Linux Server iptables firewall configuration detailed

Introduction: A Good article about the Linux firewall iptables configuration, Linux firewall configuration is critical, this involves the server security, security is not absolute, but iptables with good can eliminate most of the gray temptation. Reprinted to the Old Bar blog, more iptables configuration can consult the author.

Linux iptables firewall configuration is very important, let's configure a filter table firewall.

View the settings of this institution in Iptables

1. [Email protected] ~]# iptables-l-N

If you did not choose to start the firewall when installing Linux, there are no rules. That's it.

How to clear the original rules of Linux server iptables firewall

Whether or not you start a firewall when you install Linux, if you want to configure your own firewall, clear all the rules for the filter now.

1. [[Email protected] ~] #iptables-F clears all rule chains in the preset table filter
2. [[email protected] ~]# iptables-x clear Preset table rules in user-defined chains in filter

Nothing at all, and we did not start the firewall when installing Linux is the same. (in advance, these configurations are like using commands to configure the IP, the restart will lose its effect), how to save.

1. [Email protected] ~]#/etc/rc.d/init.d/iptables Save

This will allow you to write to the/etc/sysconfig/iptables file. Remember to re-start the firewall after writing to make it work.

1. [Email protected] ~]# service iptables restart

Now that the Iptables configuration table does not have any configuration, let's start our configuration.

Linux Firewall iptables Configuration

I suggest to read it first and then configure it according to the instructions.

1. [[email protected] ~]# iptables-pinput Drop NOTE: If you are logged in remotely ssh, you should drop it when you run the command to enter. Because no rules have been set yet. What to do, this machine operation Bai!
2. [[email protected] ~]# iptables-p OUTPUT ACCEPT
3. [Email protected] ~]# iptables-p FORWARD DROP

The above means that when the two chain rules (Input,forward) in the Iptables filter table are exceeded, the packets that are not in the two rules are handled, and that is the drop (abort). It should be said that this configuration is very safe. We want to control the incoming packets.

And for the output chain, that is, out of the package we do not have to do too much restriction, but to take the accept, that is, not in the rules of the package how to do it, that is through.

It can be seen that the Input,forward two chain uses what packets are allowed to pass through, and the output chain is not allowed by what packets pass.

This setting is quite reasonable, of course, you can also drop three chains, but I think it is not necessary, and to write the rules will increase. But if you only want a few rules that are limited, such as just Web servers. It is recommended that all three chains are drop.

First add the input chain, the default rule of the input chain is drop, so we write the need to ACCETP (through) the chain

In order to be able to use remote SSH login, we have to turn on port 22.

1. [[email protected] ~]# iptables-a input-p TCP--dport 22-j ACCEPT
2. [[email protected] ~]# iptables-a output-p tcp--sport 22-j ACCEPT (Note: This rule, if you set the OUTPUT to drop it will be written on this part, many people are looking at the writing of this rule resulted in, always Unable to ssh. In the remote, is not good.

The other port is the same, if the Web server is turned on, output is set to drop, you also add a chain:

1. [[email protected] ~]# iptables-a output-p TCP--sport 80-j ACCEPT, other similarly.)

If you made a Web server, turn on port 80.

1. [[email protected] ~]# iptables-a input-p TCP--dport 80-j ACCEPT

If you do a mail server, turn on port 25,110.

1. [[email protected] ~]# iptables-a input-p TCP--dport 110-j ACCEPT
2. [[email protected] ~]# iptables-a input-p TCP--dport 25-jaccept

If you do an FTP server, turn on port 21

1. [[email protected] ~]# iptables-a input-p TCP--dport 21-j ACCEPT
2. [[email protected] ~]# iptables-a input-p TCP--dport 20-j ACCEPT

If you do a DNS server, turn on port 53

1. [[email protected] ~]# iptables-a input-p TCP--dport 53-j ACCEPT

If you also do other servers, which port you need to open, just write it.

The main writing above is the input chain, usually not in the above rules, all drop

Allow ICMP packets to pass, that is, to allow pings,

1. [[email protected] ~]# iptables-a output-p icmp-j ACCEPT (OUTPUT set to drop)
2. [[email protected] ~]# iptables-a input-p icmp-j ACCEPT (INPUT set to drop)

Allow loopback! (This may cause problems such as DNS not shutting down properly)

1. Iptables-a input-i lo-p all-j ACCEPT (if INPUT DROP)
2. Iptables-a output-o lo-p all-j ACCEPT (if OUTPUT DROP)

Write the output chain below, the default rule for the output chain is accept, so we write a chain that needs drop (discard).

Reduce insecure port connections

1. [[email protected] ~]# iptables-a output-p TCP--sport 31337-j DROP
2. [[email protected] ~]# iptables-a output-p TCP--dport 31337-j DROP

Some Trojans scan services on ports 31337 through 31340 (that is, the elite ports in the hacker language). Since legitimate services do not use these non-standard ports to communicate, blocking these ports can effectively reduce the chance that your network may be infected by the machine and their remote primary server for independent communication.

There are other ports as well, like: 31335, 27444, 27665, 20034 NetBus, 9704, 137-139 (SMB), 2049 (NFS) port should also be banned, I write in this is not all, interested friends should go to check the relevant information.

