How to Prevent DDOS attacks in linux

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Squid is a port ing function that can be used to convert port 80. In fact, common DDOS attacks can be used to modify the parameters in/proc/sys/net/ipv4/tcp_max_syn_backlog, the default parameters are usually very small and set to more than 8000. Generally, DDOS attacks can be solved. If it reaches the timeout stage, set/proc/sys/net/ipv4/tcp_fin_timeout to a smaller value.

 

Everyone is discussing DDOS. I personally think there is no real solution at present. I just want to expand the buffer and defense capabilities and play a psychological tactic with hackers to see who sticks to the end, there are also many practices on the Internet, such as syncookies, which are complex points.

 

Sysctl-w net. ipv4.icmp _ echo_ignore_all = 1

Echo 1>/proc/sys/net/ipv4/tcp_syncookies

Sysctl-w net. ipv4.tcp _ max_syn_backlog = "2048"

Sysctl-w net. ipv4.tcp _ synack_retries = "3"

 

Iptables-a input-I eth0-p tcp -- syn-j syn-flood

# Limit 12 connections per second (burst to 24)

Iptables-A syn-flood-m limit -- limit 12/s -- limit-burst 24-j RETURN

In this case, you can try:

Iptbales-a forward-p tcp -- syn-m limit -- limit 1/s-j ACCEPT

 

VM service providers may be attacked by hackers during operation. Common attacks include SYN and DDOS attacks.

By changing the IP address, it is possible to find the attacked site to avoid the attack, but the service interruption takes a long time. Relatively thorough

The solution is to add a hardware firewall. However, hardware firewalls are expensive. You can consider using Linux

The firewall function provided by the system.

 

1. resist SYN

 

SYN attacks use the three-way handshake principle of the TCP/IP protocol to send a large number of network packets that establish connections, but they are not actually

After establishing a connection, the network queue of the attacked server is fully occupied and cannot be accessed by normal users.

The Linux Kernel provides several SYN-related configurations. Run the following command:

Sysctl-a | grep syn

See:

Net. ipv4.tcp _ max_syn_backlog = 1024

Net. ipv4.tcp _ syncookies = 0

Net. ipv4.tcp _ synack_retries = 5

Net. ipv4.tcp _ syn_retries = 5

 

Tcp_max_syn_backlog is the length of the SYN queue, and tcp_syncookies are a function to determine whether to enable SYN cookies.

Function to prevent some SYN attacks. Tcp_synack_retries and tcp_syn_retries define SYN

Number of retries.

 

Increasing the length of the SYN queue can accommodate more network connections waiting for connection. Enabling the SYN Cookie function can block some

SYN attacks can also reduce the number of retries.

 

To adjust the preceding settings, follow these steps:

Increase the SYN queue length to 2048:

Sysctl-w net. ipv4.tcp _ max_syn_backlog = 2048

Enable the syn cookie function:

Sysctl-w net. ipv4.tcp _ syncookies = 1

Reduce the number of retries:

Sysctl-w net. ipv4.tcp _ synack_retries = 3

Sysctl-w net. ipv4.tcp _ syn_retries = 3

 

To maintain the preceding configuration during system restart, you can add the preceding command to the/etc/rc. d/rc. local file.

 

2. Resist DDOS attacks

 

DDOS, distributed denial of access attack, refers to the hacker organization from different sources of many hosts, to common ports, such as 80,

25. Send a large number of connections, but these clients only establish connections, not normal access. Generally, the Apache configuration accepts connections.

A limited number (usually 256). These "fake" access will fill up Apache, and normal access will fail.

 

Linux provides a firewall tool called ipchains to shield connections from specific IP addresses or IP address segments to specific ports.

To use ipchains to defend against DDOS attacks, you must first use the netstat command to find the source address of the attack, and then use the ipchains command to block the attack.

Attack. One block is found.

 

* ** Enable the ipchains Function

First, check whether the ipchains service is set to auto start:

Chkconfig -- list ipchains

The output is generally:

Ipchains 0ff 1ff 2n 3n 4n 5n 6ff

If the column 345 is on, the ipchains service has been set to auto start.

If not, run the following command:

Chkconfig -- add ipchains

Set ipchains to auto start

Next, check whether the ipchains configuration file/etc/sysconfig/ipchains exists. If this file does not exist, ipchains

It does not take effect even if it is set to automatic start. The default ipchains configuration file is as follows:

 

 

