$KAME: getnameinfo.3,v 1.37 2005/01/05 03:23:05 itojun Exp $
$OpenBSD: getnameinfo.3,v 1.36 2004/12/21 09:48:20 jmc Exp $

Copyright (C) 2004 Internet Systems Consortium, Inc. ("ISC")
Copyright (C) 2000, 2001 Internet Software Consortium.

Permission to use, copy, modify, and distribute this software for any
purpose with or without fee is hereby granted, provided that the above
copyright notice and this permission notice appear in all copies.

THE SOFTWARE IS PROVIDED "AS IS" AND ISC DISCLAIMS ALL WARRANTIES WITH
REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
AND FITNESS. IN NO EVENT SHALL ISC BE LIABLE FOR ANY SPECIAL, DIRECT,
INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE
OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
PERFORMANCE OF THIS SOFTWARE.

$FreeBSD$

.Dd March 15, 2018 .Dt GETNAMEINFO 3 .Os .Sh NAME .Nm getnameinfo .Nd socket address structure to hostname and service name .Sh SYNOPSIS n sys/types.h n sys/socket.h n netdb.h .Ft int .Fo getnameinfo .Fa "const struct sockaddr *sa" "socklen_t salen" "char *host" .Fa "size_t hostlen" "char *serv" "size_t servlen" "int flags" .Fc .Sh DESCRIPTION The .Fn getnameinfo function is used to convert a .Li sockaddr structure to a pair of host name and service strings. It is a replacement for and provides more flexibility than the .Xr gethostbyaddr 3 and .Xr getservbyport 3 functions and is the converse of the .Xr getaddrinfo 3 function.

p If a link-layer address or UNIX-domain address is passed to .Fn getnameinfo , its ASCII representation will be stored in .Fa host . The string pointed to by .Fa serv will be set to the empty string if non-NULL; .Fa flags will always be ignored. For a link-layer address, this can be used as a replacement of the legacy .Xr link_ntoa 3 function.

p The .Li sockaddr structure .Fa sa should point to either a .Li sockaddr_in , .Li sockaddr_in6 , .Li sockaddr_dl , or .Li sockaddr_un structure

o for IPv4 , IPv6, link-layer, or UNIX-domain respectively

c that is .Fa salen bytes long. If .Fa salen is shorter than the length corresponding to the specified address family or longer than .Fn sizeof "struct sockaddr_storage" , it returns .Er EAI_FAMILY . Note that .Va sa->sa_len should be consistent with .Fa salen though the value of .Va sa->sa_len is not directly used in this function.

p The host and service names associated with .Fa sa are stored in .Fa host and .Fa serv which have length parameters .Fa hostlen and .Fa servlen . The maximum value for .Fa hostlen is .Dv NI_MAXHOST and the maximum value for .Fa servlen is .Dv NI_MAXSERV , as defined by .Aq Pa netdb.h . If a length parameter is zero, no string will be stored. Otherwise, enough space must be provided to store the host name or service string plus a byte for the NUL terminator.

p The .Fa flags argument is formed by .Tn OR Ns 'ing the following values: l -tag -width "NI_NUMERICSCOPEXX" t Dv NI_NOFQDN A fully qualified domain name is not required for local hosts. The local part of the fully qualified domain name is returned instead. t Dv NI_NUMERICHOST Return the address in numeric form, as if calling .Xr inet_ntop 3 , instead of a host name. t Dv NI_NAMEREQD A name is required. If the host name cannot be found in DNS and this flag is set, a non-zero error code is returned. If the host name is not found and the flag is not set, the address is returned in numeric form. t NI_NUMERICSERV The service name is returned as a digit string representing the port number. t NI_NUMERICSCOPE The scope identifier is returned as a digit string. t NI_DGRAM Specifies that the service being looked up is a datagram service, and causes .Xr getservbyport 3 to be called with a second argument of .Dq udp instead of its default of .Dq tcp . This is required for the few ports (512-514) that have different services for .Tn UDP and .Tn TCP . .El

