Case scenario
The SCADA system in the Drainage Group's online business area needs to collect data from the I/O Server in the DMZ area. The SCADA system can use some IPs to collect data from the I/O Server normally, but another part of the IP cannot be normal from the I/O. Data is collected on the server, indicating an exception and disconnecting.
For example: 10.2.103.8 is the IP address of the SCADA system. It can collect data from the I/O Server of 10.2.0.51 and 10.2.0.52 normally, but the IP of the SCADA system is changed to: 10.2.103.10, which cannot be normal from 10.2. Data is collected on the I/O Servers of .0.51 and 10.2.0.52.
case study
Network topology diagram (simplified)
The following figure shows a simplified topology diagram. We show the communication link between the SCADA system and the I/O Server. The port mirroring is used to bypass the deployment of the network to the SCADA system and the I/O Server. The analysis system collects communication packets between the SCADA system and the I/O Server.
Figure 1 network topology
Troubleshooting
We collect communication data from the interactive machine in the DMZ area and the interactive machine on the online business area, and carry out comparative analysis to see what causes the failure of the business system.
DMZ zone switch data
In the DMZ zone switch data, you can see that a large number of RST (reset) packets are sent to 10.2.0.52 in the TCP session, as shown in Figure 2. These connections are released by these reset packets, but why are there so many reset packets? Who sent these packets?
Figure 2 TCP session captured in the DMZ area
By looking at the transaction timing diagram of the Kelai network backtracking analysis system, it can be found that the TTL (time to live) value of the reset packet is 127. When the data is transmitted normally, the TTL (time to live) value is 61, and when it is abnormal. Obviously different, the reset packet is not sent from 10.2.103.10, but an intermediate device sends a reset packet to interrupt the normal application session.
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