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Neither the name of the University nor the names of its contributors .\" may be used to endorse or promote products derived from this software .\" without specific prior written permission. .\" .\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE .\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF .\" SUCH DAMAGE. .\" .\" From: @(#)gethostbyname.3 8.4 (Berkeley) 5/25/95 .\" $FreeBSD$ .\" .Dd June 27, 2022 .Dt GETHOSTBYNAME 3 .Os .Sh NAME .Nm gethostbyname , .Nm gethostbyname2 , .Nm gethostbyaddr , .Nm gethostent , .Nm sethostent , .Nm endhostent , .Nm herror , .Nm hstrerror , .Nm gethostbyname_r , .Nm gethostbyname2_r , .Nm gethostbyaddr_r .Nd get network host entry .Sh LIBRARY .Lb libc .Sh SYNOPSIS .In netdb.h .Vt int h_errno ; .Ft struct hostent * .Fn gethostbyname "const char *name" .Ft struct hostent * .Fn gethostbyname2 "const char *name" "int af" .Ft struct hostent * .Fn gethostbyaddr "const void *addr" "socklen_t len" "int af" .Ft struct hostent * .Fn gethostent void .Ft void .Fn sethostent "int stayopen" .Ft void .Fn endhostent void .Ft void .Fn herror "const char *string" .Ft const char * .Fn hstrerror "int err" .Ft int .Fn gethostbyname_r "const char *name" "struct hostent *he" "char *buffer" "size_t buflen" "struct hostent **result" "int *h_errnop" .Ft int .Fn gethostbyname2_r "const char *name" "int af" "struct hostent *he" "char *buffer" "size_t buflen" "struct hostent **result" "int *h_errnop" .Ft int .Fn gethostbyaddr_r "const void *addr" "socklen_t len" "int af" "struct hostent *hp" "char *buf" "size_t buflen" "struct hostent **result" "int *h_errno"p .Sh DESCRIPTION .Bf -symbolic The .Xr getaddrinfo 3 and .Xr getnameinfo 3 functions are preferred over the .Fn gethostbyname , .Fn gethostbyname2 , and .Fn gethostbyaddr functions. .Ef .Pp The .Fn gethostbyname , .Fn gethostbyname2 and .Fn gethostbyaddr functions each return a pointer to an object with the following structure describing an internet host referenced by name or by address, respectively. .Pp The .Fa name argument passed to .Fn gethostbyname or .Fn gethostbyname2 should point to a .Dv NUL Ns -terminated hostname. The .Fa addr argument passed to .Fn gethostbyaddr should point to an address which is .Fa len bytes long, in binary form (i.e., not an IP address in human readable .Tn ASCII form). The .Fa af argument specifies the address family (e.g.\& .Dv AF_INET , AF_INET6 , etc.) of this address. .Pp The structure returned contains either the information obtained from the name server, broken-out fields from a line in .Pa /etc/hosts , or database entries supplied by the .Xr yp 8 system. The order of the lookups is controlled by the .Sq hosts entry in .Xr nsswitch.conf 5 . .Bd -literal struct hostent { char *h_name; /* official name of host */ char **h_aliases; /* alias list */ int h_addrtype; /* host address type */ int h_length; /* length of address */ char **h_addr_list; /* list of addresses from name server */ }; #define h_addr h_addr_list[0] /* address, for backward compatibility */ .Ed .Pp The members of this structure are: .Bl -tag -width h_addr_list .It Va h_name Official name of the host. .It Va h_aliases A .Dv NULL Ns -terminated array of alternate names for the host. .It Va h_addrtype The type of address being returned; usually .Dv AF_INET . .It Va h_length The length, in bytes, of the address. .It Va h_addr_list A .Dv NULL Ns -terminated array of network addresses for the host. Host addresses are returned in network byte order. .It Va h_addr The first address in .Va h_addr_list ; this is for backward compatibility. .El .Pp When using the nameserver, .Fn gethostbyname and .Fn gethostbyname2 will search for the named host in the current domain and its parents unless the name ends in a dot. If the name contains no dot, and if the environment variable .Dq Ev HOSTALIASES contains the name of an alias file, the alias file will first be searched for an alias matching the input name. See .Xr hostname 7 for the domain search procedure and the alias file format. .Pp The .Fn gethostbyname2 function is an evolution of .Fn gethostbyname which is intended to allow lookups in address families other than .Dv AF_INET , for example .Dv AF_INET6 . .Pp The .Fn sethostent function may be used to request