getaddrinfo - translate between node name and address
#include <sys/socket.h>
#include <netdb.h>
int getaddrinfo(const char *restrict nodename,
const char *restrict servname,
const struct addrinfo *restrict hints,
struct addrinfo **restrict res);
The getaddrinfo() function translates the name of a service location (for example, a host name) and/or a service name and returns a set of socket addresses and associated information to be used in creating a socket with which to address the specified service. The getaddrinfo() function performs the node name to address translation in a protocol-independent manner.
The nodename and servname arguments are pointers to null-terminated strings or NULL. One or both of these arguments must be a non-null pointer. In the normal client scenario, both the nodename and servname are specified. In the normal server scenario, only the servname is specified.
The format of a valid name depends on the address family or families. If a specific family is not given and the name could be interpreted as valid within multiple supported families, the implementation attempts to resolve the name in all supported families and, in absence of errors, one or more results are returned.
If nodename is not null, the requested service location is named by nodename; otherwise, the requested service location is local to the caller.
If the nodename argument is not null, it can be a descriptive name or an address string. If the specified address family is AF_INET, AF_INET6, or AF_UNSPEC, valid descriptive names include host names. If the specified address family is AF_INET or AF_UNSPEC, address strings using Internet standard dot notation as specified in inet_addr(3C) are valid.
If the specified address family is AF_INET6 or AF_UNSPEC, standard IPv6 text forms described in inet_ntop(3C) are valid. The nodename can also be an IPv6 zone-id in the form:
address%zone-id
where the address is the literal IPv6 link-local address or host name of the destination, and the zone-id is the interface ID of the IPv6 link used to send the packet. The zone-id can either be a numeric value, indicating a literal zone value, or an interface name such as net0.
If servname is null, the call returns network-level addresses for the specified nodename. If servname is not null, it is a null-terminated character string identifying the requested service. This string can be either a descriptive name or a numeric representation suitable for use with the address family or families. If the specified address family is AF_INET, AF_INET6, or AF_UNSPEC, the service can be specified as a string specifying a decimal port number.
The caller can optionally pass an addrinfo structure, pointed to by the hints argument, to provide hints concerning the type of socket that the caller supports.
The addrinfo structure is defined as:
struct addrinfo {
int ai_flags; /* AI_PASSIVE, AI_CANONNAME,
AI_NUMERICHOST, AI_NUMERICSERV
AI_V4MAPPED, AI_ALL,
AI_ADDRCONFIG */
int ai_family; /* PF_xxx */
int ai_socktype; /* SOCK_xxx */
int ai_protocol; /* 0 or IPPROTO_xxx for IPv4 & IPv6 */
socklen_t ai_addrlen; /* length of ai_addr */
char *ai_canonname; /* canonical name for nodename */
struct sockaddr *ai_addr; /* binary address */
struct addrinfo *ai_next; /* next structure in linked list */
};
In this hints structure, all members other than ai_flags, ai_family, ai_socktype, and ai_protocol must be 0 or a null pointer. A value of PF_UNSPEC for ai_family indicates that the caller will accept any protocol family. A value of 0 for ai_socktype indicates that the caller will accept any socket type. A value of 0 for ai_protocol indicates that the caller will accept any protocol. For example, if the caller handles only TCP and not UDP, then the ai_socktype member of the hints structure should be set to SOCK_STREAM when getaddrinfo() is called. If the caller handles only IPv4 and not IPv6, then the ai_family member of the hints structure should be set to PF_INET when getaddrinfo() is called. If the third argument to getaddrinfo() is a null pointer, it is as if the caller had filled in an addrinfo structure initialized to 0 with ai_family set to PF_UNSPEC.
Upon success, a pointer to a linked list of one or more addrinfo structures is returned through the final argument. The caller can process each addrinfo structure in this list by following the ai_next pointer, until a null pointer is encountered. In each returned addrinfo structure the three members ai_family, ai_socktype, and ai_protocol are the corresponding arguments for a call to the socket(3C) function. In each addrinfo structure the ai_addr member points to a filled-in socket address structure whose length is specified by the ai_addrlen member.
