C / C++ / MFC

Re: How to find multiple servers on LAN using TCP socket [modified]

10-May-10 8:43

Fixed the low votes on your posts (and marked the query as answered).

PS: It seem to work on my LAN and I'm in agreement with you.

“Follow your bliss.” – Joseph Campbell

Moak10-May-10 9:10

Re: How to find multiple servers on LAN using TCP socket

10-May-10 9:10

Ouch... voting up a user who gives wrong technical advice is not a good idea!

There is a difference in LANs that consist of one subnet and those with multiple subnets (where ARP traffic will not be forwarded but IP traffic will be) as explained by Michel Godfroid.

Chat in Europe Java | [Coffee]

Now with 24% more Twitter

modified on Monday, May 10, 2010 3:42 PM

Randor 10-May-10 9:22

Randor

10-May-10 9:22

Hi Moak,

Moak wrote:
There is a difference in LANs that consist of one subnet and those with multiple subnets (where APR traffic will not be forwarded but IP traffic will be) as explained by Michel Godfroid.

This is not correct. The switch *will* forward the ARP broadcast. The network device on the other side however will not respond if it is on another subnet.

Best Wishes,
-David Delaune

Re: How to find multiple servers on LAN using TCP socket

Moak10-May-10 9:31

Re: How to find multiple servers on LAN using TCP socket

10-May-10 9:31

Have you considered network topologies with L3 routers, a campus LAN must not only consist of L2 hubs/switches?

Chat in Europe Java | [Coffee]

Now with 24% more Twitter

Randor 10-May-10 9:43

Randor

10-May-10 9:43

Hi Moak,

Absolutely. I simply wanted to add additional options on top of the advice by Michel Godfroid. I did not expect such a strong negative response.

Best Wishes,
-David Delaune

Re: How to find multiple servers on LAN using TCP socket [modified]

Moak10-May-10 10:25

Re: How to find multiple servers on LAN using TCP socket

10-May-10 10:25

Hi David!

You give a lot of helpful advice, just recently I learned something about Linux system calls form a very detailed posting of you! This time the advice you gave doesn't work in a typical campus LAN which is the original posters scenario (multiple /24 subnets). The initial replies from Michael mentioned that such a network topology probably has L3 routing and explained the background why ARP in such a scenario will not be able to enumerate all active hosts.

You can give it a try with nmap -PR -oN arpscan.txt 192.168.0.0/16, which is expected not to work in a larger network. It should be okay among us to correct technical details and give together the best technical advice. Please don't see my response as negative, it really wasn't meant bad in any way. Smile | :)

Best regards
/Moak

Chat in Europe Java | [Coffee]

Now with 24% more Twitter

modified on Tuesday, May 11, 2010 3:16 AM

Rajesh R Subramanian10-May-10 9:28

Re: How to find multiple servers on LAN using TCP socket

10-May-10 9:28

Moak wrote:
There is a difference in LANs that consist of one subnet and those with multiple subnets (where APR traffic will not be forwarded but IP traffic will be) as explained by Michel Godfroid.

While the OP hasn't been particularly clear about his setup, it's hard for someone to guess and answer. While Michel's answer is correct (and carries my "good answer" vote), I see very little reason to lynch-vote David's post.

I'd leave it to the OP to decide if it were wrong technical advice in this case by testing the suggestions offered in his setup, as he's not provided sufficient details for the responders.

“Follow your bliss.” – Joseph Campbell

Rajesh R Subramanian10-May-10 9:40

Re: How to find multiple servers on LAN using TCP socket

10-May-10 9:40

Moak wrote:
(where APR traffic will not be forwarded but IP traffic will be

Nitpicking: BTW, that ought to be ARP traffic. Smile | :)

“Follow your bliss.” – Joseph Campbell

Rajesh R Subramanian10-May-10 21:50

Re: How to find multiple servers on LAN using TCP socket

10-May-10 21:50

If you want to add a reply, keep it public. Do not delete it after you've posted something!

