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# inet_res (kernel v11.0.3)

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A rudimentary DNS client.

This module performs DNS name resolving towards recursive name servers.

See also ERTS User's Guide: Inet Configuration or more information about how to configure an Erlang runtime system for IP communication, and how to enable this DNS client by defining `'dns'` as a lookup method. The DNS client then acts as a backend for the resolving functions in `inet`.

This DNS client can resolve DNS records even if it is not used for normal name resolving in the node.

#### Warning

This is not a full-fledged resolver, only a DNS client that relies on being in trusted network shielded from DNS reply spoofing by firewalls, and on asking only trusted recursive name servers.

If the Crypto application is loaded, this implementation does its best to not be fooled by spoofed DNS replies. This is nevertheless a meager remedy compared to using encrypted and signed DNS methods like TSIG or DNSSEC, as full fledged resolvers do.

#### Note

If the Crypto application is loaded, this resolver client uses cryptographically random transaction IDs and port numbers, which should render the probability for a sucessfully brute force spoofed reply to be too low to be a usable exploit. An Erlang node that is installed in an exposed network environment should already have the Crypto application loaded for example to run the SSL application. If this is not the case, the Crypto application can be explicitly loaded with `application:load(crypto)`.

## Name Resolving

UDP queries are used unless resolver option `usevc` is `true`, 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.

When EDNS is enabled (resolver option `edns` is set to the EDNS version (that is; `0` instead of `false`), resolver option `udp_payload_size` sets the payload size limit. If a name server replies with the TC bit set (truncation), indicating that the answer is incomplete, the query is retried towards the same name server using TCP. Resolver option `udp_payload_size` 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.

For UDP queries, resolver options `timeout` and `retry` control retransmission. Each name server in the `nameservers` list is tried with a time-out of `timeout`/`retry`. Then all name servers are tried again, doubling the time-out, for a total of `retry` times.

But before all name servers are tried again, there is a (user configurable) time-out, `servfail_retry_timeout`. The point of this is to prevent the new query to be handled by a server's servfail cache (a client that is too eager will actually only get what is in the servfail cache). If there is too little time left of the resolver call's time-out to do a retry, the resolver call may return before the call's time-out has expired.

For queries not using the `search` list, if the query to all `nameservers` results in `{error,nxdomain}` or an empty answer, the same query is tried for `alt_nameservers`.

If randomization of transaction ID:s and port numbers is too costly for an Erlang node in a trusted network shielded from DNS reply spoofing, the legacy behaviour to reuse the UDP socket on retries and use sequential transaction ID:s can be configured by setting the resolver option `random` to `false`.

## Resolver Types

The following data types concern the resolver:

## DNS Types

The following data types concern the DNS client:

## Example

This access functions example shows how `lookup/3` can be implemented using `resolve/3` from outside the module:

    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.

## Summary

### Types

dns_class()
dns_data()

DNS record data (content)

dns_msg()

A DNS message.

dns_name()

A string with no adjacent dots.

dns_rr_type()
hostent()

Extended variant of `inet:hostent/0`.

nameserver()
res_error()
res_option()

### Legacy Functions

nnslookup(Name, Class, Type, Nameservers)

Equivalent to `nnslookup(Name, Class, Type, NSs, infinity)`.

nnslookup(Name, Class, Type, Nameservers, Timeout)

Resolve a DNS query.

nslookup(Name, Class, Type)

Equivalent to `nslookup(Name, Class, Type, infinity)`.

nslookup/4

Resolve a DNS query.

### Functions

getbyname(Name, Type)

Equivalent to `getbyname(Name, Type, [], infinity)`.

getbyname(Name, Type, Timeout)

Equivalent to `getbyname(Name, Type, [], Timeout)`.

getbyname(Name, Type, Opts, Timeout)

Resolve a DNS query.

gethostbyaddr(Address)

Equivalent to `gethostbyaddr(Address, infinity)`.

gethostbyaddr(Address, Timeout)

Backend function used by `inet:gethostbyaddr/1`.

gethostbyaddr(Address, Opts, Timeout)

Backend function used by `inet:gethostbyaddr/1`.

gethostbyname(Name)

