.TH inet_res 3erl "kernel 7.2" "Ericsson AB" "Erlang Module Definition"
.SH NAME
inet_res \- A rudimentary DNS client.
.SH DESCRIPTION
.LP
This module performs DNS name resolving to recursive name servers\&.
.LP
See also ERTS User\&'s Guide: Inet Configuration for more information about how to configure an Erlang runtime system for IP communication, and how to enable this DNS client by defining \fI\&'dns\&'\fR\& as a lookup method\&. The DNS client then acts as a backend for the resolving functions in \fIinet\fR\&\&.
.LP
This DNS client can resolve DNS records even if it is not used for normal name resolving in the node\&.
.LP
This is not a full-fledged resolver, only a DNS client that relies on asking trusted recursive name servers\&.
.SH "NAME RESOLVING"

.LP
UDP queries are used unless resolver option \fIusevc\fR\& is \fItrue\fR\&, which forces TCP queries\&. If the query is too large for UDP, TCP is used instead\&. For regular DNS queries, 512 bytes is the size limit\&.
.LP
When EDNS is enabled (resolver option \fIedns\fR\& is set to the EDNS version (that is, \fI0\fR\& instead of \fIfalse\fR\&), resolver option \fIudp_payload_size\fR\& sets the limit\&. If a name server replies with the TC bit set (truncation), indicating that the answer is incomplete, the query is retried to that name server using TCP\&. Resolver option \fIudp_payload_size\fR\& also sets the advertised size for the maximum allowed reply size, if EDNS is enabled, otherwise the name server uses the limit 512 bytes\&. If the reply is larger, it gets truncated, forcing a TCP requery\&.
.LP
For UDP queries, resolver options \fItimeout\fR\& and \fIretry\fR\& control retransmission\&. Each name server in the \fInameservers\fR\& list is tried with a time-out of \fItimeout\fR\&/\fIretry\fR\&\&. Then all name servers are tried again, doubling the time-out, for a total of \fIretry\fR\& times\&.
.LP
But before all name servers are tried again, there is a (user configurable) timeout, \fIservfail_retry_timeout\fR\&\&. The point of this is to prevent the new query to be handled by to the servfail cache (a client that is to eager will actually only get what is in the servfail cache)\&.
.LP
For queries not using the \fIsearch\fR\& list, if the query to all \fInameservers\fR\& results in \fI{error,nxdomain}\fR\& or an empty answer, the same query is tried for \fIalt_nameservers\fR\&\&.
.SH "RESOLVER TYPES"

.LP
The following data types concern the resolver:
.SH DATA TYPES
.nf

\fBres_option()\fR\& = 
.br
    {alt_nameservers, [nameserver()]} |
.br
    {edns, 0 | false} |
.br
    {inet6, boolean()} |
.br
    {nameservers, [nameserver()]} |
.br
    {recurse, boolean()} |
.br
    {retry, integer()} |
.br
    {timeout, integer()} |
.br
    {udp_payload_size, integer()} |
.br
    {usevc, boolean()}
.br
.fi
.nf

\fBnameserver()\fR\& = {inet:ip_address(), Port :: 1\&.\&.65535}
.br
.fi
.nf

\fBres_error()\fR\& = 
.br
    formerr | qfmterror | servfail | nxdomain | notimp | refused |
.br
    badvers | timeout
.br
.fi
.SH "DNS TYPES"

.LP
The following data types concern the DNS client:
.SH DATA TYPES
.nf

\fBdns_name()\fR\& = string()
.br
.fi
.RS
.LP
A string with no adjacent dots\&.
.RE

.nf

\fBrr_type()\fR\& = 
.br
    a | aaaa | cname | gid | hinfo | ns | mb | md | mg | mf |
.br
    minfo | mx | naptr | null | ptr | soa | spf | srv | txt |
.br
    uid | uinfo | unspec | wks
.br
.fi
.nf

