I am currently working on a project that involves the use of sockets. The program was working just fine, but after a windows 8.1 update on my computer, either the client socket is sending out a signal to be accepted or the server socket isn't receiving the signal so it can accept. I tried to use it on my emulator and it worked just fine. In addition, I have updated my GPU drivers to see if this could be fixed, but they still won't connect. The program doesn't crash and give me an error; it just sits there and waits. Does anyone have any ideas? Perhaps the update is blocking my peer to peer connection? Any help is much appreciated. Thanks.
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Summary:
I am guessing that the issue here is something to do with how Windows and Linux handle TCP connections, or sockets, but I have no idea what it is. I'm initiating a TCP connection to a piece of custom hardware that someone else has developed and I am trying to understand its behaviour. In doing so, I've created a .Net core 2.2 application; run on a Windows system, I can initiate the connection successfully, but on Linux (latest Raspbian), I cannot.
It appears that it may be because Linux systems do not try to retry/retransmit a SYN after a RST, whereas Windows ones do - and this behaviour seems key to how this peculiar piece of hardware works..
Background:
We have a black box piece of hardware that can be controlled and queried over a network, by using a manufacturer-supplied Windows application. Data is unencrypted and requires no authentication to connect to it and the application has some other issues. Ultimately, we want to be able to relay data from it to another system, so we decided to make our own application.
I've spent quite a long time trying to understand the packet format and have created a library, which targets .net core 2.2, that can be used to successfully communicate with this kit. In doing so, I discovered that the device seems to require a kind of "request to connect" command to be sent, via UDP. Straight afterwards, I am able to initiate a TCP connection on port 16000, although the first TCP attempt always results in a RST,ACK being returned - so a second attempt needs to be made.
What I've developed works absolutely fine on both Windows (x86) and Linux (Raspberry Pi/ARM) systems and I can send and receive data. However, when run on the Raspbian system, there seems to be problems when initiating the TCP connection. I could have sworn that we had it working absolutely fine on a previous build, but none of the previous commits seem to work - so it may well be a system/kernel update that has changed something.
The issue:
When initiating a TCP connection to this device, it will - straight away - reset the connection. It does this even with the manufacturer-supplied software, which itself then immediately re-attempts the connection again and it succeeds; so this kind of reset-once-then-it-works-the-second-time behaviour in itself isn't a "problem" that I have any control over.
What I am trying to understand is why a Windows system immediately re-attempts the connection through a retransmission...
..but the Linux system just gives up after one attempt (this is the end of the packet capture..)
To prove it is not an application-specific issue, I've tried using ncat/netcat on both the Windows system and the Raspbian system, as well as a Kali system on a separate laptop to prove it isn't an ARM/Raspberry issue. Since the UDP "request" hasn't been sent, the connection will never succeed anyway, but this simply demonstrates different behaviour between the OSes.
Linux versions look pretty much the same as above, whereby they send a single packet that gets reset - whereas the Windows attempt demonstrates the multiple retransmissions..
So, does anyone have any answer for this behaviour difference? I am guessing it isn't a .net core specific issue, but is there any way I can set socket options to attempt a retransmission? Or can it be set at the OS level with systemctl commands or something? I did try and see if there are any SocketOptionNames, in .net, that look like they'd control attempts/retries, as this answer had me wonder, but no luck so far.
If anyone has any suggestions as to how to better align this behaviour across platforms, or can explain the reason for this difference is at all, I would very much appreciate it!
Nice find! According to this, Windows´ TCP will retry a connection if it receives a RST/ACK from the remote host after sending a SYN:
... Upon receiving the ACK/RST client from the target host, the client determines that there is indeed no service listening there. In the Microsoft Winsock implementation of TCP, a pending connection will keep attempting to issue SYN packets until a maximum retry value is reached (set in the registry, this value defaults to 3 extra times)...
