'\" t .\" Copyright (c) 2007, 2008 Michael Kerrisk .\" and Copyright (c) 2006 Ulrich Drepper .\" A few pieces of an earlier version remain: .\" Copyright 2000, Sam Varshavchik .\" .\" SPDX-License-Identifier: Linux-man-pages-copyleft .\" .\" References: RFC 2553 .\" .\" 2005-08-09, mtk, added AI_ALL, AI_ADDRCONFIG, AI_V4MAPPED, .\" and AI_NUMERICSERV. .\" 2006-11-25, Ulrich Drepper .\" Add text describing Internationalized Domain Name extensions. .\" 2007-06-08, mtk: added example programs .\" 2008-02-26, mtk; clarify discussion of NULL 'hints' argument; other .\" minor rewrites. .\" 2008-06-18, mtk: many parts rewritten .\" 2008-12-04, Petr Baudis .\" Describe results ordering and reference /etc/gai.conf. .\" .\" FIXME . glibc's 2.9 NEWS file documents DCCP and UDP-lite support .\" and is SCTP support now also there? .\" .TH getaddrinfo 3 2023-07-20 "Linux man-pages 6.05.01" .SH NAME getaddrinfo, freeaddrinfo, gai_strerror \- network address and service translation .SH LIBRARY Standard C library .RI ( libc ", " \-lc ) .SH SYNOPSIS .nf .B #include .B #include .B #include .PP .BI "int getaddrinfo(const char *restrict " node , .BI " const char *restrict " service , .BI " const struct addrinfo *restrict " hints , .BI " struct addrinfo **restrict " res ); .PP .BI "void freeaddrinfo(struct addrinfo *" res ); .PP .BI "const char *gai_strerror(int " errcode ); .fi .PP .RS -4 Feature Test Macro Requirements for glibc (see .BR feature_test_macros (7)): .RE .PP .BR getaddrinfo (), .BR freeaddrinfo (), .BR gai_strerror (): .nf Since glibc 2.22: _POSIX_C_SOURCE >= 200112L glibc 2.21 and earlier: _POSIX_C_SOURCE .fi .SH DESCRIPTION Given .I node and .IR service , which identify an Internet host and a service, .BR getaddrinfo () returns one or more .I addrinfo structures, each of which contains an Internet address that can be specified in a call to .BR bind (2) or .BR connect (2). The .BR getaddrinfo () function combines the functionality provided by the .\" .BR getipnodebyname (3), .\" .BR getipnodebyaddr (3), .BR gethostbyname (3) and .BR getservbyname (3) functions into a single interface, but unlike the latter functions, .BR getaddrinfo () is reentrant and allows programs to eliminate IPv4-versus-IPv6 dependencies. .PP The .I addrinfo structure used by .BR getaddrinfo () contains the following fields: .PP .in +4n .EX struct addrinfo { int ai_flags; int ai_family; int ai_socktype; int ai_protocol; socklen_t ai_addrlen; struct sockaddr *ai_addr; char *ai_canonname; struct addrinfo *ai_next; }; .EE .in .PP The .I hints argument points to an .I addrinfo structure that specifies criteria for selecting the socket address structures returned in the list pointed to by .IR res . If .I hints is not NULL it points to an .I addrinfo structure whose .IR ai_family , .IR ai_socktype , and .I ai_protocol specify criteria that limit the set of socket addresses returned by .BR getaddrinfo (), as follows: .TP .I ai_family This field specifies the desired address family for the returned addresses. Valid values for this field include .B AF_INET and .BR AF_INET6 . The value .B AF_UNSPEC indicates that .BR getaddrinfo () should return socket addresses for any address family (either IPv4 or IPv6, for example) that can be used with .I node and .IR service . .TP .I ai_socktype This field specifies the preferred socket type, for example .B SOCK_STREAM or .BR SOCK_DGRAM . Specifying 0 in this field indicates that socket addresses of any type can be returned by .BR getaddrinfo (). .TP .I ai_protocol This field specifies the protocol for the returned socket addresses. Specifying 0 in this field indicates that socket addresses with any protocol can be returned by .BR getaddrinfo (). .TP .I ai_flags This field specifies additional options, described below. Multiple flags are specified by bitwise OR-ing them together. .PP All the other