When it comes to the Linux kernel, many people find it mysterious. Adjusting kernel parameters can seem like a complex task, but in reality, it's not as difficult as it appears. Most of these settings are rarely changed and are often left at their default values. However, understanding them can be helpful, especially when dealing with performance tuning or troubleshooting network issues.
If you've ever tried to look up the role of certain kernel parameters, you might have noticed that most resources are similar, with little detailed explanation. That’s why I’ve compiled a few key network-related kernel parameters that I’ve found useful over the years. While I’m not an expert, I hope this guide helps you better understand how these settings work. If you have more insights or corrections, feel free to share!
Below is a list of some common network-related kernel parameters:
1. **net.ipv4.tcp_max_tw_buckets**
This parameter defines the maximum number of TCP connections in the TIME_WAIT state that the system will allow. When this limit is reached, older connections are removed to free up resources. On CentOS 7, the default is 32768, but you can reduce it to around 8000 if needed. However, lowering it too much may prevent reusing existing connections, which could affect performance.
2. **net.ipv4.tcp_tw_recycle = 1**
This setting allows the system to quickly reclaim TCP connections in the TIME_WAIT state, making them available for reuse. It should be used in conjunction with `tcp_tw_reuse`.
3. **net.ipv4.tcp_tw_reuse = 1**
Enables the reuse of TCP connections in the TIME_WAIT state for new connections, improving efficiency and reducing the number of open ports.
4. **net.ipv4.tcp_syncookies = 1**
This option helps protect against SYN flood attacks by using a cookie-based mechanism to handle incoming connection requests. Instead of storing information about each half-open connection, the server generates a unique sequence number, which the client must acknowledge.
5. **net.ipv4.tcp_max_syn_backlog**
This parameter controls the maximum number of pending TCP connections that can be queued during the three-way handshake. The default is 256, but it can be increased (e.g., to 30,000) to handle high traffic loads.
6. **net.ipv4.tcp_syn_retries**
Sets the number of times a client will retry sending a SYN packet before giving up. The default is 5, but it’s recommended to set it to 2 for faster failure detection.
7. **net.ipv4.tcp_synack_retries**
Controls the number of times a server will retry sending a SYN-ACK response. The default is 5, but reducing it to 2 can help mitigate SYN flood attacks.
8. **net.ipv4.ip_local_port_range**
Defines the range of local port numbers that can be used for outgoing connections. The default is 32768–61000, but adjusting it to 1025–61000 can help avoid conflicts with reserved ports.
9. **net.ipv4.tcp_fin_timeout**
Determines how long 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 faster.
10. **net.ipv4.tcp_keepalive_time**
Specifies how long a connection remains active without any data exchange before the system starts sending keepalive probes. The default is 7200 seconds (2 hours), but it’s often reduced to 30 seconds for better responsiveness.
11. **net.ipv4.tcp_keepalive_intvl**
Sets the interval between keepalive probes. The default is 75 seconds, but changing it to 3 seconds can help detect dead connections more quickly.
12. **net.ipv4.tcp_keepalive_probes**
Determines how many keepalive probes are sent before the connection is considered dead. The default is 9, but it’s often reduced to 2 for faster detection of failed connections.
These parameters can significantly impact network performance and security, especially under heavy load or during attacks. Always test changes in a staging environment before applying them to production systems. If you're unsure, consult official documentation or seek advice from experienced administrators.
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