[TriLUG] Reverse Lookup Using /etc/hosts

[Tanner Lovelace]

On 4/25/07, Joseph Mack NA3T <jmack at wm7d.net> wrote:
> On Wed, 25 Apr 2007, Alexei Znamensky wrote:
>
> > Tarus,
> >
> > nslookup/host works exclusively with DNS, it doesn't follow the system's
> > resolver order (read /etc/nsswitch.conf or host.conf).
>
> there is no resolver facility in Linux which applications
> ask for name resolution. Applications do what they want and
> can ignore files in /etc

Not true.  The programs nslookup, host, and even dig have
always gone directly to DNS since they are tools to debug
DNS!  Anything, however, that uses the gethostbyname system
call does follow what is in /etc/nsswitch.conf.  For instance,
with this line in nsswitch.conf

hosts:          files dns

If I run ping www.trilug.org and examine what files it opens I get this:

open("/etc/ld.so.preload", O_RDONLY)    = -1 ENOENT (No such file or directory)
open("/etc/ld.so.cache", O_RDONLY)      = 3
open("/lib/tls/i686/cmov/libc.so.6", O_RDONLY) = 3
open("/etc/nsswitch.conf", O_RDONLY)    = 3
open("/etc/ld.so.cache", O_RDONLY)      = 3
open("/lib/libnss_db.so.2", O_RDONLY)   = 3
open("/lib/tls/i686/cmov/libnss_files.so.2", O_RDONLY) = 3
open("/usr/lib/libdb3.so.3", O_RDONLY)  = 3
open("/var/lib/misc/protocols.db", O_RDWR|O_LARGEFILE) = -1 ENOENT (No
such file or directory)
open("/var/lib/misc/protocols.db", O_RDONLY|O_LARGEFILE) = -1 ENOENT
(No such file or directory)
open("/etc/protocols", O_RDONLY)        = 3
open("/etc/resolv.conf", O_RDONLY)      = 4
open("/etc/host.conf", O_RDONLY)        = 4
open("/etc/hosts", O_RDONLY)            = 4
open("/etc/ld.so.cache", O_RDONLY)      = 4
open("/lib/tls/i686/cmov/libnss_dns.so.2", O_RDONLY) = 4
open("/lib/tls/i686/cmov/libresolv.so.2", O_RDONLY) = 4

Note that it does go to /etc/hosts first, as specified by nsswitch.conf.
If I then change the line in nsswitch.conf to be this instead:

hosts:          dns

and rerun the same test I get this:

open("/etc/ld.so.preload", O_RDONLY)    = -1 ENOENT (No such file or directory)
open("/etc/ld.so.cache", O_RDONLY)      = 3
open("/lib/tls/i686/cmov/libc.so.6", O_RDONLY) = 3
open("/etc/nsswitch.conf", O_RDONLY)    = 3
open("/etc/ld.so.cache", O_RDONLY)      = 3
open("/lib/libnss_db.so.2", O_RDONLY)   = 3
open("/lib/tls/i686/cmov/libnss_files.so.2", O_RDONLY) = 3
open("/usr/lib/libdb3.so.3", O_RDONLY)  = 3
open("/var/lib/misc/protocols.db", O_RDWR|O_LARGEFILE) = -1 ENOENT (No
such file or directory)
open("/var/lib/misc/protocols.db", O_RDONLY|O_LARGEFILE) = -1 ENOENT
(No such file or directory)
open("/etc/protocols", O_RDONLY)        = 3
open("/etc/resolv.conf", O_RDONLY)      = 4
open("/etc/ld.so.cache", O_RDONLY)      = 4
open("/lib/tls/i686/cmov/libnss_dns.so.2", O_RDONLY) = 4
open("/lib/tls/i686/cmov/libresolv.so.2", O_RDONLY) = 4
open("/etc/host.conf", O_RDONLY)        = 4

Note that it does not look in /etc/hosts.

So, to say "there is no resolver facility in Linux which applications
ask for name resolution" is just wrong.  It isn't ping that's searching
these files, it's gethostbyname in the C library calling into libnss_*.
The libnss libraries are the resolver.

To answer Tarus's question specifically, try this:  Add this
line to /etc/hosts:

192.168.0.1    test-host

Then compile and run this program:

import java.net.InetAddress;

public class reversedns
{

  public static void main(String args[])
  {
    try {
      InetAddress addr = InetAddress.getByName("192.168.0.1");
      System.out.println(addr.getHostName());
    }
    catch (Exception e)
    {
      System.err.println("Error: " + e.getMessage());
    }
  }
}

Running it using "java reversedns" should correctly print out

test-host

assuming that the nsswitch.conf line is like this:

hosts:          files dns

If you remove "files" from that line and re-run the test it will print out

192.168.0.1

So, yes, the java name resolver should follow the settings in nsswitch.conf.

Ok, this e-mail is long and rambling enough, but hopefully it is somewhat
informative. :-)

Cheers,
Tanner

-- 
Tanner Lovelace
clubjuggler at gmail dot com
http://wtl.wayfarer.org/
(fieldless) In fess two roundels in pale, a billet fesswise and an
increscent, all sable.