libc/net/resolver.3

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.\"	@(#)resolver.3	6.5 (Berkeley) 6/23/90
.\"	Id: resolver.man3,v 1.2 2009/01/21 00:12:34 each Exp
.\"
.Dd January 22, 2016
.Dt RESOLVER 3
.Os
.Sh NAME
.Nm res_ninit ,
.Nm res_ourserver_p ,
.Nm fp_resstat ,
.Nm res_hostalias ,
.Nm res_pquery ,
.Nm res_nquery ,
.Nm res_nsearch ,
.Nm res_nquerydomain ,
.Nm res_nmkquery ,
.Nm res_nsend ,
.Nm res_nupdate ,
.Nm res_nmkupdate ,
.Nm res_nclose ,
.Nm res_nsendsigned ,
.Nm res_findzonecut ,
.Nm res_getservers ,
.Nm res_setservers ,
.Nm res_ndestroy ,
.Nm dn_comp ,
.Nm dn_expand ,
.\" .Nm hstrerror ,
.Nm res_init ,
.Nm res_isourserver ,
.Nm fp_nquery ,
.Nm p_query ,
.Nm hostalias ,
.Nm res_query ,
.Nm res_search ,
.Nm res_querydomain ,
.Nm res_mkquery ,
.Nm res_send ,
.Nm res_update ,
.Nm res_close ,
.\" .Nm herror
.Nd resolver routines
.Sh LIBRARY
.Lb libc
.Lb libresolv
.Sh SYNOPSIS
.In resolv.h
.In res_update.h
.Vt typedef struct __res_state *res_state ;
.Pp
.Ft int
.Fn res_ninit "res_state statp"
.Ft int
.Fn res_ourserver_p "const res_state statp" "const struct sockaddr_in *addr"
.Ft void
.Fn fp_resstat "const res_state statp" "FILE *fp"
.Ft "const char *"
.Fn res_hostalias "const res_state statp" "const char *name" "char *buf" "size_t buflen"
.Ft int
.Fn res_pquery "const res_state statp" "const u_char *msg" "int msglen" "FILE *fp"
.Ft int
.Fn res_nquery "res_state statp" "const char *dname" "int class" "int type" "u_char *answer" "int anslen"
.Ft int
.Fn res_nsearch "res_state statp" "const char *dname" "int class" "int type" "u_char * answer" "int anslen"
.Ft int
.Fn res_nquerydomain "res_state statp" "const char *name" "const char *domain" "int class" "int type" "u_char *answer" "int anslen"
.Ft int
.Fo res_nmkquery
.Fa "res_state statp"
.Fa "int op"
.Fa "const char *dname"
.Fa "int class"
.Fa "int type"
.Fa "const u_char *data"
.Fa "int datalen"
.Fa "const u_char *newrr"
.Fa "u_char *buf"
.Fa "int buflen"
.Fc
.Ft int
.Fn res_nsend "res_state statp" "const u_char *msg" "int msglen" "u_char *answer" "int anslen"
.Ft int
.Fn res_nupdate "res_state statp" "ns_updrec *rrecp_in"
.Ft int
.Fn res_nmkupdate "res_state statp" "ns_updrec *rrecp_in" "u_char *buf" "int buflen"
.Ft void
.Fn res_nclose "res_state statp"
.Ft int
.Fn res_nsendsigned "res_state statp" "const u_char *msg" "int msglen" "ns_tsig_key *key" "u_char *answer" "int anslen"
.Ft int
.Fn res_findzonecut "res_state statp" "const char *dname" "ns_class class" "int options" "char *zname" "size_t zsize" "struct in_addr *addrs" "int naddrs"
.Ft int
.Fn res_getservers "res_state statp" "union res_sockaddr_union *set" "int cnt"
.Ft void
.Fn res_setservers "res_state statp" "const union res_sockaddr_union *set" "int cnt"
.Ft void
.Fn res_ndestroy "res_state statp"
.Ft int
.Fn dn_comp "const char *exp_dn" "u_char *comp_dn" "int length" "u_char **dnptrs" "u_char **lastdnptr"
.Ft int
.Fn dn_expand "const u_char *msg" "const u_char *eomorig" "const u_char *comp_dn" "char *exp_dn" "int  length"
.\" .Ft "const char *"
.\" .Fn hstrerror "int err"
.Ss DEPRECATED
.In sys/types.h
.In netinet/in.h
.In arpa/nameser.h
.In resolv.h
.In res_update.h
.Ft int
.Fn res_init "void"
