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 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. If the zone-id is
an interface name, the interface's index will be stored in the
sin6_scope_id field of the struct sockaddr_in6. If the interface does not
exist, the error EAI_NONAME will be returned. If the zone-id is a numeric
value, it will be placed in sin6_scope_id.


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 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().

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 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.

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 |
| | 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 | 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 | |
|ipaddrsel(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 | |
|table. 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 |
| | destination 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 |
+--------------------+-------------------+
|Interface Stability | Committed |
+--------------------+-------------------+
|MT-Level | MT-Safe |
+--------------------+-------------------+
|Standard | See standards(5). |
+--------------------+-------------------+

SEE ALSO


ipaddrsel(1M), gethostbyname(3NSL), getipnodebyname(3SOCKET), htonl(3C),
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.

NOTES


IPv4-mapped addresses are not recommended.


February 25, 2017 GETADDRINFO(3SOCKET)