.\" $NetBSD: getaddrinfo.3,v 1.60.6.1 2024/07/20 15:25:50 martin Exp $ .\" $KAME: getaddrinfo.3,v 1.36 2005/01/05 03:23:05 itojun Exp $ .\" $OpenBSD: getaddrinfo.3,v 1.35 2004/12/21 03:40:31 jaredy 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. .\" .Dd May 9, 2024 .Dt GETADDRINFO 3 .Os .Sh NAME .Nm getaddrinfo , .Nm freeaddrinfo , .Nm allocaddrinfo .Nd host and service name to socket address structure .Sh SYNOPSIS .In netdb.h .Ft int .Fn getaddrinfo "const char * restrict hostname" \ "const char * restrict servname" \ "const struct addrinfo * restrict hints" "struct addrinfo ** restrict res" .Ft void .Fn freeaddrinfo "struct addrinfo *ai" .Ft struct addrinfo * .Fn allocaddrinfo "socklen_t len" .Sh DESCRIPTION The .Fn getaddrinfo function is used to get a list of .Tn IP addresses and port numbers for host .Fa hostname and service .Fa servname . It is a replacement for and provides more flexibility than the .Xr gethostbyname 3 and .Xr getservbyname 3 functions. .Pp The .Fa hostname and .Fa servname arguments are either pointers to NUL-terminated strings or the null pointer. An acceptable value for .Fa hostname is either a valid host name or a numeric host address string consisting of a dotted decimal IPv4 address or an IPv6 address. The .Fa servname is either a decimal port number or a service name listed in .Xr services 5 . At least one of .Fa hostname and .Fa servname must be non-null. .Pp .Fa hints is an optional pointer to a .Li struct addrinfo , as defined by .In netdb.h : .Bd -literal struct addrinfo { int ai_flags; /* input flags */ int ai_family; /* address family for socket */ int ai_socktype; /* socket type */ int ai_protocol; /* protocol for socket */ socklen_t ai_addrlen; /* length of socket-address */ char *ai_canonname; /* canonical name for service location */ struct sockaddr *ai_addr; /* socket-address for socket */ struct addrinfo *ai_next; /* pointer to next in list */ }; .Ed .Pp This structure can be used to provide hints concerning the type of socket that the caller supports or wishes to use. The caller can supply the following structure elements in .Fa hints : .Bl -tag -width "ai_socktypeXX" .It Fa ai_family The address .Pq and protocol family that should be used. When .Fa ai_family is set to .Dv AF_UNSPEC , it means the caller will accept any address family supported by the operating system. Note that while address families .Pq Dv AF_* and protocol families .Pq Dv PF_* are theoretically distinct, in practice the distinction has been lost. .\" (.Dv !? Consistent with usage below though...) .Dv "RFC 3493" defines .Fn getaddrinfo in terms of the address family constants .Dv AF_* even though .Fa ai_family is to be passed as a protocol family to .Xr socket 2 . .It Fa ai_socktype Denotes the type of socket that is wanted: .Dv SOCK_STREAM , .Dv SOCK_DGRAM , or .Dv SOCK_RAW . When .Fa ai_socktype is zero the caller will accept any socket type. .It Fa ai_protocol Indicates which transport protocol is desired, .Dv IPPROTO_UDP or .Dv IPPROTO_TCP . If .Fa ai_protocol is zero the caller will accept any protocol. .It Fa ai_flags .Fa ai_flags is formed by .Tn OR Ns 'ing the following values: .Bl -tag -width "AI_CANONNAMEXX" .It Dv AI_CANONNAME If the .Dv AI_CANONNAME bit is set, a successful call to .Fn getaddrinfo will return a NUL-terminated string containing the canonical name of the specified hostname in the .Fa ai_canonname element of the first .Li addrinfo structure returned. .It Dv AI_NUMERICHOST If the .Dv AI_NUMERICHOST bit is set, it indicates that .Fa hostname should be treated as a numeric string defining an IPv4 or IPv6 address and no name resolution should be