'\" t .\" %%%LICENSE_START(PUBLIC_DOMAIN) .\" This page is in the public domain. .\" %%%LICENSE_END .\" .\" Almost all details are from RFC 2553. .\" .\" 2004-12-14, mtk, Added EAI_OVERFLOW error .\" 2004-12-14 Fixed description of error return .\" .TH getnameinfo 3 2023-03-30 "Linux man-pages 6.04" .SH NAME getnameinfo \- address-to-name translation in protocol-independent manner .SH LIBRARY Standard C library .RI ( libc ", " \-lc ) .SH SYNOPSIS .nf .B #include .B #include .PP .BI "int getnameinfo(const struct sockaddr *restrict " addr \ ", socklen_t " addrlen , .BI " char " host "[_Nullable restrict ." hostlen ], .BI " socklen_t " hostlen , .BI " char " serv "[_Nullable restrict ." servlen ], .BI " socklen_t " servlen , .BI " int " flags ); .fi .PP .RS -4 Feature Test Macro Requirements for glibc (see .BR feature_test_macros (7)): .RE .PP .BR getnameinfo (): .nf Since glibc 2.22: _POSIX_C_SOURCE >= 200112L glibc 2.21 and earlier: _POSIX_C_SOURCE .fi .SH DESCRIPTION The .BR getnameinfo () function is the inverse of .BR getaddrinfo (3): it converts a socket address to a corresponding host and service, in a protocol-independent manner. It combines the functionality of .BR gethostbyaddr (3) and .BR getservbyport (3), but unlike those functions, .BR getnameinfo () is reentrant and allows programs to eliminate IPv4-versus-IPv6 dependencies. .PP The .I addr argument is a pointer to a generic socket address structure (of type .I sockaddr_in or .IR sockaddr_in6 ) of size .I addrlen that holds the input IP address and port number. The arguments .I host and .I serv are pointers to caller-allocated buffers (of size .I hostlen and .I servlen respectively) into which .BR getnameinfo () places null-terminated strings containing the host and service names respectively. .PP The caller can specify that no hostname (or no service name) is required by providing a NULL .I host (or .IR serv ) argument or a zero .I hostlen (or .IR servlen ) argument. However, at least one of hostname or service name must be requested. .PP The .I flags argument modifies the behavior of .BR getnameinfo () as follows: .TP .B NI_NAMEREQD If set, then an error is returned if the hostname cannot be determined. .TP .B NI_DGRAM If set, then the service is datagram (UDP) based rather than stream (TCP) based. This is required for the few ports (512\[en]514) that have different services for UDP and TCP. .TP .B NI_NOFQDN If set, return only the hostname part of the fully qualified domain name for local hosts. .TP .B NI_NUMERICHOST If set, then the numeric form of the hostname is returned. .\" For example, by calling .\" .BR inet_ntop () .\" instead of .\" .BR gethostbyaddr (). (When not set, this will still happen in case the node's name cannot be determined.) .\" POSIX.1-2001 TC1 has NI_NUMERICSCOPE, but glibc doesn't have it. .TP .B NI_NUMERICSERV If set, then the numeric form of the service address is returned. (When not set, this will still happen in case the service's name cannot be determined.) .SS Extensions to getnameinfo() for Internationalized Domain Names Starting with glibc 2.3.4, .BR getnameinfo () has been extended to selectively allow hostnames to be transparently converted to and from the Internationalized Domain Name (IDN) format (see RFC 3490, .IR "Internationalizing Domain Names in Applications (IDNA)" ). Three new flags are defined: .TP .B NI_IDN If this flag is used, then the name found in the lookup process is converted from IDN format to the locale's encoding if necessary. ASCII-only names are not affected by the conversion, which makes this flag usable in existing programs and environments. .TP .BR NI_IDN_ALLOW_UNASSIGNED ", " NI_IDN_USE_STD3_ASCII_RULES Setting these flags will enable the IDNA_ALLOW_UNASSIGNED (allow unassigned Unicode code points) and IDNA_USE_STD3_ASCII_RULES (check output to make sure it is a STD3 conforming hostname) flags respectively to be used in the IDNA handling. .SH RETURN VALUE .\" FIXME glibc defines the following additional errors, some which .\" can probably be returned by getnameinfo(); they need to .\" be documented. .\" .\" #ifdef __USE_GNU .\" #define EAI_INPROGRESS -100 /* Processing request in progress. */ .