JSAMP hub and localhost hostname

[boch at vizir.u-strasbg.fr]

Mark,

Thank you for this useful reply.

Let me summarize what I understood from the 2 Bugzilla tickets you are 
referring to plus some tests of my own :

It seems that VODesktop default behaviour is to use '127.0.0.1' as the 
default localhost, whereas JSAMP hub first tries to resolve it using 
InetAdress.getLocalHost()... (in both cases, this can be overriden 
using a dedicated system property).

What would be the actual risks or troubles caused by using '127.0.0.1' 
as the default value for localhost in the SAMP XML-RPC url written in 
the lockfile ? In other words, which percentage of potential SAMP users 
might have a setup in which 127.0.0.1 is not the localhost ?
Would these troubles be more annoying that the ones we have when 
connected to a network constantly going up and down (very common when 
giving a demonstration !), making connections to the hub unreliable 
because of the xml-rpc url being unreachable ?


Thomas



Quoting Mark Taylor <m.b.taylor at bristol.ac.uk>:

> On Tue, 2 Dec 2008, boch at vizir.u-strasbg.fr wrote:
>
>> Hi Mark,
>>
>> I've been testing and playing with (J)SAMP the last days, and I had 
>> the following problem :
>>
>> - let's pretend I am connected to a wireless network whose DHCP 
>> server gives me hcp-131-176-166-130.esac.esa.int as hostname
>>
>> - if I launch a JSAMP hub, the lockfile will have the following line :
>> samp.hub.xmlrpc.url=http://dhcp-131-176-166-130.esac.esa.int:55823/
>>
>> - now, if I disconnect from this network, and startup Aladin, it 
>> won't be able to connect to the hub because it won't be able to 
>> resolve the XML-RPC URL hostname.
>>
>> So, my question is : wouldn't it be safer to use the IP address 
>> instead of the hostname ?
>>
>> Subsidiary question : I noticed that sometimes, the Java method 
>> InetAddress.getLocalHost() throws an UnknownHostException (I use 
>> this method to set the callback URL of Aladin). In such a case, how 
>> safe is it to consider 127.0.0.1 to be the localhost IP address ?
>> Though I am not a network expert at all, I feel this would do the 
>> right thing most of the time. Any opinion on that ?
>
> Thomas,
>
> good questions, not all of which I have good answers to.
> I'm bouncing this onto the apps-samp list in case others have thoughts
> or benefit from the discussion.
>
> First, this is an issue that we've come up against before in
> AstroGrid in relation to the AG PLASTIC hub and the VODesktop/AR
> XML-RPC server.  The consensus seems to be that there is no best
> way to do it - any single approach will fail under some network
> setups (see the followup bugzilla tickets in the snippet below).
> With that in mind, I've tried to cover all bases in JSAMP
> by making this configurable.  When JSAMP writes the hostname into
> the hub lockfile, it uses the SampUtils.getLocalhost() utility method,
> whose javadocs and source currently read like this:
>
>      public static final String LOCALHOST_PROP = "samp.localhost";
>      ...
>
>      /**
>       * Returns a string denoting the local host to be used for communicating
>       * local server endpoints and so on.
>       *
>       * <p>This is normally obtained by calling
>       * <pre>
>       *    java.net.InetAddress.getLocalHost().getCanonicalHostName()
>       * </pre>
>       * but this behaviour can be overridden by setting the
>       * {@link #LOCALHOST_PROP} system property to the string which should
>       * be returned instead.  Sometimes local network issues make it
>       * advantageous to use some non-standard string such as "127.0.0.1".
>       * See, for instance, AstroGrid bugzilla tickets
>       * <a href="http://www.astrogrid.org/bugzilla/show_bug.cgi?id=1799"
>       *    >1799</a>,
>       * <a href="http://www.astrogrid.org/bugzilla/show_bug.cgi?id=2151"
>       *    >2151</a>.
>       *
>       * @return  local host name
>       */
>      public static String getLocalhost() {
>          String hostname = System.getProperty( LOCALHOST_PROP, "" );
>          if ( hostname.length() == 0 ) {
>              try {
>                  hostname = 
> InetAddress.getLocalHost().getCanonicalHostName();
>              }
>              catch ( UnknownHostException e ) {
>                  hostname = "127.0.0.1";
>              }
>          }
>          return hostname;
>      }
>
> So, if you understand the problem you can fix it by running the hub
> with the system property samp.localhost=131.176.166.130 or whatever
> you reckon will work.
>
> This is a good start, but the trouble is people who don't understand
> the problem (=most users) won't know what's up or how to fix it.
> So there might be improvements to be made in advertising this
> functionality or in choosing a safer default.  Ideas from anyone who,
> after reading the bugzilla tickets referenced above, thinks they might
> have a contribution, are welcome.
>
> Mark
>
> -- 
> Mark Taylor   Astronomical Programmer   Physics, Bristol University, UK
> m.b.taylor at bris.ac.uk +44-117-928-8776 http://www.star.bris.ac.uk/~mbt/
>