/etc/passwd On almost all linux distributions (and commercial *nixes as well), user information is stored in /etc/passwd , a text file which contains the user's login, their encrypted password, a unique numerical user id (called the uid), a numerical group id (called the gid), an optional comment field (usually containing such items as their real name, phone number, etc.), their home directory, and their preferred shell. A typical entry in /etc/passwd looks something like this:
No one likes typing passwords. If people had their way, computers would simply know who they were and what they should have access to without us proving it at every turn.  In my last article I showed you how to create SSH Identities/Pubkeys, which can be used as an alternative to password authentication. However, I then went right back and told you to passphrase protect them, so now you were substituting one password for another, seemingly gaining nothing.
The Secure Shell is widely used to provide secure access to remote systems, and everybody who uses it is familiar with routine password access. This is the easiest to set up, is available by default, but suffers from a number of limitations. These include both security and usability issues, and we hope to cover them here. In this paper, we'll present the various forms of authentication available to the Secure Shell user and contrast the security and usability tradeoffs of each.
To get your OpenSSH keys set up, do the following: Create your SSH Keys - From a command prompt run: ssh-keygen This will prompt you for the key name and path and you are encouraged to accept the default name of id_rsa by just hitting Return/Enter when prompted. You will also be prompted for a passphrase that will be used to generate your public (~/.ssh/id_dsa.pub) and private (~/.ssh/id_dsa) SSH DSA keys. You are strongly encouraged not to use an empty passphrase. Treat your SSH passphrase with the same level of confidentiality that you do with your account password. The passphrase you enter will be used to login to your account and to load your keys using ssh-add (as described below).
Now this is all well and good, but who wants to run their whole life from a single bash instance? If you use an X window system, you can type your passphrase once when you fire up X and all subprocesses will have your keys stored. Make yourself another key: Just press return when it asks you to assign it a passphrase- this will make a key with no passphrase required. If this works right you will get two files called and in your dir. We want to work on it a little.
sshdfilter blocks the frequent brute force attacks on ssh daemons, it does this by directly reading the sshd logging output (or syslog output) and generating iptables (or ipfw ) rules, the process can be quick enough to block an attack before they get a chance to enter any password at all. The blocking policy is defined by a list of blockrules largely by user name or by the type of user name. There are two install routes, the original style sshdfilter starts sshd itself, having started sshd with the -e and -D options. The newer style uses a syslog configuration line that writes sshd messages to a dedicated named pipe, normally /var/run/sshd.fifo . Regardless, this means sshdfilter can see events as they happen and act on them as they happen. sshdfilter then looks for lines something like: Did not receive identification string from x.x.x.x Illegal user x from x.x.x.x Failed password for illegal user x from x.x.x.x port x ssh2 Failed password for x from x.x.x.x port x ssh2