使用 Jenkins 配置完遠程 SSH 伺服器，發生無法連接問題，查看 Log 後找到
[SSH] Exception negotiation fail
com.jcraft.jsch.JSchException: Algorithm negotiation fail
at com.jcraft.jsch.Session.receive_kexinit(Session.java:520)
at com.jcraft.jsch.Session.connect(Session.java:286)
at com.jcraft.jsch.Session.connect(Session.java:150)
at org.jvnet.hudson.plugins.SSHSite.createSession(SSHSite.java:141)
at org.jvnet.hudson.plugins.SSHSite.executeCommand(SSHSite.java:151)
at org.jvnet.hudson.plugins.SSHBuildWrapper.executePreBuildScript(SSHBuildWrapper.java:75)
at org.jvnet.hudson.plugins.SSHBuildWrapper.setUp(SSHBuildWrapper.java:59)
at hudson.model.Build$BuildExecution.doRun(Build.java:154)
at hudson.model.AbstractBuild$AbstractBuildExecution.run(AbstractBuild.java:533)
at hudson.model.Run.execute(Run.java:1754)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:43)
at hudson.model.ResourceController.execute(ResourceController.java:89)
at hudson.model.Executor.run(Executor.java:240)
Finished: FAILURE
原因分析： OpenSSH 和 JSch 支援的交換演算法不同，需要一方打開另一方支援的交換演算法。
OpenSSH 僅預設啟用以下金鑰交換演算法：
- curve25519-sha256@libssh.org
- ecdh-sha2-nistp256
- ecdh-sha2-nistp384
- ecdh-sha2-nistp521
- diffie-hellman-group-exchange-sha256
- diffie-hellman-group14-sha1
而 JSch 聲稱支援以下金鑰交換演算法：
- diffie-hellman-group-exchange-sha1
- diffie-hellman-group1-sha1
解決辦法： 在 SSH 的配置檔 /etc/ssh/sshd_config 增加以下兩行，讓 SSH 支援相應的演算法和 MACs
KexAlgorithms curve25519-sha256@libssh.org,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group14-sha1,diffie-hellman-group-exchange-sha1,diffie-hellman-group1-sha1
MACs hmac-sha2-512-etm@openssh.com,hmac-sha2-256-etm@openssh.com,hmac-ripemd160-etm@openssh.com,umac-128-etm@openssh.com,hmac-sha2-512,hmac-sha2-256,hmac-ripemd160,umac-128@openssh.com,hmac-md5,hmac-sha1,hmac-sha1-96,hmac-md5-96
完成後重啟 SSH 即可解決問題
參考 http://stackoverflow.com/questions/26424621/algorithm-negotiation-fail-ssh-in-jenkins