fields in the structure pointed to by .I hints must contain either 0 or a null pointer, as appropriate. .PP Specifying .I hints as NULL is equivalent to setting .I ai_socktype and .I ai_protocol to 0; .I ai_family to .BR AF_UNSPEC ; and .I ai_flags to .BR "(AI_V4MAPPED\ |\ AI_ADDRCONFIG)" . (POSIX specifies different defaults for .IR ai_flags ; see NOTES.) .I node specifies either a numerical network address (for IPv4, numbers-and-dots notation as supported by .BR inet_aton (3); for IPv6, hexadecimal string format as supported by .BR inet_pton (3)), or a network hostname, whose network addresses are looked up and resolved. If .I hints.ai_flags contains the .B AI_NUMERICHOST flag, then .I node must be a numerical network address. The .B AI_NUMERICHOST flag suppresses any potentially lengthy network host address lookups. .PP If the .B AI_PASSIVE flag is specified in .IR hints.ai_flags , and .I node is NULL, then the returned socket addresses will be suitable for .BR bind (2)ing a socket that will .BR accept (2) connections. The returned socket address will contain the "wildcard address" .RB ( INADDR_ANY for IPv4 addresses, .B IN6ADDR_ANY_INIT for IPv6 address). The wildcard address is used by applications (typically servers) that intend to accept connections on any of the host's network addresses. If .I node is not NULL, then the .B AI_PASSIVE flag is ignored. .PP If the .B AI_PASSIVE flag is not set in .IR hints.ai_flags , then the returned socket addresses will be suitable for use with .BR connect (2), .BR sendto (2), or .BR sendmsg (2). If .I node is NULL, then the network address will be set to the loopback interface address .RB ( INADDR_LOOPBACK for IPv4 addresses, .B IN6ADDR_LOOPBACK_INIT for IPv6 address); this is used by applications that intend to communicate with peers running on the same host. .PP .I service sets the port in each returned address structure. If this argument is a service name (see .BR services (5)), it is translated to the corresponding port number. This argument can also be specified as a decimal number, which is simply converted to binary. If .I service is NULL, then the port number of the returned socket addresses will be left uninitialized. If .B AI_NUMERICSERV is specified in .I hints.ai_flags and .I service is not NULL, then .I service must point to a string containing a numeric port number. This flag is used to inhibit the invocation of a name resolution service in cases where it is known not to be required. .PP Either .I node or .IR service , but not both, may be NULL. .PP The .BR getaddrinfo () function allocates and initializes a linked list of .I addrinfo structures, one for each network address that matches .I node and .IR service , subject to any restrictions imposed by .IR hints , and returns a pointer to the start of the list in .IR res . The items in the linked list are linked by the .I ai_next field. .PP There are several reasons why the linked list may have more than one .I addrinfo structure, including: the network host is multihomed, accessible over multiple protocols (e.g., both .B AF_INET and .BR AF_INET6 ); or the same service is available from multiple socket types (one .B SOCK_STREAM address and another .B SOCK_DGRAM address, for example). Normally, the application should try using the addresses in the order in which they are returned. The sorting function used within .BR getaddrinfo () is defined in RFC\ 3484; the order can be tweaked for a particular system by editing .I /etc/gai.conf (available since glibc 2.5). .PP If .I hints.ai_flags includes the .B AI_CANONNAME flag, then the .I ai_canonname field of the first of the .I addrinfo structures in the returned list is set to point to the official name of the host. .\" Prior to glibc 2.3.4, the ai_canonname of each addrinfo .\" structure was set pointing to the canonical name; that was .