inet_res

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.

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.