\fBdns_class()\fR\& = in | chaos | hs | any
.br
.fi
.nf

\fBdns_msg()\fR\& = term()
.br
.fi
.RS
.LP
This is the start of a hiearchy of opaque data structures that can be examined with access functions in \fIinet_dns\fR\&, which return lists of \fI{Field,Value}\fR\& tuples\&. The arity 2 functions only return the value for a specified field\&.
.LP
.nf

dns_msg() = DnsMsg
    inet_dns:msg(DnsMsg) ->
        [ {header, dns_header()}
        | {qdlist, dns_query()}
        | {anlist, dns_rr()}
        | {nslist, dns_rr()}
        | {arlist, dns_rr()} ]
    inet_dns:msg(DnsMsg, header) -> dns_header() % for example
    inet_dns:msg(DnsMsg, Field) -> Value

dns_header() = DnsHeader
    inet_dns:header(DnsHeader) ->
        [ {id, integer()}
        | {qr, boolean()}
        | {opcode, query | iquery | status | integer()}
        | {aa, boolean()}
        | {tc, boolean()}
        | {rd, boolean()}
        | {ra, boolean()}
        | {pr, boolean()}
        | {rcode, integer(0..16)} ]
    inet_dns:header(DnsHeader, Field) -> Value

query_type() = axfr | mailb | maila | any | rr_type()

dns_query() = DnsQuery
    inet_dns:dns_query(DnsQuery) ->
        [ {domain, dns_name()}
        | {type, query_type()}
        | {class, dns_class()} ]
    inet_dns:dns_query(DnsQuery, Field) -> Value

dns_rr() = DnsRr
    inet_dns:rr(DnsRr) -> DnsRrFields | DnsRrOptFields
    DnsRrFields = [ {domain, dns_name()}
                  | {type, rr_type()}
                  | {class, dns_class()}
                  | {ttl, integer()}
                  | {data, dns_data()} ]
    DnsRrOptFields = [ {domain, dns_name()}
                     | {type, opt}
                     | {udp_payload_size, integer()}
                     | {ext_rcode, integer()}
                     | {version, integer()}
                     | {z, integer()}
                     | {data, dns_data()} ]
    inet_dns:rr(DnsRr, Field) -> Value
.fi
.LP
There is an information function for the types above:
.LP
.nf

inet_dns:record_type(dns_msg()) -> msg;
inet_dns:record_type(dns_header()) -> header;
inet_dns:record_type(dns_query()) -> dns_query;
inet_dns:record_type(dns_rr()) -> rr;
inet_dns:record_type(_) -> undefined.
.fi
.LP
So, \fIinet_dns:(inet_dns:record_type(X))(X)\fR\& converts any of these data structures into a \fI{Field,Value}\fR\& list\&.
.RE

.nf

\fBdns_data()\fR\& = 
.br
    dns_name() |
.br
    inet:ip4_address() |
.br
    inet:ip6_address() |
.br
    {MName :: dns_name(),
.br
     RName :: dns_name(),
.br
     Serial :: integer(),
.br
     Refresh :: integer(),
.br
     Retry :: integer(),
.br
     Expiry :: integer(),
.br
     Minimum :: integer()} |
.br
    {inet:ip4_address(), Proto :: integer(), BitMap :: binary()} |
.br
    {CpuString :: string(), OsString :: string()} |
.br
    {RM :: dns_name(), EM :: dns_name()} |
.br
    {Prio :: integer(), dns_name()} |
.br
    {Prio :: integer(),
.br
     Weight :: integer(),
.br
     Port :: integer(),
.br
     dns_name()} |
.br
    {Order :: integer(),
.br
     Preference :: integer(),
.br
     Flags :: string(),
.br
     Services :: string(),
.br
     Regexp :: string(),
.br
     dns_name()} |
.br
    [string()] |
.br
    binary()
.br
.fi
.RS
.LP
\fIRegexp\fR\& is a string with characters encoded in the UTF-8 coding standard\&.
.RE

.SH EXPORTS
.LP
.nf

.B
getbyname(Name, Type) -> {ok, Hostent} | {error, Reason}
.br
.fi
.br
.nf

.B
getbyname(Name, Type, Timeout) -> {ok, Hostent} | {error, Reason}
.br
.fi
.br
.RS
.LP
Types:

