Monthly Archives: November 2010

Using Git and Github

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To download a repository hosted on github
[bash]git clone git@github.com:account_name/repo_name.git[/bash]

The git@github.com:account_name/repo_name.git will be shown on the home page for the repository on github.com

To update your code from the master branch of the repo:

[bash]git pull origin master[/bash]

To commit the changes you have made locally:

[bash]git commit -m "the message explaining what these code changes did"[/bash]

To send your changes to the repo at github:
[bash]git push origin master[/bash]

Replacing a Host Key

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Host keys are used to security log into remote servers (such as Virtual Private Servers – VPS). With Ubuntu if you are using host keys to sign into servers securely and have asked for strict checking, if you make a change (such as rebuilding your VPS) the host key will change and you cannot login and will get a message like:

@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@ WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED! @
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY!
Someone could be eavesdropping on you right now (man-in-the-middle attack)!
It is also possible that the RSA host key has just been changed.
The fingerprint for the RSA key sent by the remote host is
…
Please contact your system administrator.
…
RSA host key for 128.0.0.128 has changed and you have requested strict checking.
Host key verification failed.

if that happens you need to remove your local host key. Then you can sign back in and you will be able to save a new copy of the host key. If you don’t know why the key has changed you should figure that out first as it maybe be an indication of an important security problem. To remove you local key, you can use ssh-keygen -R [ip address of server with the bad key] for example: ssh-keygen -R 128.0.0.128

Then when you try to sign in you will get

The authenticity of host '128.0.0.128 (128.0.0.128)' can't be established.

RSA key fingerprint is ed:...:ea.
Are you sure you want to continue connecting (yes/no)?

And if you know why (such as you made changes to the server) you can say yes and connect and save the new known host key.

Managing Users in Ubuntu

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add a user: sudo adduser [newuser]
sudo adduser john
then give password, and setup home folder,when prompted

edit the list of super users
visudo

list users
cat /etc/passwd

change password of a user – sudo passwd [username]
sudo passwd john
to change your password you can just use sudo passwd.

delete user

sudo deluser username

This will not delete the home folder (and subfolder) those must be dealt with separately. A new user created with the old user name would have access to those files.

temporarily lock a user account – Simply locking a user account will not prevent a user from logging into your server remotely if they have previously set up RSA public key authentication.

sudo passwd -l username

To unlock the account

sudo passwd -u username

Groups

Groups are used in to control permissions (see file permissions)
add user to a group
usermod -G [group-name] [username]
usermod -G basketball john
using the -G switch ads the group as a supplemental group. Using -g would make the group that users primary group.

see what groups a user is in
id john

add a new group: groupadd [new_group_name]
groupadd ruby_developers