attempted. .It Dv AI_NUMERICSERV If the .Dv AI_NUMERICSERV bit is set, it indicates that the .Fa servname string contains a numeric port number. This is used to prevent service name resolution. .It Dv AI_PASSIVE If the .Dv AI_PASSIVE bit is set it indicates that the returned socket address structure is intended for use in a call to .Xr bind 2 . In this case, if the .Fa hostname argument is the null pointer, then the IP address portion of the socket address structure will be set to .Dv INADDR_ANY for an IPv4 address or .Dv IN6ADDR_ANY_INIT for an IPv6 address. .Pp If the .Dv AI_PASSIVE bit is not set, the returned socket address structure will be ready for use in a call to .Xr connect 2 for a connection-oriented protocol or .Xr connect 2 , .Xr sendto 2 , or .Xr sendmsg 2 if a connectionless protocol was chosen. The .Tn IP address portion of the socket address structure will be set to the loopback address if .Fa hostname is the null pointer and .Dv AI_PASSIVE is not set. .It Dv AI_SRV Perform SRV (RFC 2782) record lookups first. This is a non-portable extension. .El .El .Pp All other elements of the .Li addrinfo structure passed via .Fa hints must be zero or the null pointer. .Pp If .Fa hints is the null pointer, .Fn getaddrinfo behaves as if the caller provided a .Li struct addrinfo with .Fa ai_family set to .Dv AF_UNSPEC and all other elements set to zero or .Dv NULL . .Pp After a successful call to .Fn getaddrinfo , .Fa *res is a pointer to a linked list of one or more .Li addrinfo structures. The list can be traversed by following the .Fa ai_next pointer in each .Li addrinfo structure until a null pointer is encountered. The three members .Fa ai_family , .Fa ai_socktype , and .Fa ai_protocol in each returned .Li addrinfo structure are suitable for a call to .Xr socket 2 . For each .Li addrinfo structure in the list, the .Fa ai_addr member points to a filled-in socket address structure of length .Fa ai_addrlen . .Pp By default IPv6 address entries are ordered before IPv4 ones, but the order of the entries in the list can be controlled using .Xr ip6addrctl 8 . .Pp This implementation of .Fn getaddrinfo allows numeric IPv6 address notation with scope identifier, as documented in chapter 11 of draft-ietf-ipv6-scoping-arch-02.txt. By appending the percent character and scope identifier to addresses, one can fill the .Li sin6_scope_id field for addresses. This would make management of scoped addresses easier and allows cut-and-paste input of scoped addresses. .Pp At this moment the code supports only link-local addresses with the format. The scope identifier is hardcoded to the name of the hardware interface associated with the link .Po such as .Li ne0 .Pc . An example is .Dq Li fe80::1%ne0 , which means .Do .Li fe80::1 on the link associated with the .Li ne0 interface .Dc . .Pp The current implementation assumes a one-to-one relationship between the interface and link, which is not necessarily true from the specification. .Pp All of the information returned by .Fn getaddrinfo is dynamically allocated: the .Li addrinfo structures themselves as well as the socket address structures and the canonical host name strings included in the .Li addrinfo structures. .Pp Memory allocated for the dynamically allocated structures created by a successful call to .Fn getaddrinfo is released by the .Fn freeaddrinfo function. The .Fa ai pointer should be an .Li addrinfo structure created by a call to .Fn getaddrinfo or .Fn allocaddrinfo . The .Fn allocaddrinfo function is intended primarily for authors of .Xr nsdispatch 3 plugins implementing .Fn getaddrinfo backends. .Fn allocaddrinfo allocates a .Li struct addrinfo