\" more than POSIX.1-2001 specified, or other implementations provided. .\" MTK, Aug 05 .PP The remaining fields of each returned .I addrinfo structure are initialized as follows: .IP \[bu] 3 The .IR ai_family , .IR ai_socktype , and .I ai_protocol fields return the socket creation parameters (i.e., these fields have the same meaning as the corresponding arguments of .BR socket (2)). For example, .I ai_family might return .B AF_INET or .BR AF_INET6 ; .I ai_socktype might return .B SOCK_DGRAM or .BR SOCK_STREAM ; and .I ai_protocol returns the protocol for the socket. .IP \[bu] A pointer to the socket address is placed in the .I ai_addr field, and the length of the socket address, in bytes, is placed in the .I ai_addrlen field. .PP If .I hints.ai_flags includes the .B AI_ADDRCONFIG flag, then IPv4 addresses are returned in the list pointed to by .I res only if the local system has at least one IPv4 address configured, and IPv6 addresses are returned only if the local system has at least one IPv6 address configured. The loopback address is not considered for this case as valid as a configured address. This flag is useful on, for example, IPv4-only systems, to ensure that .BR getaddrinfo () does not return IPv6 socket addresses that would always fail in .BR connect (2) or .BR bind (2). .PP If .I hints.ai_flags specifies the .B AI_V4MAPPED flag, and .I hints.ai_family was specified as .BR AF_INET6 , and no matching IPv6 addresses could be found, then return IPv4-mapped IPv6 addresses in the list pointed to by .IR res . If both .B AI_V4MAPPED and .B AI_ALL are specified in .IR hints.ai_flags , then return both IPv6 and IPv4-mapped IPv6 addresses in the list pointed to by .IR res . .B AI_ALL is ignored if .B AI_V4MAPPED is not also specified. .PP The .BR freeaddrinfo () function frees the memory that was allocated for the dynamically allocated linked list .IR res . .SS Extensions to getaddrinfo() for Internationalized Domain Names Starting with glibc 2.3.4, .BR getaddrinfo () has been extended to selectively allow the incoming and outgoing hostnames to be transparently converted to and from the Internationalized Domain Name (IDN) format (see RFC 3490, .IR "Internationalizing Domain Names in Applications (IDNA)" ). Four new flags are defined: .TP .B AI_IDN If this flag is specified, then the node name given in .I node is converted to IDN format if necessary. The source encoding is that of the current locale. .IP If the input name contains non-ASCII characters, then the IDN encoding is used. Those parts of the node name (delimited by dots) that contain non-ASCII characters are encoded using ASCII Compatible Encoding (ACE) before being passed to the name resolution functions. .\" Implementation Detail: .\" To minimize effects on system performance the implementation might .\" want to check whether the input string contains any non-ASCII .\" characters. If there are none the IDN step can be skipped completely. .\" On systems which allow not-ASCII safe encodings for a locale this .\" might be a problem. .TP .B AI_CANONIDN After a successful name lookup, and if the .B AI_CANONNAME flag was specified, .BR getaddrinfo () will return the canonical name of the node corresponding to the .I addrinfo structure value passed back. The return value is an exact copy of the value returned by the name resolution function. .IP If the name is encoded using ACE, then it will contain the .I xn\-\- prefix for one or more components of the name. To convert these components into a readable form the .B AI_CANONIDN flag can be passed in addition to .BR AI_CANONNAME . The resulting string is encoded using the current locale's encoding. .\" .\"Implementation Detail: .\"If no component of the returned name starts with xn\-\- the IDN .