Support to provide a SNMP mock server

Below is a sample code:

package main

import (
	"encoding/binary"
	"fmt"
	"net"
	"strconv"
	"strings"

	"github.com/gosnmp/gosnmp"
)

const (
	// SNMP 版本号
	SNMPVersion = 1

	// 固定的 OID 和值
	OID   = ".1.3.6.1.4.1.5105.100.1.9.4.15.0" // sysDescr
	Value = "My SNMP Server"
)

// sendSNMPRequest 发送 SNMP 请求并接收响应
func sendSNMPRequest(target string, community string, oid string) ([]byte, error) {
	// 创建 SNMP 客户端
	gosnmp.Default.Target = target
	gosnmp.Default.Port = 161
	gosnmp.Default.Community = community
	gosnmp.Default.Version = gosnmp.Version2c

	// 创建 PDU
	pdu := gosnmp.SnmpPDU{
		Name:  oid,
		Type:  gosnmp.OctetString,
		Value: Value,
	}

	// 创建请求
	request := &gosnmp.SnmpPacket{
		Version:    gosnmp.Version2c,
		Community:  community,
		Error:      gosnmp.NoError,
		ErrorIndex: 0,
		PDUType:    gosnmp.GetRequest,
		Variables:  []gosnmp.SnmpPDU{pdu},
	}

	// 编码请求
	req, err := request.MarshalMsg()
	if err != nil {
		return nil, fmt.Errorf("failed to marshal SNMP request: %v", err)
	}

	// 发送请求并接收响应
	conn, err := net.Dial("udp", fmt.Sprintf("%s:%d", target, 161))
	if err != nil {
		return nil, fmt.Errorf("failed to dial UDP: %v", err)
	}
	defer conn.Close()

	_, err = conn.Write(req)
	if err != nil {
		return nil, fmt.Errorf("failed to send SNMP request: %v", err)
	}

	buffer := make([]byte, 4096) // Increased buffer size
	n, err := conn.Read(buffer)
	if err != nil {
		return nil, fmt.Errorf("failed to read SNMP response: %v", err)
	}

	return buffer[:n], nil
}

func main() {
	pc, err := net.ListenPacket("udp", ":161")
	if err != nil {
		fmt.Println("Failed to listen on UDP port 161:", err)
		return
	}
	defer pc.Close()

	fmt.Println("SNMP server listening on UDP port 161...")

	buffer := make([]byte, 4096) // Increased buffer size
	for {
		// 读取客户端请求
		n, addr, err := pc.ReadFrom(buffer)
		if err != nil {
			fmt.Println("Failed to read UDP packet:", err)
			continue
		}

		// 解析 SNMP 请求
		req := buffer[:n]
		fmt.Printf("Received SNMP request from %s: %X\n", addr, req)

		// 生成 SNMP 响应
		resp := generateSNMPResponse(req)
		if resp == nil {
			fmt.Println("Failed to generate SNMP response")
			continue
		}

		// 发送响应
		_, err = pc.WriteTo(resp, addr)
		if err != nil {
			fmt.Println("Failed to send SNMP response:", err)
			continue
		} else {
			fmt.Printf("Sent SNMP response to %s: %X\n", addr, resp)
		}
	}

	// 测试发送 SNMP 请求
	//resp, err := sendSNMPRequest("192.168.12.109", "public", ".1.3.6.1.4.1.5105.100.1.9.4.15.0")
	//if err != nil {
	//	fmt.Println("Failed to send SNMP request:", err)
	//} else {
	//	fmt.Printf("Received SNMP response: %X\n", resp)
	//}
}

// generateSNMPResponse 生成 SNMP 响应数据
func generateSNMPResponse(req []byte) []byte {
	// 解析请求
	// 这里仅处理简单的 GET 请求
	if len(req) < 4 {
		fmt.Println("Invalid SNMP request length")
		return nil
	}

	// 提取社区字符串
	communityLen := req[6]
	community := string(req[7 : 7+communityLen]) // 提取社区字符串
	fmt.Printf("Received SNMP request with community: %s\n", community)

	// 提取请求 ID
	requestId := binary.BigEndian.Uint32(req[7+communityLen+2+2 : 7+communityLen+2+2+4])
	fmt.Printf("Received SNMP request with ID %d\n", requestId)

	// 使用 gosnmp 库构建响应
	gosnmp.Default.Target = "192.168.123.58"
	gosnmp.Default.Port = 161
	gosnmp.Default.Community = community
	gosnmp.Default.Version = gosnmp.Version2c

	// 创建 PDU
	pdu := gosnmp.SnmpPDU{
		Name:  OID,
		Type:  gosnmp.OctetString,
		Value: Value,
	}

	// 创建响应
	response := &gosnmp.SnmpPacket{
		Version:   gosnmp.Version2c,
		Community: community,
		//ErrorStatus: gosnmp.NoError,
		Error:      gosnmp.NoError,
		ErrorIndex: 0,
		RequestID:  requestId,
		PDUType:    gosnmp.GetResponse,
		Variables:  []gosnmp.SnmpPDU{pdu},
	}

	// 编码响应
	resp, err := response.MarshalMsg()
	if err != nil {
		fmt.Println("Failed to marshal SNMP response:", err)
		return nil
	}

	fmt.Printf("Generated SNMP response: %X\n", resp)
	return resp
}

// encodeOID 编码 OID
func encodeOID(oid string) []byte {
	// 使用 BER 编码来编码 OID
	parts := splitOID(oid)
	encoded := make([]byte, 0)
	first := parts[0]*40 + parts[1]
	encoded = append(encoded, byte(first))
	for _, part := range parts[2:] {
		encoded = append(encoded, encodeVarInt(part)...)
	}
	return encoded
}

// encodeVarInt 使用 BER 编码来编码一个整数
func encodeVarInt(value int) []byte {
	if value < 128 {
		return []byte{byte(value)}
	}
	encoded := make([]byte, 0)
	for value > 0 {
		encoded = append([]byte{byte(value%128 | 0x80)}, encoded...)
		value /= 128
	}
	encoded[len(encoded)-1] &= 0x7F
	return encoded
}

// encodeValue 编码值
func encodeValue(value string) []byte {
	// 使用 BER 编码来编码值
	encoded := make([]byte, 0)
	encoded = append(encoded, byte(0x04))       // 字符串类型
	encoded = append(encoded, byte(len(value))) // 字符串长度
	encoded = append(encoded, []byte(value)...) // 字符串值
	return encoded
}

// splitOID 分割 OID 字符串为整数数组
func splitOID(oid string) []int {
	parts := make([]int, 0)
	for _, part := range strings.Split(oid, ".") {
		num, _ := strconv.Atoi(part)
		parts = append(parts, num)
	}
	return parts
}

Start by locating the repository's mock-server entry points and existing protocol handlers, then compare their structure with the sample's main and generateSNMPResponse functions. Determine how SNMP requests and responses should integrate with the existing mock configuration and tests. Done means a documented, testable SNMP mock-server path that handles the requested behavior without relying on the sample's hard-coded addresses.