linux + ssh limitation + ssh at the same time from multiple machine to one machine

Question

the following script "testssh.ksh" proves that ssh have some problems when we try to perform ssh from multiple machines on the same time

in fact the target of this script is to verify the file test_file under /var/tmp in the Solaris server (10.10.18.6) , as all see in some ssh steps we can’t verify the existing of the test_file because ssh stuck or not activate from the expect

background - this script perform 15 times ssh to Solaris server with IP - 10.10.18.6 on the same time in order to verify the file_test under /var/tmp in the server

my question - how to improve the ssh process in order to avoid the following problems

Remark - sleep can help us in this situation - but I not want to add sleep before ssh process

  [root@linux /var/tmp]# more  testssh.ksh
  #!/bin/ksh



  expect=`cat << EOF
  set timeout -1
  spawn  ssh  10.10.18.6 
       expect {
                 ")?"   { send "yes\r"  ; exp_continue  }

                 word:  {send pass123\r}
              }
  expect >  {send "ls  /var/tmp/test_file\r"}
  expect >    {send exit\r}
  expect eof
  EOF`


  for i in 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
  do
     ( expect -c  "$expect"  | grep "test_file"  | grep -v ls ) &
  done

example - when we run the script testssh.ksh

     [root@linux /var/tmp]# /var/tmp/testssh.ksh
     /var/tmp/test_file
     /var/tmp/test_file
     /var/tmp/test_file
     /var/tmp/test_file
     /var/tmp/test_file
     expect: spawn id exp6 not open
     while executing
     "expect >  {send "ls  /var/tmp/test_file\r"}"
     expect: spawn id exp6 not open
     while executing
     "expect >  {send "ls  /var/tmp/test_file\r"}"
     expect: spawn id exp6 not open
     while executing
     "expect >  {send "ls  /var/tmp/test_file\r"}"
     expect: spawn id exp6 not open
     while executing
     "expect >  {send "ls  /var/tmp/test_file\r"}"
     /var/tmp/test_file
     /var/tmp/test_file
     /var/tmp/test_file
     /var/tmp/test_file
     /var/tmp/test_file
     /var/tmp/test_file

Answer 1

Have you set the MaxSession and MaxStartups in your sshd.conf (or equivalent)? 40 simultaneous SSH connections should not, I believe, be too many for your server to handle.

From man sshd_config page:

 MaxSessions
         Specifies the maximum number of open sessions permitted per net‐
         work connection.  The default is 10.

 MaxStartups
         Specifies the maximum number of concurrent unauthenticated con‐
         nections to the SSH daemon.  Additional connections will be
         dropped until authentication succeeds or the LoginGraceTime
         expires for a connection.  The default is 10.

         Alternatively, random early drop can be enabled by specifying the
         three colon separated values “start:rate:full” (e.g. "10:30:60").
         sshd(8) will refuse connection attempts with a probability of
         “rate/100” (30%) if there are currently “start” (10) unauthenti‐
         cated connections.  The probability increases linearly and all
         connection attempts are refused if the number of unauthenticated
         connections reaches “full” (60).

If you haven't changed these, your server won't handle more than 10 simultaneous connections.

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