getaddrinfo(3SOCKET) Sockets Library Functions getaddrinfo(3SOCKET)NAME
getaddrinfo, getnameinfo, freeaddrinfo, gai_strerror - translate
between node name and address
SYNOPSIS
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);
DESCRIPTION
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 transla‐
tion. The nodename and servname arguments are pointers to null-termi‐
nated 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 serv‐
name 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 pro‐
tocol 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 struc‐
tures 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 struc‐
ture 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 struc‐
ture, 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-termi‐
nated string containing the canonical name of the specified nodename.
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 shall be a numeric port string. Otherwise, an [EAI_NO‐
NAME] 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 exam‐
ple, 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 getad‐
drinfo() 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 allo‐
cated: 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 pro‐
vided 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 pro‐
viding 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().
RETURN VALUES
For getaddrinfo(), if the query is successful, a pointer to a linked
list of one or more addrinfo structures is returned by the fourth argu‐
ment and the function returns 0. The order of the addresses returned i
nthe fourth argument is discussed in the ADDRESS ORDERING section. If
the query fails, a non-zero error code will be returned. For getname‐
info(), 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 suc‐
cessful, 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.
Address Ordering
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 defini‐ │
│ │ tion 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_DEP‐ │
│ │ RECATED). │
├─────────────────────────────┼─────────────────────────────┤
│Prefer matching label. This │ Prefer a destination whose │
│rule uses labels that are │ label is equal to the label │
│obtained through the IPv6 │ of its source address. │
│default address selection │ │
│policy table. See ipad‐ │ │
│drsel(1M) for a description │ │
│of the default contents of │ │
│the table and how the table │ │
│is configured. │ │
├─────────────────────────────┼─────────────────────────────┤
│Prefer higher precedence. │ Prefer the destination │
│This rule uses precedence │ whose precedence is higher │
│values that are obtained │ than the other destination. │
│through the IPv6 default │ │
│address selection policy ta‐ │ │
│ble. See ipaddrsel(1M) for a │ │
│description of the default │ │
│contents of the table and │ │
│how the table is configured. │ │
├─────────────────────────────┼─────────────────────────────┤
│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 │ Prefer the destination │
│inet6(7P) for the definition │ whose scope is smaller than │
│of this rule. │ the other destination. │
├─────────────────────────────┼─────────────────────────────┤
│Use longest matching prefix. │ When the two destinations │
│ │ belong to the same address │
│ │ family, prefer the destina‐ │
│ │ tion that has the longer │
│ │ matching prefix with its │
│ │ source address. │
└─────────────────────────────┴─────────────────────────────┘
ERRORS
The following names are the error values returned by getaddrinfo() and
are defined in <netdb.h>:
EAI_ADDRFAMILY Address family for nodename is not supported.
EAI_AGAIN Temporary failure in name resolution has occurred .
EAI_BADFLAGS Invalid value specified for ai_flags.
EAI_FAIL Non-recoverable failure in name resolution has
occurred.
EAI_FAMILY The ai_family is not supported.
EAI_MEMORY Memory allocation failure has occurred.
EAI_NODATA No address is associated with nodename.
EAI_NONAME Neither nodename nor servname is provided or known.
EAI_SERVICE The servname is not supported for ai_socktype.
EAI_SOCKTYPE The ai_socktype is not supported.
EAI_OVERFLOW Argument buffer has overflowed.
EAI_SYSTEM System error was returned in errno.
FILES
/etc/inet/hosts local database that associates names of nodes
with IP addresses
/etc/netconfig network configuration database
/etc/nsswitch.conf configuration file for the name service switch
ATTRIBUTES
See attributes(5) for description of the following attributes:
┌─────────────────────────────┬─────────────────────────────┐
│ ATTRIBUTE TYPE │ ATTRIBUTE VALUE │
├─────────────────────────────┼─────────────────────────────┤
│MT-Level │MT-Safe │
└─────────────────────────────┴─────────────────────────────┘
SEE ALSOipaddrsel(1M), gethostbyname(3NSL), getipnodebyname(3SOCKET),
htonl(3SOCKET), inet(3SOCKET), netdb.h(3HEAD), socket(3SOCKET),
hosts(4), nsswitch.conf(4), inet6(7P)
Draves, R. RFC 3484, Default Address Selection for Internet Protocol
version 6 (IPv6). Network Working Group. February 2003.
NOTES
IPv4-mapped addresses are not recommended.
SunOS 5.10 14 Nov 2007 getaddrinfo(3SOCKET)