.Ft int
.Fn res_isourserver "const struct sockaddr_in *addr"
.Ft int
.Fn fp_nquery "const u_char *msg" "int msglen" "FILE *fp"
.Ft void
.Fn p_query "const u_char *msg" "FILE *fp"
.Ft "const char *"
.Fn hostalias "const char *name"
.Ft int
.Fn res_query "const char *dname" "int class" "int type" "u_char *answer" "int anslen"
.Ft int
.Fn res_search "const char *dname" "int class" "int type" "u_char *answer" "int anslen"
.Ft int
.Fn res_querydomain "const char *name" "const char *domain" "int class" "int type" "u_char *answer" "int anslen"
.Ft int
.Fo res_mkquery
.Fa "int op"
.Fa "const char *dname"
.Fa "int class"
.Fa "int type"
.Fa "const char *data"
.Fa "int datalen"
.Fa "struct rrec *newrr"
.Fa "u_char *buf"
.Fa "int buflen"
.Fc
.Ft int
.Fn res_send "const u_char *msg" "int msglen" "u_char *answer" "int anslen"
.Ft int
.Fn res_update "ns_updrec *rrecp_in"
.Ft void
.Fn res_close "void"
.\" .Ft void
.\" .Fn herror "const char *s"
.Sh DESCRIPTION
These routines are used for making, sending and interpreting
query and reply messages with Internet domain name servers.
.Pp
State information is kept in
.Fa statp
and is used to control the behavior of these functions.
.Fa statp
should be set to all zeros prior to the first call to any of these functions.
.Pp
The functions
.Fn res_init ,
.Fn res_isourserver ,
.Fn fp_nquery ,
.Fn p_query ,
.Fn hostalias ,
.Fn res_query ,
.Fn res_search ,
.Fn res_querydomain ,
.Fn res_mkquery ,
.Fn res_send ,
.Fn res_update ,
.Fn res_close
.\" and
.\" .Fn herror
are deprecated and are supplied for compatability with old source
code.
They use global configuration and state information that is
kept in the structure
.Ft _res
rather than that referenced through
.Ft statp .
.Pp
Most of the values in
.Ft statp
and
.Ft _res
are initialized on the first call to
.Fn res_ninit
/
.Fn res_init
to reasonable defaults and can be ignored.
Options stored in
.Ft statp->options
/
.Ft _res.options
are defined in
.Pa resolv.h
and are as follows.
Options are stored as a simple bit mask containing the bitwise
.Dq OR
of the options enabled.
.Bl -tag -width "RES_USE_INET6"
.It Dv RES_INIT
True if the initial name server address and default domain name are
initialized (i.e.,
.Fn res_ninit
/
.Fn res_init
has been called).
.It Dv RES_DEBUG
Print debugging messages.
.It Dv RES_AAONLY
Accept authoritative answers only.
Should continue until it finds an authoritative answer or finds an error.
Currently this is not implemented.
.It Dv RES_USEVC
Use TCP connections for queries instead of UDP datagrams.
.It Dv RES_STAYOPEN
Used with
.Dv RES_USEVC
to keep the TCP connection open between queries.
This is useful only in programs that regularly do many queries.
UDP should be the normal mode used.
.It Dv RES_IGNTC
Ignore truncation errors, i.e., don't retry with TCP.
.It Dv RES_RECURSE
Set the recursion-desired bit in queries.
This is the default.
(\c
.Fn res_nsend
/
.Fn res_send
does not do iterative queries and expects the name server
to handle recursion.)
.It Dv RES_DEFNAMES
If set,
.Fn res_nsearch
/
.Fn res_search
will append the default domain name to single-component names
(those that do not contain a dot).
This option is enabled by default.
