View Source inet_res (kernel v10.2)
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.
This is not a full-fledged resolver, only a DNS client that relies on asking trusted recursive name servers.
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.
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 record data (content)
A DNS message.
A string with no adjacent dots.
Extended variant of inet:hostent/0.
Legacy Functions
Equivalent to nnslookup(Name, Class, Type, NSs, infinity).
Resolve a DNS query.
Equivalent to nslookup(Name, Class, Type, infinity).
Resolve a DNS query.
Functions
Equivalent to getbyname(Name, Type, infinity).
Resolve a DNS query.
Equivalent to gethostbyaddr(Address, infinity).
Backend function used by inet:gethostbyaddr/1.
Backend functions used by inet:gethostbyname/1,2.
Equivalent to gethostbyname(Name, Family, infinity).
Backend functions used by inet:gethostbyname/1,2.
Equivalent to lookup(Name, Class, Type, [], infinity).
Look up DNS data.
Resolve a DNS query.
Types
-type dns_class() :: in | chaos | hs | any.
-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/0s
are actually Latin-1 strings.
-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) -> ValueThere 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.
-type dns_name() :: string().
A string with no adjacent dots.
-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.
-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.
-type nameserver() :: {inet:ip_address(), Port :: 1..65535}.
-type res_error() :: formerr | qfmterror | servfail | nxdomain | notimp | refused | badvers | timeout.
-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()} | {nxdomain_reply, boolean()}.
Legacy Functions
-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).
-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.
-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).
-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
-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).
-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().
Resolve a DNS query.
Resolves a DNS query of the specified Type for the specified host,
of classin. 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.
-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).
-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.
-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).
-spec gethostbyname(Name, Family) -> {ok, Hostent} | {error, Reason} when Name :: dns_name(), Hostent :: inet:hostent(), Family :: inet:address_family(), Reason :: inet:posix() | res_error().
Equivalent to gethostbyname(Name, Family, infinity).
-spec gethostbyname(Name, Family, Timeout) -> {ok, Hostent} | {error, Reason} when 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.
-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).
-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).
-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.
-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).
-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).
-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.