Backend functions used by `inet:gethostbyname/1,2`.

gethostbyname(Name, Family)

Equivalent to `gethostbyname(Name, Family, infinity)`.

gethostbyname(Name, Family, Timeout)

Equivalent to `gethostbyname(Name, Family, [], Timeout)`.

gethostbyname(Name, Family, Opts, Timeout)

Backend functions used by `inet:gethostbyname/1,2`.

lookup(Name, Class, Type)

Equivalent to `lookup(Name, Class, Type, [], infinity)`.

lookup(Name, Class, Type, Opts)

Equivalent to `lookup(Name, Class, Type, Opts, infinity)`.

lookup(Name, Class, Type, Opts, Timeout)

Look up DNS data.

resolve(Name, Class, Type)

Equivalent to `resolve(Name, Class, Type, [], infinity)`.

resolve(Name, Class, Type, Opts)

Equivalent to `resolve(Name, Class, Type, Opts, infinity)`.

resolve(Name, Class, Type, Opts, Timeout)

Resolve a DNS query.

## Types

# dns_class()

(not exported)

    -type dns_class() :: in | chaos | hs | any.

# dns_data()

(not exported)

    -type dns_data() ::
              dns_name() |
              inet:ip4_address() |
              inet:ip6_address() |
              {MName :: dns_name(),
               RName :: dns_name(),
               Serial :: integer(),
               Refresh :: integer(),
               Retry :: integer(),
               Expiry :: integer(),
               Minimum :: integer()} |
              {inet:ip4_address(), Proto :: integer(), BitMap :: binary()} |
              {CpuString :: string(), OsString :: string()} |
              {RM :: dns_name(), EM :: dns_name()} |
              {Prio :: integer(), dns_name()} |
              {Prio :: integer(), Weight :: integer(), Port :: integer(), dns_name()} |
              {Order :: integer(),
               Preference :: integer(),
               Flags :: string(),
               Services :: string(),
               Regexp :: string(),
               dns_name()} |
              [string()] |
              binary().

DNS record data (content)

The basic type of each data element is specified in this type.

`Regexp` is a UTF-8 `string/0`. The other `string/0`s are actually Latin-1 strings.

# dns_msg()

(not exported)

    -type dns_msg() :: term().

A DNS message.

This is the start of a hierarchy of opaque data structures that can be examined with access functions in `inet_dns`, which return lists of `{Field,Value}` tuples. The arity 2 functions return the value for a specified field.

    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 | dns_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, dns_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

There is an information function for the types above:

    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.

So, `inet_dns:(inet_dns:record_type(X))(X)` converts any of these data structures into a `{Field,Value}` list.

# dns_name()

(not exported)

    -type dns_name() :: string().

A string with no adjacent dots.

# dns_rr_type()

(not exported)

    -type dns_rr_type() ::
              a | aaaa | caa | cname | gid | hinfo | ns | mb | md | mg | mf | minfo | mx | naptr | null |
              ptr | soa | spf | srv | txt | uid | uinfo | unspec | uri | wks.

# hostent()

    -type hostent() ::
              inet:hostent() |
              {hostent,
               H_name :: inet:hostname(),
               H_aliases :: [inet:hostname()],
               H_addrtype :: dns_rr_type(),
               H_length :: non_neg_integer(),
               H_addr_list :: [dns_data()]}.

Extended variant of `inet:hostent/0`.

Allows `dns_rr_type/0` for the `#hostent{}.h_addrtype` field, and `[``dns_data/0``]` for the `#hostent{}.h_addr_list` field.

# nameserver()

    -type nameserver() :: {inet:ip_address(), Port :: 1..65535}.

# res_error()

    -type res_error() :: formerr | qfmterror | servfail | nxdomain | notimp | refused | badvers | timeout.

# res_option()

    -type res_option() ::
              {alt_nameservers, [nameserver()]} |
              {edns, 0 | false} |
              {inet6, boolean()} |
              {nameservers, [nameserver()]} |
              {recurse, boolean()} |
              {retry, integer()} |
              {timeout, integer()} |
              {udp_payload_size, integer()} |
              {dnssec_ok, boolean()} |
              {usevc, boolean()} |
              {random, boolean()} |
              {nxdomain_reply, boolean()}.

