[lug] root password

Rob Nagler nagler at bivio.biz
Wed Aug 2 14:35:14 MDT 2006

Sean Reifschneider writes:
> These days, we also recommend that SSH password authentication be disabled,
> because of the number of scans going on looking for weak passwords, and the
> rate of escalation of those scans.

I have always wondered about this.  It seems to me that you can't
crack passwords through the sshd; it's computationally impossible
afaik.  I don't think the "standard" GNU password program will let you
use a dictionary word as a password.  Given that, the universe of
passwords is not crackable on a single machine, that is, even if you
get a distributed attack, there's still a single machine (the computer
under attack) doing the password checking.

And, if you come under a distributed attack, I assume you might see a
high load average, and your system will issue warning alarms.  You can
also rate limit the requests per second to the ssh port, although it's
not clear this is necessary.

A couple of important things we set in the sshd_config that isn't set
by default:

    VerifyReverseMapping yes
    PermitRootLogin no

This prevents a lot.  We also limit the access to the machines which
can be accessed via certain networks for all but a couple of
machines, and those "open" machines have limited data on them.

We don't use ssh keys except for automated logins, but those logins
are sandboxed with scponly, and they aren't root logins, and they
don't have any privs except to look at the files they have already

Keys stored on disk are therefore not more secure.  Indeed, they are
more convenient to login if you use ssh-agent (see below), which most
people do.  Even if you don't use ssh-agent, they key can be subject
to a local exploit that leads to a remote exploit.  If you only use
passwords, and you require the "open" ssh machine login to have a
different password than the "inside" machines, you have higher
security, because no local information on the the "open" machine can
be exploited to gain access to the "inside" machines.  And, if you
tunnel your ssh sessions (LocalForward <port> inside.my.com:22) from
the outside machine to the the inside machines, you aren't subject to
sniffing on the "open" machine even if it is cracked.

ssh-agent is a convenience, and therefore easily exploited.  Once you
have root access on the "open" machine, all "inside" machines are
yours with nothing to stop you.  It's the "soft, chewy center"

Security is very tricky, but at the root of all secure systems is a
password.  You should have a password for different classes of access
("open", "inside", "su", imap, random website, etc.).  You should only
have a few of them so you can remember them without writing them down,
and you should change them often.

My rule of thumb is this: If you can change an access level without a
human (or crypto device) entering a new password, you have a security


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