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Test Your Knowledge in Computers #386
Difficulty: Medium
The Linux telnet command provides a user interface to a remote system using the SSH protocol.
True or False?
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pam_ssh_agent_auth(8) [centos man page]

pam_ssh_agent_auth(8)							PAM						     pam_ssh_agent_auth(8)

PAM_SSH_AGENT_AUTH
       This module provides authentication via ssh-agent.  If an ssh-agent listening at SSH_AUTH_SOCK can successfully authenticate that it has
       the secret key for a public key in the specified file, authentication is granted, otherwise authentication fails.

SUMMARY
/etc/pam.d/sudo: auth sufficient pam_ssh_agent_auth.so file=/etc/security/authorized_keys /etc/sudoers: Defaults env_keep += "SSH_AUTH_SOCK" This configuration would permit anyone who has an SSH_AUTH_SOCK that manages the private key matching a public key in /etc/security/authorized_keys to execute sudo without having to enter a password. Note that the ssh-agent listening to SSH_AUTH_SOCK can either be local, or forwarded. Unlike NOPASSWD, this still requires an authentication, it's just that the authentication is provided by ssh-agent, and not password entry. ARGUMENTS
file=<path to authorized_keys> Specify the path to the authorized_keys file(s) you would like to use for authentication. Subject to tilde and % EXPANSIONS (below) allow_user_owned_authorized_keys_file A flag which enables authorized_keys files to be owned by the invoking user, instead of root. This flag is enabled automatically whenever the expansions %h or ~ are used. debug A flag which enables verbose logging sudo_service_name=<service name you compiled sudo to use> (when compiled with --enable-sudo-hack) Specify the service name to use to identify the service "sudo". When the PAM_SERVICE identifier matches this string, and if PAM_RUSER is not set, pam_ssh_agent_auth will attempt to identify the calling user from the environment variable SUDO_USER. This defaults to "sudo". EXPANSIONS
~ -- same as in shells, a user's Home directory Automatically enables allow_user_owned_authorized_keys_file if used in the context of ~/. If used as ~user/, it would expect the file to be owned by 'user', unless you explicitely set allow_user_owned_authorized_keys_file %h -- User's Home directory Automatically enables allow_user_owned_authorized_keys_file %H -- The short-hostname %u -- Username %f -- FQDN EXAMPLES
in /etc/pam.d/sudo "auth sufficient pam_ssh_agent_auth.so file=~/.ssh/authorized_keys" The default .ssh/authorized_keys file in a user's home-directory "auth sufficient pam_ssh_agent_auth.so file=%h/.ssh/authorized_keys" Same as above. "auth sufficient pam_ssh_agent_auth.so file=~fred/.ssh/authorized_keys" If the home-directory of user 'fred' was /home/fred, this would expand to /home/fred/.ssh/authorized_keys. In this case, we have not specified allow_user_owned_authorized_keys_file, so this file must be owned by 'fred'. "auth sufficient pam_ssh_agent_auth.so file=/secure/%H/%u/authorized_keys allow_user_owned_authorized_keys_file" On a host named foobar.baz.com, and a user named fred, would expand to /secure/foobar/fred/authorized_keys. In this case, we specified allow_user_owned_authorized_keys_file, so fred would be able to manage that authorized_keys file himself. "auth sufficient pam_ssh_agent_auth.so file=/secure/%f/%u/authorized_keys" On a host named foobar.baz.com, and a user named fred, would expand to /secure/foobar.baz.com/fred/authorized_keys. In this case, we have not specified allow_user_owned_authorized_keys_file, so this file must be owned by root. v0.8 2009-08-09 pam_ssh_agent_auth(8)

Check Out this Related Man Page

SSH-KEYGEN(1)						    BSD General Commands Manual 					     SSH-KEYGEN(1)

