How can I set TTL for DNS Caching with Delphi SOAP webservices? - web-services

Question

I have a Delphi Application accessing a web service on the Internet.
The provider of said service has now changed to a cloud based infrastructure and came to me with the following request:
"Please configure networkaddress.cache.ttl in the file $JAVA_HOME/jre/lib/security/java.security as follows: networkaddress.cache.ttl=60"
Accordung to the web service provider this is necessary for the service to deliver correct results under all circumstances.
He has checked the connection of my appication and it appears to him that once started it sticks with the same IP-address.
Restarting the Applications did solve this for once.
I have been importing the WSDL using the wizard provided by the IDE so I am using an instance of THTTPRIO to access the web service.
The THTTPRIO-instance is created in FormCreate() of the main form.
The Application is running 24/7 and accesses several methods of the web service regularly.
Does the Delphi SOAP framwork use DNS caching at all?
If yes, is there a similar property like networkaddress.cache.ttl in Java?
Is there a good workaround for the issue (e.g. reinstancing the wrapper class just before every call to the service instead of only once in FormCreate)?
EDIT:
The Application is compiled using Delphi 10.1. Berlin (originally developed with Delphi XE7). It is running on an Windows 7 machine.

Answer 1

Have you found DnsFlushResolverCache?
In order to clear DNS resolver cache like what IPCONFIG /flushdns
does, programmatically:
Requires dnsapi.dll which requires Win2000 or later. As usual, MS does
not document the API
Source: http://brannockdevice.blogspot.de/2006/04/winxp-flushing-dns-programatically.html

Answer 2

I've been digging into the source code a little and I believe I found an answer.
The Delphi SOAP framwork doesn't seem to do any DNS caching on its own.
It's just that the connection to the web service is done with the first request via the Windows InternetConnect() function and the resulting handle ist stored and reused from then on.
Here's a little more background information on this:
The THTTPRIO class communicates with the web service through an instance of THTTPReqResp:
property HTTPWebNode: THTTPReqResp read GetHTTPWebNode write SetHTTPWebNode;
THTTPReqResp calls the Connect() method before every request which again uses a WinApi function to estabish a connection and stores the resulting handle in a field:
FInetConnect := InternetConnect(FInetRoot, PChar(FURLHost), FURLPort, PChar(FUserName), PChar(FPassword), INTERNET_SERVICE_HTTP, 0, NativeUInt(Self));
But - the Connect() method only does this if it is not already connected - the following code is found just at the beginning of the Connect() method:
if (FConnected) then
Exit;
By doing so the connection just stays open from the first request on.
Following requests are simply using the stored handle from InetConnect.
For my problem with the cloud service it should be sufficient to create my own instance of THTTPRIO and pass it to the function returning the web service interface:
RIO := THTTPRIO.Create(nil);
FM := GetFleetManagerAPISoap(FALSE, '', RIO);
Then I will just disconnect each time before calling any web service function:
RIO.HTTPWebNode.Connect(FALSE);
In theory that should finally lead to a fresh connection via InternetConnect() and hopefully my problem will be solved.
(Still have to test this, though)

Answer 3

I think that you can set it globally in Windows registry.
Open regedit.exe and navigate to:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters.
In the right pane, add the following value:
Name: DefaultTTL,
Type: REG_DWORD,
Valid Range: 1-255
Restar the computer.
https://technet.microsoft.com/en-us/library/cc957530.aspx