.RS 3
Name = dns_name()
.br
Type = rr_type()
.br
Timeout = timeout()
.br
Hostent = inet:hostent()
.br
Reason = inet:posix() | res_error()
.br
.RE
.RE
.RS
.LP
Resolves a DNS record of the specified type for the specified host, of class \fIin\fR\&\&. Returns, on success, a \fIhostent()\fR\& record with \fIdns_data()\fR\& elements in the address list field\&.
.LP
This function uses resolver option \fIsearch\fR\& that is a list of domain names\&. If the name to resolve contains no dots, it is prepended to each domain name in the search list, and they are tried in order\&. If the name contains dots, it is first tried as an absolute name and if that fails, the search list is used\&. If the name has a trailing dot, it is supposed to be an absolute name and the search list is not used\&.
.RE

.LP
.nf

.B
gethostbyaddr(Address) -> {ok, Hostent} | {error, Reason}
.br
.fi
.br
.nf

.B
gethostbyaddr(Address, Timeout) -> {ok, Hostent} | {error, Reason}
.br
.fi
.br
.RS
.LP
Types:

.RS 3
Address = inet:ip_address()
.br
Timeout = timeout()
.br
Hostent = inet:hostent()
.br
Reason = inet:posix() | res_error()
.br
.RE
.RE
.RS
.LP
Backend functions used by \fIinet:gethostbyaddr/1\fR\&\&.
.RE

.LP
.nf

.B
gethostbyname(Name) -> {ok, Hostent} | {error, Reason}
.br
.fi
.br
.nf

.B
gethostbyname(Name, Family) -> {ok, Hostent} | {error, Reason}
.br
.fi
.br
.nf

.B
gethostbyname(Name, Family, Timeout) ->
.B
                 {ok, Hostent} | {error, Reason}
.br
.fi
.br
.RS
.LP
Types:

.RS 3
Name = dns_name()
.br
Hostent = inet:hostent()
.br
Timeout = timeout()
.br
Family = inet:address_family()
.br
Reason = inet:posix() | res_error()
.br
.RE
.RE
.RS
.LP
Backend functions used by \fIinet:gethostbyname/1,2\fR\&\&.
.LP
This function uses resolver option \fIsearch\fR\& just like \fIgetbyname/2,3\fR\&\&.
.LP
If resolver option \fIinet6\fR\& is \fItrue\fR\&, an IPv6 address is looked up\&.
.RE

.LP
.nf

.B
lookup(Name, Class, Type) -> [dns_data()]
.br
.fi
.br
.nf

.B
lookup(Name, Class, Type, Opts) -> [dns_data()]
.br
.fi
.br
.nf

.B
lookup(Name, Class, Type, Opts, Timeout) -> [dns_data()]
.br
.fi
.br
.RS
.LP
Types:

.RS 3
Name = dns_name() | inet:ip_address()
.br
Class = dns_class()
.br
Type = rr_type()
.br
Opts = [res_option() | verbose]
.br
Timeout = timeout()
.br
.RE
.RE
.RS
.LP
Resolves the DNS data for the record of the specified type and class for the specified name\&. On success, filters out the answer records with the correct \fIClass\fR\& and \fIType\fR\&, and returns a list of their data fields\&. So, a lookup for type \fIany\fR\& gives an empty answer, as the answer records have specific types that are not \fIany\fR\&\&. An empty answer or a failed lookup returns an empty list\&.
.LP
Calls \fIresolve/*\fR\& with the same arguments and filters the result, so \fIOpts\fR\& is described for those functions\&.
.RE

.LP
.nf

.B
resolve(Name, Class, Type) -> {ok, dns_msg()} | Error
.br
.fi
.br
.nf

.B
resolve(Name, Class, Type, Opts) -> {ok, dns_msg()} | Error
.br
.fi
.br
.nf

.B
resolve(Name, Class, Type, Opts, Timeout) ->
.B
           {ok, dns_msg()} | Error
.br
.fi
.br
.RS
.LP
Types:

