NAME
getnameinfo
—
socket address structure to hostname
and service name
SYNOPSIS
#include
<sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
int
getnameinfo
(const
struct sockaddr *sa,
socklen_t salen,
char *host,
size_t hostlen,
char *serv,
size_t servlen,
int flags);
DESCRIPTION
The
getnameinfo
()
function is used to convert a sockaddr
structure to
a pair of host name and service strings. It is a replacement for and
provides more flexibility than the
gethostbyaddr(3) and
getservbyport(3) functions and is the converse of the
getaddrinfo(3) function.
The sockaddr
structure
sa should point to either a
sockaddr_in
or sockaddr_in6
structure (for IPv4 or IPv6 respectively) that is
salen bytes long.
The host and service names associated with
sa are stored in host and
serv which have length parameters
hostlen and servlen. The maximum
value for hostlen is
NI_MAXHOST
and the maximum value for
servlen is NI_MAXSERV
, as
defined by ⟨netdb.h⟩. If a length
parameter is zero, no string will be stored. Otherwise, enough space must be
provided to store the host name or service string plus a byte for the NUL
terminator.
The flags argument is formed by OR'ing the following values:
NI_NOFQDN
- A fully qualified domain name is not required for local hosts. The local part of the fully qualified domain name is returned instead.
NI_NUMERICHOST
- Return the address in numeric form, as if calling inet_ntop(3), instead of a host name.
NI_NAMEREQD
- A name is required. If the host name cannot be found in DNS and this flag is set, a non-zero error code is returned. If the host name is not found and the flag is not set, the address is returned in numeric form.
- NI_NUMERICSERV
- The service name is returned as a digit string representing the port number.
- NI_DGRAM
- Specifies that the service being looked up is a datagram service, and causes getservbyport(3) to be called with a second argument of “udp” instead of its default of “tcp”. This is required for the few ports (512-514) that have different services for UDP and TCP.
This implementation allows numeric IPv6 address notation with
scope identifier, as documented in RFC 4007. IPv6 link-local address will
appear as a string like “fe80::1%ne0
”.
Refer to
getaddrinfo(3) for more information.
RETURN VALUES
getnameinfo
() 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 get a numeric host name, and service name, for a given socket address. Observe that there is no hardcoded reference to a particular address family.
struct sockaddr *sa; /* input */ char hbuf[NI_MAXHOST], sbuf[NI_MAXSERV]; if (getnameinfo(sa, sa->sa_len, hbuf, sizeof(hbuf), sbuf, sizeof(sbuf), NI_NUMERICHOST | NI_NUMERICSERV)) errx(1, "could not get numeric hostname"); printf("host=%s, serv=%s\n", hbuf, sbuf);
The following version checks if the socket address has a reverse address mapping:
struct sockaddr *sa; /* input */ char hbuf[NI_MAXHOST]; if (getnameinfo(sa, sa->sa_len, hbuf, sizeof(hbuf), NULL, 0, NI_NAMEREQD)) errx(1, "could not resolve hostname"); printf("host=%s\n", hbuf);
SEE ALSO
gai_strerror(3), getaddrinfo(3), gethostbyaddr(3), getservbyport(3), inet_ntop(3), resolver(3), hosts(5), resolv.conf(5), services(5), hostname(7), named(8)
Craig Metz, Protocol Independence Using the Sockets API, Proceedings of the Freenix Track: 2000 USENIX Annual Technical Conference, June 2000.
STANDARDS
The getnameinfo
() function is defined by
the IEEE Std 1003.1g-2000 (“POSIX.1g”)
draft specification and documented in RFC 2553.
R. Gilligan, S. Thomson, J. Bound, and W. Stevens, Basic Socket Interface Extensions for IPv6, RFC 2553, March 1999.
S. Deering, B. Haberman, T. Jinmei, E. Nordmark, and B. Zill, IPv6 Scoped Address Architecture, RFC 4007, March 2005.
CAVEATS
getnameinfo
() can return both numeric and
FQDN forms of the address specified in sa. There is no
return value that indicates whether the string returned in
host is a result of binary to numeric-text translation
(like inet_ntop(3)), or is the result of a DNS reverse lookup.
Because of this, malicious parties could set up a PTR record as follows:
1.0.0.127.in-addr.arpa. IN PTR 10.1.1.1
and trick the caller of getnameinfo
() into
believing that sa is 10.1.1.1
when it is actually 127.0.0.1
.
To prevent such attacks, the use of
NI_NAMEREQD
is recommended when the result of
getnameinfo
() is used for access control
purposes:
struct sockaddr *sa; char addr[NI_MAXHOST]; struct addrinfo hints, *res; int error; error = getnameinfo(sa, sa->sa_len, addr, sizeof(addr), NULL, 0, NI_NAMEREQD); if (error == 0) { memset(&hints, 0, sizeof(hints)); hints.ai_socktype = SOCK_DGRAM; /*dummy*/ hints.ai_flags = AI_NUMERICHOST; if (getaddrinfo(addr, "0", &hints, &res) == 0) { /* malicious PTR record */ freeaddrinfo(res); printf("bogus PTR record\n"); return -1; } /* addr is FQDN as a result of PTR lookup */ } else { /* addr is numeric string */ error = getnameinfo(sa, sa->sa_len, addr, sizeof(addr), NULL, 0, NI_NUMERICHOST); }
BUGS
The implementation of getnameinfo
() is not
thread-safe.
OpenBSD intentionally uses a different
NI_MAXHOST
value from what RFC 2553 suggests, to
avoid buffer length handling mistakes.