in a way that is compatible with being returned from .Fn getaddrinfo and being ultimately freed by .Fn freeaddrinfo . The returned structure is zeroed, except for the .Fa ai_addr field, which will point to .Fa len bytes of memory for storage of a socket address. It is safe to allocate memory separately for .Fa ai_canonname with .Xr malloc 3 , or in any other way that is compatible with deallocation by .Xr free 3 . .Sh RETURN VALUES .Fn getaddrinfo 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 connect to .Dq Li www.kame.net service .Dq Li http via a stream socket. It loops through all the addresses available, regardless of address family. If the destination resolves to an IPv4 address, it will use an .Dv AF_INET socket. Similarly, if it resolves to IPv6, an .Dv AF_INET6 socket is used. Observe that there is no hardcoded reference to a particular address family. The code works even if .Fn getaddrinfo returns addresses that are not IPv4/v6. .Bd -literal -offset indent struct addrinfo hints, *res, *res0; int error; int s; const char *cause = NULL; memset(&hints, 0, sizeof(hints)); hints.ai_family = AF_UNSPEC; hints.ai_socktype = SOCK_STREAM; error = getaddrinfo("www.kame.net", "http", &hints, &res0); if (error) { errx(1, "%s", gai_strerror(error)); /*NOTREACHED*/ } s = -1; for (res = res0; res; res = res->ai_next) { s = socket(res->ai_family, res->ai_socktype, res->ai_protocol); if (s < 0) { cause = "socket"; continue; } if (connect(s, res->ai_addr, res->ai_addrlen) < 0) { cause = "connect"; close(s); s = -1; continue; } break; /* okay we got one */ } if (s < 0) { err(1, "%s", cause); /*NOTREACHED*/ } freeaddrinfo(res0); .Ed .Pp The following example tries to open a wildcard listening socket onto service .Dq Li http , for all the address families available. .Bd -literal -offset indent struct addrinfo hints, *res, *res0; int error; int s[MAXSOCK]; int nsock; const char *cause = NULL; memset(&hints, 0, sizeof(hints)); hints.ai_family = AF_UNSPEC; hints.ai_socktype = SOCK_STREAM; hints.ai_flags = AI_PASSIVE; error = getaddrinfo(NULL, "http", &hints, &res0); if (error) { errx(1, "%s", gai_strerror(error)); /*NOTREACHED*/ } nsock = 0; for (res = res0; res && nsock < MAXSOCK; res = res->ai_next) { s[nsock] = socket(res->ai_family, res->ai_socktype, res->ai_protocol); if (s[nsock] < 0) { cause = "socket"; continue; } if (bind(s[nsock], res->ai_addr, res->ai_addrlen) < 0) { cause = "bind"; close(s[nsock]); continue; } (void) listen(s[nsock], 5); nsock++; } if (nsock == 0) { err(1, "%s", cause); /*NOTREACHED*/ } freeaddrinfo(res0); .Ed .Sh SEE ALSO .Xr bind 2 , .Xr connect 2 , .Xr send 2 , .Xr socket 2 , .Xr gai_strerror 3 , .Xr gethostbyname 3 , .Xr getnameinfo 3 , .Xr getservbyname 3 , .Xr resolver 3 , .Xr hosts 5 , .Xr resolv.conf 5 , .Xr services 5 , .Xr hostname 7 , .Xr ip6addrctl 8 , .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 internet draft .%N draft-ietf-ipv6-scoping-arch-02.txt .%O work in progress material .Re .Rs .%A Craig Metz .%T "Protocol Independence Using the Sockets API" .%I USENIX Association .%B Proceedings of the FREENIX Track: 2000 USENIX Annual Technical Conference .%D June 18-23, 2000 .%P 99-108 .%U http://www.usenix.org/events/usenix2000/freenix/metzprotocol/metzprotocol.pdf .Re .Sh STANDARDS The .Fn getaddrinfo function is defined by the .St -p1003.1g-2000 draft specification and documented in .Dv "RFC 3493" , .Dq Basic Socket Interface Extensions for IPv6 . .Sh NOTES In the current implementation, .Dl freeaddrinfo(NULL); will cause a segmentation fault. .Po Tn RFC 3493 does not specify what should happen, .St -xns5.2 says .Dv NULL is an invalid argument.)