When it comes to the Linux kernel, many people find it quite mysterious. Adjusting kernel parameters can seem like a dark art, but in reality, it's not as complicated as it might appear. Most of these settings are rarely modified and are usually left at their default values.
If you've ever tried to understand what some of these kernel parameters do, you might have spent hours searching online only to find that most resources repeat the same information without much depth. That’s why I’ve compiled a few key network-related kernel parameters that I’ve found useful. While I’m not an expert, I hope this helps you get started. If you have more insights or corrections, feel free to share!
Here are some important network-related kernel parameters:
1. **net.ipv4.tcp_max_tw_buckets**
This parameter sets the maximum number of TIME_WAIT sockets the system will allow. When a large number of connections are closed, the number of TIME_WAIT sockets can grow quickly, consuming system resources. The default is 32768 on CentOS 7, but you can lower it to something like 8000 if needed. However, reducing it too much may prevent reusing existing connections, which can slow down performance for repeated client-server interactions.
2. **net.ipv4.tcp_tw_recycle = 1**
This enables faster recycling of TIME_WAIT sockets. It works best when used with `tcp_tw_reuse`, allowing the system to reuse these connections more efficiently.
3. **net.ipv4.tcp_tw_reuse = 1**
This allows the system to reuse TIME_WAIT sockets for new connections, improving performance by avoiding the need to wait for the full timeout period.
4. **net.ipv4.tcp_syncookies = 1**
This feature helps protect against SYN flood attacks by using a cookie-based mechanism instead of maintaining state for each incoming connection. When enabled, the server sends a SYN-ACK with a sequence number that acts as a cookie. If the client doesn’t respond correctly, the connection is dropped without further processing.
5. **net.ipv4.tcp_max_syn_backlog**
This defines the maximum number of pending connections in the SYN_RECV state. A higher value can help handle bursts of traffic, especially during DDoS attacks. On CentOS 7, the default is 256, but it can be increased to around 30,000 for high-traffic environments.
6. **net.ipv4.tcp_syn_retries**
This controls how many times a client will retry sending a SYN packet. The default is 5, but it's often recommended to set it to 2 for faster failure detection.
7. **net.ipv4.tcp_synack_retries**
This determines how many times the server will retry sending a SYN-ACK in response to a client's initial request. Setting it to 2 can help reduce the risk of being overwhelmed by malicious traffic.
8. **net.ipv4.ip_local_port_range**
This defines the range of ports that the system can use for outgoing connections. The default is 32768–61000, but you can adjust it to 1025–61000 to increase the available port space, especially for high-traffic applications.
9. **net.ipv4.tcp_fin_timeout**
This sets the time a connection remains in the FIN-WAIT-2 state before being closed. The default is 60 seconds, but reducing it to 6 can help free up resources more quickly.
10. **net.ipv4.tcp_keepalive_time**
This defines how long a connection remains active before the system starts sending keepalive probes. The default is 7200 seconds (2 hours), but setting it to 30 seconds ensures that inactive connections are detected more quickly.
11. **net.ipv4.tcp_keepalive_intvl**
This specifies the interval between keepalive probes. The default is 75 seconds, but reducing it to 3 seconds can speed up the detection of dead connections.
12. **net.ipv4.tcp_keepalive_probes**
This determines how many keepalive probes are sent before the connection is considered dead. The default is 9, but changing it to 2 can help avoid unnecessary delays in detecting failed connections.
These parameters can significantly impact network performance and security, especially under heavy load or during attacks. Always test changes in a controlled environment before applying them to production systems.
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