如何搭建并使用便携式 4G/LTE 伪基站研究移动安全

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0x00 前言

在移动互联网深入普及和物联网开始规模应用的今天，网络安全公司怎能不研究移动安全，要研究移动安全，怎能没有4G／LTE伪基站研究测试环境？

本文介绍如何利用开源项目和SDR，合法的搭建并使用便携式4G/LTE伪基站，用于实验室的安全研究，或者用于用户授权的物理渗透测试。

0x01 合法开展研究活动

根据国家无线电管理法规，我们需要先取得频率、设备和人员许可才能合法的在通信业务频段进行收发。而已经分配给运营商的频段，需要该运营商同意才能使用。因此，我们做通信实验时，通常的做法不是申请许可，而是构造一个封闭的电磁空间，即法拉第笼，在这个空间里进行无线信号的发射和接收不会影响到外部世界。

法拉第笼是由金属或金属网包围形成的一个空间，大可以到一个大房子，小可能就是一个小箱子。测试时，如果法拉第笼比较小，我们就只把发射和接收无线电信号的天线放在法拉第笼里面，其它部分仍可放在外面，内外通过馈线连接。

0x02 硬件准备

SBC：UP Board， Intel Atom x5-Z8350 四核CPU，4GB RAM，64GB eMMC

SDR：USRP B200mini + USB电缆

天线+馈线+双工器

充电宝：5V 3A（非常重要）

0x04 软件准备

1.Ubuntu 16.04 LTS

安装之后更新

sudo apt-get update

sudo apt-get upgrade

2.必备软件：Git

sudo apt-get install git

国内专享：Shadowsocks、Privoxy

sudo apt-get install privoxy python-gevent python-pip

sudo pip install shadowsocks

科学上网配置略。

0x05 安装步骤

1.eNodeB: OpenAirInterface

git clone https://gitlab.eurecom.fr/oai/openairinterface5g.git

cd openairinterface5g

git checkout develop

source oaienv

cd cmake_targets

./build_oai -I --eNB -x --install-system-files -w USRP

2.EPC: OpenAir-CN

git clone https://gitlab.eurecom.fr/oai/openair-cn.git

cd openair-cn

git checkout develop

cd scripts

./build_hss -i

./build_mme -i

./build_spgw -i

./build_hss

./build_mme

./build_spgw

0x06 配置

1.eNodeB： 

找到 ~/openairinterface5g/targets/PROJECTS/GENERIC-LTE-EPC/CONF/enb.band3.tm1.usrpb210.conf 进行如下修改，并另存为enb.band3.tm1.usrpb200.conf。

tracking_area_code  =  "";

    mobile_country_code =  "";

    mobile_network_code =  "";

    Nid_cell                          = ;

    ////////// MME parameters:
    mme_ip_address      = ( { ipv4       = "127.0.1.10";
                              ipv6       = "192:168:30::17";
                              active     = "yes";
                              preference = "ipv4";
                            }
                          );
    NETWORK_INTERFACES :
    {
        ENB_INTERFACE_NAME_FOR_S1_MME         = "lo";
        ENB_IPV4_ADDRESS_FOR_S1_MME           = "127.0.1.2/8";

        ENB_INTERFACE_NAME_FOR_S1U            = "lo";
        ENB_IPV4_ADDRESS_FOR_S1U              = "127.0.6.2/8";
        ENB_PORT_FOR_S1U                      = ; # Spec
    };

2.EPC:

修改hosts文件和hostname，然后重新启动。

sudo gedit /etc/hosts

127.0.0.1   localhost
127.0.1.1   mini.openair4G.eur mini
127.0.33.1  hss.openair4G.eur hss
sudo gedit /etc/hostname

mini

复制EPC配置文件：

sudo mkdir -p /usr/local/etc/oai/freeDiameter
sudo cp ~/openair-cn/etc/mme.conf /usr/local/etc/oai
sudo cp ~/openair-cn/etc/hss.conf /usr/local/etc/oai
sudo cp ~/openair-cn/etc/spgw.conf /usr/local/etc/oai
sudo cp ~/openair-cn/etc/acl.conf /usr/local/etc/oai/freeDiameter
sudo cp ~/openair-cn/etc/mme_fd.conf /usr/local/etc/oai/freeDiameter
sudo cp ~/openair-cn/etc/hss_fd.conf /usr/local/etc/oai/freeDiameter

