GETNAMEINFO(3) | Library Functions Manual | GETNAMEINFO(3) |
getnameinfo
—
socket address structure to hostname and service
name
#include
<sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
int
getnameinfo
(const struct sockaddr
*sa, socklen_t salen, char
*host, socklen_t hostlen, char
*serv, socklen_t servlen, int
flags);
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.
If a link-layer address is passed to
getnameinfo
(),
its ASCII representation will be stored in host. The
string pointed to by serv will be set to the empty
string if non-NULL; flags will always be ignored. This
is intended as a replacement for the legacy
link_ntoa(3) function.
The sockaddr
structure
sa should point to either a
sockaddr_in
, sockaddr_in6
or
sockaddr_dl
structure (for IPv4, IPv6 or link-layer
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
NI_NUMERICHOST
NI_NAMEREQD
This implementation allows numeric IPv6 address notation with
scope identifier, as documented in chapter 11 of
draft-ietf-ipv6-scoping-arch-02.txt. IPv6 link-local address will appear as
a string like “fe80::1%ne0
”. Refer to
getaddrinfo(3) for more
information.
getnameinfo
() returns zero on success or
one of the error codes listed in
gai_strerror(3) if an error
occurs.
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"); /*NOTREACHED*/ } 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"); /*NOTREACHED*/ } printf("host=%s\n", hbuf);
gai_strerror(3), getaddrinfo(3), gethostbyaddr(3), getservbyport(3), inet_ntop(3), link_ntoa(3), resolver(3), hosts(5), resolv.conf(5), services(5), hostname(7), named(8)
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, internet draft, draft-ietf-ipv6-scoping-arch-02.txt, work in progress material.
Craig Metz, Protocol Independence Using the Sockets API, Proceedings of the freenix track: 2000 USENIX annual technical conference, June 2000.
The getnameinfo
() function is defined by
the IEEE Std 1003.1g-2000 (“POSIX.1g”)
draft specification and documented in RFC 2553, “Basic Socket
Interface Extensions for IPv6”.
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; socklen_t salen; char addr[NI_MAXHOST]; struct addrinfo hints, *res; int error; error = getnameinfo(sa, salen, 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, salen, addr, sizeof(addr), NULL, 0, NI_NUMERICHOST); }
February 28, 2007 | macOS 14.6 |