped to qso_gethostbyname98().

Related Information

_XOPEN_SOURCE--Using _XOPEN_SOURCE for the UNIX 98 compatible interface
hstrerror()--Retrieve Resolver Error Message
res_hostalias()--Retrieve the host alias
gethostbyaddr()--Get Host Information for IP Address
gethostent()--Get Next Entry from Local Host Table
sethostent()--Open Local Host Table
endhostent()--Close Local Host Table
getaddrinfo()--Get Address Information
getnameinfo()--Get Name Information for Socket Address

API introduced: V3R1

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