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# inet_res

### MODULE

inet_res

### MODULE SUMMARY

A Rudimentary DNS Client

### DESCRIPTION

Performs DNS name resolving towards recursive name servers

See also ERTS User's Guide: Inet configuration for more information on how to configure an Erlang runtime system for IP communication and how to enable this DNS client by defining 'dns' as a lookup method. It 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.

This is not a full-fledged resolver. It is just a DNS client that relies on asking trusted recursive nameservers.

### Name Resolving

UDP queries are used unless resolver option usevc is true, which forces TCP queries. If the query is to 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 i.e 0 instead of false), resolver option udp_payload_size sets the limit. If a nameserver replies with the TC bit set (truncation), indicating the answer is incomplete, the query is retried to that nameserver using TCP. The resolver option udp_payload_size also sets the advertised size for the max allowed reply size, if EDNS is enabled, otherwise the nameserver uses the limit 512 byte. If the reply is larger it gets truncated, forcing a TCP re-query.

For UDP queries, the resolver options timeout and retry control retransmission. Each nameserver in the nameservers list is tried with a timeout of timeout / retry. Then all nameservers are tried again doubling the timeout, for a total of retry times.

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

### DATA TYPES

As defined in the module inet:

    hostent() = #hostent{}
    posix() = some atom()s
    ip_address() = tuple of integers of arity 4 or 8

Resolver types:

    These correspond to resolver options:

    res_option() =
        [ {alt_nameservers, [ nameserver() ]}
        | {edns, 0 | false}               % Use EDNS
        | {inet6, bool()}                 % Return IPv6 addresses
        | {nameservers, [ nameserver() ]} % List of nameservers
        | {recurse, bool()}               % Request server recursion
        | {retry, integer()}              % UDP retries
        | {timeout, integer()}            % UDP query timeout
        | {udp_payload_size, integer()}   % EDNS payload size
        | {usevc, bool()} ]               % Use TCP (Virtual Circuit)

    nameserver() = {ip_address(),Port}
        Port = integer(1..65535)

    res_error() =
        formerr |
        qfmterror |
        servfail |
        nxdomain |
        notimp |
        refused |
        badvers |
        timeout

DNS types:

    dns_name() = string() with no adjacent dots

    rr_type() = a | aaaa | cname | gid | hinfo | ns | mb | md | mg | mf
              | minfo | mx | naptr | null | ptr | soa | spf | srv | txt
              | uid | uinfo | unspec | wks

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

    dns_class() = in | chaos | hs | any

    dns_msg() = DnsMsg
        This is the start of a hiearchy of opaque data structures
        that can be examined with access functions in inet_dns
        that return lists of {Field,Value} tuples. The arity 2
        functions just return the value for a given field.

        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

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

    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

    dns_data() =             % for dns_type()
        [ dns_name()         % ns, md, mf, cname, mb, mg, mr, ptr
        | ip_address(v4)     % a
        | ip_address(v6)     % aaaa
        | {MName,RName,Serial,Refresh,Retry,Expiry,Minimum} % soa
        | {ip_address(v4),Proto,BitMap} % wks
        | {CpuString,OsString} % hinfo
        | {RM,EM}            % minfo
        | {Prio,dns_name()}  % mx
        | {Prio,Weight,Port,dns_name()} % srv
        | {Order,Preference,Flags,Services,Regexp,dns_name()} % naptr
        | [ string() ]         % txt, spf
        | binary() ]           % null, integer()
    MName, RName = dns_name()
    Serial, Refresh, Retry, Expiry, Minimum = integer(),
    Proto = integer()
    BitMap = binary()
    CpuString, OsString = string()
    RM = EM = dns_name()
    Prio, Weight, Port = integer()
    Order, Preference = integer()
    Flags, Services = string(),
    Regexp = string(utf8)

    There is an info 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) will convert
    any of these data structures into a {Field,Value} list.

### EXPORTS

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

Types:

Name = dns_name()
Type = rr_type()
Timeout = integer() >= 0 | infinity
Reason = posix() | res_error()

Resolve a DNS record of the given type for the given host, of class in. On success returns a hostent() record with dns_data() elements in the address list field.

This function uses the 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 simply 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 = ip_address()
Timeout = integer() >= 0 | infinity
Reason = posix() | res_error()

Backend functions used by inet:gethostbyaddr/1 .

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

Types:

Name = dns_name()
Timeout = integer() >= 0 | infinity
Reason = posix() | res_error()

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

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

If the resolver option inet6 is true, an IPv6 address is looked up, and if that fails the IPv4 address is looked up and returned on IPv6 mapped IPv4 format.

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() | ip_address()
Type = rr_type()
Opts = res_option() | verbose
Timeout = integer() >= 0 | infinity
Reason = posix() | res_error()

Resolve the DNS data for the record of the given type and class for the given 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 will give an empty answer since the answer records have specific types that are not any. An empty answer as well as a failed lookup returns an empty list.

Calls resolve/2..4 with the same arguments and filters the result, so Opts is explained there.

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() | ip_address()
Type = rr_type()
Opts = res_option() | verbose | atom()
Timeout = integer() >= 0 | infinity
Error = {error,Reason} | {error,{Reason,dns_msg()}}
Reason = posix() | res_error()

Resolve a DNS record of the given type and class for the given name. The returned dns_msg() can be examined using access functions in inet_db as described 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 override the corresponding resolver options. If the option nameservers is given, it is also assumed that it is the complete list of nameserves, so the resolver option alt_nameserves is ignored. Of course, if that option is also given to this function, it is used.

The verbose option (or rather {verbose,true}), causes diagnostics printout through io:format/2 of queries, replies retransmissions, etc, similar to from utilities like dig, nslookup et.al.

If Opt is an arbitrary 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 an alias for {usevc,false}.

The inet6 option currently has no effect on this function. You probably want to use Type = a | aaaa instead.

### Examples

Access functions example: how lookup/3 could have been 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 have been deprecated due to the annoying double meaning of the nameservers/timeout argument, and because they had 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() | ip_address()
Type = rr_type()
Nameservers = [ nameserver() ]
Timeout = integer() >= 0 | infinity
Reason = posix() | res_error()

Resolve a DNS record of the given type and class for the given name.

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

Types:

Name = dns_name() | ip_address()
Type = rr_type()
Nameservers = [ nameserver() ]
Timeout = integer() >= 0 | infinity
Reason = posix() | res_error()

Resolve a DNS record of the given type and class for the given name.