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?
Related
I have a server with multiple clients. The simulated network is in heavy congestion. What I found is that the server reset some TCP connections after received the ACK segment of three-way handshake. But it doesn't happen when the network is in good condition.
What I found is that the ACK of three-way handshake is received about 3.5s later than the SYN-ACK.
Is that because the three-way handshake SYN-ACK time-out? If SYN-ACK time out, why not resend SYN-ACK.
Thank you for any suggestions.
This looks like related to SYN cookies.
SYN cookies
When a Linux host receives too much SYN traffic, it activates the SYN cookies mechanism.
When SYN cookies is enabled, a server answers to SYN by issuing a SYN-ACK segment with specific data encoded in the TCP sequence field. In that field it encodes the timestamp, the MSS and a cryptographic hash of the two endpoints (local and remote IPs and ports) plus the timestamp.
This is done so that the server does not have to store anything about the connection at this point, it simply send the answer and forget about it.
Then, when the client answer with its ACK, the server checks the hash in the ack field (the ack of the client is the sequence of the server). If it is correct, it creates the connection with the data stored in the field.
SYN cookies explain why the server does not resend SYN-ACK packets when they timeout.
But, why the reset after receiving the ACK?
Maybe clients (or server) are behind a NAT that modifies ports and the NAT also gets congested, so that it cannot link the final ACK to the previous SYN, and assigns a new source port. When the server receives it, it resets the connection (it does not matter if SYN cookies are enabled or not).
Or maybe the server process is not accepting connections at the same speed they are arriving, the kernel queue has filled and newer ones are discarded that way.
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
What is TCP response packets?
How to meet this requirement in access-list on a router?
You probably want to look up stateful firewalling for whatever router you're using.
TCP response packets are basically any related TCP packets that come back after an initial SYN has been sent. Typically this would be either a packet with SYN+ACK set, or one with RST if the connection was refused.
Stateful firewalls keep track of not just the source and destination of individual packets, but what connection the packets belong to. By doing this they are able to distinguish between expected, legitimate replies to SYN packets (and others) and random or malicious unrequested "replies".
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.