процедуры распознавания (resolver routines)
Описание (Description)
Note: This page is incomplete (various resolver functions
provided by glibc are not described) and likely out of date.
The functions described below make queries to and interpret the
responses from Internet domain name servers.
The API consists of a set of more modern, reentrant functions and
an older set of nonreentrant functions that have been superseded.
The traditional resolver interfaces such as res_init() and
res_query() use some static (global) state stored in the _res
structure, rendering these functions non-thread-safe. BIND 8.2
introduced a set of new interfaces res_ninit(), res_nquery(), and
so on, which take a res_state as their first argument, so you can
use a per-thread resolver state.
The res_ninit() and res_init() functions read the configuration
files (see resolv.conf(5)) to get the default domain name and
name server address(es). If no server is given, the local host
is tried. If no domain is given, that associated with the local
host is used. It can be overridden with the environment variable
LOCALDOMAIN. res_ninit() or res_init() is normally executed by
the first call to one of the other functions. Every call to
res_ninit() requires a corresponding call to res_nclose() to free
memory allocated by res_ninit() and subsequent calls to
res_nquery().
The res_nquery() and res_query() functions query the name server
for the fully qualified domain name name of specified type and
class. The reply is left in the buffer answer of length anslen
supplied by the caller.
The res_nsearch() and res_search() functions make a query and
waits for the response like res_nquery() and res_query(), but in
addition they implement the default and search rules controlled
by RES_DEFNAMES and RES_DNSRCH (see description of _res options
below).
The res_nquerydomain() and res_querydomain() functions make a
query using res_nquery()/res_query() on the concatenation of name
and domain.
The following functions are lower-level routines used by
res_nquery()/res_query().
The res_nmkquery() and res_mkquery() functions construct a query
message in buf of length buflen for the domain name dname. The
query type op is one of the following (typically QUERY):
QUERY Standard query.
IQUERY Inverse query. This option was removed in glibc 2.26,
since it has not been supported by DNS servers for a very
long time.
NS_NOTIFY_OP
Notify secondary of SOA (Start of Authority) change.
newrr is currently unused.
The res_nsend() and res_send() function send a preformatted query
given in msg of length msglen and returns the answer in answer
which is of length anslen. They will call res_ninit()/res_init()
if it has not already been called.
The dn_comp() function compresses the domain name exp_dn and
stores it in the buffer comp_dn of length length. The
compression uses an array of pointers dnptrs to previously
compressed names in the current message. The first pointer
points to the beginning of the message and the list ends with
NULL. The limit of the array is specified by lastdnptr. If
dnptr is NULL, domain names are not compressed. If lastdnptr is
NULL, the list of labels is not updated.
The dn_expand() function expands the compressed domain name
comp_dn to a full domain name, which is placed in the buffer
exp_dn of size length. The compressed name is contained in a
query or reply message, and msg points to the beginning of the
message.
The resolver routines use configuration and state information
contained in a __res_state structure (either passed as the statep
argument, or in the global variable _res, in the case of the
older nonreentrant functions). The only field of this structure
that is normally manipulated by the user is the options field.
This field can contain the bitwise "OR" of the following options:
RES_INIT
True if res_ninit() or res_init() has been called.
RES_DEBUG
Print debugging messages. This option is available only
if glibc was built with debugging enabled, which is not
the default.
RES_AAONLY (unimplemented; deprecated in glibc 2.25)
Accept authoritative answers only. res_send() continues
until it finds an authoritative answer or returns an
error. This option was present but unimplemented in glibc
until version 2.24; since glibc 2.25, it is deprecated,
and its usage produces a warning.
RES_USEVC
Use TCP connections for queries rather than UDP datagrams.
RES_PRIMARY (unimplemented; deprecated in glibc 2.25)
Query primary domain name server only. This option was
present but unimplemented in glibc until version 2.24;
since glibc 2.25, it is deprecated, and its usage produces
a warning.
