I am trying to create a simple TCP client/server application. So, the server will wait for a command from the client. Once the command received, the server will perform some tasks which could take up to 60 seconds. During this time, the TCP client should wait for the result of those tasks.
The sending and receiving of the command have been done using the "WriteLn" and "ReadLn" method.
But, I don't know how to do the "waiting" part for the TCP client. Because I don't see any event in the TidTCPClient that can capture this.
I tried using "ReadLn" with timeout, but it seems doesn't work.
recv := '';
recv := TCPClient.ReadLn(#0, 30*1000);
From the server, before 30 seconds, I sent something:
AThread.Connection.WriteLn('SUCCESS');
But that has never been received.
Any advice?
Apparently, I should use this:
recv := TCPClient.ReadLn('', 30*1000);
Instead of:
recv := TCPClient.ReadLn(#0, 30*1000);
Then the issue is resolved.
Related
I am writing a Golang ssh/sftp client which connects to a sftp server with a slowness in connecting and writing files, using golang.org/x/crypto/ssh package. I need to set Connection timeout and SO timeout (as we do in Java JSCH library).
First to achieve Connection timeout I was using ssh.ClientConfig.Timeout, but only worked for nanosecond and microsecond values, not for milliseconds and above, where I needed to set 5 seconds. According to the API doc also I assume ssh.ClientConfig.Timeout is used only for TCP socket connection creation and ssh handshake is not included there.
So then I tried net.Conn.SetDeadline() and it was for end-to-end connection creation + writing file + connection closing. Since this is also not fine, tried net.Conn.SetWriteDeadline() which looks like SO timeout (applied in TCP packet level) but timeout error is not appeared just after the duration elasped, instead comes out after the server's late reply or subsequent write operation starts.
So can someone please show the correct way of setting Connection timeout and SO timeout in Golang ssh package or tell whether this is supported or not?
I have Delphi application that uses Indy HTTP components (that uses Windows socket). And from time to time I am receiving #10060 socket error (WSAETIMEDOUT - An attempt to connect timed out without establishing a connection) upon execution of Indy procedure:
CheckForSocketError(IdWinsock2.Connect(ASocket, #LAddr, SizeOf(LAddr)));
...
connect : TconnectProc;
...
TconnectProc = function ( const s: TSocket; const name: PSockAddr; const namelen: Integer): Integer; stdcall;
Actually all this is just wrap around Windows connect function https://learn.microsoft.com/en-us/windows/win32/api/winsock2/nf-winsock2-connect that gives error and message WSAETIMEDOUT. So - my question is - can this be programming error at all? Even if I have server running on the other computer and even that server has trouble serving request, even in such cases the low level connect should execute normally, if the server can not serve the GET/POST request, then, of course, the errors should be but those errors should spring in only during execution of other Socket functions not in the connect function, isn't so?
I am trying to solve my problem https://serverfault.com/questions/973648/is-it-possible-that-unencrypted-traffic-can-cause-windows-socket-10057-10060?noredirect=1#comment1266907_973648 and now I am seeking whats happening in my code.
My server side code is very simple - it is just TIdHttpServer component with implemented (I provide event name only here):
MyForm.IdHTTPServerCommandGet(AContext: TIdContext;
ARequestInfo: TIdHTTPRequestInfo; AResponseInfo: TIdHTTPResponseInfo);
So - what can be worng with my implemention, what can lead to the appearance of WSAETIMEDOUT for connect? Yes, my procedure can be long sometimes, but it for years it returned the answer sucessfully and there were no communication errors. And I guess, that connect function can even not depend (does not use/raise) OnCommandGet event, so, I have no control how the server side handles socket connect function from the client?
It may be possible that this is connected with TCP (not HTTP) keepalive, maybe some Windows updates have reduced client-side settings of Windows TCP keepalive for the clients and now this manifests as such error.
Indy TCP clients, like TIdHTTP, have a public ConnectTimeout property, which is set to 0 (infinite) by default. If no timeout is specified, a hard-coded 2 minute timeout is used if the client's TIdTCPClient.Connect() method is called in the main UI thread and TIdAntiFreeze is active, otherwise no timeout is used.
If a timeout is used, Indy calls Winsock's connect() function in a worker thread and waits for that thread to terminate. If Indy's timeout elapses, the socket is closed to abort the connect(), and then EIdConnectTimeout is raised to the caller. If connect() exits before Indy's timeout elapses, an exception is raised to the caller only if connect() failed.
If no timeout is used, Indy calls Winsock's connect() directly, waits for it to exit on its own accord, and then raises an exception only if failed.
So, the ONLY way you can get a WSATIMEDOUT error from Indy when it is calling Winsock's connect() function is if Winsock itself timed out internally before Indy's own timeout elapses. That does not necessarily indicate a problem in your code. It just means that the Host you are trying to connect to is simply not reachable at that moment in time. If the server were reachable, but could not accept your connection, you would get a different error, such as WSAECONNREFUSED.
If your server is behind a firewall or router, make sure it is not blocking connections from reaching your server. Try running a packet sniffer on the server machine, such as WireShark, and make sure the 3-way TCP handshake from TIdHTTP is reaching the server machine correctly.
