STUN

STUN探测类型

探测方式

根据 RFC 3489 / 5780：

维度	含义
Primary IP	A
Alternate IP	B
Primary Port	P1
Alternate Port	P2

STUN Server 必须能从不同的 (IP, Port) 组合回应：

测试	源 IP	源 Port
Test I	A	P1
Test II	A	P2
Test III	B	P1
Test IV	B	P2

客户端通过 是否能收到这些回应 判断：

• NAT filtering behavior
• NAT mapping behavior

STUN server

    stunserver \
    --mode full \
    --primaryinterface 128.34.56.78 \
    --altinterface     128.34.56.79 \
    --primaryport 3478 \
    --altport     3479   

STUN client

    stunclient --mode full 128.34.56.78

Mapping行为（NatBehavior）

决定：公网映射端口是否稳定、是否与对端有关

Behavior 枚举	标准术语	含义
DirectMapping	无 NAT	无地址/端口转换
EndpointIndependentMapping	EIM	同一内网 IP:Port → 同一公网 IP:Port（与对端无关）
AddressDependentMapping	ADM	映射依赖对端 IP
AddressAndPortDependentMapping	APDM	映射依赖对端 IP + Port

Filtering行为（FilterBehavior）

决定：外部“谁”能主动打进来

Filtering 枚举	标准术语	含义
DirectConnectionFiltering	无过滤	任意外部可直连
EndpointIndependentFiltering	EIF	任意端口，只要 IP 匹配即可
AddressDependentFiltering	ADF	需 IP 匹配
AddressAndPortDependentFiltering	APDF	需 IP + Port 都匹配

映射到四大NAT类型

Mapping（Behavior）	Filtering	NAT 类型
EndpointIndependentMapping	EndpointIndependentFiltering	Full Cone NAT
EndpointIndependentMapping	AddressDependentFiltering	Restricted Cone NAT
EndpointIndependentMapping	AddressAndPortDependentFiltering	Port Restricted Cone NAT
AddressDependentMapping	任意	Symmetric NAT
AddressAndPortDependentMapping	任意	Symmetric NAT
DirectMapping	DirectConnectionFiltering	Open Internet（无 NAT）

只要 Mapping 不是 Endpoint Independent，一律判定为 Symmetric NAT
过滤方式在 Symmetric NAT 场景下已经“无实用价值”

Full Cone NAT

特征

• 映射固定
• 无过滤（任何外部都能打进）

Mapping = EndpointIndependentMapping
Filtering = EndpointIndependentFiltering

Restricted Cone NAT

特征

• 映射固定
• 只限制对端 IP，不限制端口

Mapping = EndpointIndependentMapping
Filtering = AddressDependentFiltering

Port Restricted Cone NAT

特征

• 映射固定
• 同时限制对端 IP + Port

Mapping = EndpointIndependentMapping
Filtering = AddressAndPortDependentFiltering

Symmetric NAT

特征

• 映射随对端变化,映射依赖对端 IP + Port

Mapping = AddressDependentMapping
Mapping = AddressAndPortDependentMapping

代码

enum NatType {
    Nat_OpenInternet,
    Nat_FullCone,
    Nat_RestrictedCone,
    Nat_PortRestrictedCone,
    Nat_Symmetric,
    Nat_Unknown
};

NatType DetectNatType(NatBehavior mapping, NatFiltering filtering)
{
    if (mapping == DirectMapping)
        return Nat_OpenInternet;

    if (mapping != EndpointIndependentMapping)
        return Nat_Symmetric;

    switch (filtering)
    {
        case EndpointIndependentFiltering:
            return Nat_FullCone;
        case AddressDependentFiltering:
            return Nat_RestrictedCone;
        case AddressAndPortDependentFiltering:
            return Nat_PortRestrictedCone;
        default:
            return Nat_Unknown;
    }
}