## Legacy Functions

# nnslookup(Name, Class, Type, Nameservers)

    -spec nnslookup(Name, Class, Type, Nameservers) -> {ok, dns_msg()} | {error, Reason}
                       when
                           Name :: dns_name() | inet:ip_address(),
                           Class :: dns_class(),
                           Type :: dns_rr_type(),
                           Nameservers :: [nameserver()],
                           Reason :: inet:posix().

Equivalent to `nnslookup(Name, Class, Type, NSs, infinity)`.

# nnslookup(Name, Class, Type, Nameservers, Timeout)

    -spec nnslookup(Name, Class, Type, Nameservers, Timeout) -> {ok, dns_msg()} | {error, Reason}
                       when
                           Name :: dns_name() | inet:ip_address(),
                           Class :: dns_class(),
                           Type :: dns_rr_type(),
                           Timeout :: timeout(),
                           Nameservers :: [nameserver()],
                           Reason :: inet:posix().

Resolve a DNS query.

Like `nslookup/4` but calls `resolve/5` with both the arguments `Opts = [{nameservers, Nameservers}]` and `Timeout`.

# nslookup(Name, Class, Type)

    -spec nslookup(Name, Class, Type) -> {ok, dns_msg()} | {error, Reason}
                      when
                          Name :: dns_name() | inet:ip_address(),
                          Class :: dns_class(),
                          Type :: dns_rr_type(),
                          Reason :: inet:posix() | res_error().

Equivalent to `nslookup(Name, Class, Type, infinity)`.

# nslookup/4

    -spec nslookup(Name, Class, Type, Timeout) -> {ok, dns_msg()} | {error, Reason}
                      when
                          Name :: dns_name() | inet:ip_address(),
                          Class :: dns_class(),
                          Type :: dns_rr_type(),
                          Timeout :: timeout(),
                          Reason :: inet:posix() | res_error();
                  (Name, Class, Type, Nameservers) -> {ok, dns_msg()} | {error, Reason}
                      when
                          Name :: dns_name() | inet:ip_address(),
                          Class :: dns_class(),
                          Type :: dns_rr_type(),
                          Nameservers :: [nameserver()],
                          Reason :: inet:posix() | res_error().

Resolve a DNS query.

This function is a legacy wrapper to `resolve/5` that simplifies errors matching `{error, {Reason, _}}` into `{error, Reason}` or `{error, einval}`.

With argument `Timeout` calls `resolve/5` with `Opts = []`.

With argument `Nameservers` calls `resolve/5` with `Opts = [{nameservers, Nameservers}]` and `Timeout = infinity`.

## Functions

# getbyname(Name, Type)

    -spec getbyname(Name, Type) -> {ok, Hostent} | {error, Reason}
                       when
                           Name :: dns_name(),
                           Type :: dns_rr_type(),
                           Hostent :: inet:hostent() | hostent(),
                           Reason :: inet:posix() | res_error().

Equivalent to `getbyname(Name, Type, [], infinity)`.

# getbyname(Name, Type, Timeout)

    -spec getbyname(Name, Type, Timeout) -> {ok, Hostent} | {error, Reason}
                       when
                           Name :: dns_name(),
                           Type :: dns_rr_type(),
                           Timeout :: timeout(),
                           Hostent :: inet:hostent() | hostent(),
                           Reason :: inet:posix() | res_error().

Equivalent to `getbyname(Name, Type, [], Timeout)`.

# getbyname(Name, Type, Opts, Timeout)

(since OTP 28.1)

    -spec getbyname(Name, Type, Opts, Timeout) -> {ok, Hostent} | {error, Reason}
                       when
                           Name :: dns_name(),
                           Type :: dns_rr_type(),
                           Opts :: [Opt],
                           Opt :: res_option() | verbose | atom(),
                           Timeout :: timeout(),
                           Hostent :: inet:hostent() | hostent(),
                           Reason :: inet:posix() | res_error().

Resolve a DNS query.

Resolves a DNS query of the specified `Type` for the specified host, of class`in`. Returns, on success, when resolving a `Type = a|aaaa` DNS record, a `#hostent{}` record with `#hostent.h_addrtype = inet|inet6`, respectively; see `inet:hostent/0`.

