背景
由于服务器端的重新密钥协商的开销至少是客户端的10倍，因此攻击者可利用这个过程向服务器发起拒绝服务攻击。OpenSSL 1.0.2及以前版本受影响。
方法
使用OpenSSL（linux系统基本都自带）连接服务器进行测试：
- openssl s_client -connect ip:port
- HEAD / HTTP/1.0
- R
示例
服务器443端口关闭重协商，使用openssl s_client -connect 172.31.0.22:443 连接测试（删除了部分证书信息）：
[root@localhost ~]# openssl s_client -connect 172.31.0.22:443 CONNECTED(00000003) depth=0 CN = HTTPS-Self-Signed-Certificate verify error:num=18:self signed certificate verify return:1 depth=0 CN = HTTPS-Self-Signed-Certificate verify return:1 --- Certificate chain 0 s:/CN=HTTPS-Self-Signed-Certificate i:/CN=HTTPS-Self-Signed-Certificate --- Server certificate -----BEGIN CERTIFICATE----- ...... -----END CERTIFICATE----- subject=/CN=HTTPS-Self-Signed-Certificate issuer=/CN=HTTPS-Self-Signed-Certificate --- No client certificate CA names sent Server Temp Key: DH, 1024 bits --- SSL handshake has read 1254 bytes and written 463 bytes --- New, TLSv1/SSLv3, Cipher is DHE-RSA-AES256-SHA Server public key is 1024 bit Secure Renegotiation IS supported Compression: NONE Expansion: NONE SSL-Session: Protocol : TLSv1.2 Cipher : DHE-RSA-AES256-SHA Session-ID: AAF98A92D700189C29EEFE766769E6E5641BAC6A9AB96BC7D1302AE79D21CA06 Session-ID-ctx: Master-Key: 36FC13A9ADBC82EB9E0CC60F9981E2A3D6A2BEC093A0415AFB2A843880174709BB1A87946AA698D95DA3788C72D621CB Key-Arg : None Krb5 Principal: None PSK identity: None PSK identity hint: None TLS session ticket lifetime hint: 3600 (seconds) TLS session ticket: Start Time: 1567604880 Timeout : 300 (sec) Verify return code: 18 (self signed certificate) --- HEAD / HTTP/1.0 R RENEGOTIATING 140432695093152:error:14094153:SSL routines:SSL3_READ_BYTES:no renegotiation:s3_pkt.c:1242:
SSL握手完成后，输入HEAD / HTTP/1.0，然后回车，输入‘R’触发重协商，此时服务器报错并断开连接：
140432695093152:error:14094153:SSL routines:SSL3_READ_BYTES:no renegotiation:s3_pkt.c:1242:
说明服务器重协商功能被关闭。
服务器4443端口关闭重协商，使用openssl s_client -connect 172.31.0.22:4443 连接测试（删除了部分证书信息）：
[root@localhost ~]# openssl s_client -connect 172.31.0.22:4443 CONNECTED(00000003) depth=0 CN = HTTPS-Self-Signed-Certificate verify error:num=18:self signed certificate verify return:1 depth=0 CN = HTTPS-Self-Signed-Certificate verify return:1 --- Certificate chain 0 s:/CN=HTTPS-Self-Signed-Certificate i:/CN=HTTPS-Self-Signed-Certificate --- Server certificate -----BEGIN CERTIFICATE----- ...... -----END CERTIFICATE----- subject=/CN=HTTPS-Self-Signed-Certificate issuer=/CN=HTTPS-Self-Signed-Certificate --- No client certificate CA names sent --- SSL handshake has read 845 bytes and written 463 bytes --- New, TLSv1/SSLv3, Cipher is AES256-SHA Server public key is 1024 bit Secure Renegotiation IS supported Compression: NONE Expansion: NONE SSL-Session: Protocol : TLSv1.2 Cipher : AES256-SHA Session-ID: 6D0DF6EFC8491C9DEEB0161B85A47C101CF5DA9A9CD4EAA4EFCEEF46571A2A2F Session-ID-ctx: Master-Key: B3BBD776EA24230B37E7EF4B2EAF02D6D66185F12D3C87640308FB1996E0BDA4A94CDB35455D0E98A5C34AAAF6EA1C7F Key-Arg : None Krb5 Principal: None PSK identity: None PSK identity hint: None TLS session ticket lifetime hint: 3600 (seconds) TLS session ticket: ...... Start Time: 1567605522 Timeout : 300 (sec) Verify return code: 18 (self signed certificate) --- HEAD / HTTP/1.0 R RENEGOTIATING depth=0 CN = HTTPS-Self-Signed-Certificate verify error:num=18:self signed certificate verify return:1 depth=0 CN = HTTPS-Self-Signed-Certificate verify return:1 HEAD / HTTP/1.0 HTTP/1.1 400 Bad request Date: Wed, 04 Sep 2019 05:43:05 Content-Length: 0 ^C
使用和上方所述相同的操作，发送‘R’触发重协商，可以看到重协商成功，连接正常，此时再次发送HEAD / HTTP/1.0 ，敲两次回车，得到服务器响应400。说明服务器重协商功能开启。
总结
通过OpenSSL连接服务器测试重协商功能，如果服务器重协商功能关闭则终端发送‘R’后会报错并断开连接。
参考资料
https://mailarchive.ietf.org/arch/msg/tls/wdg46VE_jkYBbgJ5yE4P9nQ-8IU
https://blog.ivanristic.com/2009/12/testing-for-ssl-renegotiation.html