If the AI_PASSIVE bit is set in the ai_flags member of the hints structure, the caller plans to use the returned socket address structure in a call to bind(3C). In this case, if the nodename argument is a null pointer, the IP address portion 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 in the ai_flags member of the hints structure, then the returned socket address structure will be ready for a call to connect(3C) (for a connection-oriented protocol) or either connect(3C), sendto(3C), or sendmsg(3C) (for a connectionless protocol). If the nodename argument is a null pointer, the IP address portion of the socket address structure will be set to the loopback address.
If the AI_CANONNAME bit is set in the ai_flags member of the hints structure, then upon successful return the ai_canonname member of the first addrinfo structure in the linked list will point to a null-terminated string containing the canonical name of the specified nodename. A numeric host address string is not a name, and thus does not have a canonical name form; no address to host name translation is performed.
If the AI_NUMERICHOST bit is set in the ai_flags member of the hints structure, then a non-null nodename string must be a numeric host address string. Otherwise an error of EAI_NONAME is returned. This flag prevents any type of name resolution service (such as DNS) from being called.
If the AI_NUMERICSERV flag is specified, then a non-null servname string supplied will be a numeric port string. Otherwise, an [EAI_NONAME] error is returned. This flag prevents any type of name resolution service (for example, NIS) from being invoked.
If the AI_V4MAPPED flag is specified along with an ai_family of AF_INET6, then getaddrinfo() returns IPv4-mapped IPv6 addresses on finding no matching IPv6 addresses (ai_addrlen shall be 16). For example, if no AAAA records are found when using DNS, a query is made for A records. Any found records are returned as IPv4-mapped IPv6 addresses.
The AI_V4MAPPED flag is ignored unless ai_family equals AF_INET6.
If the AI_ALL flag is used with the AI_V4MAPPED flag, then getaddrinfo() returns 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 results of both queries. Any IPv4 addresses found are returned as IPv4-mapped IPv6 addresses.
The AI_ALL flag without the AI_V4MAPPED flag is ignored.
When ai_family is not specified (AF_UNSPEC), AI_V4MAPPED and AI_ALL flags are used only if AF_INET6 is supported.
If the AI_ADDRCONFIG flag is specified, IPv4 addresses are returned only if an IPv4 address is configured on the local system, and IPv6 addresses are returned only if an IPv6 address is configured on the local system. For this case, the loopback address is not considered to be as valid as a configured address. For example, when using the DNS, a query for AAAA records should occur only if the node has at least one IPv6 address configured (other than IPv6 loopback) and a query for A records should occur only if the node has at least one IPv4 address configured (other than the IPv4 loopback).
The ai_socktype member to which argument hints points specifies the socket type for the service, as defined in socket(3C). If a specific socket type is not given (for example, a value of 0) and the service name could be interpreted as valid with multiple supported socket types, the implementation attempts to resolve the service name for all supported socket types and, in the absence of errors, all possible results are returned. A non-zero socket type value limits the returned information to values with the specified socket type.
If the ai_family member to which hints points has the value AF_UNSPEC, addresses are returned for use with any address family that can be used with the specified nodename and/or servname. Otherwise, addresses are returned for use only with the specified address family. If ai_family is not AF_UNSPEC and ai_protocol is not 0, then addresses are returned for use only with the specified address family and protocol; the value of ai_protocol is interpreted as in a call to the socket() function with the corresponding values of ai_family and ai_protocol.
All of the information returned by getaddrinfo() is dynamically allocated: the addrinfo structures as well as the socket address structures and canonical node name strings pointed to by the addrinfo structures. The freeaddrinfo(3C) function is called to return this memory to the system. For freeaddrinfo(), the addrinfo structure pointed to by the ai argument is freed, along with any dynamic storage pointed to by the structure. This operation is repeated until a null ai_next pointer is encountered.
