inet_res
(kernel)A Rudimentary DNS Client
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.
Functions
getbyname(Name, Type) -> {ok,hostent()} | {error,Reason}
getbyname(Name, Type, Timeout) -> {ok,hostent()} | {error,Reason}
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}
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}
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() ]
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
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.
Functions
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}
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()}
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.