NAME
ssh-keygen -- authentication key generation, management and conversion SYNOPSIS
ssh-keygen [-q] [-b bits] -t type [-N new_passphrase] [-C comment] [-f output_keyfile] ssh-keygen -p [-P old_passphrase] [-N new_passphrase] [-f keyfile] ssh-keygen -i [-f input_keyfile] ssh-keygen -e [-f input_keyfile] ssh-keygen -y [-f input_keyfile] ssh-keygen -c [-P passphrase] [-C comment] [-f keyfile] ssh-keygen -l [-f input_keyfile] ssh-keygen -B [-f input_keyfile] ssh-keygen -D reader ssh-keygen -U reader [-f input_keyfile] DESCRIPTION
ssh-keygen generates, manages and converts authentication keys for ssh(1). ssh-keygen can create RSA keys for use by SSH protocol version 1 and RSA or DSA keys for use by SSH protocol version 2. The type of key to be generated is specified with the -t option. Normally each user wishing to use SSH with RSA or DSA authentication runs this once to create the authentication key in $HOME/.ssh/identity, $HOME/.ssh/id_dsa or $HOME/.ssh/id_rsa. Additionally, the system administrator may use this to generate host keys, as seen in /etc/rc. Normally this program generates the key and asks for a file in which to store the private key. The public key is stored in a file with the same name but ``.pub'' appended. The program also asks for a passphrase. The passphrase may be empty to indicate no passphrase (host keys must have an empty passphrase), or it may be a string of arbitrary length. A passphrase is similar to a password, except it can be a phrase with a series of words, punctuation, numbers, whitespace, or any string of characters you want. Good passphrases are 10-30 characters long, are not simple sentences or otherwise easily guessable (English prose has only 1-2 bits of entropy per character, and provides very bad passphrases), and contain a mix of upper and lowercase letters, numbers, and non-alphanumeric characters. The passphrase can be changed later by using the -p option. There is no way to recover a lost passphrase. If the passphrase is lost or forgotten, a new key must be generated and copied to the corre- sponding public key to other machines. For RSA1 keys, there is also a comment field in the key file that is only for convenience to the user to help identify the key. The comment can tell what the key is for, or whatever is useful. The comment is initialized to ``user@host'' when the key is created, but can be changed using the -c option. After a key is generated, instructions below detail where the keys should be placed to be activated. The options are as follows: -b bits Specifies the number of bits in the key to create. Minimum is 512 bits. Generally 1024 bits is considered sufficient, and key sizes above that no longer improve security but make things slower. The default is 1024 bits. -c Requests changing the comment in the private and public key files. This operation is only supported for RSA1 keys. The program will prompt for the file containing the private keys, for the passphrase if the key has one, and for the new comment. -e This option will read a private or public OpenSSH key file and print the key in a 'SECSH Public Key File Format' to stdout. This option allows exporting keys for use by several commercial SSH implementations. -f filename Specifies the filename of the key file. -i This option will read an unencrypted private (or public) key file in SSH2-compatible format and print an OpenSSH compatible private (or public) key to stdout. ssh-keygen also reads the 'SECSH Public Key File Format'. This option allows importing keys from several commercial SSH implementations. -l Show fingerprint of specified public key file. Private RSA1 keys are also supported. For RSA and DSA keys ssh-keygen tries to find the matching public key file and prints its fingerprint. -p Requests changing the passphrase of a private key file instead of creating a new private key. The program will prompt for the file containing the private key, for the old passphrase, and twice for the new passphrase. -q Silence ssh-keygen. Used by /etc/rc when creating a new key. -y This option will read a private OpenSSH format file and print an OpenSSH public key to stdout. -t type Specifies the type of the key to create. The possible values are ``rsa1'' for protocol version 1 and ``rsa'' or ``dsa'' for protocol version 2. -B Show the bubblebabble digest of specified private or public key file. -C comment Provides the new comment. -D reader Download the RSA public key stored in the smartcard in reader. -N new_passphrase Provides the new passphrase. -P passphrase Provides the (old) passphrase. -U reader Upload an existing RSA private key into the smartcard in reader. FILES
$HOME/.ssh/identity Contains the protocol version 1 RSA authentication identity of the user. This file should not be readable by anyone but the user. It is possible to specify a passphrase when generating the key; that passphrase will be used to encrypt the private part of this file using 3DES. This file is not automatically accessed by ssh-keygen but it is offered as the default file for the private key. ssh(1) will read this file when a login attempt is made. $HOME/.ssh/identity.pub Contains the protocol version 1 RSA public key for authentication. The contents of this file should be added to $HOME/.ssh/authorized_keys on all machines where the user wishes to log in using RSA authentication. There is no need to keep the contents of this file secret. $HOME/.ssh/id_dsa Contains the protocol version 2 DSA authentication identity of the user. This file should not be readable by anyone but the user. It is possible to specify a passphrase when generating the key; that passphrase will be used to encrypt the private part of this file using 3DES. This file is not automatically accessed by ssh-keygen but it is offered as the default file for the private key. ssh(1) will read this file when a login attempt is made. $HOME/.ssh/id_dsa.pub Contains the protocol version 2 DSA public key for authentication. The contents of this file should be added to $HOME/.ssh/authorized_keys on all machines where the user wishes to log in using public key authentication. There is no need to keep the contents of this file secret. $HOME/.ssh/id_rsa Contains the protocol version 2 RSA authentication identity of the user. This file should not be readable by anyone but the user. It is possible to specify a passphrase when generating the key; that passphrase will be used to encrypt the private part of this file using 3DES. This file is not automatically accessed by ssh-keygen but it is offered as the default file for the private key. ssh(1) will read this file when a login attempt is made. $HOME/.ssh/id_rsa.pub Contains the protocol version 2 RSA public key for authentication. The contents of this file should be added to $HOME/.ssh/authorized_keys on all machines where the user wishes to log in using public key authentication. There is no need to keep the contents of this file secret. AUTHORS
OpenSSH is a derivative of the original and free ssh 1.2.12 release by Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo de Raadt and Dug Song removed many bugs, re-added newer features and created OpenSSH. Markus Friedl contributed the support for SSH protocol versions 1.5 and 2.0. SEE ALSO
ssh(1), ssh-add(1), ssh-agent(1), sshd(8) J. Galbraith and R. Thayer, SECSH Public Key File Format, draft-ietf-secsh-publickeyfile-01.txt, March 2001, work in progress material. BSD
September 25, 1999 BSD

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