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• What happens if a server never receives the RST packet?

  - by Rob

  Someone recently decided to show me a POC of a new Denial of Service method using SYN/TCP he's figured out. I thought it was complete nonsense, but after explaining to him about SYN-SYN/ACK-RST, he left me speechless. He told me "what if the server you're using to trick into sending the SYN/ACK packets can't receive the RST packet?" I have no idea. He claims that the server will continue trying to send SYN/ACK packets, and that the packetrate will continue to build up. Is there any truth to this? Can anyone elaborate? Apparently, the way it works is this: He spoofs the IP of the SYN packet to the target's IP. He then sends the SYN packet to a handful of random servers They all reply with their SYN/ACK packet to the target IP, of course The target responds with RST, as we know BUT somehow he keeps the target from sending the RST or keeps the random servers from processing it With this, apparently the servers will continue trying to send the SYN/ACK packets, thus producing a somewhat of a "snowball" effect.

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• How to make sure you server NIC performance is at best on Windows?

  - by Bobb

  I realised that I followed some obscure paper on setting NICs on Windows for too long. It might be outdated with new hardware released in past couple of years and with W2008R2. I read a bit about offloading and RSS settings on Windows and I realiased that it is very much circumstantial. Noone can really say - enable that and disable this. etc. So what I really want is for my next server try and setup testing environment and measure how my particular application will behave with different settings. The target is going to be latency of TCP primarily. Please note I am talking about latency inside the box. Are there precision tools for Windows to measure latency (down to microseconds)? P.S. I know this is not easy question. Windows time drift is awful problem for any precision test but still I am sure I am not the fist person to need that... Please share your experience

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• Can a network interface be configured to have a default gateway for UDP packets?

  - by Vaibhav

  It is quite possible that my question may not make a lot of sense. I apologize, but I am not a networking guy, and that's my excuse. To elaborate, WikiPedia defines "Default Gateway" as a node on a "TCP/IP" network. And the way it works is that if a network interface is sending a packet to an IP address not present on its subnet, it sends it out to the default gateway (which then knows what to do with that packet). Is this true if a UDP packet (datagram) is involved? I mean, if my network interface is sending a UDP packet to an IP address that is not present on its subnet, would it automatically send it to the Default Gateway as well?

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• iptables redirect single website traffic to port 8080

  - by Luke John Southard

  My goal is to be able to make a connection to one, and only one, website through a proxy. Everything else should be dropped. I have been able to do this successfully without a proxy with this code: ./iptables -I INPUT 1 -i lo -j ACCEPT ./iptabels -A OUTPUT -p udp --dport 53 -j ACCEPT ./iptables -A OUTPUT -p tcp -d www.website.com --dport 80 -j ACCEPT ./iptables -A INPUT -m conntrack --cstate ESTABLISHED,RELATED -j ACCEPT ./iptables -P INPUT DROP ./iptables -P OUTPUT DROP How could I do the same thing except redirect the traffic to port 8080 somewhere? I've been trying to redirect in the PREROUTING chain in the nat table. I'm unsure if this is the proper place to do that tho. Thanks for your help!

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• How to tune Windows 2008r2 and IIS to maximize single file download speeds?

  - by uSlackr

  We recently put up an IIS site (on WinSvr 2008r2) that is used almost exclusively for downloading files over the internet. The data exists as a large collection of .zip files ranging from 1MB - 35GB in size. We want to allow a lot of downloads during a day (more than 500GB) but have implemented an outbound ASA throttle at 60mbps in order to preserve bandwidth for other uses. The total link speed is 100mbps. Here's the interesting part: While we can serve up multiple downloads to hit the 60mbps cap, we cannot get any single download to exceed 2.5M bytes/sec (20 Mbits/s). Is there any TCP or IIS tuning we can do to push up individual download speeds? Or something else to look at?

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• SAT Thread and Process output capture in c#

  - by alex

  Hi: This is a strange problem I encountered. I have an window application written in c# to do testing. It has a MDI parent form that is hosting a few children forms. One of the forms launch test cripts by creating processes and capture the scripts output to a text box. Another form open serial port and monitoring the status of the device I am working on(like a shell). If I ran both of them together, the output of the script seems only appear in the text box after the test is done. However, If I don't open the serial port form, the output of the script is captured in real time. Does anyone knows what's causing the problem? I notice the onDataReceived even handler for serial port form has a [SAThread] header to it. Will this cause the serial port thread having higher priority than other processes? Thanks in advance.