When resolving other `Type = dns_rr_type()`:s (of class `in`), also returns a `#hostent{}` record but with `dns_rr_type/0` in `#hostent.h_addrtype`, and the resolved `dns_data/0` in `#hostent.h_addr_list`; see `hostent/0`.

This function uses resolver option `search` 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.

See `resolve/5` about `Opts`.

# gethostbyaddr(Address)

    -spec gethostbyaddr(Address) -> {ok, Hostent} | {error, Reason}
                           when
                               Address :: inet:ip_address(),
                               Hostent :: inet:hostent(),
                               Reason :: inet:posix() | res_error().

Equivalent to `gethostbyaddr(Address, infinity)`.

# gethostbyaddr(Address, Timeout)

    -spec gethostbyaddr(Address, Timeout) -> {ok, Hostent} | {error, Reason}
                           when
                               Address :: inet:ip_address(),
                               Timeout :: timeout(),
                               Hostent :: inet:hostent(),
                               Reason :: inet:posix() | res_error().

Backend function used by `inet:gethostbyaddr/1`.

# gethostbyaddr(Address, Opts, Timeout)

(since OTP 28.1)

    -spec gethostbyaddr(Address, Opts, Timeout) -> {ok, Hostent} | {error, Reason}
                           when
                               Address :: inet:ip_address(),
                               Opts :: [Opt],
                               Opt :: res_option() | verbose | atom(),
                               Timeout :: timeout(),
                               Hostent :: inet:hostent(),
                               Reason :: inet:posix() | res_error().

Backend function used by `inet:gethostbyaddr/1`.

# gethostbyname(Name)

    -spec gethostbyname(Name) -> {ok, Hostent} | {error, Reason}
                           when
                               Name :: dns_name(),
                               Hostent :: inet:hostent(),
                               Reason :: inet:posix() | res_error().

Backend functions used by `inet:gethostbyname/1,2`.

If resolver option `inet6` is `true`, equivalent to `gethostbyname(Name, inet6, infinity)`, otherwise `gethostbyname(Name, inet, infinity)`.

# gethostbyname(Name, Family)

    -spec gethostbyname(Name, Family) -> {ok, Hostent} | {error, Reason}
                           when
                               Name :: dns_name(),
                               Family :: inet:address_family(),
                               Hostent :: inet:hostent(),
                               Reason :: inet:posix() | res_error().

Equivalent to `gethostbyname(Name, Family, infinity)`.

# gethostbyname(Name, Family, Timeout)

    -spec gethostbyname(Name, Family, Timeout) -> {ok, Hostent} | {error, Reason}
                           when
                               Name :: dns_name(),
                               Family :: inet:address_family(),
                               Timeout :: timeout(),
                               Hostent :: inet:hostent(),
                               Reason :: inet:posix() | res_error().

Equivalent to `gethostbyname(Name, Family, [], Timeout)`.

# gethostbyname(Name, Family, Opts, Timeout)

(since OTP 28.1)

    -spec gethostbyname(Name, Family, Opts, Timeout) -> {ok, Hostent} | {error, Reason}
                           when
                               Name :: dns_name(),
                               Family :: inet:address_family(),
                               Opts :: [Opt],
                               Opt :: res_option() | verbose | atom(),
                               Timeout :: timeout(),
                               Hostent :: inet:hostent(),
                               Reason :: inet:posix() | res_error().

Backend functions used by `inet:gethostbyname/1,2`.

This function uses resolver option `search` just like `getbyname/2,3`.

# lookup(Name, Class, Type)

    -spec lookup(Name, Class, Type) -> [dns_data()]
                    when Name :: dns_name() | inet:ip_address(), Class :: dns_class(), Type :: dns_rr_type().

Equivalent to `lookup(Name, Class, Type, [], infinity)`.

# lookup(Name, Class, Type, Opts)

    -spec lookup(Name, Class, Type, Opts) -> [dns_data()]
                    when
                        Name :: dns_name() | inet:ip_address(),
                        Class :: dns_class(),
                        Type :: dns_rr_type(),
                        Opts :: [res_option() | verbose].