.RS 3
Name = dns_name() | inet:ip_address()
.br
Class = dns_class()
.br
Type = rr_type()
.br
Opts = [Opt]
.br
Opt = res_option() | verbose | atom()
.br
Timeout = timeout()
.br
Error = {error, Reason} | {error, {Reason, dns_msg()}}
.br
Reason = inet:posix() | res_error()
.br
.RE
.RE
.RS
.LP
Resolves a DNS record of the specified type and class for the specified name\&. The returned \fIdns_msg()\fR\& can be examined using access functions in \fIinet_db\fR\&, as described in section in DNS Types\&.
.LP
If \fIName\fR\& is an \fIip_address()\fR\&, the domain name to query for is generated as the standard reverse \fI"\&.IN-ADDR\&.ARPA\&."\fR\& name for an IPv4 address, or the \fI"\&.IP6\&.ARPA\&."\fR\& name for an IPv6 address\&. In this case, you most probably want to use \fIClass = in\fR\& and \fIType = ptr\fR\&, but it is not done automatically\&.
.LP
\fIOpts\fR\& overrides the corresponding resolver options\&. If option \fInameservers\fR\& is specified, it is assumed that it is the complete list of name serves, so resolver option \fIalt_nameserves\fR\& is ignored\&. However, if option \fIalt_nameserves\fR\& is also specified to this function, it is used\&.
.LP
Option \fIverbose\fR\& (or rather \fI{verbose,true}\fR\&) causes diagnostics printout through \fIio:format/2\fR\& of queries, replies retransmissions, and so on, similar to from utilities, such as \fIdig\fR\& and \fInslookup\fR\&\&.
.LP
If \fIOpt\fR\& is any atom, it is interpreted as \fI{Opt,true}\fR\& unless the atom string starts with \fI"no"\fR\&, making the interpretation \fI{Opt,false}\fR\&\&. For example, \fIusevc\fR\& is an alias for \fI{usevc,true}\fR\& and \fInousevc\fR\& is an alias for \fI{usevc,false}\fR\&\&.
.LP
Option \fIinet6\fR\& has no effect on this function\&. You probably want to use \fIType = a | aaaa\fR\& instead\&.
.RE

.SH "EXAMPLE"

.LP
This access functions example shows how \fIlookup/3\fR\& can be implemented using \fIresolve/3\fR\& from outside the module:
.LP
.nf

example_lookup(Name, Class, Type) ->
    case inet_res:resolve(Name, Class, Type) of
        {ok,Msg} ->
            [inet_dns:rr(RR, data)
             || RR <- inet_dns:msg(Msg, anlist),
                 inet_dns:rr(RR, type) =:= Type,
                 inet_dns:rr(RR, class) =:= Class];
        {error,_} ->
            []
     end.
.fi
.SH "LEGACY FUNCTIONS"

.LP
These are deprecated because the annoying double meaning of the name servers/time-out argument, and because they have no decent place for a resolver options list\&.
.SH EXPORTS
.LP
.nf

.B
nslookup(Name, Class, Type) -> {ok, dns_msg()} | {error, Reason}
.br
.fi
.br
.nf

.B
nslookup(Name, Class, Type, Timeout) ->
.B
            {ok, dns_msg()} | {error, Reason}
.br
.fi
.br
.nf

.B
nslookup(Name, Class, Type, Nameservers) ->
.B
            {ok, dns_msg()} | {error, Reason}
.br
.fi
.br
.RS
.LP
Types:

.RS 3
Name = dns_name() | inet:ip_address()
.br
Class = dns_class()
.br
Type = rr_type()
.br
Timeout = timeout()
.br
Nameservers = [nameserver()]
.br
Reason = inet:posix() | res_error()
.br
.RE
.RE
.RS
.LP
Resolves a DNS record of the specified type and class for the specified name\&.
.RE

.LP
.nf

.B
nnslookup(Name, Class, Type, Nameservers) ->
.B
             {ok, dns_msg()} | {error, Reason}
.br
.fi
.br
.nf

.B
nnslookup(Name, Class, Type, Nameservers, Timeout) ->
.B
             {ok, dns_msg()} | {error, Reason}
.br
.fi
.br
.RS
.LP
Types:

.RS 3
Name = dns_name() | inet:ip_address()
.br
Class = dns_class()
.br
Type = rr_type()
.br
Timeout = timeout()
.br
Nameservers = [nameserver()]
.br
Reason = inet:posix()
.br
.RE
.RE
.RS
.LP
Resolves a DNS record of the specified type and class for the specified name\&.
.RE