RES_IGNTC
Ignore truncation errors. Don't retry with TCP.
RES_RECURSE
Set the recursion desired bit in queries. Recursion is
carried out by the domain name server, not by res_send().
[Enabled by default].
RES_DEFNAMES
If set, res_search() will append the default domain name
to single component names—that is, those that do not
contain a dot. [Enabled by default].
RES_STAYOPEN
Used with RES_USEVC to keep the TCP connection open
between queries.
RES_DNSRCH
If set, res_search() will search for hostnames in the
current domain and in parent domains. This option is used
by gethostbyname(3). [Enabled by default].
RES_INSECURE1
Accept a response from a wrong server. This can be used
to detect potential security hazards, but you need to
compile glibc with debugging enabled and use RES_DEBUG
option (for debug purpose only).
RES_INSECURE2
Accept a response which contains a wrong query. This can
be used to detect potential security hazards, but you need
to compile glibc with debugging enabled and use RES_DEBUG
option (for debug purpose only).
RES_NOALIASES
Disable usage of HOSTALIASES environment variable.
RES_USE_INET6
Try an AAAA query before an A query inside the
gethostbyname(3) function, and map IPv4 responses in IPv6
"tunneled form" if no AAAA records are found but an A
record set exists. Since glibc 2.25, this option is
deprecated, and its usage produces a warning; applications
should use getaddrinfo(3), rather than gethostbyname(3).
RES_ROTATE
Causes round-robin selection of name servers from among
those listed. This has the effect of spreading the query
load among all listed servers, rather than having all
clients try the first listed server first every time.
RES_NOCHECKNAME (unimplemented; deprecated in glibc 2.25)
Disable the modern BIND checking of incoming hostnames and
mail names for invalid characters such as underscore (_),
non-ASCII, or control characters. This option was present
in glibc until version 2.24; since glibc 2.25, it is
deprecated, and its usage produces a warning.
RES_KEEPTSIG (unimplemented; deprecated in glibc 2.25)
Do not strip TSIG records. This option was present but
unimplemented in glibc until version 2.24; since glibc
2.25, it is deprecated, and its usage produces a warning.
RES_BLAST (unimplemented; deprecated in glibc 2.25)
Send each query simultaneously and recursively to all
servers. This option was present but unimplemented in
glibc until version 2.24; since glibc 2.25, it is
deprecated, and its usage produces a warning.
RES_USEBSTRING (glibc 2.3.4 to 2.24)
Make reverse IPv6 lookups using the bit-label format
described in RFC 2673; if this option is not set (which is
the default), then nibble format is used. This option was
removed in glibc 2.25, since it relied on a backward-
incompatible DNS extension that was never deployed on the
Internet.
RES_NOIP6DOTINT (glibc 2.24 and earlier)
Use ip6.arpa zone in IPv6 reverse lookup instead of
ip6.int, which is deprecated since glibc 2.3.4. This
option is present in glibc up to and including version
2.24, where it is enabled by default. In glibc 2.25, this
option was removed.
RES_USE_EDNS0 (since glibc 2.6)
Enables support for the DNS extensions (EDNS0) described
in RFC 2671.
RES_SNGLKUP (since glibc 2.10)
By default, glibc performs IPv4 and IPv6 lookups in
parallel since version 2.9. Some appliance DNS servers
cannot handle these queries properly and make the requests
time out. This option disables the behavior and makes
glibc perform the IPv6 and IPv4 requests sequentially (at
the cost of some slowdown of the resolving process).
RES_SNGLKUPREOP
When RES_SNGLKUP option is enabled, opens a new socket for
the each request.
RES_USE_DNSSEC
Use DNSSEC with OK bit in OPT record. This option implies
RES_USE_EDNS0.
RES_NOTLDQUERY
Do not look up unqualified name as a top-level domain
(TLD).
RES_DEFAULT
Default option which implies: RES_RECURSE, RES_DEFNAMES,
RES_DNSRCH, and RES_NOIP6DOTINT.
