User Management on Linux boxes

The following command examples demonstrate user management functions on Linux systems.  The command is bolded with the parameters italized.  Type the bolded parts as they appear and replace the italized sections with your data.  For more information on each command, use man commandname.

Create a user

adduser username

Create a group

addgroup groupname

Show all users on a system

cat /etc/passwd

Show all groups on a system

cat /etc/group

Show which groups a user belongs to

groups username

Add a user to an additional group

adduser username groupname

Remove a user from a group

gpasswd -d username groupname

Remove an account from a system

userdel username

To remove the home folder at the same time

userdel -r username 

To delete a group

groupdel groupname

Posted in Linux, OSX

Introductory iptables (Linux Firewall)

Iptables is a program that allows an administrator to configure the tables provided by the Linux kernel firewall and the chains and rules it stores. Different kernel modules and programs are currently used for different protocols; iptables applies to IPv4, ip6tables to IPv6, arptables to ARP, and ebtables to Ethernet frames.

iptables requires elevated privileges to operate and must be executed by user root, otherwise it fails to function. On most Linux systems, iptables is installed as /usr/sbin/iptables and documented in its man pages which can be opened using man iptables when installed. It may also be found in /sbin/iptables, but since iptables is more like a service rather than an “essential binary”, the preferred location remains /usr/sbin.

More at:

Example Usage (run as root)

Show all iptables:

iptables  –L

Remove a rule from table “ input “ where 5 is the rule number

iptables -D INPUT  5

The following will add 3 rules to the table named “input”.  The first allows inbound traffic on port 80.  The second allows inbound traffic on port 443, the last is a deny all that drops traffic that hasn’t previously matched any rules in the table.  This rule must be last – if it isn’t, delete it and add it back to the table so it appears last. 

iptables -A INPUT -p tcp –dport 80 -j ACCEPT
iptables -A INPUT -p tcp –dport 443 -j ACCEPT
iptables -P INPUT DROP

The drop all rule above must appear last in the table, any subsequent accept rules will be ignored as rules are processed in order .


Additional Resources

Ubuntu iptables howto:

CentOS iptables howto:


Posted in Linux, Networking