Workout progress:

Current arm size: 14.4in

Desired arm size: 18in

Next Target: 15.4in by Dec 2010

Current training method: HIT

Randor 10-May-10 9:11

Randor

10-May-10 9:11

Hi Rajesh,

Thanks for the support. To be fair the recommendation made by Michel Godfroid is not incorrect. I was simply giving an alternative and optimized solution for scanning a local network. Allow me to give a more complete explanation.

Here is an exercise for the readers:

1.) Download a packet sniffer such as WireShark[^]

2.) Follow the advice Michel Godfroid gave and scan the entire local subnet. Monitor the Wireshark output.

Here is what you will see:

1.) You will first see an ARP broadcast generated by TCPIP.SYS for each attempted connection to a local IP address (if not already cached). If there is something there it will respond with an ARP reply.

2.) If a network device is on the other side you will then see the TCP handshake and connection attempt. If there is no device there winsock will return a Winsock Error Code[^].

The response I gave was cutting out the TCP protocol from the equation... Since we know the NDIS layer TCPIP.SYS will ARP for the address anyway... when scanning the local subnet.

Best Wishes,
-David Delaune

Re: How to find multiple servers on LAN using TCP socket

Rajesh R Subramanian10-May-10 9:24

Re: How to find multiple servers on LAN using TCP socket [modified]

10-May-10 9:24

Randor wrote:
To be fair the recommendation made by Michel Godfroid is not incorrect.

I realise that, and actually his answer carries my 5 vote!

“Follow your bliss.” – Joseph Campbell

Michel Godfroid10-May-10 9:04

Michel Godfroid

10-May-10 9:04

Sure, the ARP broadcast is there
- to find the MAC address of the DHCP or BOOTP server in order to get an IP address if auto address configuration is enabled.
- to detect an available address if no DHCP or bootp servers are there, and automatic address assignment is in effect (169.254/16 network)
- to see if the assigned address is already in use on the network.
- if netbios is enabled, to get at the network neighbourhood machines (netbios is basically a layer 2 protocol)
- to find a master browser, and start the master browser election
- to find the default gateway

Otherwise I'm a bit bothered by your definition of LAN environment: can a LAN not include routers? In that case my home environment does not qualify as a LAN. Each virtual machine host includes a router to effect separation between in host-traffic and LAN Traffic. Any corporate environment, even in the 192.168 subrange will include routers, just because it's usually bad practice to put more than a couple of hundred hosts on a broadcast segment (and the default netmask is /24).
I use a 10/8 network at home, which I divide in /24 networks because of
- Megalomania
- I can't be bothered to calculate subnets which are not a multiple of 8 Smile | :)

I suspects network admins out there are as lazy as I am.

modified on Monday, May 10, 2010 3:14 PM

Re: How to find multiple servers on LAN using TCP socket

Randor 10-May-10 9:29

Randor

10-May-10 9:29

Hi Michel,

As I stated... your advice is not incorrect. I simply gave an alternative solution for scanning the local subnet for networked devices.

Lets have a beer and work on calculating subnets which are not a multiple of 8. Smile | :)

Best Wishes,
-David Delaune

Re: How to find multiple servers on LAN using TCP socket

Michel Godfroid10-May-10 9:44

Michel Godfroid

10-May-10 9:44

I admit I got a bit carried away. I have a thing with MAC addresses, and not using them for what they were designed Mad | :mad:

. A childhood trauma involving X25 probably.
Given the OP's initial statement (and not knowing the OP's level of knowledge), I thought your initial answer might confuse the issue. I concentrated on an answer which would work in every case.

Randor wrote:
Lets have a beer

As soon as the ash clouds leave Europe...