编辑hss.conf

## MySQL mandatory options
MYSQL_server = "127.0.0.1";
MYSQL_user   = "root";
MYSQL_pass   = "linux";
MYSQL_db     = "oai_db";

编辑mme.conf

REALM = "openair4G.eur";

    S6A :
    {
        S6A_CONF                   = "/usr/local/etc/oai/freeDiameter/mme_fd.conf"; # YOUR MME freeDiameter config file path
        HSS_HOSTNAME               = "hss";                                         # THE HSS HOSTNAME
    };

GUMMEI_LIST = (
        {MCC="" ; MNC=""; MME_GID="" ; MME_CODE=""; }                   # YOUR GUMMEI CONFIG HERE
     );

TAI_LIST = (
{MCC="" ; MNC="";  TAC = ""; }                              # YOUR PLMN CONFIG HERE
);

   NETWORK_INTERFACES :
    {
        # MME binded interface for S1-C or S1-MME  communication (S1AP), can be ethernet interface, virtual ethernet interface, we don't advise wireless interfaces
        MME_INTERFACE_NAME_FOR_S1_MME         = "lo";                        # YOUR NETWORK CONFIG HERE
        MME_IPV4_ADDRESS_FOR_S1_MME           = "127.0.1.10/8";            # YOUR NETWORK CONFIG HERE

        # MME binded interface for S11 communication (GTPV2-C)
        MME_INTERFACE_NAME_FOR_S11_MME        = "lo";                          # YOUR NETWORK CONFIG HERE
        MME_IPV4_ADDRESS_FOR_S11_MME          = "127.0.8.11/8";                # YOUR NETWORK CONFIG HERE
        MME_PORT_FOR_S11_MME                  = ;                          # YOUR NETWORK CONFIG HERE
    };

S-GW :
{
    # S-GW binded interface for S11 communication (GTPV2-C), if none selected the ITTI message interface is used
    SGW_IPV4_ADDRESS_FOR_S11                = "127.0.8.1/8";            # YOUR NETWORK CONFIG HERE

};

编辑spgw.conf

S-GW :
{

    NETWORK_INTERFACES :
    {
        # S-GW binded interface for S11 communication (GTPV2-C), if none selected the ITTI message interface is used
        SGW_INTERFACE_NAME_FOR_S11              = "lo";                    # YOUR NETWORK CONFIG HERE
        SGW_IPV4_ADDRESS_FOR_S11                = "127.0.8.1/8";            # YOUR NETWORK CONFIG HERE

        # S-GW binded interface for S1-U communication (GTPV1-U) can be ethernet interface, virtual ethernet interface, we don't advise wireless interfaces
        SGW_INTERFACE_NAME_FOR_S1U_S12_S4_UP    = "lo";                       # YOUR NETWORK CONFIG HERE, USE "lo" if S-GW run on eNB host
        SGW_IPV4_ADDRESS_FOR_S1U_S12_S4_UP      = "127.0.6.1/8";           # YOUR NETWORK CONFIG HERE
        SGW_IPV4_PORT_FOR_S1U_S12_S4_UP         = ;                         # PREFER NOT CHANGE UNLESS YOU KNOW WHAT YOU ARE DOING

        # S-GW binded interface for S5 or S8 communication, not implemented, so leave it to none
        SGW_INTERFACE_NAME_FOR_S5_S8_UP         = "none";                       # DO NOT CHANGE (NOT IMPLEMENTED YET)
        SGW_IPV4_ADDRESS_FOR_S5_S8_UP           = "0.0.0.0/24";                 # DO NOT CHANGE (NOT IMPLEMENTED YET)
    };

...
}

P-GW =
{
    NETWORK_INTERFACES :
    {
        # P-GW binded interface for S5 or S8 communication, not implemented, so leave it to none
        PGW_INTERFACE_NAME_FOR_S5_S8          = "none";                         # DO NOT CHANGE (NOT IMPLEMENTED YET)
        PGW_IPV4_ADDRESS_FOR_S5_S8            = "0.0.0.0/24";                   # DO NOT CHANGE (NOT IMPLEMENTED YET)

