[erlang-questions] {error,timeout} with gethostbyname

[Raimo Niskanen]

First some words about the history of these dark corners
of name resolution in Erlang, especially for Windows.

In the beginning Erlang tried to mimic how Unix OS:es
does name resolution by parsing files in the same way
as the OS. This turned out to be very hard to get
correct for all dialects, and impossible for Windows,
so the native resolver was introduced and is now
default for Erlang. It uses the native OS calls
gethosbyname(), gethostbyaddr(), getipnodebyname()
getipnodebyaddr(), getaddrinfo(), getnameinfo() et.al
on different OS:es so Erlang will behave as any other
application on the OS.

The building blocks used in the old way are still there,
inet_hosts, inet_res+inet_dns, and inet_db plus inet_config
and more does a lot of strange things to both be backwards
compatible and still useful...

OK, then, let's move on.


On Sun, Sep 27, 2009 at 07:26:49PM +0200, info wrote:
> Please find a lot of questions in order to undertand the process for understanding how erlang is working with the DNS data.
> 
> When erl is starting, the inet_db is built: correct ?

Yes.

> When erl stops, the inet_db is destroyed: correct ?

Well of course; it is in memory only, and it is not written anywhere.

> Q: from which information does erl build this db ? from the DNS data ? if yes, which file or database or other source ?

For short name distribution mode and no distribution mode,
almost nothing is initialized.  inet:gethostname() is
called, which calls the OS function gethostname().
If it returns a FQDN, the domain is taken from that.
The lookup method is set to 'native'.
Then a check is made; if the hostname can not be
looked up through the 'native' lookup method:
   Add 'file' as a lookup method to use after 'native'.
   If the domain is not set; add 127.0.0.1 as an
      address of the hostname to the 'file' method.
   If the domain is set; try to look up the FQDN
   through the 'native' lookup method, if that succeedes;
      add the resolved IP address and aliases to the 'file' method.
   If the FQDN lookup fails, add 127.0.0.1 as above.
This fixup of the 'host' method is done to ensure
that the own hostname can be looked up.

For long name distribution mode (windows), load configuration from
windows registry:
\hklm\system\CurrentControlSet\Services\TcpIp\Parameters\
varibles:
DataBasePath, Domain, DhcpDomain, EnableDNS, NameServer and SearchList.
The DataBasePath is used to read a Unix style "hosts" file.
There are more variables that should be read, I guess, 
for example DhcpNameServer.
After this the same check as for short name mode is done,
to maybe patch in the 'file' lookup.

> Q:which module in erlang does that ?

inet_config through inet_db.

> Q:is it possible to "follow" it ? 

Er,... what do you mean "follow".

> Q:In my case  inet_db doesn't contain the domain name: normal ?

Since gethostname() on Windows apparently returns the short
hostname, it is normal.

> Q:With the native option, which default information I should find in this db ?

As above. With the -sname or no name option, you should only
get 'hostname' initialized. With the -name option you should
get more, probably res_domain maybe res_search, if this data
is in the registry.

> You suggested this workaround:
>  inet_db:add_ns({127,0,0,1}).
>  inet_db:set_domain("my_domain.local").
>  inet_db:add_search("my_domain.local").
>  inet_db:set_lookup([file,dns]).
> Q:Does it mean that this information are missing in my inet_db ?
> *******
> Correct:
> inet:gethostbyname("my_host").
> {ok,{hostent,"my_host",[],inet,4,[{127,0,0,1}]}}

Well, since 'native' lookup is supposed to work, not really,
but yes... for 'dns' lookup it is missing. In the best of
worlds this could be read from the registry, but on neither
of my (Windows 2003, Windows XP) machines is the nameserver
in the registry...

Oh, and you got the wrong answer there, the lookup order
should probably be [dns,file] (my mistake). You got the
loopback address from the 'file' method instead of
the external IP address from the 'dns' method.

Does inet:gethostbyname("www.google.com") work with
this configuration?

> 
> In fact I try to use erlang in order to find what is wrong in my DNS data. There are too much info on google about name resolution ...

It appears DNS is working for you as both nslookup and
a sufficiently configured inet_res can look up names
through DNS. It is some other part of the name server
in your OS configuration that fails.

Have you looked at this:
http://support.microsoft.com/kb/323380
esp. "How to Configure Forwarders".

...or...

I just looked at my 2003 server, anid under Start -> My Computer
(right click) -> Properties [Computer Name] there is a
"Full computer name:" and a "Domain:" configured.

Under Start -> Control Panel -> Network Connections ->
Local Area Connection (right click) -> Status,
Properties, Internet Protocol (TCP/IP), Properties,
I have set which DNS servers to use. This might
be what makes native lookup work for me.

:

-- 

/ Raimo Niskanen, Erlang/OTP, Ericsson AB