.It Dv RES_DNSRCH
If this option is set,
.Fn res_nsearch
/
.Fn res_search
will search for host names in the current domain and in parent domains; see
.Xr hostname 7 .
This is used by the standard host lookup routine
.Xr gethostbyname 3 .
This option is enabled by default.
.It Dv RES_USE_INET6
Enables support for IPv6-only applications.
This causes IPv4 addresses to be returned as an IPv4 mapped address.
For example, 10.1.1.1 will be returned as ::ffff:10.1.1.1.
The option is meaningful with certain kernel configuration only.
.It Dv RES_USE_EDNS0
Enables support for OPT pseudo-RR for EDNS0 extension.
With the option, resolver code will attach OPT pseudo-RR into DNS queries,
to inform of our receive buffer size.
The option will allow DNS servers to take advantage of non-default receive
buffer size, and to send larger replies.
DNS query packets with EDNS0 extension is not compatible with
non-EDNS0 DNS servers.
.It Dv RES_NOALIASES
This option turns off the user level aliasing feature controlled by
the
.Ev HOSTALIASES
environment variable.
Network daemons should set this option.
.It Dv RES_ROTATE
This options causes
.Fn res_nsend
/
.Fn res_send
to rotate the list of nameservers in
.Fa statp->nsaddr_list
/
.Fa _res.nsaddr_list .
.It Dv RES_KEEPTSIG
This option causes
.Fn res_nsendsigned
to leave the message unchanged after TSIG verification; otherwise the TSIG
record would be removed and the header updated.
.It Dv RES_NOTLDQUERY
This option causes
.Fn res_nsearch
to not attempt to resolve an unqualified name as if it were a top level
domain (TLD).
This option can cause problems if the site has "localhost" as a TLD rather
than having localhost on one or more elements of the search list.
This option has no effect if neither
.Dv RES_DEFNAMES
or
.Dv RES_DNSRCH
are set.
.El
.Pp
The
.Fn res_ninit
/
.Fn res_init
routines
read the configuration file (if any; see
.Xr resolv.conf 5 )
to get the default domain name, search list and
the Internet address of the local name server(s).
If no server is configured, the host running the resolver is tried.
The current domain name is defined by the hostname
if not specified in the configuration file;
it can be overridden by the environment variable
.Ev LOCALDOMAIN .
This environment variable may contain several blank-separated
tokens if you wish to override the
.Fa search list
on a per-process basis.
This is similar to the
.Fa search
command in the configuration file.
Another environment variable
.Ev RES_OPTIONS
can be set to override certain internal resolver options which are otherwise
set by changing fields in the
.Ft statp
/
.Ft _res
structure or are inherited from the configuration file's
.Fa options
command.
The syntax of the
.Ev RES_OPTIONS
environment variable is explained in
.Xr resolv.conf 5 .
Initialization normally occurs on the first call
to one of the other resolver routines.
.Pp
In
.Nx
the initialization code also sets up a
.Xr kqueue 2
and creates a
.Xr kevent 2
watching a file descriptor that points to the resolver file.
Every resolver function calls the internal function
.Fn __res_check
which checks for a new
.Xr kevent 2
related to the
.Xr resolv.conf 5
file, and reloads the file if necessary.
This does not work if the file is accessed through a symlink and the symlink
changes to point to a different file.
To fix the symlink issue one could add a system call per resolver call to
get the current time, and reload every so often.
This is not done currently, but it is under consideration.
.Pp
The memory referred to by
.Ft statp
must be set to all zeros prior to the first call to
.Fn res_ninit .
.Fn res_ndestroy
should be called to free memory allocated by
.Fn res_ninit
after last use.
.Pp
The
.Fn res_nquery
/
.Fn res_query
functions provide interfaces to the server query mechanism.
They construct a query, send it to the local server,
await a response, and make preliminary checks on the reply.
The query requests information of the specified
.Fa type
and
.Fa class
for the specified fully-qualified domain name
.Fa dname .
The reply message is left in the
.Fa answer
buffer with length
.Fa anslen
supplied by the caller.
.Fn res_nquery
/
.Fn res_query
return \-1 on error or the length of the answer.