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• ssms cannot connect to default sql server instance without specifying port number

  - by Oliver

  I have multiple SQL Server 2005 instances on a box. From SSMS on my desktop I can connect to that box's named instances with no problem. After some recent network configuration changes, when I want to connect to the default instance from SSMS on my desktop, I have to specify the port number. Before the network changes, I did not have to specify the port number of the default instance. If I remote to any other box (including the one in question), and use that box's SSMS to connect to that default instance, success. From my desktop, and only from my desktop, I have to specify the port number. Is it a SQL Server configuration that I've missed? Is it possible something in my PC's configuration is getting in the way? Where would I look, or what could I pass on to the network folks to help them resolve this? Any help is appreciated.

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• Getting Serial Port Information in C#

  - by Jim Fell

  I have some code that loads the serial ports into a combo-box: List<String> tList = new List<String>(); comboBoxComPort.Items.Clear(); foreach (string s in SerialPort.GetPortNames()) { tList.Add(s); } tList.Sort(); comboBoxComPort.Items.Add("Select COM port..."); comboBoxComPort.Items.AddRange(tList.ToArray()); comboBoxComPort.SelectedIndex = 0; I would like to add the port descriptions (similar to what are shown for the COM ports in the Device Manager) to the list and sort the items in the list that are after index 0 (solved: see above snippet). Does anyone have any suggestions for adding the port descriptions? I am using Microsoft Visual C# 2008 Express Edition (.NET 2.0). Any thoughts you may have would be appreciated. Thanks.

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• STAThread and Process output capture in c#

  - by alex

  Hi: This is a strange problem I encountered. I have an window application written in c# to do testing. It has a MDI parent form that is hosting a few children forms. One of the forms launch test cripts by creating processes and capture the scripts output to a text box. Another form open serial port and monitoring the status of the device I am working on(like a shell). If I ran both of them together, the output of the script seems only appear in the text box after the test is done. However, If I don't open the serial port form, the output of the script is captured in real time. Does anyone knows what's causing the problem? I notice the onDataReceived even handler for serial port form has a [STAThread] header to it. Will this cause the serial port thread having higher priority than other processes? Thanks in advance.

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• Add / remove a port number to/from a URL with REGEX in PHP

  - by SuperDuck

  Hello guys, I've searched but was unable to find an existing regex function. Has anybody done this before? I wish to add a port number, or remove a potantially existing one from a url in php. To use in some functions which translate a given url to the secure one, unsecure one, etc. Now I need a second SSL secured site on the server so I need to dynamically add a port number while converting http to https, and remove any port number while converting from https to http. Thanks, Duck

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• TcpListener.Start() does not open the port

  - by SoMoS

  Hello, I have a class that inherits from the TcpListener, this class Shadows the Start method just to call the base Start() and the base BeginAcceptTcpClient(). From time to time the method is called but the port is not opened (netstat does not show the port open). The class looks like this Public Class ExtendedTcpListener Inherits System.Net.Sockets.TcpListener Public Shadows Sub Start() SyncLock (m_stopLock) MyBase.Start() MyBase.BeginAcceptTcpClient(AddressOf Me.CompleteAcceptTcpClient, Me) My.Application.Log.WriteEntry("Extended Tcp Listener started ...", TraceEventType.Verbose) End SyncLock End Sub Any idea on what's happening or how to debug the issue? As the Start() is called without exception I expected to find the port always opened (the log is always written). Extra information: when the Start method works fine it works each time until app is restarted. When the Start method does not work it won't work again until the app is restarted.

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• Ubuntu 8.04 LTS MySQL port remote access

  - by Stig Christian

  Hi! I have tried everything I know, but still no solution to this problem. MySQL works perfectly on localhost, but it seems port 3306 is blocked from the outside... I have commented out "bind-address" in /etc/mysql/my.cnf, and there is no iptables rules that deny this port. (I have even opened the port in iptables just to be sure) Still I can`t connect to the server from a remote connection (telnet or portscan returns negative results). Where is the problem?

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• How to stop listening on an HTTP::Daemon port in Perl

  - by Trevor

  I have a basic perl HTTP server using HTTP::Daemon. When I stop and start the script, it appears that the port is still being listened on and I get an error message saying that my HTTP::Daemon instance is undefined. If I try to start the script about a minute after it has stopped, it works fine and can bind to the port again. Is there any way to stop listening on the port when the program terminates instead of having to wait for it to timeout? use HTTP::Daemon; use HTTP::Status; my $d = new HTTP::Daemon(LocalAddr => 'localhost', LocalPort => 8000); while (my $c = $d->accept) { while (my $r = $c->get_request) { $c->send_error(RC_FORBIDDEN) } $c->close; undef($c); }

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• Java: How to clear socket bindings

  - by Matt1776

  I am having a few issues with sockets within my Java SIP client. When I bind to an address and port, if something goes wrong I have to attempt to reconnect, usually after I've stopped and restarted the process. Problem with that is then the port is bound and I am forced to increment the local port. How can I remove the binding to the port I am targeting before binding to it? If that isnt possible, then how can I trap the process just before it ends so that I can locate the socket binding and close it manually?

