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GETADDRINFO(3) Library Functions Manual GETADDRINFO(3) NAME getaddrinfo, freeaddrinfo -- socket address structure to host and service name SYNOPSIS #include <sys/types.h> #include <sys/socket.h> #include <netdb.h> int getaddrinfo(const char *hostname, const char *servname, const struct addrinfo *hints, struct addrinfo **res); void freeaddrinfo(struct addrinfo *ai); DESCRIPTION The getaddrinfo() function is used to get a list of addresses and port num- bers for host hostname and service servname. It is a replacement for and provides more flexibility than the gethostbyname(3) and getservbyname(3) functions. The hostname and servname arguments are either pointers to NUL-terminated strings or the null pointer. An acceptable value for hostname is either a valid host name or a numeric host address string consisting of a dotted decimal IPv4 address, an IPv6 address, or a UNIX-domain address. The servname is either a decimal port number or a service name listed in ser- vices(5). At least one of hostname and servname must be non-null. hints is an optional pointer to a struct addrinfo, as defined by <netdb.h>: struct addrinfo { int ai_flags; /* AI_PASSIVE, AI_CANONNAME, .. */ int ai_family; /* AF_xxx */ int ai_socktype; /* SOCK_xxx */ int ai_protocol; /* 0 or IPPROTO_xxx for IPv4 and IPv6 */ socklen_t ai_addrlen; /* length of ai_addr */ char *ai_canonname; /* canonical name for hostname */ struct sockaddr *ai_addr; /* binary address */ struct addrinfo *ai_next; /* next structure in linked list */ }; 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 fol- lowing structure elements in hints: ai_family The address family that should be used. When ai_family is set to AF_UNSPEC, it means the caller will accept any ad- dress family supported by the operating system. ai_socktype Denotes the type of socket that is wanted: SOCK_STREAM, SOCK_DGRAM, SOCK_SEQPACKET, or SOCK_RAW. When ai_socktype is zero the caller will accept any socket type. ai_protocol Indicates which transport protocol is desired, IPPROTO_UDP, IPPROTO_TCP, IPPROTO_SCTP, or IPPROTO_UDPLITE. If ai_protocol is zero the caller will accept any protocol. ai_flags The ai_flags field to which the hints parameter points shall be set to zero or be the bitwise-inclusive OR of one or more of the values AI_ADDRCONFIG, AI_ALL, AI_CANONNAME, AI_NUMERICHOST, AI_NUMERICSERV, AI_PASSIVE and AI_V4MAPPED. For a UNIX-domain address, ai_flags is ignored. AI_ADDRCONFIG If the AI_ADDRCONFIG bit is set, IPv4 ad- dresses shall be returned only if an IPv4 address is configured on the local system, and IPv6 addresses shall be returned only if an IPv6 address is configured on the local system. AI_ALL If the AI_ALL flag is used with the AI_V4MAPPED flag, then getaddrinfo() shall return all matching IPv6 and IPv4 addresses. For example, when using the DNS, queries are made for both AAAA records and A records, and getaddrinfo() returns the combined re- sults of both queries. Any IPv4 addresses found are returned as IPv4-mapped IPv6 ad- dresses. The AI_ALL flag without the AI_V4MAPPED flag is ignored. AI_CANONNAME If the AI_CANONNAME bit is set, a successful call to getaddrinfo() will return a NUL-ter- minated string containing the canonical name of the specified hostname in the ai_canonname element of the first addrinfo structure returned. AI_NUMERICHOST If the AI_NUMERICHOST bit is set, it indi- cates that hostname should be treated as a numeric string defining an IPv4 or IPv6 ad- dress and no name resolution should be at- tempted. AI_NUMERICSERV If the AI_NUMERICSERV bit is set, then a non-null servname string supplied shall be a numeric port string. Otherwise, an EAI_NONAME error shall be returned. This bit shall prevent any type of name resolu- tion service (for example, NIS+) from being invoked. AI_PASSIVE If the AI_PASSIVE bit is set it indicates that the returned socket address structure is intended for use in a call to bind(2). In this case, if the hostname argument is the null pointer, then the IP address por- tion of the socket address structure will be set to INADDR_ANY for an IPv4 address or IN6ADDR_ANY_INIT for an IPv6 address. If the AI_PASSIVE bit is not set, the re- turned socket address structure will be ready for use in a call to connect(2) for a connection-oriented protocol or connect(2), sendto(2), or sendmsg(2) if a connectionless protocol was chosen. The IP address portion of the socket address structure will