\"step can be skipped, therefore avoiding unnecessary slowdowns. .TP .BR AI_IDN_ALLOW_UNASSIGNED ", " AI_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 getaddrinfo(); 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 .BR getaddrinfo () returns 0 if it succeeds, or one of the following nonzero error codes: .TP .B EAI_ADDRFAMILY .\" Not in SUSv3 The specified network host does not have any network addresses in the requested address family. .TP .B EAI_AGAIN The name server returned a temporary failure indication. Try again later. .TP .B EAI_BADFLAGS .I hints.ai_flags contains invalid flags; or, .I hints.ai_flags included .B AI_CANONNAME and .I name was NULL. .TP .B EAI_FAIL The name server returned a permanent failure indication. .TP .B EAI_FAMILY The requested address family is not supported. .TP .B EAI_MEMORY Out of memory. .TP .B EAI_NODATA .\" Not in SUSv3 The specified network host exists, but does not have any network addresses defined. .TP .B EAI_NONAME The .I node or .I service is not known; or both .I node and .I service are NULL; or .B AI_NUMERICSERV was specified in .I hints.ai_flags and .I service was not a numeric port-number string. .TP .B EAI_SERVICE The requested service is not available for the requested socket type. It may be available through another socket type. For example, this error could occur if .I service was "shell" (a service available only on stream sockets), and either .I hints.ai_protocol was .BR IPPROTO_UDP , or .I hints.ai_socktype was .BR SOCK_DGRAM ; or the error could occur if .I service was not NULL, and .I hints.ai_socktype was .B SOCK_RAW (a socket type that does not support the concept of services). .TP .B EAI_SOCKTYPE The requested socket type is not supported. This could occur, for example, if .I hints.ai_socktype and .I hints.ai_protocol are inconsistent (e.g., .B SOCK_DGRAM and .BR IPPROTO_TCP , respectively). .TP .B EAI_SYSTEM Other system error; .I errno is set to indicate the error. .PP The .BR gai_strerror () function translates these error codes to a human readable string, suitable for error reporting. .SH FILES .I /etc/gai.conf .SH ATTRIBUTES For an explanation of the terms used in this section, see .BR attributes (7). .TS allbox; lbx lb lb l l l. Interface Attribute Value T{ .na .nh .BR getaddrinfo () T} Thread safety MT-Safe env locale T{ .na .nh .BR freeaddrinfo (), .BR gai_strerror () T} Thread safety MT-Safe .TE .sp 1 .SH VERSIONS According to POSIX.1, specifying .\" POSIX.1-2001, POSIX.1-2008 .I hints as NULL should cause .I ai_flags to be assumed as 0. The GNU C library instead assumes a value of .B (AI_V4MAPPED\~|\~AI_ADDRCONFIG) for this case, since this value is considered an improvement on the specification. .SH STANDARDS POSIX.1-2008. .TP .BR getaddrinfo () RFC\ 2553. .SH HISTORY POSIX.1-2001. .TP .B AI_ADDRCONFIG .TQ .B AI_ALL .TQ .B AI_V4MAPPED glibc 2.3.3. .TP .B AI_NUMERICSERV glibc 2.3.4. .SH NOTES .BR getaddrinfo () supports the .IB address % scope-id notation for specifying the IPv6 scope-ID. .SH EXAMPLES .\" getnameinfo.3 refers to this example .\" socket.2 refers to this example .\" bind.2 refers to this example .\" connect.2 refers to this example .\" recvfrom.2 refers to this example .\" sendto.2 refers to this example The following programs demonstrate the use of .BR getaddrinfo (), .BR gai_strerror (), .BR freeaddrinfo (), and .BR getnameinfo (3). The programs are an echo server and client for UDP datagrams. .SS Server program \& .