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• fwrite() not writing. Error with my code, or the remote client?

  - by Rob

  Trying to set up a script to send commands to a remote client on a Win32 system. Here is the code: $command = $_POST['command']; $host = $_POST['host']; $port = $_POST['port']; $fp = @fsockopen($host, $port, $e, $s, 15); if (!$fp) { echo 'Error! Here\'s your problem: ' . $e . ': ' . $s; }else{ $fw = fwrite($fp, $command); if (!$fw){ echo 'Failed sending command.'; fclose($fp); }else{ fclose($fp); echo 'Successfully sent: ' . $command; } } My buddy is working on the remote client, and he says that this script is sending '' However, my script is echoing Successfully sent: test Am I doing something wrong, or is it a problem on his end?

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• How to turn one server into many servers? (Virtualization/VMWare)

  - by user1229962

  I'm hoping for a high level discussion of this problem I know is quickly approaching my application. I have a server that binds on a specific port and manages TCP Sockets from my clients. I know that it is common practice to use VMWare to virtualize servers and run multiple servers at once. How can a single server design be changed to support multiple servers? Multiple servers can't bind to the same port. If I had to guess I would say a proxy server binds to the port and then sends connections off to the other servers to be handled as if it was still a single server application. I'm wondering what options there are and what are the common practices for solving this problem? Thanks in advance!

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• Creating a simple command line interface (CLI) using a python server (TCP sock) and few scripts

  - by VN44CA

  I have a Linux box and I want to be able to telnet into it (port 77557) and run few required commands without having to access to the whole Linux box. So, I have a server listening on that port, and echos the entered command on the screen. (for now) Telnet 192.168.1.100 77557 Trying 192.168.1.100... Connected to 192.168.1.100. Escape character is '^]'. hello<br /> You typed: "hello"<br /> NOW: I want to create lot of commands that each take some args and have error codes. Anyone has done this before? It would be great if I can have the server upon initialization go through each directory and execute the init.py file and in turn, the init.py file of each command call into a main template lib API (e.g. RegisterMe()) and register themselves with the server as function call backs. At least this is how I would do it in C/C++. But I want the best Pythonic way of doing this. /cmd/ /cmd/myreboot/ /cmd/myreboot/ini.py (note underscore don't show for some reason) /cmd/mylist/ /cmd/mylist/init.py ... etc IN: /cmd/myreboot/_ini_.py: from myMainCommand import RegisterMe RegisterMe(name="reboot",args=Arglist, usage="Use this to reboot the box", desc="blabla") So, repeating this creates a list of commands and when you enter the command in the telnet session, then the server goes through the list, matches the command and passed the args to that command and the command does the job and print the success or failure to stdout. Thx

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• tcp/ip accept not returning, but client does

  - by paquetp

  server: vxworks 6.3 calls the usual socket, bind, listen, then: for (;;) { client = accept(sfd,NULL,NULL); // pass client to worker thread } client: .NET 2.0 TcpClient constructor to connect to server that takes the string hostname and int port, like: TcpClient client = new TcpClient(server_ip, port); This is working fine when the server is compiled and executed in windows (native c++). intermittently, the constructor to TcpClient will return the instance, without throwing any exception, but the accept call in vxWorks does not return with the client fd. tcpstatShow indicates no accept occurred. What could possibly make the TcpClient constructor (which calls 'Connect') return the instance, while the accept call on the server not return? It seems to be related to what the system is doing in the background - it seems more likely to get this symptom to occur when the server is busy persisting data to flash or an NFS share when the client attempts to connect, but can happen when it isn't also. I've tried adjusting priority of the thread running accept I've looked at the size of the queue in 'listen'. There's enough. The total number of file descriptors available should be enough (haven't validated this yet though, first thing in the morning)

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• Remoting connection over TCP

  - by Voyager Systems

  I've got a remoting server and client built. The server is set up as such: BinaryServerFormatterSinkProvider serverProv = new BinaryServerFormatterSinkProvider(); serverProv.TypeFilterLevel = TypeFilterLevel.Full; BinaryClientFormatterSinkProvider clientProv = new BinaryClientFormatterSinkProvider(); IDictionary props = new Hashtable(); props["port"] = port; TcpChannel channel = new TcpChannel( props, clientProv, serverProv ); ChannelServices.RegisterChannel( channel, false ); RemotingConfiguration.RegisterWellKnownServiceType( typeof( Controller ), "Controller", WellKnownObjectMode.Singleton ); The client is set up as such: ChannelServices.RegisterChannel( new TcpChannel( 0 ), false ); m_Controller = (Controller)RemotingServices.Connect( typeof( Controller ), "tcp://" + ip + ":2594/Controller" ); When I try to connect to the server from the same computer with the IP specified as 'localhost,' it connects fine. However, when I try to connect from a remote computer, not on the LAN, given the server's IP address, I receive the following exception on the client after a long wait: A connection attempt failed because the connected party did not properly respond after a period of time How can I resolve this? I'm sure it's a configuration problem because the ports are forwarded properly on the server and there is no firewall active. Thanks