To aid applications in printing error messages based on the EAI_* codes returned by getaddrinfo(), the gai_strerror(3C) function is defined. The argument is one of the EAI_* values defined below and the return value points to a string describing the error. If the argument is not one of the EAI_* values, the function still returns a pointer to a string whose contents indicate an unknown error.
A 0 return value for getaddrinfo() indicates successful completion; a non-zero return value indicates failure. The possible values for the failures are listed in the ERRORS section.
Upon successful return of getaddrinfo(), the location to which res points refers to a linked list of addrinfo structures, each of which specifies a socket address and information for use in creating a socket with which to use that socket address. The list includes at least one addrinfo structure. The ai_next member of each structure contains a pointer to the next structure on the list, or a null pointer if it is the last structure on the list. Each structure on the list includes values for use with a call to the socket function, and a socket address for use with the connect function or, if the AI_PASSIVE flag was specified, for use with the bind(3C) function. The ai_family, ai_socktype, and ai_protocol members are usable as the arguments to the socket() function to create a socket suitable for use with the returned address. The ai_addr and ai_addrlen members are usable as the arguments to the connect() or bind() functions with such a socket, according to the AI_PASSIVE flag.
If nodename is not null, and if requested by the AI_CANONNAME flag, the ai_canonname member of the first returned addrinfo structure points to a null-terminated string containing the canonical name corresponding to the input nodename. If the canonical name is not available, then ai_canonname refers to the nodename argument or a string with the same contents. The contents of the ai_flags member of the returned structures are undefined.
All members in socket address structures returned by getaddrinfo() that are not filled in through an explicit argument (for example, sin6_flowinfo) are set to 0, making it easier to compare socket address structures.
AF_INET6 addresses returned by the fourth argument of getaddrinfo() are ordered according to the algorithm described in RFC 3484, Default Address Selection for Internet Protocol version 6 (IPv6). The addresses are ordered using a list of pair-wise comparison rules which are applied in order. If a rule determines that one address is better than another, the remaining rules are irrelevant to the comparison of those two addresses. If two addresses are equivalent according to one rule, the remaining rules act as a tie-breaker. The address ordering list of pair-wise comparison rules follow below:
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The following names are the error values returned by getaddrinfo() and are defined in <netdb.h>:
Address family for nodename is not supported.
Temporary failure in name resolution has occurred.
Invalid value specified for ai_flags.
Non-recoverable failure in name resolution has occurred.
The ai_family is not supported.
Memory allocation failure has occurred.
No address is associated with nodename.
Neither nodename nor servname is provided or known.
The servname is not supported for ai_socktype.
The ai_socktype is not supported.
Argument buffer has overflowed.
System error was returned in errno.
If the caller handles only TCP and not UDP, for example, then the ai_protocol member of the hints structure should be set to IPPROTO_TCP when getaddrinfo() is called.
If the caller handles only IPv4 and not IPv6, then the ai_family member of the hints structure should be set to AF_INET when getaddrinfo() is called.
local database that associates names of nodes with IP addresses
network configuration database
configuration file for the name service switch
See attributes(7) for description of the following attributes:
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bind(3C), connect(3C), freeaddrinfo(3C), gai_strerror(3C), gethostbyname(3C), getipnodebyname(3C), getnameinfo(3C), getservbyname(3C), htonl(3C), inet(3C), inet_addr(3C), inet_ntop(3C), sendmsg(3C), sendto(3C), socket(3C), netdb.h(3HEAD), inet6(4P), hosts(5), nss(5), nsswitch.conf(5), attributes(7), standards(7), ipaddrsel(8)
Draves, R. RFC 3484, Default Address Selection for Internet Protocol version 6 (IPv6). Network Working Group. February 2003. https://tools.ietf.org/html/rfc3484
Gilligan, R. RFC 3493, Basic Socket Interface Extensions for IPv6. Network Working Group. February 2003. https://tools.ietf.org/html/rfc3493
IPv4-mapped addresses are not recommended.
The getaddrinfo() function was added to Oracle Solaris in the Solaris 8 release.