A few weeks ago I wrote a small program which created a socket to an apache webserver and made a request.
Back then I did not know that this web server had a KeepAliveTimeout of 5 seconds.
After my first request I waited 1 minute. After this I wanted to reuse my first socket for another webserver request, but got an error.
From Beej's Guide to Network Programming I learned that if recv returns 0, then the other side has closed its connection:
Wait! recv() can return 0. This can mean only one thing: the remote side has closed
the connection on you! A return value of 0 is recv()'s way of letting you know this
has occurred.
My questions are now:
What does Apache send when the KeepAliveTimeout is over - a FIN or a RST packet?
I know that using a TCP connection for 2 unrelated HTTP requests like in this scenario might
not be the best thing. But in order to understand TCP more the next question is:
After my first successful http request, and before sending the next HTTP request over the same socket, would there be somehow a possibility to get informed about this keepalivetimeout TCPsocket termination of the server other than receiving 0 from the next recv() call?
It will send a FIN. If you write a request to the server after that, send() will return -1 with errno/WSAGetLastError() = ECONNRESET.
would there be somehow a possibility to get informed about this keepalivetimeout tcp socket termination of the server
Yes, by reading the proper response header parameter, namely Keep-Alive: timeout=delta-seconds:
'timeout' Parameter
A host sets the value of the timeout parameter to the time that the host will allows an idle connection to remain open before it is closed. A connection is idle if no data is sent or received by a host.
The value of the timeout parameter is a single integer in seconds.
A host MAY keep an idle connection open for longer than the time that it indicates, but it SHOULD attempt to retain a connection for at least as long as indicated.
As you can see, it's up to the host to decide. Given it only SHOULD try to keep the connection open as long as promised, but it isn't required that it does in order to conform to the spec, so the server might decide to close and reuse the connection to serve another pending client.
I need to close UDP socket which has unsent data immediately.
There is SO_LINGER parameter for TCP sockets but I didn't find out anything for UDP.
It's on Windows.
Thanks in advance.
Update 0:
I give background of this question. I have application 1st thread opens/binds/closes socket, 2nd thread sends datagrams to it.
In some cases after closing the socket (errorcode = 0) bind function returns errorcode 10048 "Address already in use". I found out after close() execution port is still used (via netstat command). Maybe I ask incorrect question and the reason of such behavior is something else?
For all application purposes once your send() returns, the packet is "sent". There's no send-buffer like in TCP, and you have no control over the NIC packet queue. Normal close() is all you need.
Edit 0:
#EJP, here's a quote from UNP for you (Section 2.11 "UDP Output"):
This time, we show the socket send buffer as a dashed box bacause it
doesn't really exist. A UDP socket has a send buffer size (which we
can change with the SO_SNDBUF socket option, Section 7.5), but this
is simply an upper limit on the maximum-sized UDP datagram that can
be written to the socket. If an application writes a datagram larget
than the socket send buffer size, EMSGSIZE is returned. Since UDP is
unreliable, it does not need tp keep a copy of the application's data
and does not need an actual send buffer. (The application data is
normally copied into a kernel buffer of some form as it passes down
the protocol stack, but this copy is discarded by the datalink layer
after the data is transmitted.)
This is what I meant in my answer - you have no control over the send buffer - , so "for all application purposes" it does not exist.
I was having this problem with a windows UDP socket as well. After hours of trying everything I finally found my problem was that I was calling socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP) on the main thread to create the socket, calling bind(...) and recvfrom() on a worker thread, then after closing the worker thread I called closesocket(...) on the main thread. None of the functions returned an error but tor some reason, doing this leaves the UDP address/port combination in use (so a future call to bind() triggers error 10048 WSAEADDRINUSE and netstat -abot -p UDP also shows the port still in use until the whole application is closed). The solution was to move socket(...) and closesocket(...) calls into the worker thread.
Other than weird issues like the case above, there is normally no way that a UDP server socket can be left open after calling closesocket() on it. Microsoft explains that there is no connection maintained with a UDP socket and no need to call shutdown() or any other function. Usually the reason a TCP socket is left open after calling closesocket() is that it wasn't disconnected gracefully and it's waiting for about 4 minutes in TCP_WAIT state for possible additional data to come in before it actually closes. In the case above, netstat showed the UDP socket never closed until the application was closed even if I waited 30+ minutes.
If you're using a wrapper around winsock like the .NET framework, I've also read some features like setting up async callbacks can leave a UDP socket bound open if you don't clean up the callbacks correctly, but I don't think there are any such features in the win32 winsock API that can cause that.
Just close it. There's nothing in UDP that says that pending data will be sent, unlike TCP.
I have a weird behaviour on windows. I have 2 processes that are talking to each other on UDP protocol.
Scenario: 1 of the proceeses is up and the other is not. The process try to send udp message towards the one that is down. The one that is up gets from OS or soemthing else a signal for the socket to read as it got message from the other process.
How come ?
It sends a signal on the same port, but it not a real message that was sent from the other application. When trying to read it u get an excpetion of number 10054, connection reset.