.Pp
The
.Fn res_nsearch
/
.Fn res_search
routines make a query and awaits a response like
.Fn res_nquery
/
.Fn res_query ,
but in addition, they implement the default and search rules
controlled by the
.Dv RES_DEFNAMES
and
.Dv RES_DNSRCH
options.
They return the length of the first successful reply which is stored in
.Ft answer
or \-1 on error.
.Pp
The remaining routines are lower-level routines used by
.Fn res_nquery
/
.Fn res_query .
The
.Fn res_nmkquery
/
.Fn res_mkquery
functions construct a standard query message and place it in
.Fa buf .
They return the size of the query, or \-1 if the query is
larger than
.Fa buflen .
The query type
.Fa op
is usually
.Dv QUERY ,
but can be any of the query types defined in
.Aq Pa arpa/nameser.h .
The domain name for the query is given by
.Fa dname .
.Fa newrr
is currently unused but is intended for making update messages.
.Pp
The
.Fn res_nsend
/
.Fn res_send
/
.Fn res_nsendsigned
routines
send a pre-formatted query and return an answer.
They will call
.Fn res_ninit
/
.Fn res_init
if
.Dv RES_INIT
is not set, send the query to the local name server, and
handle timeouts and retries.
Additionally,
.Fn res_nsendsigned
will use TSIG signatures to add authentication to the query and verify the
response.
In this case, only one nameserver will be contacted.
The length of the reply message is returned, or \-1 if there were errors.
.Pp
.Fn res_nquery
/
.Fn res_query ,
.Fn res_nsearch
/
.Fn res_search
and
.Fn res_nsend
/
.Fn res_send
return a length that may be bigger than
.Fa anslen .
In that case the query should be retried with a bigger buffer.
.Em NOTE :
The answer to the second query may be larger still so supplying
a buffer that bigger that the answer returned by the previous
query is recommended.
.Pp
.Fa answer
.Em MUST
be big enough to receive a maximum UDP response from the server or
parts of the answer will be silently discarded.
The default maximum UDP response size is 512 bytes.
.Pp
The function
.Fn res_ourserver_p
returns true when
.Fa inp
is one of the servers in
.Fa statp->nsaddr_list
/
.Fa _res.nsaddr_list .
.Pp
The functions
.Fn fp_nquery
/
.Fn p_query
print out the query and any answer in
.Fa msg
on
.Fa fp .
.Fn p_query
is equivalent to
.Fn fp_nquery
with
.Fa msglen
set to 512.
.Pp
The function
.Fn fp_resstat
prints out the active flag bits in
.Fa statp->options
preceeded by the text ";; res options:" on
.Fa file .
.Pp
The functions
.Fn res_hostalias
/
.Fn hostalias
look up name in the file referred to by the
.Ev HOSTALIASES
files and return a fully qualified hostname if found or
.Dv NULL
if not found or an error occurred.
.Fn res_hostalias
uses
.Fa buf
to store the result in,
.Fn hostalias
uses a static buffer.
.Pp
The functions
.Fn res_getservers
and
.Fn res_setservers
are used to get and set the list of server to be queried.
.Pp
The functions
.Fn res_nupdate
/
.Fn res_update
take a list of ns_updrec
.Fa rrecp_in .
They identify the containing zone for each record and group the records
according to containing zone maintaining in zone order then send an update
request to the servers for these zones.
The number of zones updated is returned or \-1 on error.
Note that
.Fn res_nupdate
will perform TSIG authenticated dynamic update operations if the key is not
.Dv NULL .
.Pp
The function
.Fn res_findzonecut
discovers the closest enclosing zone cut for a specified domain name,
and finds the IP addresses of the zone's master servers.
.Pp
The functions
.Fn res_nmkupdate
/
.Fn res_mkupdate
take a linked list of ns_updrec
.Fa rrecp_in
and construct an UPDATE message in
.Fa buf .
.Fn res_nmkupdate
/
.Fn res_mkupdate
return the length of the constructed message on no error or one of the
following error values.
.Bl -inset -width "-5"
.It \-1
An error occurred parsing
.Fa rrecp_in .
