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inet_res - A rudimentary DNS client.
Contents
Data Types
dns_name() = string()
A string with no adjacent dots.
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 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.
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.
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.
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.
Ericsson AB kernel 8.2 inet_res(3erl)
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.
Name
inet_res - A rudimentary DNS client.
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 to 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.
Resolver Types
The following data types concern the resolver:
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