The value used to limit those retries is set in HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\TcpMaxConnectRetransmissions according to the same article. At least in Win10 Pro it doesn´t seem to be present by default.
Although this is a conveniece for Windows machines, an application still should determine its own criteria for handling a failed connect attempt IMO (i. e number of attempts, timeouts etc).
Anyhow, as I said, surprising fact! Living and learning I guess ...
Cristian.
I have the following problem: I am using sensu to monitor some raspberry pis. Im using standalone checks which works just fine. Now sometimes it might happen that one of the pis lost its wifi connection or just gets restarted manual and dhcp fails or for some other reason has no internet connection. The idea is to let the pi check it self for a internet connection and if the check fails it should solve the problem by it self like restarting wifi or reboot the pi.
Of course a simple bash script with a cronjob should do the job but I want to do the check with sensu. The problem is obvious if the check fails i don't have a internet connection and therefore can't send the check result to the sensu server.
Long story short ;) is it possible to implement something like the remediation feature just on the client? So that a handler on the client it self starts the script which should resolve the problem.
I don't think this is possible. Standalone checks are scheduled by the client but the check result us still published to the server. The result is then handled by the handler which resides on the server.
You could write a standalone "check" plugin which monitors the wifi and if it is off then it will turn it on. It isn't using a handler though.
I have written TCP/IP server using Netty4.0 running on a Linux machine listening to small GPS tracking devices. I have been facing weird problem, which is server stops listening to them in a sudden several hours after I starts it. There is any error log I can see and still server is running. It looks like only channel is not working. When I run a client to do health check, the client socket is still alive and keep sending packet to the server but server does not get it.
If you have any idea how to solve it, please tell me about it. It would be appreciated.
It is impossible to tell without more informations. I would check different things like if there was an OOM exception or with telnet if the server really refuse connections etc. Also jstack may show you if there is some deadlock etc.
I am experiencing a strange issue with connecting to my web server from my Iphone4.
I have an app that connects to my web serer and downloads a chunk of JSON. This works fine when i'm connected via WIFI, but fails through 3G. I don't think it's my app, as Safari also fails to retrieve any page on my web server when using 3G.
My web server has a fixed i.p and is running apache/Centos pretty fresh out of the box, I have not messed with the firewall or iptables settings.
The error obtained from the iphone is "The network connection was lost"
It feels like a firewall issue, but i have not explicitly denied any ips. Is there some aspect of a 3g connection that gets denied by default on Linux?
I realise this is all vague at this time, but does anyone have any wisdom to offer?
Many thanks in advance.
Here is a topic related to iphone connection loss through 3G cellular data whereas WiFi works fine, also giving the error "The network connection was lost"
Couple suggestions:
Ensure that your web server is
responding in a timely fashion. If
it is taking a little too much time,
the 3g connection might give up.
Check your web server logs and see if the connection is reaching
your web server at all, and if any
errors are being recorded. If so,
investigate.
There isn't anything special about a 3g connection wrt you web server. Only difference would be that it is less reliable or slower.
How are you actually trying to connect to your web server? When its over wifi you are local on your network and the name of the server will probably work. But when its over 3g its on the interwebs and will probably not respond to the name of the server.
I have written a small client server socket application. It is a proof of concept for some socket programming that I want to apply to a much bigger project.
For the moment I want to user wireshark to analyse the traffic that goes between them. They are both running on my local machine.
I have installed a loopback interface, and have tried to use wireshark with it.
No joy. Any ideas?
I have successfully analysed traffic between my machine and other machines no problems.
I have had a look here,
http://wiki.wireshark.org/CaptureSetup/Loopback
And I am not using the address 127.0.0.1 which they mention saying you can't capture traffic on 127.0.0.1
Thanks.
You might try creating a virtual machine to run your application and using wireshark on it.
Save yourself some grief and download Microsoft Network Monitor.
As good as Wireshark is on Unixen, Windows is a "special" case :)