From c21c3b0befeb46a51b6bf3758ffa30813bea0ff0 Mon Sep 17 00:00:00 2001 From: Daniel Baumann Date: Sat, 9 Mar 2024 14:19:22 +0100 Subject: Adding upstream version 1.44.3. Signed-off-by: Daniel Baumann --- health/guides/tcp/10s_ipv4_tcp_resets_sent.md | 43 +++++++++++++++++++++++++++ 1 file changed, 43 insertions(+) create mode 100644 health/guides/tcp/10s_ipv4_tcp_resets_sent.md (limited to 'health/guides/tcp/10s_ipv4_tcp_resets_sent.md') diff --git a/health/guides/tcp/10s_ipv4_tcp_resets_sent.md b/health/guides/tcp/10s_ipv4_tcp_resets_sent.md new file mode 100644 index 000000000..9a941694e --- /dev/null +++ b/health/guides/tcp/10s_ipv4_tcp_resets_sent.md @@ -0,0 +1,43 @@ +### Understand the alert + +TCP reset is an abrupt closure of the session. It causes the resources allocated to the connection to be immediately released and all other information about the connection is erased. + +The Netdata Agent monitors the average number of sent TCP RESETS over the last 10 seconds. This can indicate a port scan or that a service running on the system has crashed. Additionally, it's a result of a high number of sent TCP RESETS. Furthermore, it can also indicate a SYN reset attack. + +### More about TCP Resets + +TCP uses a three-way handshake to establish a reliable connection. The connection is full duplex, and both sides synchronize (SYN) and acknowledge (ACK) each other. The exchange of these four flags +is performed in three steps: SYN, SYN-ACK, and ACK. + +When an unexpected TCP packet arrives at a host, that host usually responds by sending a reset packet back on the same connection. A reset packet is one with no payload and with the RST bit set in the TCP header flags. There are a few circumstances in which a TCP packet might not be expected. The most common cases are: + +1. A TCP packet received on a port that is not open. +2. An aborting connection +3. Half opened connections +4. Time wait assassination +5. Listening endpoint Queue is Full +6. A TCP Buffer Overflow + +Basically, A TCP Reset usually occurs when a system receives data which doesn't agree with its view of the connection. + +When your system cannot establish a connection it will retry by default `net.ipv4.tcp_syn_retries` times. + +### Troubleshoot the alert + +- Use tcpdump to capture the traffic and use Wireshark to inspect the network packets. You must stop the capture after a certain observation period (60s up to 5 minutes). This command will create a dump file which can be interpreted by Wireshark that contains all the TCP packets with RST flag set. + ``` + tcpdump -i any 'tcp[tcpflags] & (tcp-rst) == (tcp-rst)' -s 65535 -w output.pcap + ``` + +- Identify which application sends TCP resets + +1. Check the instances of `RST` events of the TCP protocol. Wireshark also displays the ports on which the two systems tried to establish the TCP connection, (XXXXXX -> XXXXXX). +2. To check which application is using this port, run the following code: + ``` + lsof -i:XXXXXX -P -n + ``` +### Useful resources + +1. [TCP reset explanation](https://www.pico.net/kb/what-is-a-tcp-reset-rst/) +2. [TCP 3-way handshake on wikipedia](https://en.wikipedia.org/wiki/Handshaking) +3. [Read more about Wireshark here](https://www.wireshark.org/) \ No newline at end of file -- cgit v1.2.3