        # P-GW binded interface for SGI (egress/ingress internet traffic)
        PGW_INTERFACE_NAME_FOR_SGI            = "eth0";                         # YOUR NETWORK CONFIG HERE
        PGW_IPV4_ADDRESS_FOR_SGI              = "192.168.12.82/24";             # YOUR NETWORK CONFIG HERE
        PGW_MASQUERADE_SGI                    = "yes";                          # YOUR NETWORK CONFIG HERE
    };
...
   # DNS address communicated to UEs
    DEFAULT_DNS_IPV4_ADDRESS     = "192.168.106.12";                            # YOUR NETWORK CONFIG HERE
    DEFAULT_DNS_SEC_IPV4_ADDRESS = "192.168.12.100";                            # YOUR NETWORK CONFIG HERE

...
}

编辑HSS freediameter配置文件 (/usr/local/etc/oai/freeDiameter/hss_fd.conf):

Identity = "hss.openair4G.eur";
Realm = "openair4G.eur";

编辑MME freediameter配置文件 (/usr/local/etc/oai/freeDiameter/mme_fd.conf):

Identity = "mini.openair4G.eur";
Realm = "openair4G.eur";
ConnectPeer= "hss.openair4G.eur" { ConnectTo = "127.0.33.1"; No_SCTP ; No_IPv6; Prefer_TCP; No_TLS; port = ;  realm = "openair4G.eur";};

0x07 运行

安装证书：

cd ~/openair-cn/scripts
./check_hss_s6a_certificate /usr/local/etc/oai/freeDiameter/ hss.openair4G.eur
./check_mme_s6a_certificate /usr/local/etc/oai/freeDiameter/ mini.openair4G.eur

首次运行HSS：

cd ~/openair-cn/scripts
./run_hss -i ~/openair-cn/src/oai_hss/db/oai_db.sql

之后每次运行HSS：

cd ~/openair-cn/scripts
./run_hss

运行MME:

cd ~/openair-cn/scripts
./run_mme

安全测试通常不需要运行SP-GW。

运行eNB:

cd ~/openairinterface5g
source oaienv
cd cmake_targets/lte_build_oai/build
sudo -E ./lte-softmodem -O $OPENAIR_DIR/targets/PROJECTS/GENERIC-LTE-EPC/CONF/enb.band3.tm1.usrpb200.conf

0x08 高级玩法

1.对TD-LTE的支持

　　OAI本身支持TD-LTE。但是因为TD系统全网收发同步的要求，伪基站要正常工作，就必须与运营商现网同步。OAI已含有部分通过空中接口信号实现TD同步的代码，需要进一步修改才能完成与现网同步。

2.精简代码

　　如果我们只做网络安全研究，不做通信研究，就不需要完整的EPC。大部分公司只是研究空口的DoS攻击和RRC重定向攻击，每次只有少量固定格式的网络报文从MME返回，因此可以修改代码，直接返回我们想要的报文，或者执行我们想要的逻辑，就可以精简掉EPC，只运行lte-softmodem一个进程就好了。

3.连接运营商核心网

　　如果能从运营商核心网获得安全认证四元组：Kasme、AUTN、RAND、XRES，就能通过双向鉴权，让LTE手机相信我们的基站是真基站。这需要修改MME的代码和freeDiameter的配置来实现。

4.同一硬件集成LTE+GSM伪基站

5.架设自己的测试网

如果你对某一种高级玩法感兴趣，并且知道大体怎么实现，欢迎找我组队，可以一起做点有意思的（开源）通信安全项目，Seeker老师的微信：70772177。

0x09 结束语

在万物互联时代，了解LTE可用性和安全性的边界，事关我们每个人的人身安全和数据安全。

不管你是在开发无人驾驶汽车、医疗设备、智能电表，还是普通的移动应用，过于相信运营商网络的可用性和安全性都会带来安全威胁。不只是网络安全公司需要研究无线通信的安全，产品开发者同样需要。