**First, the detailed explanation of SNMP programming**
SNMP (Simple Network Management Protocol) is a remote monitoring application that operates over TCP/UDP. It is used to monitor various aspects of networked devices, such as memory usage, CPU utilization, and disk space. SNMP is divided into two main components: the SNMP server, which resides on the host being monitored, and the SNMP client, which runs on the monitoring system. The server collects data from the host and organizes it in a hierarchical structure known as a Management Information Base (MIB). The client sends queries to retrieve real-time information about the monitored device.
The SNMP server organizes data in a tree-like structure, similar to a file system or the Windows registry, but much simpler. Each node in the tree is identified by an object identifier (OID), which is a sequence of numbers separated by dots. For example, `.1.3.6.1.2.1.2` represents a specific branch in the MIB tree. This OID can be used to locate and retrieve specific information from the server.
The SNMP client interacts with the server by sending different types of requests. The most common are GET, which retrieves a specific value, and GETNEXT, which fetches the next available variable in the MIB tree. In addition, the SNMP server can send TRAP messages to the client when certain events occur, such as an error or threshold being exceeded. These traps allow for proactive monitoring and alerting.
**Second, SNMP installation**
Most Linux distributions come with SNMP pre-installed. You can check if it's already installed using the command `whereis snmpd`. If not, you can install it via the package manager or compile it from source.
For source installation:
- Run `./configure`, followed by `make` and `make install`.
For package-based installation on CentOS:
- Use `yum install net-snmp` to install the core SNMP service.
- Install `net-snmp-devel` for development purposes.
- Install `net-snmp-utils` for testing and utility tools.
**Third, SNMP configuration**
After installation, the SNMP server needs to be configured. A sample configuration file (`snmpd.conf`) is usually included in the source code. Copy this file to a suitable location, such as `/usr/local/etc/snmp/`, and rename it to `snmpd.conf`.
Inside the configuration file, you may need to modify several parameters:
- To allow access to all MIB nodes, add a line like `view systemonly included .1`.
- Change the `agentAddress` from `udp:127.0.0.1:161` to the external IP address of the machine so that it can be accessed remotely.
**Fourth, testing and troubleshooting**
Once the configuration is complete, start the SNMP service:
- For package installations, use `systemctl start snmpd` or `service snmpd start`.
- For source installations, run the binary with the configuration file: `snmpd -c /usr/local/etc/snmp/snmpd.conf`.
Test the setup using `snmpwalk`:
- On the monitored machine: `snmpwalk -v 1 localhost -c public .1`
- On the monitoring machine: `snmpwalk -v 1 [IP] -c public .1`
If remote access fails, disable the firewall (`service iptables stop`) and set SELinux to permissive mode (`setenforce 0`).
**Fifth, SNMP C programming**
The NET-SNMP library provides a powerful API for integrating custom programs with SNMP. You can define custom OIDs in the `snmpd.conf` file, which will trigger a script or program when queried.
For example, adding the following line:
```
Extend .1.3.6.1.4.1.2021.50 monitor /bin/sh /tmp/monitor.sh
```
will execute `/tmp/monitor.sh` whenever the OID `.1.3.6.1.4.1.2021.50` is queried.
On the monitoring side, you can write a C program to interact with the SNMP server. The program uses functions like `snmp_pdu_create`, `snmp_add_null_var`, and `snmp_synch_response` to send queries and process responses. It also includes logic to handle multiple variables and extract their values from the response.
This approach allows developers to create custom monitoring scripts and integrate them seamlessly with existing SNMP infrastructure.
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