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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.
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 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.
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:
DATA TYPES
res_option() =
{alt_nameservers, [nameserver()]} |
{edns, 0 | false} |
{inet6, boolean()} |
{nameservers, [nameserver()]} |
{recurse, boolean()} |
{retry, integer()} |
{timeout, integer()} |
{udp_payload_size, integer()} |
{usevc, 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.
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
dns_class() = in | chaos | hs | any
dns_msg() = term()
This is the start of a hiearchy 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 | 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, 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.
EXPORTS
getbyname(Name, Type) -> {ok, Hostent} | {error, Reason}
getbyname(Name, Type, Timeout) -> {ok, Hostent} | {error, Reason}
Types:
Name = dns_name()
Type = rr_type()
Timeout = timeout()
Hostent = inet:hostent()
Reason = inet:posix() | res_error()
Resolves a DNS record of the specified type for the specified host, of class in. Returns, on
success, a hostent() record with dns_data() elements in the address list field.
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. 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() | inet:ip_address()
Class = dns_class()
Type = 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 = 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.
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 = 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 = rr_type()
Timeout = timeout()
Nameservers = [nameserver()]
Reason = inet:posix()
Resolves a DNS record of the specified type and class for the specified name.