Equivalent to `lookup(Name, Class, Type, Opts, infinity)`.

# lookup(Name, Class, Type, Opts, Timeout)

    -spec lookup(Name, Class, Type, Opts, Timeout) -> [dns_data()]
                    when
                        Name :: dns_name() | inet:ip_address(),
                        Class :: dns_class(),
                        Type :: dns_rr_type(),
                        Opts :: [res_option() | verbose],
                        Timeout :: timeout().

Look up DNS data.

Resolves the DNS data for the record `Name` of the specified `Type` and `Class`. On success, filters out the answer records with the correct `Class` and `Type`, and returns a list of their data fields. So, a lookup for type `any` gives an empty answer, as the answer records have specific types that are not `any`. An empty answer or a failed lookup returns an empty list.

Calls `resolve/*` with the same arguments and filters the result, so `Opts` is described for those functions.

# resolve(Name, Class, Type)

    -spec resolve(Name, Class, Type) -> {ok, dns_msg()} | Error
                     when
                         Name :: dns_name() | inet:ip_address(),
                         Class :: dns_class(),
                         Type :: dns_rr_type(),
                         Error :: {error, Reason} | {error, {Reason, dns_msg()}},
                         Reason :: inet:posix() | res_error().

Equivalent to `resolve(Name, Class, Type, [], infinity)`.

# resolve(Name, Class, Type, Opts)

    -spec resolve(Name, Class, Type, Opts) -> {ok, dns_msg()} | Error
                     when
                         Name :: dns_name() | inet:ip_address(),
                         Class :: dns_class(),
                         Type :: dns_rr_type(),
                         Opts :: [Opt],
                         Opt :: res_option() | verbose | atom(),
                         Error :: {error, Reason} | {error, {Reason, dns_msg()}},
                         Reason :: inet:posix() | res_error().

Equivalent to `resolve(Name, Class, Type, Opts, infinity)`.

# resolve(Name, Class, Type, Opts, Timeout)

    -spec resolve(Name, Class, Type, Opts, Timeout) -> {ok, dns_msg()} | Error
                     when
                         Name :: dns_name() | inet:ip_address(),
                         Class :: dns_class(),
                         Type :: dns_rr_type(),
                         Opts :: [Opt],
                         Opt :: res_option() | verbose | atom(),
                         Timeout :: timeout(),
                         Error :: {error, Reason} | {error, {Reason, dns_msg()}},
                         Reason :: inet:posix() | res_error().

Resolve a DNS query.

Resolves a DNS query for the specified `Type`, `Class`, and `Name`, into a DNS message possibly containing Resource Records. The returned `dns_msg/0` can be examined using access functions in `inet_db`, as described in section in DNS Types.

If `Name` is an `ip_address()`, the domain name to query about is generated as the standard reverse `".IN-ADDR.ARPA."` name for an IPv4 address, or the `".IP6.ARPA."` name for an IPv6 address. In this case, you most probably want to use `Class = in` and `Type = ptr`, but it is not done automatically.

`Opts` overrides the corresponding resolver options. If option `nameservers` is specified, it is assumed that it is the complete list of name serves, so resolver option `alt_nameserves` is ignored. However, if option `alt_nameserves` is also specified to this function, it is used.

Option `verbose` (or rather `{verbose,true}`) causes diagnostics printout through `io:format/2` of queries, replies, retransmissions, and so on, similar to utilities such as `dig` and `nslookup`.

Option `nxdomain_reply` (or rather `{nxdomain_reply, true}`) causes NXDOMAIN errors from DNS servers to be returned as `{error, {nxdomain, dns_msg()}}`. `dns_msg/0` contains the additional sections that where included by the answering server. This is mainly useful to inspect the SOA record to get the TTL for negative caching.

If `Opt` is any atom, it is interpreted as `{Opt,true}` unless the atom string starts with `"no"`, making the interpretation `{Opt,false}`. For example, `usevc` is an alias for `{usevc, true}` and `nousevc` is an alias for `{usevc, false}`.

Option `inet6` has no effect on this function. You probably want to use `Type = a | aaaa` instead.