192.168.1.18 master
192.168.1.80 slave

ssh don't from “Known hosts file Verification Strategy”
ssh success “Non verifying Verification Strategy” and "Manually trusted key Verification Strategy"

master login in success
su -u jenkins ssh ftpuser@192.168.1.80

Warning: no key algorithms provided; JENKINS-42959 disabled
[01/26/18 18:58:40] [SSH] Opening SSH connection to 192.168.1.80:22.
[01/26/18 18:58:40] [SSH] WARNING: No entry currently exists in the Known Hosts file for this host. Connections will be denied until this new host and its associated key is added to the Known Hosts file.
Key exchange was not finished, connection is closed.
java.io.IOException: There was a problem while connecting to 192.168.1.80:22
at com.trilead.ssh2.Connection.connect(Connection.java:818)
at com.trilead.ssh2.Connection.connect(Connection.java:687)
at com.trilead.ssh2.Connection.connect(Connection.java:601)
at hudson.plugins.sshslaves.SSHLauncher.openConnection(SSHLauncher.java:1284)
at hudson.plugins.sshslaves.SSHLauncher$2.call(SSHLauncher.java:804)
at hudson.plugins.sshslaves.SSHLauncher$2.call(SSHLauncher.java:793)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)
Caused by: java.io.IOException: Key exchange was not finished, connection is closed.
at com.trilead.ssh2.transport.KexManager.getOrWaitForConnectionInfo(KexManager.java:93)
at com.trilead.ssh2.transport.TransportManager.getConnectionInfo(TransportManager.java:230)
at com.trilead.ssh2.Connection.connect(Connection.java:770)
... 9 more
Caused by: java.io.IOException: The server hostkey was not accepted by the verifier callback
at com.trilead.ssh2.transport.KexManager.handleMessage(KexManager.java:535)
at com.trilead.ssh2.transport.TransportManager.receiveLoop(TransportManager.java:777)
at com.trilead.ssh2.transport.TransportManager$1.run(TransportManager.java:489)
... 1 more
[01/26/18 18:58:40] Launch failed - cleaning up connection
[01/26/18 18:58:40] [SSH] Connection closed.

 01/26/18 18:56:26] [SSH] Opening SSH connection to 192.168.1.80:22.
[01/26/18 18:56:26] [SSH] WARNING: SSH Host Keys are not being verified. Man-in-the-middle attacks may be possible against this connection.
[01/26/18 18:56:26] [SSH] Authentication successful.
[01/26/18 18:56:27] [SSH] The remote user's environment is:
BASH=/usr/bin/bash
BASHOPTS=cmdhist:extquote:force_fignore:hostcomplete:interactive_comments:progcomp:promptvars:sourcepath
BASH_ALIASES=()
BASH_ARGC=()
BASH_ARGV=()
BASH_CMDS=()
BASH_EXECUTION_STRING=set
BASH_LINENO=()
BASH_SOURCE=()
BASH_VERSINFO=([0]="4" [1]="2" [2]="46" [3]="1" [4]="release" [5]="x86_64-redhat-linux-gnu")
BASH_VERSION='4.2.46(1)-release'
DIRSTACK=()
EUID=1000
GROUPS=()
HOME=/home/ftpuser
HOSTNAME=jenkins-node
HOSTTYPE=x86_64
IFS=$' \t\n'
LANG=zh_CN.UTF-8
LESSOPEN='||/usr/bin/lesspipe.sh %s'
LOGNAME=ftpuser
MACHTYPE=x86_64-redhat-linux-gnu
MAIL=/var/mail/ftpuser
OPTERR=1
OPTIND=1
OSTYPE=linux-gnu
PATH=/usr/local/bin:/usr/bin
PIPESTATUS=([0]="0")
PPID=4227
PS4='+ '
PWD=/home/ftpuser
SELINUX_LEVEL_REQUESTED=
SELINUX_ROLE_REQUESTED=
SELINUX_USE_CURRENT_RANGE=
SHELL=/bin/bash
SHELLOPTS=braceexpand:hashall:interactive-comments
SHLVL=1
SSH_CLIENT='192.168.1.18 47772 22'
SSH_CONNECTION='192.168.1.18 47772 192.168.1.80 22'
TERM=dumb
UID=1000
USER=ftpuser
XDG_RUNTIME_DIR=/run/user/1000
XDG_SESSION_ID=30
_=/etc/bashrc
[01/26/18 18:56:27] [SSH] Checking java version of java
[01/26/18 18:56:27] [SSH] java -version returned 1.8.0_141.
[01/26/18 18:56:27] [SSH] Starting sftp client.
[01/26/18 18:56:27] [SSH] Copying latest slave.jar...
[01/26/18 18:56:27] [SSH] Copied 529,326 bytes.
Expanded the channel window size to 4MB
[01/26/18 18:56:27] [SSH] Starting slave process: cd "/opt/jenkins_slave_work" && java -jar slave.jar
<===[JENKINS REMOTING CAPACITY]===>channel started
Slave.jar version: 2.60
This is a Unix agent
Evacuated stdout
Agent successfully connected and online