Is there any way to delay the Syn-ACK when the SYN is send by the client to the server and server has reply with SYN-ACK, in the three-way handshake protocol, is there any specific tuning required in sysctl.cnf file or whether it can be done by iptables settings ?
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When using the -T option to use TCP SYN for probes, (1.)is this how it works: SYN is sent and ACK is received so only a two way handshake occurs so connection with server is made but no confirmation is sent back to server? and it still tracks the server address and data received through traceroute,(2.) is this what keeps the probes from being identified by server firewall/ applications?
The question is we have the following setup and we have noticed sometime client sends RST packet to terminate initial TCP handshake connection and application gets a timeout.
[10.5.8.30]------[Linux FW]-------[10.5.16.20]
Wireshark:
You can see in Wireshark RST packet, I thought its FW sending RST but in capture packet coming from 10.5.8.30 so what could be wrong here? why connection getting reset randomly, if I try next time then it will work.
The fact that the source IP for the RST packet is 10.5.8.30 doesn't mean that it really came from 10.5.8.30.
There are firewalls and various other intermediary devices that forge such packets. Try capturing on both ends to check whether 10.5.8.30 did, in fact, send the RST. It doesn't make sense for a client to send a TCP Syn and then a RST.
I an simulating http client traffic with RAW socket.
I send a SYN packet then get the SYN-ACK from the server.
Finally I send an ACK+request packet and waits for the response.
I noticed that when using wget or curl,
the first ACK and the request are sent in two different packets.
why is that, and is that relevant to anything?
A client application that uses a TCP socket typically calls socket() then connect() then send(). The connect() function establishes the TCP connection, and to do this the TCP protocol requires 3 packets: SYN, SYN+ACK, ACK. After that the send() call sends the first data. Therefore the ACK and data are sent separately.
I think your packet flow probably does satisfy the TCP protocol (see https://www.rfc-editor.org/rfc/rfc793), but it is unusual.
I'm attempting to craft a raw TCP packet to send over Ether in a raw socket on a linux client and server. The special part of the TCP packet is that I'm attempting to use the raw data field of the TCP SYN packet and RST packet to send data back and forth (for a proof of concept about an unused part of the TCP protocol).
I've disabled RST packets from my iptables on the server.
In short, here's my current situation:
Client sends SYN with data is sent to server
Server receives a SYN packet without data
Server responds with a RST packet with data
Client receives a RST packet without data
But, using the same socket, I can successfully do this:
SYN without data sent to server
Server receives a SYN packet
Server responds with a SYN ACK packet with data
Client receives a SYN ACK packet without data
Client receives a PSH ACK packet with data
Can someone explain to me why the packets I send don't seem to make it to the server in the same way I send them?
Why am I receiving two packets (one with SYN ACK and one with PSH ACK) in my successful attempts?
SYN and RST packets seem to lose their data, but SYN ACK packets don't. Is this a firewall issue?
If so, how can I debug what's intercepting my packets?
Thanks!
Turns out the VMWare virtual adapter was modifying the packets in transit. When I did a packet capture on the host operating system, there were no issues transmitting data.
We have experienced an issue with a CORBA server. A client host connected to this server was sending some CORBA commands that did not get any response from the server. when we took a tcpdump (it is a Linux machine), we saw the following:
client sent SYN
server replied with SYN,ACK
client then sent ACK
server sent again and again (3 times) the SYN,ACK and eventually after three * attempts it sent RST.
Does anyone know why this happened? Why would syn, ack be sent after client ACK?