Cheers! Thumbs Up | :thumbsup:

Have anybody catch errors of CrystalCtrl ?

mesajflaviu10-May-10 1:50

mesajflaviu

10-May-10 1:50

I try to launch an crystal report with CrystalCtrl , and succeed in this way :

#include "CrystalCtrl.h"

void CMainFrame::OnHelpLaunchreport()
{
	// TODO: Add your command handler code here
	
	try
	{
		GetSystemMenu(FALSE)->EnableMenuItem(SC_CLOSE,MF_GRAYED);
		m_CrystalCtrl.SetReportFileName("autocolant.rpt");
		CString sTitle = m_CrystalCtrl.GetReportFileName();
		m_CrystalCtrl.SetWindowTitle(sTitle);
		m_CrystalCtrl.SetWindowShowExportBtn(FALSE);
		m_CrystalCtrl.SetWindowShowPrintSetupBtn(TRUE);
		m_CrystalCtrl.SetWindowState(2);	// 2 Maximized, 1 Minimized, 0 Normal
		m_CrystalCtrl.SetAction(TRUE);
		GetSystemMenu(FALSE)->EnableMenuItem(SC_CLOSE,MF_BYCOMMAND);
	}
	catch(CException* pException)
	{
		pException->ReportError();
		pException->Delete();
	}
}

My questions is : if something bad is happening during try section , programs stop without any warning .... How can I catch exception ?

mesajflaviu10-May-10 4:51

mesajflaviu

10-May-10 4:51

When is talk about Crystal Report , is silence , avoid by everybody ... Laugh | :laugh:

David Crow10-May-10 4:55

David Crow

10-May-10 4:55

mesajflaviu wrote:
When is talk about Crystal Report , is silence...

Perhaps that's an indication that few, if any, use it.

"One man's wage rise is another man's price increase." - Harold Wilson

"Fireproof doesn't mean the fire will never come. It means when the fire comes that you will be able to withstand it." - Michael Simmons

"Man who follows car will be exhausted." - Confucius

mesajflaviu10-May-10 6:29

mesajflaviu

10-May-10 6:29

I don't mean something bad , is just an ... thought ... if I upset on somebody , forgive me ! You are always very kind , so I don't want to harm anybody !

read/write function working in linux

David Crow10-May-10 6:37

David Crow

10-May-10 6:37

I doubt anyone's upset. I was just allaying your concern about not getting a response. There can be a multitude of reasons why folks do not answer questions.

rupeshkp72810-May-10 0:56

rupeshkp728

10-May-10 0:56

I want to know how a read/write calls is accomplished in linux?
I mean how a function say read() will read the data from a file.
What all will be the steps from user mode read() function call to kernel mode?

Will anybody explain the steps or let me know any site from where I can get this info?

Michel Godfroid10-May-10 1:18

Michel Godfroid

10-May-10 1:18

1) I suggest you post in the linux forum.
2) Since this is all open source stuff, I suggest you have a look at the stdio library source code.

rupeshkp72810-May-10 1:56

rupeshkp728

10-May-10 1:56

Thanks for the information.

Randor 10-May-10 6:13

Randor

10-May-10 6:13

Hi,

All modern operating systems in protected mode [^]work nearly the same when executing privileged instructions. They all pass through a call gate[^] and allow the kernel to do the system call[^]. The modern system call instructions are SYSENTER/SYSEXIT. In the old days it was interrupt 0x2e in Windows and 0x80 in Linux.

Here is an example of a Win32 function which is written with both 0x2e and SYSENTER syntax.

http://www.codeproject.com/Messages/2468550/Re-Send-Key-Input.aspx[^]

To answer your question I would summarize as follows:

1.) Glibc simply pushes your arguments onto the stack and invokes the sysenter instruction. (An older version of Linux will invoke interrupt 0x80)

2.) When the CPU sees the SYSENTER instruction it will JMP to the address listed in the registered SYSCALL table. It will execute the instructions there. (Here is where permissions and actual I/O is performed)

Best Wishes,
-David Delaune

Moak10-May-10 10:26