Question

我在Go中弄乱了SNMP库，并想出了一种类型Field，该类型根据this document定义了SNMP BER编码的字段。每个字段都由类型，长度和值组成，其中类型是ASN.1类型，长度是字段值的长度，值可以是另一个字段，字段序列或字节序列。这使我考虑了递归定义SNMP消息的可能性。这是我想出的一些代码，但是我一直试图将其转换为递归结构：

package main

import "fmt"

// ASN.1 BER encoded types.
type ASN1BER byte

const (
    Integer          ASN1BER = 0x02
    BitString                = 0x03
    OctetString              = 0x04
    Null                     = 0x05
    ObjectIdentifier         = 0x06
    Sequence                 = 0x30
    GetRequest               = 0xa0
)

// SNMP versions.
type Version byte

const (
    Version1  Version = 0x00
    Version2c         = 0x01
)

// SNMP message field.
type Field struct {
    Type  ASN1BER
    Len   int
    Value interface{}
}

func main() {
    // SNMP Message
    msg := &Field{Type: Sequence, Len: 44, Value: []*Field{
        // SNMP Version
        {Type: Integer, Len: 1, Value: Version2c},
        // Community String
        {Type: OctetString, Len: 7, Value: []byte("private")},
        // Get-Request PDU
        {Type: GetRequest, Len: 30, Value: []*Field{
            // Request ID
            {Type: Integer, Len: 1, Value: []byte{0x01}},
            // Error Status
            {Type: Integer, Len: 1, Value: []byte{0x00}},
            // Error Index
            {Type: Integer, Len: 1, Value: []byte{0x00}},
            // Varbind List
            {Type: Sequence, Len: 19, Value: []*Field{
                // Varbind
                {Type: Sequence, Len: 17, Value: []*Field{
                    // OID 1.3.6.1.4.1.2680.1.2.7.3.2.0
                    {Type: ObjectIdentifier, Len: 13, Value: []byte{0x2b, 0x06, 0x01, 0x04, 0x01, 0x94, 0x78, 0x01, 0x02, 0x07, 0x03, 0x02, 0x00}},
                    // Value
                    {Type: Null, Len: 0},
                }}},
            }},
        }},
    }
    fmt.Println(msg)
}

是否可以递归表示SNMP消息？如果是这样，那么基本案例和递归案例将是什么？目的是能够递归地打印，编码和解码SNMP消息。

Answer 1

解决方案是拥有一个 type switch 会根据值的实际类型（可能是递归的）分为不同的代码路径。

package main

import (
    "fmt"
    "log"
    "reflect"
)

// ASN.1 BER encoded types.
type ASN1BER byte

const (
    Integer          ASN1BER = 0x02
    BitString                = 0x03
    OctetString              = 0x04
    Null                     = 0x05
    ObjectIdentifier         = 0x06
    Sequence                 = 0x30
    GetRequest               = 0xa0
)

// SNMP versions.
type Version byte

const (
    Version1  Version = 0x00
    Version2c         = 0x01
)

// SNMP message field.
type Field struct {
    Type  ASN1BER
    Len   int
    Value interface{}
}

func walk(f *Field, indent string) error {
    switch f.Type {
    case GetRequest, Sequence:
        indent += "\t"
        switch v := f.Value.(type) {
        case *Field:
            return walk(v, indent)
        case []*Field:
            for _, f := range v {
                err := walk(f, indent)
                if err != nil {
                    return err
                }
            }
        default:
            return &ValueTypeError{
                ASNType:   f.Type,
                ValueType: reflect.TypeOf(v),
            }
        }
    default:
        fmt.Printf("%sType: %d; value: %v\n", indent, f.Type, f.Value)
    }
    return nil
}

type ValueTypeError struct {
    ASNType   ASN1BER
    ValueType reflect.Type
}

func (e *ValueTypeError) Error() string {
    return fmt.Sprintf("invalid Go type (%s) for ASN1BER type %d",
        e.ValueType.Name(), e.ASNType)
}

func main() {
    // SNMP Message
    msg := Field{Type: Sequence, Len: 44, Value: []*Field{
        // SNMP Version
        {Type: Integer, Len: 1, Value: Version2c},
        // Community String
        {Type: OctetString, Len: 7, Value: []byte("private")},
        // Get-Request PDU
        {Type: GetRequest, Len: 30, Value: []*Field{
            // Request ID
            {Type: Integer, Len: 1, Value: []byte{0x01}},
            // Error Status
            {Type: Integer, Len: 1, Value: []byte{0x00}},
            // Error Index
            {Type: Integer, Len: 1, Value: []byte{0x00}},
            // Varbind List
            {Type: Sequence, Len: 19, Value: []*Field{
                // Varbind
                {Type: Sequence, Len: 17, Value: []*Field{
                    // OID 1.3.6.1.4.1.2680.1.2.7.3.2.0
                    {Type: ObjectIdentifier, Len: 13, Value: []byte{0x2b, 0x06, 0x01, 0x04, 0x01, 0x94, 0x78, 0x01, 0x02, 0x07, 0x03, 0x02, 0x00}},
                    // Value
                    {Type: Null, Len: 0},
                }}},
            }},
        }},
    }

    bad := Field{
        Type:  Sequence,
        Value: 42,
    }

    for i, f := range [...]*Field{&msg, &bad} {
        log.Println("walking:", i)
        err := walk(f, "")
        if err != nil {
            log.Println("error:", err)
        }
    }
}

Playground link。

递归定义SNMP消息

1 个答案: