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getaddrinfo, getnameinfo, freeaddrinfo, gai_strerror - translate between node name and address

cc [ flag.\|.\|. ] file .\|.\|. -lsocket  -lnsl  [ library .\|.\|. ]
#include <sys/socket.h>
#include <netdb.h>

int getaddrinfo(const char *nodename, const char *servname,
 const struct addrinfo *hints, struct addrinfo **res);

int getnameinfo(const struct sockaddr *sa, socklen_t salen,
 char *host, size_t hostlen, char *serv, size_t servlen,
 int flags);

void freeaddrinfo(struct addrinfo *ai);

char *gai_strerror(int errcode);

These functions perform translations from node name to address and from address to node name in a protocol-independent manner.

The getaddrinfo() function performs the node name to address translation. 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.

A non-null nodename string can be a node name or a numeric host address string. The nodename can also be an IPv6 zone-id in the form:

<address>%<zone-id>

The address is the literal IPv6 link-local address or host name of the destination. 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 hme0.

A non-null servname string can be either a service name or 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(3SOCKET) 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(3SOCKET). 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(3SOCKET) (for a connection-oriented protocol) or either connect(3SOCKET), sendto(3SOCKET), or sendmsg(3SOCKET) (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).

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() function is called to return this information 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() 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.

The getnameinfo() function looks up an IP address and port number provided by the caller in the name service database and system-specific database, and returns text strings for both in buffers provided by the caller. The function indicates successful completion by a 0 return value; a non-zero return value indicates failure.

The first argument, sa, points to either a sockaddr_in structure (for IPv4) or a sockaddr_in6 structure (for IPv6) that holds the IP address and port number. The salen argument gives the length of the sockaddr_in or sockaddr_in6 structure.

The function returns the node name associated with the IP address in the buffer pointed to by the host argument.

The function can also return the IPv6 zone-id in the form:

<address>%<zone-id>

The caller provides the size of this buffer with the hostlen argument. The service name associated with the port number is returned in the buffer pointed to by serv, and the servlen argument gives the length of this buffer. The caller specifies not to return either string by providing a 0 value for the hostlen or servlen arguments. Otherwise, the caller must provide buffers large enough to hold the node name and the service name, including the terminating null characters.

To aid the application in allocating buffers for these two returned strings, the following constants are defined in <netdb.h>:

#define NI_MAXHOST 1025
#define NI_MAXSERV 32

The final argument is a flag that changes the default actions of this function. By default, the fully-qualified domain name (FQDN) for the host is looked up in the name service database and returned. If the flag bit NI_NOFQDN is set, only the node name portion of the FQDN is returned for local hosts.

If the flag bit NI_NUMERICHOST is set, or if the host's name cannot be located in the name service, the numeric form of the host's address is returned instead of its name, for example, by calling inet_ntop() (see inet(3SOCKET)) instead of getipnodebyname(3SOCKET). If the flag bit NI_NAMEREQD is set, an error is returned if the host's name cannot be located in the name service database.

If the flag bit NI_NUMERICSERV is set, the numeric form of the service address is returned (for example, its port number) instead of its name. The two NI_NUMERIC* flags are required to support the -n flag that many commands provide.

A fifth flag bit, NI_DGRAM, specifies that the service is a datagram service, and causes getservbyport(3SOCKET) to be called with a second argument of udp instead of the default tcp. This is required for the few ports (for example, 512-514) that have different services for UDP and TCP.

These NI_* flags are defined in <netdb.h> along with the AI_* flags already defined for getaddrinfo().

For getaddrinfo(), if the query is successful, a pointer to a linked list of one or more addrinfo structures is returned by the fourth argument and the function returns 0. The order of the addresses returned in the fourth argument is discussed in the ADDRESS ORDERING section. If the query fails, a non-zero error code will be returned. For getnameinfo(), if successful, the strings hostname and service are copied into host and serv, respectively. If unsuccessful, zero values for either hostlen or servlen will suppress the associated lookup; in this case no data is copied into the applicable buffer. If gai_strerror() is successful, a pointer to a string containing an error message appropriate for the EAI_* errors is returned. If errcode is not one of the EAI_* values, a pointer to a string indicating an unknown error is returned.

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:

Avoid unusable destinations.

Prefer a destination that is reachable through the IP routing table.

Prefer matching scope.

Prefer a destination whose scope is equal to the scope of its source address. See inet6(7P) for the definition of scope used by this rule.

Avoid link-local source.

Avoid selecting a link-local source address when the destination address is not a link-local address.

Avoid deprecated addresses.

Prefer a destination that is not deprecated (IFF_DEPRECATED).

Prefer matching label. This rule uses labels that are obtained through the IPv6 default address selection policy table. See ipaddrsel(1M) for a description of the default contents of the table and how the table is configured.

Prefer a destination whose label is equal to the label of its source address.

Prefer higher precedence. This rule uses precedence values that are obtained through the IPv6 default address selection policy table. See ipaddrsel(1M) for a description of the default contents of the table and how the table is configured.

Prefer the destination whose precedence is higher than the other destination.

Prefer native transport.

Prefer a destination if the interface that is used for sending packets to that destination is not an IP over IP tunnel.

Prefer smaller scope. See inet6(7P) for the definition of this rule.

Prefer the destination whose scope is smaller than the other destination.

Use longest matching prefix.

When the two destinations belong to the same address family, prefer the destination that has the longer matching prefix with its source address.

The following names are the error values returned by getaddrinfo() and are defined in <netdb.h>: EAI_ADDRFAMILY

EAI_AGAIN EAI_BADFLAGS EAI_FAIL EAI_FAMILY EAI_MEMORY EAI_NODATA EAI_NONAME EAI_SERVICE EAI_SOCKTYPE EAI_OVERFLOW EAI_SYSTEM /etc/inet/hosts /etc/netconfig /etc/nsswitch.conf

See attributes(5) for description of the following attributes:

ATTRIBUTE TYPE ATTRIBUTE VALUE

Interface Stability Committed

MT-Level MT-Safe

Standard See standards(5).

ipaddrsel(1M), gethostbyname(3NSL), getipnodebyname(3SOCKET), htonl(3SOCKET), inet(3SOCKET), sockaddr(3SOCKET), netdb.h(3HEAD), socket(3SOCKET), hosts(4), nsswitch.conf(4), attributes(5), standards(5), inet6(7P)

Draves, R. RFC 3484, Default Address Selection for Internet Protocol version 6 (IPv6). Network Working Group. February 2003.

IPv4-mapped addresses are not recommended.