# Firewall configuration written by lokkit # Manual customization of this file is not recommended. # Note: ifup-post will punch the current nameservers through the # firewall; such entries will ** not * be listed here.: input ACCEPT: forward ACCEPTutput ACCEPT-A input-s 0/0-d 0/0-I lo-j ACCEPT # allow http, ftp, smtp, ssh, domain via tcp; domain via udp-A input-p tcp-s 0/0-d 0/0 pop3-y-j ACCEPT-A input-p tcp-s 0/0-d 0/0 http-y-j ACCEPT-A input-p tcp-s 0/0-d 0/0 https-y-j ACCEPT-A input-p tcp-s 0/0-d 0/0 ftp-y-j ACCEPT-A input-p tcp-s 0/0-d 0/0 smtp -y-j ACCEPT-A input-p tcp-s 0/0-d 0/0 ssh-y-j ACCEPT-A input-p tcp-s 0/0-d 0/0 domain-y-j ACCEPT-A input-p udp-s 0/0-d 0/0 domain-j ACCEPT # deny icmp packet #-A input-p icmp-s 0/0-d 0/0-j DENY # default rules-A input-p tcp -s 0/0-d 0/0-y-j REJECT-A input-p tcp-s 0/0-d 0/0 2049-y-j REJECT-A input-p udp-s 0/0-d 0/0- j REJECT-A input-p udp-s 0/0-d 0/0 2049-j REJECT-A input-p tcp-s 0/0-d 0/0 6000: 6009-y-j REJECT-A input-p tcp-s 0/0-d 0/0-y-j REJECT

 

If the/etc/sysconfig/ipchains file does not exist, you can use the above content to create it. After creation, start the ipchains service:

/Etc/init. d/ipchains start

 

* ** Use the netstat command to find the attack source

If the hacker attacks port 80 on the Web, view the IP address and port of the Client Connected to port 80. The command is as follows:

Netstat-an-t tcp | grep ": 80" | grep ESTABLISHED | awk {printf "% s", $5, $6} | sort

Output:

161.2.8.9: 123 FIN_WAIT2

161.2.8.9: 124 FIN_WAIT2

61.233.85.253: 23656 FIN_WAIT2

...

The first column is the Client IP address and port, and the second column is the connection status.

If there are many connections from the same IP address (more than 50 connections) and they are continuous ports, it is likely to be an attack.

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If you only want to view the established connection, run the following command:

Netstat-an-t tcp | grep ": 80" | grep ESTABLISHED | awk {printf "% s", $5, $6} | sort

 

* ** Use ipchains to block attack sources

You can use ipchains to block attack sources in two ways. One is to add it to/etc/sysconfig/ipchains, and then restart

Ipchains service. The other is to directly use the ipchains command. After blocking, you may need to restart the attacked service,

Is an established attack connection invalid

 

* Add/etc/sysconfig/ipchains

Assume that the connection from 218.202.8.151 to 80 is blocked. Edit the/etc/sysconfig/ipchains file in the utput ACCEPT

Add the following lines:

-A input-s 218.202.8.151-d 0/0 http-y-j REJECT

Save the changes and restart ipchains:

/Etc/init. d/ipchains restart

If you want to block the entire network segment of 218.202.8, add:

-A input-s 218.202.8.0/255.255.255.0-d 0/0 http-y-j REJECT

 

* Directly use the command line

Add the/etc/sysconfig/ipchains file and restart the ipchains method,

Some connections may be drilled in. The most convenient method is to directly use the ipchains command.

If the connection from 218.202.8.151 to 80 is blocked, run the following command:

Ipchains-I input 1-p tcp-s 218.202.8.151-d 0/0 http-y-j REJECT

If you want to block the entire network segment of 218.202.8, run the following command:

Ipchains-I input 1-p tcp-s 218.202.8.0/255.255.255.0-d 0/0 http-y-j REJECT

Here,-I indicates insertion, input indicates rule connection, and 1 indicates adding to the first one.

 

You can edit a shell script to make it easier. The command is as follows:

 

Vi blockit

Content:

#! /Bin/sh

If [! -Z "$1"]; then

Echo "Blocking: $1"

Ipchains-I input 1-p tcp-s "$1"-d 0/0 http-y-j REJECT

Else

Echo "which ip to block? "

Fi

Save, and then:

Chmod 700 blockit

Usage:

./Blockit 218.202.8.151

./Blockit 218.202.8.0/255.255.255.0

 

The rules created by the preceding command line method will expire after the restart. You can use the ipchains-save command to print the rules:

Ipchains-save

Output:

 

 

: Input ACCEPT: forward ACCEPTutput ACCEPTSaving 'input. -A input-s 0.0.0.0/0.0.0.0-d 0.0.0.0/0.0.0.0-I lo-j ACCEPT-A input-s 0.0.0.0.0/0.0.0.0-d 0.0.0.0/0.0.0.0 110:110-p 6-j ACCEPT-y- A input-s 0.0.0.0/0.0.0.0-d 0.0.0.0/0.0.0.0 80: 80-p 6-j ACCEPT-y-A input-s 0.0.0.0/0.0.0.0-d 0.0.0.0/0.0.0.0 22:22-p 6-j ACCEPT-y-A input-s 0.0.0.0.0/0.0.0.0- d 0.0.0.0/0.0.0.0 88: 88-p 6-j ACCE