inet_res(3)

NAME
inet_res - A rudimentary DNS client.

DESCRIPTION

This module performs DNS name resolving to recursive name servers.

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 '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 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 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) timeout, 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 timeout to do a retry, the resolver call may return before the call's timeout 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:

DATA TYPES
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()}
nameserver() = {inet:ip_address(), Port :: 1..65535}
res_error() =
    formerr | qfmterror | servfail | nxdomain | notimp | refused |
    badvers | timeout

DNS TYPES

The following data types concern the DNS client:

DATA TYPES
dns_name() = string()

    A string with no adjacent dots.
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
dns_class() = in | chaos | hs | any
dns_msg() = term()

    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 only 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_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()

    Regexp is a string with characters encoded in the UTF-8 coding standard.
hostent() =
    {hostent,
     H_name :: inet:hostname(),
     H_aliases :: [inet:hostname()],
     H_addrtype :: dns_rr_type(),
     H_length :: integer() >= 0,
     H_addr_list :: [dns_data()]}

EXPORTS

getbyname(Name, Type) -> {ok, Hostent} | {error, Reason}
getbyname(Name, Type, Timeout) -> {ok, Hostent} | {error, Reason}

    Types:
        Name = dns_name()
        Type = dns_rr_type()
        Timeout = timeout()
        Hostent = inet:hostent() | hostent()
        Reason = inet:posix() | res_error()

    Resolves a DNS record 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().

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

    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.

gethostbyaddr(Address) -> {ok, Hostent} | {error, Reason}
gethostbyaddr(Address, Timeout) -> {ok, Hostent} | {error, Reason}

    Types:
        Address = inet:ip_address()
        Timeout = timeout()
        Hostent = inet:hostent()
        Reason = inet:posix() | res_error()

    Backend functions used by inet:gethostbyaddr/1.

gethostbyname(Name) -> {ok, Hostent} | {error, Reason}
gethostbyname(Name, Family) -> {ok, Hostent} | {error, Reason}
gethostbyname(Name, Family, Timeout) ->
                 {ok, Hostent} | {error, Reason}

    Types:
        Name = dns_name()
        Hostent = inet:hostent()
        Timeout = timeout()
        Family = inet:address_family()
        Reason = inet:posix() | res_error()

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

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

    If resolver option inet6 is true, an IPv6 address is looked up.

lookup(Name, Class, Type) -> [dns_data()]
lookup(Name, Class, Type, Opts) -> [dns_data()]
lookup(Name, Class, Type, Opts, Timeout) -> [dns_data()]

    Types:
        Name = dns_name() | inet:ip_address()
        Class = dns_class()
        Type = dns_rr_type()
        Opts = [res_option() | verbose]
        Timeout = timeout()

    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 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) -> {ok, dns_msg()} | Error
resolve(Name, Class, Type, Opts) -> {ok, dns_msg()} | Error
resolve(Name, Class, Type, Opts, Timeout) ->
           {ok, dns_msg()} | Error

    Types:
        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()

    Resolves a DNS record of the specified type and class for the specified name. The returned dns_msg() 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 for 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 from 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() 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.

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.

LEGACY FUNCTIONS

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.

EXPORTS

nslookup(Name, Class, Type) -> {ok, dns_msg()} | {error, Reason}
nslookup(Name, Class, Type, Timeout) ->
            {ok, dns_msg()} | {error, Reason}
nslookup(Name, Class, Type, Nameservers) ->
            {ok, dns_msg()} | {error, Reason}

    Types:
        Name = dns_name() | inet:ip_address()
        Class = dns_class()
        Type = dns_rr_type()
        Timeout = timeout()
        Nameservers = [nameserver()]
        Reason = inet:posix() | res_error()

    Resolves a DNS record of the specified type and class for the specified name.

nnslookup(Name, Class, Type, Nameservers) ->
             {ok, dns_msg()} | {error, Reason}
nnslookup(Name, Class, Type, Nameservers, Timeout) ->
             {ok, dns_msg()} | {error, Reason}

    Types:
        Name = dns_name() | inet:ip_address()
        Class = dns_class()
        Type = dns_rr_type()
        Timeout = timeout()
        Nameservers = [nameserver()]
        Reason = inet:posix()

    Resolves a DNS record of the specified type and class for the specified name.