DDOS attack

How to prevent DDOS attack by CSF firewall?

Ananda Raj Pandey8 Comments

Preventing DDOS attack from csf firewall.

First make sure DDOS attack is not from open recursive DNS settings. To check and fix that issue please read this article – Preventing DDOS aplification open resolver attack

This article is to set CSF firewall so that any DDOS intentional attack to your server can be prevented.

 

Step 1: open and edit CSF config file. 

vi /etc/csf/csf.conf

 

Settings: 

Enable connection tracking.
CT_LIMIT is max number of connection allowed from one IP, you can set this value as per your server requirement.

CT_LIMIT =100

Set connection tracking interval.

CT_INTERVAL =30

If you want to get possible ddos attack email then enable it.

CT_EMAIL_ALERT =1

If you want to make IP blocks permanent then set this to 1, otherwise blocks
will be temporary and will be cleared after CT_BLOCK_TIME seconds

CT_PERMANENT = 1

If you opt for temporary IP blocks for CT, then the following is the interval
in seconds that the IP will remained blocked for (e.g. 1800 = 30 mins)

CT_BLOCK_TIME = 1800

If you only want to count specific ports (e.g. 80,443) then add the ports
to the following as a comma separated list. E.g. “80,443”

CT_PORTS = 80,23,443

 

These settings will be enough for DDOS attacks but if you are getting more attacks even you have above option configured then we can set few more options.

 

Step 2: Enable distributed attacks

LF_DISTATTACK = 1  

Set the following to the minimum number of unique IP addresses that trigger
LF_DISTATTACK

LF_DISTATTACK_UNIQ = 2

 

Step 3: Enable distributed FTP attacks

LF_DISTFTP = 1

 

Set the following to the minimum number of unique IP addresses that trigger
LF_DISTFTP. LF_DISTFTP_UNIQ must be <= LF_DISTFTP for this to work

LF_DISTFTP_UNIQ = 3

 

If this option is set to 1 the blocks will be permanent
If this option is > 1, the blocks will be temporary for the specified number
of seconds

LF_DISTFTP_PERM =1

 

Step 4: Enable distributed SMTP attacks.

 

LF_DISTSMTP =1

 

Set the following to the minimum number of unique IP addresses that trigger
LF_DISTSMTP. LF_DISTSMTP_UNIQ must be <= LF_DISTSMTP for this to work

LF_DISTSMTP_UNIQ =4

 

If this option is set to 1 the blocks will be permanent
If this option is > 1, the blocks will be temporary for the specified number
of seconds

LF_DISTSMTP_PERM =1

 

This is the interval during which a distributed FTP or SMTP attack is
measured

LF_DIST_INTERVAL = 300

Preventing DDOS aplification open resolver attack

Ananda Raj Pandey3 Comments

DDOS Attack  by open DNS resolver:

Open dns resolver provides name resolution to any network outside your network. This means any one can use your
server to resolve the host name and also use it to attack other server by spoofing as your server. This in return
consume your server bandwidth and also cpu + memory resources making your server slow or even result to crash.
(settings suggested below is for BIND server)

Disable open recursive requests:

If we dont need open recursive on our system then we can completly disable it by following method.

vi /etc/named.conf
recursion no; // turn off recursion
allow-transfer {none;};
allow-query-cache {none;};

after modifing the named.conf file DNS server must be restarted.

service named restart

If we need to enable dns recursion then we can specify the ips, so that only these ips can do recursions.

acl ourips {
192.168.0.0/24; // change ip as required
localhost;
};
options {
allow-recursion {ourips;};
allow-query-cache{ourips;};
allow-query{any;}; // for web servers
recursion yes;
...
}

after midifing the named.conf restart the bind server

service named restart

To check whether recursion is turn off run the following command:

host google.com <your name server>
and result will be 
Host google.com not found

If you are getting too much request to your named server, you will get big log file with entry of dined request. This
will slow down the server, so to disable failed request to be written to the log
add “category security {null;}; to named file.

vi /etc/named.conf
logging {
category security {null;};
channel default_debug {
......
};