I'm wondering if there's a way to see when the client sends a packet, what header it sends, and how I can deal with it. If in instance, a client sends a header like : 0xAA, how would I reply to that?
This is what I currently have.
socket = require("socket")
server = assert(socket.bind("*", 11000))
ip, port = server:getsockname()
while true do
local client = server:accept()
client:settimeout(0)
client:send(string.char(0xEC,0x2C,0x4A,0x00)) -- Reply
end
What I'm looking for is when the connected client sends an packet, the server reads the header, and I can choose what packet I wish to send back (the reply is sent instantly when a client connects)
Related
I understand that the client and the server are connected after client receives SYN ACK messages from the server during 3 way handshake and sending ACK messages to the Server. After they are connected, when the client sends the other messages to the server, what happens if that messages arrives at the server before the ACK message that client sent when doing the 3 way handshake?
what happens if that messages arrives at the server before the ACK message that client sent when doing the 3 way handshake?
ACK is just the flag in the TCP header together with the sequence number of the latest received data. It can be contained in an empty packet (i.e. no payload, just TCP header) but also in a packet with payload. It does not matter if a specific packet with an ACK is received as long as an ACK covering the data is received at all.
This means it is sufficient if the initial data send by the client cover the final ACK for the TCP handshake. In fact there is not even a need to send a standalone ACK (without payload) from the client at all to finish the TCP handshake but the client can start sending data as soon as the client has received the SYN and ACK from the server.
My unit is supposed to send CSR request to a CA server and get the new certificate. When checked packets captured via Wireshark, TCP packet is sent with SYN but received RST,ACK. When tried to identify the root cause for this, I see many possibilities in the web. Now, how how to identify the actual reason for this failure?
when a TCP client wants to establish a tcp connection with a tcp server
it needs to send SYN and then ACK
while tcp server only sends SYN/ACK
so they are different
but , after the 3_way handshaking,
is this connection symmetric, namely, are TCP client and server in equal status
for example, after the 3-way handshake, usually the client send packet first,
can TCP server send packet first?
No, the procedure is not different at all, but instead of sending a SYN then an ACK in two different packets, the servers concatenate them by sending them via a single packet!
In the other hand, remember always that the client/server nomenclature is relative. The server is the party that remains in listening mode, while the client is the party that initiates the connection ...
After the establishment of the connection, both parties are equivalent (same status as you said: ESTABLISHED). For that reason, both can send the FIN statement to close the connection ...
After the connection is established, both ends are indeed "symmetric". Who sends first is decided by the underlying protocol and differes amongst them.
For example, HTTP starts with the GET <path> HTTP/1.0 command, while other protocols let the server give a greeting line first, and only then the client sends its request.
So in general, both ends are free to send their stuff first.
I have TCP client application and trying to connect with Server located at remote machine.
I am able to connect it.
when I Send Message Called Hello packet the Server should respond with data and time info.
but to my surprise recv returns 0 at client.
since I can't Debug code at Server.
I am not sure but may be there is problem in encoding the message format hello packet at th client upon receiving the wrong packet server is clsoing the connection
I wanted to confirm the meaning of following sequence is
I got following info from wire shark
src IP------>dst ip SYN
dst ip ----->src ip SYN,ACK
src IP------>dst ip ACK
src IP------>dst ip continuation or non http traffic "Hello Packet"
dst ip------>ACK
dst ip------>FIN, ACK
Does this means Server is closing the connection once it receives the hello packet?
Yeah, the FIN,ACK sequence is sent by one of the entity connected when they want to close the connection
Connect function returns after sending the last ACK(3rd segment of 3-way handshake of initiating TCP connection). What happens if this 3rd segment is lost because listen is still waiting for ACK at server but there is no one at client to send that ACK again ?
If the client sends its ACK with a data packet, and it gets lost, the client will notice that the data hasn't been ACKd by the server and resend the packet.
If the client sends its ACK in a separate packet, and it gets lost, the server will notice that the SYN/ACK hasn't been ACKd by the client and resend the packet. Client will respond by resending the ACK.
Whether the application's connect call is still blocking at that time doesn't matter, since the ACKing is done by the OS's TCP/IP implementation.