Security Paranoia - restricting ssh access
I had email from someone today whose system was hacked,
apparently by a dictionary attack over ssh. They overwrote most of
his hard drive with "DIE SCO DIE". He said there were "over 8000
root login attempts and another 6000 for various other names".
Several things come to mind immediately. First, you shouldn't
allow root logins over ssh anyway. The
in /etc/ssh/sshd_config should be uncommented and changed to no.
Second, you should have an AllowUsers line, and if possible, limit
that to names that aren't obvious. For example, I wouldn't use
"tony" or "apl". I might use "mghtrey" (though I don't), because
that's a completely made up name. With that line, only the listed
users are allowed to login with ssh at all. That includes root even
if PermitRootLogin isn't turned off, but I set both anyway.
Exclusionary duplication never hurts.
Next, don't have passwords susceptible to dictionary attack.
Letters, numbers, and punctuation are a must. Do NOT use the same
passwords on multiple servers and do not allow user equivalency or
ssh_agent unless you absolutely have to have it.
Finally, don't let someone have 8,000 chances at guessing your
password. I have two limitations set. One is the
which (from the man page):
Specifies the maximum number of concurrent unauthenticated con-
nections to the sshd 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 will refuse connection attempts with a proba-
bility of ``rate/100'' (30%) if there are currently ``start''
(10) unauthenticated connections. The probability increases lin-
early and all connection attempts are refused if the number of
unauthenticated connections reaches ``full'' (60).
But I don't stop there on my most critical servers. I also
auth required /lib/security/$ISA/pam_tally.so no_magic_root onerr=fail
account required /lib/security/$ISA/pam_tally.so onerr=fail
file=/var/log/faillog deny=1 no_magic_root even_deny_root_account
in my /etc/pam.d/system-auth file. That kills login if you type
a bad password just twice. I reset it with a cron job every hour
during the day when I'm working in case I screw up twice ("
/sbin/pam_tally --reset") but not at night.
So, for someone to break in with ssh, they first have to know
the one account that allows ssh at all and they have exactly two
chances to guess the password - miss those two and pam locks them
out. Keep on trying, and ssh itself will also lock you out
You really cannot be too careful now. If you don't need ssh
outside of working hours, turn it off with a cron job. The "bad
guys" are constantly hammering at our servers; there's no reason to
make it easy for them.
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© 2011-03-22 Tony Lawrence