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• iptables firewall rules not allowing ssh from lan to DMZ

  - by ageis23

  Chain INPUT (policy ACCEPT) target prot opt source destination REJECT tcp -- anywhere anywhere tcp dpt:www reject-with tcp-reset REJECT tcp -- anywhere anywhere tcp dpt:telnet reject-with tcp-reset ACCEPT 0 -- anywhere anywhere state RELATED,ESTABLISHED DROP udp -- anywhere anywhere udp dpt:route DROP udp -- anywhere anywhere udp dpt:route ACCEPT udp -- anywhere anywhere udp dpt:route logdrop icmp -- anywhere anywhere logdrop igmp -- anywhere anywhere ACCEPT udp -- anywhere anywhere udp dpt:5060 ACCEPT 0 -- anywhere anywhere state NEW logaccept 0 -- anywhere anywhere state NEW ACCEPT 0 -- anywhere anywhere ACCEPT 0 -- anywhere anywhere ACCEPT 0 -- anywhere anywhere logdrop 0 -- anywhere anywhere Chain FORWARD (policy ACCEPT) target prot opt source destination REJECT 0 -- 192.168.0.0/24 192.168.2.0/24 reject-with icmp-port-unreachable ACCEPT tcp -- choister 192.168.2.142 tcp dpt:ssh state NEW REJECT 0 -- 192.168.0.0/24 192.168.3.0/24 reject-with icmp-port-unreachable ACCEPT gre -- 192.168.1.0/24 anywhere ACCEPT tcp -- 192.168.1.0/24 anywhere tcp dpt:1723 ACCEPT 0 -- anywhere anywhere ACCEPT 0 -- anywhere anywhere ACCEPT 0 -- anywhere anywhere ACCEPT 0 -- anywhere anywhere TCPMSS tcp -- anywhere anywhere tcp flags:SYN,RST/SYN TCPMSS clamp to PMTU lan2wan 0 -- anywhere anywhere ACCEPT 0 -- anywhere anywhere state RELATED,ESTABLISHED logaccept tcp -- anywhere choister tcp dpt:www TRIGGER 0 -- anywhere anywhere TRIGGER type:in match:0 relate:0 trigger_out 0 -- anywhere anywhere logaccept 0 -- anywhere anywhere state NEW logdrop 0 -- anywhere anywhere The ssh server I'm trying to connect to is in the DMZ(192.168.0.145). It's mainly used as a web server. I need access to it from my room 192.168.2.142. I don't get why ssh can't forward onto the 192.168.2.0 subnet? I'm sure it's the reject rule that causing this because it works without it.

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• Question regarding TCP Connection Forcefully shut down.

  - by Tara Singh

  Hi All, I am designing a Client Server Chat application in Java which uses TCP connection between them. I am not able to figure out how to detect at server side when a client forcefully closes down. I need this as i am maintaining a list of online clients and i need to remove user from the list when he forcefully closes the connection. Any help will be highly appreciated. Thanks Tara Singh

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• Why Does RTP use UDP instead of TCP?

  - by mahesh

  I wanted to know why UDP is used in RTP rather than TCP ?. Major VoIP Tools used only UDP as i hacked some of the VoIP OSS.

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• Sources of latency in sending-receiving tcp/udp packets in linux

  - by osgx

  Hello What are sources of latency in process of sending/receiving tcp/udp packets in linux 2.6 ? I want to know a latency sources in "ping-pong" latency tests. There are some rather good papers of ethernet latency, but they cover only latency sources in the wire and switch (and rather cursory, only for specific switch). What steps of processing does follow a packet? Papers with deep latency analysis of usual ping (icmp) will be useful too. I rely on community :)

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• Transmitting complex objects using TCP in C#

  - by Rakesh K

  Hi, I have a client server application in which I need to transmit a user defined object from Client to Server using TCP connection. My object is of the following structure: class Conversation { private string convName, convOwner; public ArrayList convUsers; public string getConvName() { return this.convName; } public string getConvOwner() { return this.convOwner; } } Please help me how to trnasmit this object at from client and again de-serialize it into appropriate object at server side. Thanks, Rakesh K.

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• Emulator TCP Packet Size

  - by jpspringall

  Has anyone tried to do a tcp client server app using the emulator using the pc as a server and the phone as the client? I've got a bit of an issue where its only sending one packet, ie 1491 bytes of data regardless of how much there actually is to send, from the client(Phone) to the server(PC) Thanks James

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