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Top Forums Shell Programming and Scripting Steps after username and password is entered ! Post 302820495 by Corona688 on Wednesday 12th of June 2013 06:49:06 PM
Old 06-12-2013
If you login to a local terminal, you talk to /bin/login, if you login with sshd it goes its own way, but these days everything talks to the same login system -- pam. (pluggable authentication modules) It's not a program, it's a library, and it's very very picky about what it talks to, all controlled by configuration files under /etc/pam.d/

It's very configurable, but you mostly see it used the traditional way, where it checks /etc/passwd for login information and /etc/shadow for passwords.

/etc/passwd is where things like the location of your home folder and your default shell are stored. It can be read by anyone. /etc/shadow, readable only by root, is where hashed passwords are stored -- passwords go through a one-way function like md5 or sha to scramble them. You can't turn it back into a password, you can only compare two hashes to see if they're the same. (And even that takes some effort, because pam 'salts' them with a small random string.)

Still, hashed passwords aren't invulnerable. With enough computing power, you can hash millions of possible passwords until you find a match. This is why the 'passwd' file doesn't contain passwords anymore; as computers became more powerful, the hashed passwords required better protection.

So pam compares the hash of the password you typed with the stored hash to see if they match, and if they do, makes the setuid() system call to change its process ID into a different user, and following that, executes your shell.
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rlogin(1)						      General Commands Manual							 rlogin(1)

NAME
rlogin - Connects the local host with a remote host SYNOPSIS
rlogin [-8L] [-e character] [-l user] remote_host The remote login command (rlogin) logs into remote_host and connects your local terminal to the remote host. OPTIONS
Allows an 8-bit data path at all times. Otherwise, unless the Stop and Continue key sequences on the remote host are not standard, rlogin uses a 7-bit data path and the eighth (high) bit of each byte is stripped. Changes the Escape character. Substitute the character you choose for character. Changes the remote username to the one you specify. Otherwise, your local username is used at the remote host. Allows the rlogin session to be run in litout mode. In this mode, the escape sequence ~. (where ~ is the escape character) disconnects you from the remote host and the escape sequence ~^Z (where ^Z, or Ctrl-Z, is the suspend character) suspends the rlogin session if you are using csh. DESCRIPTION
The remote terminal type is the same as that given in the local TERM environment variable. The terminal or window size is also the same, if the remote host supports them, and any changes in size are transferred. All echoing takes place at the remote host, so except for delays, the terminal connection is transparent. Pressing the Stop and Continue key sequences stops and starts the flow of information, and the input and output buffers are flushed on Interrupts. The rlogin command can only be used to connect to systems that are running the rlogind daemon. On systems that do not support rlogin, you can use telnet (if supported) as an alternative. If you do not specify the -l option, the local username is used at the remote host. If -l user is specified, the username entered is used at the remote host. In either case, the remote host allows access only if one or both of the following conditions is satisfied: The local host is included in the remote host's /etc/hosts.equiv file, the local user is not the superuser, and the -l user option is not specified. The local host is included in a $HOME/.rhosts file in the home directory of the remote user account. If -l user is specified, the local username must also be included in the file. If neither of these conditions is met and a password is defined for the remote user account, the remote host prompts for a password. The remote password file is checked to verify the password entered, and the login prompt is displayed if the password is not correct. Pressing the End-of-File key sequence at the login prompt ends the remote login attempt. For security reasons, any $HOME/.rhosts file must be owned by either the remote user or the root user and should have permissions set to 600 (read and write by owner only). In addition to the preceding conditions, rlogin also allows access to the remote host if the remote user account does not have a password defined. However, for security reasons, use of a password on all user accounts is recommended. Unless otherwise modified by the -e option, the standard Escape character for disconnecting from the remote host is a ~ (tilde). The Escape character is only recognized by the remote host if it occurs at the beginning of a line. Otherwise, the Escape character is sent to the remote host as a normal character. To send the Escape character to the remote host as a normal character at the beginning of a line, press the Escape character twice. Pressing the Escape character and a (dot) (for example, ~.) immediately disconnects the local terminal from the remote host. EXAMPLES
In the following examples, the local host is listed in the /etc/hosts.equiv file at the remote host: To log in to a remote host with your local username, enter: $ rlogin host2 Password: <Enter password> To log off the remote host and close the connection, enter the End-of-File key sequence. To log in to a remote host with a differ- ent username, enter: $ rlogin host2 -l dale You are prompted to enter your password and then are logged in to the remote host host2 with the username dale. To log in to host2 with the your local username and change the Escape character to (backslash), enter: $ rlogin host2 -e\ FILES
Specifies remote hosts from which users can execute commands on the local host (provided these users have an account on the local host). Specifies remote users who can use a local user account. SEE ALSO
Commands: rcp(1), rsh(1), rlogind(8), telnet(1) Files: rhosts(4) rlogin(1)
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