The gethostbyname() function returns a structure of type hostent for the given host name. Here name is either a host name, or an IPv4 address in standard dot notation, or an IPv6 address in colon (and possibly dot) notation. (See RFC 1884 for the description of IPv6 addresses.) If name is an IPv4 or IPv6 address, no lookup is performed and gethostbyname() simply copies name into the h_name field and its struct in_addr equivalent into the h_addr_list[0? field of the returned hostent structure. If name doesn't end in a dot and the environment variable HOSTALIASES is set, the alias file pointed to by HOSTALIASES will first be searched for name (see hostname(7) for the file format). The current domain and its parents are searched unless name ends in a dot.
The gethostbyaddr() function returns a structure of type hostent for the given host address addr of length len and address type type. The only valid address type is currently AF_INET.
The sethostent() function specifies, if stayopen is true (1), that a connected TCP socket should be used for the name server queries and that the connection should remain open during successive queries. Otherwise, name server queries will use UDP datagrams.
The endhostent() function ends the use of a TCP connection for name server queries.
The (obsolete) herror() function prints the error message associated with the current value of h_errno on stderr.
The (obsolete) hstrerror() function takes an error number (typically h_errno) and returns the corresponding message string.
The domain name queries carried out by gethostbyname() and gethostbyaddr() use a combination of any or all of the name server named(8), a broken out line from /etc/hosts, and the Network Information Service (NIS or YP), depending upon the contents of the order line in /etc/host.conf. (See resolv+(8)). The default action is to query named(8), followed by /etc/hosts.
The hostent structure is defined in as follows:
The members of the hostent structure are:
The official name of the host.
A zero-terminated array of alternative names for the host.
The type of address; always AF_INET at present.
The length of the address in bytes.
A zero-terminated array of network addresses for the host in network byte order.
The variable h_errno can have the following values:
The specified host is unknown.
NO_ADDRESS or NO_DATA
The requested name is valid but does not have an IP address.
A non-recoverable name server error occurred.
resolver configuration file
The functions gethostbyname() and gethostbyaddr() may return pointers to static data, which may be overwritten by later calls. Copying the struct hostent does not suffice, since it contains pointers - a deep copy is required.
The SUS-v2 standard is buggy and declares the len parameter to be of type size_t. (That is wrong, because it has to be int, and size_t is not. The Austin draft makes it socklen_t, which is OK.)
Glibc2 also has a
struct hostent *gethostbyname2(const char *name, int af); that works like gethostbyname(), but permits to specify the address family to which the address must belong.
The Austin draft marks gethostbyaddr() and gethostbyname() legacy, and introduces