the use of a connected .Tn TCP socket for queries. Queries will by default use .Tn UDP datagrams. If the .Fa stayopen flag is non-zero, a .Tn TCP connection to the name server will be used. It will remain open after calls to .Fn gethostbyname , .Fn gethostbyname2 or .Fn gethostbyaddr have completed. .Pp The .Fn endhostent function closes the .Tn TCP connection. .Pp The .Fn herror function writes a message to the diagnostic output consisting of the string argument .Fa string , the constant string .Qq Li ":\ " , and a message corresponding to the value of .Va h_errno . .Pp The .Fn hstrerror function returns a string which is the message text corresponding to the value of the .Fa err argument. .Pp Functions with the .Em _r suffix provide reentrant versions of their respective counterparts. The caller must supply five additional parameters: a .Vt struct hostent variable to be filled on success, a .Va buffer of .Va buflen bytes in size, a .Vt struct hostent .Va result variable that will point to the result on success or be set to .Dv NULL on failure or if the name is not found. The .Va h_errnop variable will be filled with the error code if any. All these functions return 0 on success. .Sh FILES .Bl -tag -width /etc/nsswitch.conf -compact .It Pa /etc/hosts .It Pa /etc/nsswitch.conf .It Pa /etc/resolv.conf .El .Sh EXAMPLES Print out the hostname associated with a specific IP address: .Bd -literal -offset indent const char *ipstr = "127.0.0.1"; struct in_addr ip; struct hostent *hp; if (!inet_aton(ipstr, &ip)) errx(1, "can't parse IP address %s", ipstr); if ((hp = gethostbyaddr((const void *)&ip, sizeof ip, AF_INET)) == NULL) errx(1, "no name associated with %s", ipstr); printf("name associated with %s is %s\en", ipstr, hp->h_name); .Ed .Sh DIAGNOSTICS Error return status from .Fn gethostbyname , .Fn gethostbyname2 and .Fn gethostbyaddr is indicated by return of a .Dv NULL pointer. The integer .Va h_errno may then be checked to see whether this is a temporary failure or an invalid or unknown host. The routine .Fn herror can be used to print an error message describing the failure. If its argument .Fa string is .Pf non- Dv NULL , it is printed, followed by a colon and a space. The error message is printed with a trailing newline. .Pp The variable .Va h_errno can have the following values: .Bl -tag -width HOST_NOT_FOUND .It Dv HOST_NOT_FOUND No such host is known. .It Dv 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. .It Dv NO_RECOVERY Some unexpected server failure was encountered. This is a non-recoverable error. .It Dv NO_DATA The requested name is valid but does not have an IP address; this is not a temporary error. This means that the name is known to the name server but there is no address associated with this name. Another type of request to the name server using this domain name will result in an answer; for example, a mail-forwarder may be registered for this domain. .El .Sh SEE ALSO .Xr getaddrinfo 3 , .Xr getnameinfo 3 , .Xr inet_aton 3 , .Xr resolver 3 , .Xr hosts 5 , .Xr hostname 7 .Sh HISTORY The .Fn herror function appeared in .Bx 4.3 . The .Fn endhostent , .Fn gethostbyaddr , .Fn gethostbyname , .Fn gethostent , and .Fn sethostent functions appeared in .Bx 4.2 . The .Fn gethostbyname2 function first appeared in .Tn BIND version 4.9.4. The .Fn gethostbyname_r function first appeared in .Fx 6.2 . .Sh CAVEATS The .Fn gethostent function is defined, and .Fn sethostent and .Fn endhostent are redefined, when .Lb libc is built to use only the routines to lookup in .Pa /etc/hosts and not the name server. .Pp The .Fn gethostent function reads the next line of .Pa /etc/hosts , opening the file if necessary. .Pp The .Fn sethostent function opens and/or rewinds the file .Pa /etc/hosts . If the .Fa stayopen argument is non-zero, the file will not be closed after each call to .Fn gethostbyname , .Fn gethostbyname2 or .Fn gethostbyaddr . .Pp The .Fn endhostent function closes the file. .Sh BUGS These functions use a thread-specific data storage; if the data is needed for future use, it should be copied before any subsequent calls overwrite it. .Pp Though these functions are thread-safe, still it is recommended to use the .Xr getaddrinfo 3 family of functions, instead. .Pp Only the Internet address format is currently understood.