Linux Firewall prevents DOS and DDOS attacks

Article Title: Linux system Firewall prevents DOS and DDOS attacks. Linux is a technology channel of the IT lab in China. Includes basic categories such as desktop applications, Linux system management, kernel research, embedded systems, and open source.

Use the firewall function of Linux to defend against Network Attacks

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. A thorough solution is to add a hardware firewall. However, hardware firewalls are expensive. You can consider using the firewall function provided by the Linux system to defend against attacks .?

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 do not actually establish connections. As a result, the network queue of the attacked server is full, 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. Whether to enable the SYN Cookie function can prevent some SYN attacks. Tcp_synack_retries and tcp_syn_retries define the number of retries of SYN.

Increasing the SYN queue length can accommodate more network connections waiting for connection. Enabling the SYN Cookie function can prevent some SYN attacks and 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 and distributed denial of access (DDOS) attacks mean that hackers send a large number of connections to common ports, such as 80 and 25, to many hosts from different sources. However, these clients only establish connections, not normal access. Generally, due to the limited number of accepted connections configured by Apache (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. 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 0: off 1: off 2: on 3: on 4: on 5: on 6: off

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
# Firewall; such entries will ** not * be listed here.
: Input ACCEPT
: Forward ACCEPT
: Output 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/IP 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 7100-y-j REJECT

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

/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 \ n", $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.

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

Netstat-an-t tcp | grep ": 80" | grep ESTABLISHED | awk '{printf "% s \ n", $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 restart the ipchains service. The other is to directly use the ipchains command. After the attack is blocked, you may need to restart the attacked service. The attack connection has expired.

* Add/etc/sysconfig/ipchains

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

The method of adding the/etc/sysconfig/ipchains file and restarting ipchains is slow, and some connections may be drilled in when ipchains are restarted. 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 ACCEPT
: Output ACCEPT
Saving '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-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-d 0.0.0.0/0.0.0.0 88: 88-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 89: 89-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 90: 90-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 91: 91-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 8180: 8180-p 6-j ACCEPT-y
-A input-s 0.0.0.0/0.0.0.0-d 0.0.0.0/0.0.0.0 443: 443-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 21:21-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 25:25-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-d 0.0.0.0/0.0.0.0 53: 53-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 9095: 9095-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 8007: 8007-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 53: 53-p 17-j ACCEPT
-A input-s 0.0.0.0/0.0.0.0-d 0.0.0.0/0.0.0.0 0: 1023-p 6-j REJECT-y
-A input-s 0.0.0.0/0.0.0.0-d 0.0.0.0/0.0.0.0 2049: 2049-p 6-j REJECT-y
-A input-s 0.0.0.0/0.0.0.0-d 0.0.0.0/0.0.0.0 :1023-p 17-j REJECT
-A input-s 0.0.0.0/0.0.0.0-d 0.0.0.0/0.0.0.0 2049: 2049-p 17-j REJECT
-A input-s 0.0.0.0/0.0.0.0-d 0.0.0.0/0.0.0.0 6000: 6009-p 6-j REJECT-y
-A input-s 0.0.0.0/0.0.0.0-d 0.0.0.0/0.0.0.0 7100: 7100-p 6-j REJECT-y

You need to set "Saving 'input '. "Remove and save the other content to the/etc/sysconfig/ipchains file. In this way, after the next restart, the created rule will take effect again.

3. If iptables is used

In RH 8.0 and later versions, iptables is enabled to replace ipchains. The two are very similar and different.

* Enable iptables

If the iptables file does not exist in/etc/sysconfig/, you can create:

# Firewall configuration written by lokkit
# Manual customization of this file is not recommended.
# Note: ifup-post will punch the current nameservers through
# Firewall; such entries will ** not * be listed here.

* Filter
: Input accept [0: 0]
: Forward accept [0: 0]
: Output accept [0: 0]
: RH-Lokkit-0-50-INPUT-[0: 0]
-A input-j RH-Lokkit-0-50-INPUT
-A RH-Lokkit-0-50-INPUT-I lo-j ACCEPT
-A RH-Lokkit-0-50-INPUT-p tcp-m tcp -- dport ftp-j ACCEPT
-A RH-Lokkit-0-50-INPUT-p tcp-m tcp -- dport ssh-j ACCEPT
-A RH-Lokkit-0-50-INPUT-p tcp-m tcp -- dport http-j ACCEPT
-A RH-Lokkit-0-50-INPUT-p tcp-m tcp -- dport smtp-j ACCEPT
-A RH-Lokkit-0-50-INPUT-p tcp-m tcp -- dport pop3-j ACCEPT
-A RH-Lokkit-0-50-INPUT-p tcp-m tcp -- dport mysql-j ACCEPT
-A RH-Lokkit-0-50-INPUT-p tcp-m tcp -- dport 2001-j ACCEPT
-A RH-Lokkit-0-50-INPUT-p tcp-m tcp -- dport domain-j ACCEPT
-A RH-Lokkit-0-50-INPUT-p udp-m udp -- dport domain-j ACCEPT
-A RH-Lokkit-0-50-INPUT-p tcp-m tcp -- dport -- syn-j REJECT
-A RH-Lokkit-0-50-INPUT-p tcp-m tcp -- dport 2049 -- syn-j REJECT
-A RH-Lokkit-0-50-INPUT-p udp-m udp -- dport-j REJECT
-A RH-Lokkit-0-50-INPUT-p udp-m udp -- dport 2049-j REJECT
-A RH-Lokkit-0-50-INPUT-p tcp-m tcp -- dport 6000: 6009 -- syn-j REJECT
-A RH-Lokkit-0-50-INPUT-p tcp-m tcp -- dport 7100 -- syn-j REJECT
COMMIT

The preceding configuration allows ftp, ssh, http, smtp, pop3, mysql, 2001 (Prim @ Hosting ACA port), and domain port.

* Start iptables

/Etc/init. d/iptables start

* Set iptables to auto start

Chkconfig -- level 2345 iptables on

* Use iptables to shield IP addresses

Iptables-I RH-Lokkit-0-50-INPUT 1-p tcp-m tcp-s 213.8.166.227 -- dport 80 -- syn-j REJECT

Note that the difference with ipchains is:

-The parameters of the rule name after I are different from those defined in ipchains. They are not uniform input, but defined in/etc/sysconfig/iptables.

-M tcp

The parameter of the specified port is -- dport 80.

More -- syn parameter, which can automatically detect sync attacks

Disable ping using iptables: -a input-p icmp-m icmp -- icmp-type 8-m limit -- limit 6/min -- limit-burst 2-j ACCEPT-A INPUT-p icmp-m icmp -- icmp- type 8-j REJECT -- reject-with icmp-port-unreachable