Block UDP Traffic via IPTables

While testing an application to verify all code flow paths, one of the scenarios demanded that the application handle dropped UDP packet stream. Now we had a recorded UDP stream at hand which we could play at will.

To simulate UDP packet drop, we added a rule in IPTables to drop/block all UDP packets destined for our UDP destination port 10222. The following command does it for you. 

To Block all udp traffic destined for port 10222

[root@paragpc ~]# iptables -A INPUT -p udp -i eth1 --dport 10222 -j DROP

where:

• -A is for add/Append iptable rule
• -p is for protocol
• -i  = --in-interface eth1  (there is similarly -o =--out-interface
• -j = JUMP

Refer the man page of iptables for more details. 

However don't forget to remove the rule once your test case is over.

Remove iptable rule dropping udp traffic destined for port 10222 [-D is for Delete iptable rule]

[root@paragpc ~]# iptables -D INPUT -p udp -i eth1 --dport 10222 -j DROP

However if you wish to permanently save the rule run the following command(source).

service iptables save

Disable/Ban certain users to connect from ssh

Generally after turning on the sshd service on our machines, we fail to authorize or rather restrict a set of users only to access (white-listing) the machine. The following setting in sshd_config helps you to restrict access to a select few users. The settings will take effect after restarting the sshd demon on your machine if already running.

[parag@paragcentos:/home/parag]sudo cat /etc/ssh/sshd_config  | grep -i Allow
...
##Allow the following users only to remote via ssh
AllowUsers root parag
...
...
[parag@paragcentos:/home/parag]

Also please read this post on more tips on how to secure your ssh server.