\" #define EAI_CANCELED -101 /* Request canceled. */ .\" #define EAI_NOTCANCELED -102 /* Request not canceled. */ .\" #define EAI_ALLDONE -103 /* All requests done. */ .\" #define EAI_INTR -104 /* Interrupted by a signal. */ .\" #define EAI_IDN_ENCODE -105 /* IDN encoding failed. */ .\" #endif On success, 0 is returned, and node and service names, if requested, are filled with null-terminated strings, possibly truncated to fit the specified buffer lengths. On error, one of the following nonzero error codes is returned: .TP .B EAI_AGAIN The name could not be resolved at this time. Try again later. .TP .B EAI_BADFLAGS The .I flags argument has an invalid value. .TP .B EAI_FAIL A nonrecoverable error occurred. .TP .B EAI_FAMILY The address family was not recognized, or the address length was invalid for the specified family. .TP .B EAI_MEMORY Out of memory. .TP .B EAI_NONAME The name does not resolve for the supplied arguments. .B NI_NAMEREQD is set and the host's name cannot be located, or neither hostname nor service name were requested. .TP .B EAI_OVERFLOW The buffer pointed to by .I host or .I serv was too small. .TP .B EAI_SYSTEM A system error occurred. The error code can be found in .IR errno . .PP The .BR gai_strerror (3) function translates these error codes to a human readable string, suitable for error reporting. .SH FILES .I /etc/hosts .br .I /etc/nsswitch.conf .br .I /etc/resolv.conf .SH ATTRIBUTES For an explanation of the terms used in this section, see .BR attributes (7). .ad l .nh .TS allbox; lbx lb lb l l l. Interface Attribute Value T{ .BR getnameinfo () T} Thread safety MT-Safe env locale .TE .hy .ad .sp 1 .SH STANDARDS POSIX.1-2008. RFC\ 2553. .SH HISTORY glibc 2.1. POSIX.1-2001. .PP Before glibc 2.2, the .I hostlen and .I servlen arguments were typed as .IR size_t . .SH NOTES In order to assist the programmer in choosing reasonable sizes for the supplied buffers, .I defines the constants .PP .in +4n .EX #define NI_MAXHOST 1025 #define NI_MAXSERV 32 .EE .in .PP Since glibc 2.8, these definitions are exposed only if suitable feature test macros are defined, namely: .BR _GNU_SOURCE , .B _DEFAULT_SOURCE (since glibc 2.19), or (in glibc versions up to and including 2.19) .B _BSD_SOURCE or .BR _SVID_SOURCE . .PP The former is the constant .B MAXDNAME in recent versions of BIND's .I header file. The latter is a guess based on the services listed in the current Assigned Numbers RFC. .SH EXAMPLES The following code tries to get the numeric hostname and service name, for a given socket address. Note that there is no hardcoded reference to a particular address family. .PP .in +4n .EX struct sockaddr *addr; /* input */ socklen_t addrlen; /* input */ char hbuf[NI_MAXHOST], sbuf[NI_MAXSERV]; if (getnameinfo(addr, addrlen, hbuf, sizeof(hbuf), sbuf, sizeof(sbuf), NI_NUMERICHOST | NI_NUMERICSERV) == 0) printf("host=%s, serv=%s\en", hbuf, sbuf); .EE .in .PP The following version checks if the socket address has a reverse address mapping. .PP .in +4n .EX struct sockaddr *addr; /* input */ socklen_t addrlen; /* input */ char hbuf[NI_MAXHOST]; if (getnameinfo(addr, addrlen, hbuf, sizeof(hbuf), NULL, 0, NI_NAMEREQD)) printf("could not resolve hostname"); else printf("host=%s\en", hbuf); .EE .in .PP An example program using .BR getnameinfo () can be found in .BR getaddrinfo (3). .SH SEE ALSO .BR accept (2), .BR getpeername (2), .BR getsockname (2), .BR recvfrom (2), .BR socket (2), .BR getaddrinfo (3), .BR gethostbyaddr (3), .BR getservbyname (3), .BR getservbyport (3), .BR inet_ntop (3), .BR hosts (5), .BR services (5), .BR hostname (7), .BR named (8) .PP R.\& Gilligan, S.\& Thomson, J.\& Bound and W.\& Stevens, .IR "Basic Socket Interface Extensions for IPv6" , RFC\ 2553, March 1999. .PP Tatsuya Jinmei and Atsushi Onoe, .IR "An Extension of Format for IPv6 Scoped Addresses" , internet draft, work in progress .UR ftp://ftp.ietf.org\:/internet\-drafts\:/draft\-ietf\-ipngwg\-scopedaddr\-format\-02.txt .UE . .PP Craig Metz, .IR "Protocol Independence Using the Sockets API" , Proceedings of the freenix track: 2000 USENIX annual technical conference, June 2000 .ad l .UR http://www.usenix.org\:/publications\:/library\:/proceedings\:/usenix2000\:/freenix\:/metzprotocol.html .UE .