p This implementation allows numeric IPv6 address notation with scope identifier, as documented in chapter 11 of RFC 4007. IPv6 link-local address will appear as a string like .Dq Li fe80::1%ne0 . Refer to .Xr getaddrinfo 3 for more information. .Sh RETURN VALUES .Fn getnameinfo returns zero on success or one of the error codes listed in .Xr gai_strerror 3 if an error occurs. .Sh EXAMPLES The following code tries to get a numeric host name, and service name, for a given socket address. Observe that there is no hardcoded reference to a particular address family. d -literal -offset indent struct sockaddr *sa; /* input */ char hbuf[NI_MAXHOST], sbuf[NI_MAXSERV]; if (getnameinfo(sa, sa->sa_len, hbuf, sizeof(hbuf), sbuf, sizeof(sbuf), NI_NUMERICHOST | NI_NUMERICSERV)) { errx(1, "could not get numeric hostname"); /* NOTREACHED */ } printf("host=%s, serv=%s\en", hbuf, sbuf); .Ed

p The following version checks if the socket address has a reverse address mapping: d -literal -offset indent struct sockaddr *sa; /* input */ char hbuf[NI_MAXHOST]; if (getnameinfo(sa, sa->sa_len, hbuf, sizeof(hbuf), NULL, 0, NI_NAMEREQD)) { errx(1, "could not resolve hostname"); /* NOTREACHED */ } printf("host=%s\en", hbuf); .Ed .Sh SEE ALSO .Xr gai_strerror 3 , .Xr getaddrinfo 3 , .Xr gethostbyaddr 3 , .Xr getservbyport 3 , .Xr inet_ntop 3 , .Xr link_ntoa 3 , .Xr resolver 3 , .Xr inet 4 , .Xr inet6 4 , .Xr unix 4 , .Xr hosts 5 , .Xr resolv.conf 5 , .Xr services 5 , .Xr hostname 7 , .Xr named 8 .Rs .%A R. Gilligan .%A S. Thomson .%A J. Bound .%A J. McCann .%A W. Stevens .%T Basic Socket Interface Extensions for IPv6 .%R RFC 3493 .%D February 2003 .Re .Rs .%A S. Deering .%A B. Haberman .%A T. Jinmei .%A E. Nordmark .%A B. Zill .%T "IPv6 Scoped Address Architecture" .%R RFC 4007 .%D March 2005 .Re .Rs .%A Craig Metz .%T Protocol Independence Using the Sockets API .%B "Proceedings of the freenix track: 2000 USENIX annual technical conference" .%D June 2000 .Re .Sh STANDARDS The .Fn getnameinfo function is defined by the .St -p1003.1-2004 specification and documented in .Tn "RFC 3493" , .Dq Basic Socket Interface Extensions for IPv6 . .Sh CAVEATS .Fn getnameinfo can return both numeric and FQDN forms of the address specified in .Fa sa . There is no return value that indicates whether the string returned in .Fa host is a result of binary to numeric-text translation (like .Xr inet_ntop 3 ) , or is the result of a DNS reverse lookup. Because of this, malicious parties could set up a PTR record as follows: d -literal -offset indent 1.0.0.127.in-addr.arpa. IN PTR 10.1.1.1 .Ed

p and trick the caller of .Fn getnameinfo into believing that .Fa sa is .Li 10.1.1.1 when it is actually .Li 127.0.0.1 .

p To prevent such attacks, the use of .Dv NI_NAMEREQD is recommended when the result of .Fn getnameinfo is used for access control purposes: d -literal -offset indent struct sockaddr *sa; socklen_t salen; char addr[NI_MAXHOST]; struct addrinfo hints, *res; int error; error = getnameinfo(sa, salen, addr, sizeof(addr), NULL, 0, NI_NAMEREQD); if (error == 0) { memset(&hints, 0, sizeof(hints)); hints.ai_socktype = SOCK_DGRAM; /*dummy*/ hints.ai_flags = AI_NUMERICHOST; if (getaddrinfo(addr, "0", &hints, &res) == 0) { /* malicious PTR record */ freeaddrinfo(res); printf("bogus PTR record\en"); return -1; } /* addr is FQDN as a result of PTR lookup */ } else { /* addr is numeric string */ error = getnameinfo(sa, salen, addr, sizeof(addr), NULL, 0, NI_NUMERICHOST); } .Ed .Pp
.Ox
intentionally uses a different
.Dv NI_MAXHOST
value from what
.Tn "RFC 2553"
suggests, to avoid buffer length handling mistakes.