\" SRC BEGIN (server.c) .EX #include #include #include #include #include #include #include \& #define BUF_SIZE 500 \& int main(int argc, char *argv[]) { int sfd, s; char buf[BUF_SIZE]; ssize_t nread; socklen_t peer_addrlen; struct addrinfo hints; struct addrinfo *result, *rp; struct sockaddr_storage peer_addr; \& if (argc != 2) { fprintf(stderr, "Usage: %s port\en", argv[0]); exit(EXIT_FAILURE); } \& memset(&hints, 0, sizeof(hints)); hints.ai_family = AF_UNSPEC; /* Allow IPv4 or IPv6 */ hints.ai_socktype = SOCK_DGRAM; /* Datagram socket */ hints.ai_flags = AI_PASSIVE; /* For wildcard IP address */ hints.ai_protocol = 0; /* Any protocol */ hints.ai_canonname = NULL; hints.ai_addr = NULL; hints.ai_next = NULL; \& s = getaddrinfo(NULL, argv[1], &hints, &result); if (s != 0) { fprintf(stderr, "getaddrinfo: %s\en", gai_strerror(s)); exit(EXIT_FAILURE); } \& /* getaddrinfo() returns a list of address structures. Try each address until we successfully bind(2). If socket(2) (or bind(2)) fails, we (close the socket and) try the next address. */ \& for (rp = result; rp != NULL; rp = rp\->ai_next) { sfd = socket(rp\->ai_family, rp\->ai_socktype, rp\->ai_protocol); if (sfd == \-1) continue; \& if (bind(sfd, rp\->ai_addr, rp\->ai_addrlen) == 0) break; /* Success */ \& close(sfd); } \& freeaddrinfo(result); /* No longer needed */ \& if (rp == NULL) { /* No address succeeded */ fprintf(stderr, "Could not bind\en"); exit(EXIT_FAILURE); } \& /* Read datagrams and echo them back to sender. */ \& for (;;) { char host[NI_MAXHOST], service[NI_MAXSERV]; \& peer_addrlen = sizeof(peer_addr); nread = recvfrom(sfd, buf, BUF_SIZE, 0, (struct sockaddr *) &peer_addr, &peer_addrlen); if (nread == \-1) continue; /* Ignore failed request */ \& s = getnameinfo((struct sockaddr *) &peer_addr, peer_addrlen, host, NI_MAXHOST, service, NI_MAXSERV, NI_NUMERICSERV); if (s == 0) printf("Received %zd bytes from %s:%s\en", nread, host, service); else fprintf(stderr, "getnameinfo: %s\en", gai_strerror(s)); \& if (sendto(sfd, buf, nread, 0, (struct sockaddr *) &peer_addr, peer_addrlen) != nread) { fprintf(stderr, "Error sending response\en"); } } } .EE .\" SRC END .SS Client program \& .\" SRC BEGIN (client.c) .EX #include #include #include #include #include #include #include \& #define BUF_SIZE 500 \& int main(int argc, char *argv[]) { int sfd, s; char buf[BUF_SIZE]; size_t len; ssize_t nread; struct addrinfo hints; struct addrinfo *result, *rp; \& if (argc < 3) { fprintf(stderr, "Usage: %s host port msg...\en", argv[0]); exit(EXIT_FAILURE); } \& /* Obtain address(es) matching host/port. */ \& memset(&hints, 0, sizeof(hints)); hints.ai_family = AF_UNSPEC; /* Allow IPv4 or IPv6 */ hints.ai_socktype = SOCK_DGRAM; /* Datagram socket */ hints.ai_flags = 0; hints.ai_protocol = 0; /* Any protocol */ \& s = getaddrinfo(argv[1], argv[2], &hints, &result); if (s != 0) { fprintf(stderr, "getaddrinfo: %s\en", gai_strerror(s)); exit(EXIT_FAILURE); } \& /* getaddrinfo() returns a list of address structures. Try each address until we successfully connect(2). If socket(2) (or connect(2)) fails, we (close the socket and) try the next address. */ \& for (rp = result; rp != NULL; rp = rp\->ai_next) { sfd = socket(rp\->ai_family, rp\->ai_socktype, rp\->ai_protocol); if (sfd == \-1) continue; \& if (connect(sfd, rp\->ai_addr, rp\->ai_addrlen) != \-1) break; /* Success */ \& close(sfd); } \& freeaddrinfo(result); /* No longer needed */ \& if (rp == NULL) { /* No address succeeded */ fprintf(stderr, "Could not connect\en"); exit(EXIT_FAILURE); } \& /* Send remaining command\-line arguments as separate datagrams, and read responses from server. */ \& for (size_t j = 3; j < argc; j++) { len = strlen(argv[j]) + 1; /* +1 for terminating null byte */ \& if (len > BUF_SIZE) { fprintf(stderr, "Ignoring long message in argument %zu\en", j); continue; } \& if (write(sfd, argv[j], len) != len) { fprintf(stderr, "partial/failed write\en"); exit(EXIT_FAILURE); } \& nread = read(sfd, buf, BUF_SIZE); if (nread == \-1) { perror("read"); exit(EXIT_FAILURE); } \& printf("Received %zd bytes: %s\en", nread, buf); } \& exit(EXIT_SUCCESS); } .EE .\" SRC END .SH SEE ALSO .\" .BR getipnodebyaddr (3), .\" .BR getipnodebyname (3), .BR getaddrinfo_a (3), .BR gethostbyname (3), .BR getnameinfo (3), .BR gai.conf (5), .BR inet (3), .BR gai.conf (5), .BR hostname (7), .BR ip (7)