.It \-2
The buffer
.Fa buf
was too small.
.It \-3
The first record was not a zone section or there was a section order problem.
The section order is S_ZONE, S_PREREQ and S_UPDATE.
.It \-4
A number overflow occurred.
.It \-5
Unknown operation or no records.
.El
.Pp
The functions
.Fn res_nclose
/
.Fn res_close
close any open files referenced through
.Fa statp
/
.Fa _res .
.Pp
The function
.Fn res_ndestroy
calls
.Fn res_nclose
then frees any memory allocated by
.Fn res_ninit .
.Pp
The
.Fn dn_comp
function
compresses the domain name
.Fa exp_dn
and stores it in
.Fa comp_dn .
The size of the compressed name is returned or \-1 if there were errors.
The size of the array pointed to by
.Fa comp_dn
is given by
.Fa length .
The compression uses
an array of pointers
.Fa dnptrs
to previously-compressed names in the current message.
The first pointer points to
the beginning of the message and the list ends with
.Dv NULL .
The limit to the array is specified by
.Fa lastdnptr .
A side effect of
.Fn dn_comp
is to update the list of pointers for labels inserted into the message
as the name is compressed.
If
.Fa dnptr
is
.Dv NULL ,
names are not compressed.
If
.Fa lastdnptr
is
.Dv NULL ,
the list of labels is not updated.
.Pp
The
.Fn dn_expand
entry expands the compressed domain name
.Fa comp_dn
to a full domain name.
The compressed name is contained in a query or reply message;
.Fa msg
is a pointer to the beginning of the message.
.Fa eomorig
is a pointer to the first location after the message.
The uncompressed name is placed in the buffer indicated by
.Fa exp_dn
which is of size
.Fa length .
The size of compressed name is returned or \-1 if there was an error.
.Pp
The variables
.Ft statp->res_h_errno
/
.Ft _res.res_h_errno
and external variable
.Ft h_errno
are set whenever an error occurs during resolver operation.
The following
definitions are given in
.Aq Pa netdb.h :
.Bd -literal
#define NETDB_INTERNAL -1
/* see errno */
#define NETDB_SUCCESS  0
/* no problem */
#define HOST_NOT_FOUND 1
/* Authoritative Answer Host not found */
#define TRY_AGAIN      2
/* Non-Authoritative not found, or SERVFAIL */
#define NO_RECOVERY    3
/* Non-Recoverable: FORMERR, REFUSED, NOTIMP */
#define NO_DATA        4
/* Valid name, no data for requested type */
.Ed
.\" .Pp
.\" The
.\" .Fn herror
.\" function writes a message to the diagnostic output consisting of the string
.\" parameter
.\" .Fa s ,
.\" the constant string ": ", and a message corresponding to the value of
.\" .Ft h_errno .
.\" .Pp
.\" The
.\" .Fn hstrerror
.\" function returns a string which is the message text corresponding to the
.\" value of the
.\" .Fa err
.\" parameter.
.Pp
The following functions are only in
.Dv libresolv :
.Fn res_findzonecut ,
.Fn res_nmkupdate ,
.Fn res_nsendsigned ,
and
.Fn res_nupdate .
All the rest are in both
.Dv libc
and
.Dv libresolv .
.Sh FILES
.Bl -tag -width "/etc/resolv.conf    "
.It Pa /etc/resolv.conf
The configuration file, see
.Xr resolv.conf 5 .
.El
.Sh SEE ALSO
.Xr getaddrinfo 3 ,
.Xr gethostbyaddr 3 ,
.Xr gethostbyname 3 ,
.Xr getnameinfo 3 ,
.Xr resolv.conf 5 ,
.Xr hostname 7 ,
.Xr named 8
.Pp
.%T RFC 974 ,
.%T RFC 1032 ,
.%T RFC 1033 ,
.%T RFC 1034 ,
.%T RFC 1035 ,
.%T RFC 1535
.Rs
.%T "Name Server Operations Guide for BIND"
.Re
.Sh HISTORY
The
.Nm
function appeared in
.Bx 4.3 .