linux ssh How to Hide the OpenSSH Version Details when Telnet to Port 22

Information

How to hide the OpenSSH version details when telnet to port 22 in Red Hat Enterprise Linux?

Details

OpenSSH version details are displayed from the file /usr/sbin/sshd. The only method to hide the version detail is to edit the binary file /usr/sbin/sshd and remove/overwrite the appropriate line.

Steps:

  1. Copy the file /usr/sbin/sshd to /tmp.

    #cp /usr/sbin/sshd /tmp         
    #ksh -o vi         

  2. Find the location of text OpenSSH in it using strings command.

    Look for text SSH-2.0-OpenSSH_4.x since it is the version showed in telnet output.

    #cd /tmp  
    #strings -t d -a -n 7 sshd | grep -i openssh 
    308104 OpenSSH_4.3p2-FC-4.3p2-36.el5_4.2
    310376 hmac-md5,hmac-sha1,hmac-ripemd160,hmac-ripemd160
    @openssh.com,hmac-sha1-96,hmac-md5-96
    310768 OpenSSH_4.3p2
    311452 OpenSSH_4.3

  3. Remove 311452 OpenSSH_4.3 line from the above output using dd command.

    #dd if=./sshd bs=1 skip=311452 count=11 | od -A n -c
    11+0 records in
    11+0 records out
    11 bytes (11 B) copied, 0.000929952 seconds, 11.8 kB/s
      O   p   e   n   S   S   H   _   4   .   3  
    #dd if=./sshd bs=1 count=311452 of=sshd.1
    311452+0 records in
    311452+0 records out
    311452 bytes (311 kB) copied, 3.77178 seconds, 82.6 kB/s
    #dd if=./sshd bs=1 skip=311452 count=11 of=sshd.2
    11+0 records in
    11+0 records out
    11 bytes (11 B) copied, 0.000568408 seconds, 19.4 kB/s
    #dd if=./sshd bs=1 skip=311463 count=999999999 of=sshd.3
    96889+0 records in
    96889+0 records out
    96889 bytes (97 kB) copied, 1.69652 seconds, 57.1 kB/s
    OpenSSH_4.3 is now cut and copied to sshd.2 file.

  4. Check the content of sshd.2 file using the od command.

    #od -A n -c sshd.2
      O   p   e   n   S   S   H   _   4   .   3  

  5. Write the text ItsHidden to sshd.2 and check the file size change before and after.

    #ls -l sshd.2
    -rw-r--r-- 1 root root 11 May  6 14:11 sshd.2
    #print -n ItsHidden > sshd.2
    #ls -l sshd.2
    -rw-r--r-- 1 root root 11 May  6 14:12 sshd.2

  6. Combine all the above sshd files to sshd.new.

    #cat sshd.* > sshd.new

  7. Give execute permission sshd.new and replace the sshd binary with sshd.new.

    #chmod 755 ./sshd.new    
    #cp /usr/sbin/sshd /usr/sbin/sshd.bak  
    #rm /usr/sbin/sshd  
    #cp /tmp/sshd.new /usr/sbin/sshd

  8. Restart sshd service and test the outcome with telnet command.

    #service sshd stop  
    #ps aux | grep -i sshd  
    #kill -9 <pid_sshd>  
    #service sshd restart  
    #telnet localhost 22  

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