be set to the loopback address if hostname is the null pointer and AI_PASSIVE is not set. AI_V4MAPPED If the AI_V4MAPPED flag is specified along with an ai_family of AF_INET6, then getad- drinfo() shall return IPv4-mapped IPv6 ad- dresses on finding no matching IPv6 ad- dresses ( ai_addrlen shall be 16). For example, when using the DNS, if no AAAA records are found then a query is made for A records and any found are returned as IPv4-mapped IPv6 addresses. The AI_V4MAPPED flag shall be ignored unless ai_family equals AF_INET6. All other elements of the addrinfo structure passed via hints must be zero or the null pointer. If hints is the null pointer, getaddrinfo() behaves as if the caller pro- vided a struct addrinfo with ai_family set to AF_UNSPEC and all other ele- ments set to zero or NULL. After a successful call to getaddrinfo(), *res is a pointer to a linked list of one or more addrinfo structures. The list can be traversed by fol- lowing the ai_next pointer in each addrinfo structure until a null pointer is encountered. Each returned addrinfo structure contains three members that are suitable for a call to socket(2): ai_family, ai_socktype, and ai_protocol. For each addrinfo structure in the list, the ai_addr member points to a filled-in socket address structure of length ai_addrlen. This implementation of getaddrinfo() allows numeric IPv6 address notation with scope identifier, as documented in chapter 11 of RFC 4007. By append- ing the percent character and scope identifier to addresses, one can fill the sin6_scope_id field for addresses. This would make management of scoped addresses easier and allows cut-and-paste input of scoped addresses. 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 as- sociated with the link (such as ne0). An example is "fe80::1%ne0", which means "fe80::1 on the link associated with the ne0 interface". The current implementation assumes a one-to-one relationship between the interface and link, which is not necessarily true from the specification. All of the information returned by getaddrinfo() is dynamically allocated: the addrinfo structures themselves as well as the socket address structures and the canonical host name strings included in the addrinfo structures. Memory allocated for the dynamically allocated structures created by a suc- cessful call to getaddrinfo() is released by the freeaddrinfo() function. The ai pointer should be a addrinfo structure created by a call to getad- drinfo(). IMPLEMENTATION NOTES The behavior of freeaddrinfo(NULL) is left unspecified by both Version 4 of the Single UNIX Specification ("SUSv4") and RFC 3493. The current imple- mentation ignores a NULL argument for compatibility with programs that rely on the implementation details of other operating systems. RETURN VALUES getaddrinfo() returns zero on success or one of the error codes listed in gai_strerror(3) if an error occurs. EXAMPLES The following code tries to connect to "www.kame.net" service "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 AF_INET socket. Similarly, if it resolves to IPv6, an AF_INET6 socket is used. Observe that there is no hardcoded reference to a particu- lar address family. The code works even if getaddrinfo() returns addresses that are not IPv4/v6. 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); The following example tries to open a wildcard listening socket onto ser- vice "http", for all the address families available. 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); SEE ALSO bind(2), connect(2), send(2), socket(2), gai_strerror(3), gethostbyname(3), getnameinfo(3), getservbyname(3), resolver(3), inet(4), inet6(4), unix(4), hosts(5), resolv.conf(5), services(5), hostname(7) R. Gilligan, S. Thomson, J. Bound, J. McCann, and W. Stevens, Basic Socket Interface Extensions for IPv6, RFC 3493, February 2003. S. Deering, B. Haberman, T. Jinmei, E. Nordmark, and B. Zill, IPv6 Scoped Address Architecture, RFC 4007, March 2005. Craig Metz, "Protocol Independence Using the Sockets API", Proceedings of the freenix track: 2000 USENIX annual technical conference, June 2000. STANDARDS The getaddrinfo() function is defined by the IEEE Std 1003.1-2004 ("POSIX.1") specification and documented in RFC 3493, "Basic Socket Interface Extensions for IPv6". FreeBSD 14.3 June 27, 2022 GETADDRINFO(3)
NAME | SYNOPSIS | DESCRIPTION | IMPLEMENTATION NOTES | RETURN VALUES | EXAMPLES | SEE ALSO | STANDARDS
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