Of course, more secure. You can also set the output chain to drop, then you add more rules, like adding above, allow SSH login. Just write it.

Tighter iptables settings that restrict remote connections to a computer

For example: We only allow SSH connection for 192.168.0.3 machines.

1. [[email protected] ~]# iptables-a input-s 192.168.0.3-p tcp--dport 22-j ACCEPT

If you want to allow or restrict a certain IP address, you can use 192.168.0.0/24 to represent all IPs on the 192.168.0.1-255 side.

24 indicates the number of subnet masks. But remember to delete the line in the/etc/sysconfig/iptables.

1. -A input-p tcp-m TCP--dport 22-j ACCEPT because it means all addresses can be logged in.

or using the command mode:

1. [[email protected] ~]# iptables-d input-p TCP--dport 22-j ACCEPT

And then save, I say one side, the reverse is the way of the command, only at the time of entry into force, if you want to restart the role, it should be saved. Write to the/etc/sysconfig/iptables file.

1. [Email protected] ~]#/etc/rc.d/init.d/iptables Save

This write!192.168.0.3 indicates that the IP address except the 192.168.0.3

Other regular connections are set up as well.

The following is the forward chain, the default rule of the forward chain is drop, we write the need to ACCETP (through) the chain, the monitoring of the forwarding chain.

Turn on forwarding, (when doing NAT, forward default rule is drop, must be done)

1. [Email protected] ~]# iptables-a forward-i eth0-o eth1-m State--state related,established-j ACCEPT
2. [Email protected] ~]# iptables-a forward-i eth1-o eh0-j ACCEPT

Discard Bad TCP packets

1. [[Email protected] ~] #iptables-A forward-p TCP! --syn-m State--state New-j DROP

Handle the number of IP fragments, prevent attacks, allow 100 per second

1. [[Email protected] ~] #iptables-A forward-f-M limit--limit 100/s--limit-burst 100-j ACCEPT

Set ICMP packet filtering to allow 1 packets per second, limiting the trigger condition to 10 packets.

1. [[Email protected] ~] #iptables-A forward-p icmp-m limit--limit 1/s--limit-burst 10-j ACCEPT

I'm in front only so allow ICMP packets to pass because I have restrictions here.

Configuring a NAT Table Firewall

1. View the settings of this agency on NAT

1. [Email protected] rc.d]# iptables-t nat-l
2. Chain prerouting (Policy ACCEPT)
3. Target Prot opt source destination
4. Chain postrouting (Policy ACCEPT)
5. Target Prot opt source destination
6. SNAT All-192.168.0.0/24 anywhere to:211.101.46.235
7. Chain OUTPUT (Policy ACCEPT)
8. Target Prot opt source destination

My NAT has been configured (just to provide the simplest proxy Internet access, and no firewall rules have been added). For more information on how to configure NAT, refer to my other article

Of course you don't have to clear the rules if you haven't configured NAT, because Nat doesn't have anything by default.

If you want to clear, the command is

1. [[email protected] ~]# iptables-f-t NAT
2. [[email protected] ~]# Iptables-x-t NAT
3. [[email protected] ~]# Iptables-z-t NAT

2, adding rules

Add basic NAT Address translation (see my other article on how to configure NAT),

To add a rule, we only add the drop chain. Because the default chain is all accept.

Prevention of intranet IP spoofing for external network

1. [Email protected] sysconfig]# iptables-t nat-a prerouting-i eth0-s 10.0.0.0/8-j DROP
2. [Email protected] sysconfig]# iptables-t nat-a prerouting-i eth0-s 172.16.0.0/12-j DROP
3. [Email protected] sysconfig]# iptables-t nat-a prerouting-i eth0-s 192.168.0.0/16-j DROP

If we want to, for example, stop MSN,QQ,BT and so on, need to find the port or IP they use, (personally think not too much necessary)

Cases:

Prohibit all connections to the 211.101.46.253

1. [Email protected] ~]# iptables-t nat-a prerouting-d 211.101.46.253-j DROP

Disabling the FTP (21) port

1. [[email protected] ~]# iptables-t nat-a prerouting-p tcp--dport 21-j DROP

The scope of the writing is so large that we can define it more precisely.

1. [Email protected] ~]# iptables-t nat-a prerouting-ptcp--dport 21-d 211.101.46.253-j DROP

This disables only FTP connections for 211.101.46.253 addresses, and other connections are available. such as Web (80 port) connections.

As I wrote, you just have to find the IP address of other software such as QQ,MSN, and the port, and based on what protocol, just write it.

Last: Drop illegal connection

1. [Email protected] ~]# iptables-a input-m state--state invalid-j DROP
2. [Email protected] ~]# iptables-a output-m State--state invalid-j DROP
3. [Email protected] ~]# iptables-a forward-m State--state invalid-j DROP

Allow all established and related connections

1. [Email protected] ~]# iptables-a input-m State--state established,related-j ACCEPT
2. [Email protected] ~]# iptables-a output-m State--state established,related-j ACCEPT
3. [Email protected] ~]#/etc/rc.d/init.d/iptables Save

This can be written in the/etc/sysconfig/iptables file. After writing, remember to restart the firewall iptables to function.

1. [[Email protected] ~] #service iptables restart

Do not forget to save, do not write a step to save a step. You can save, experiment and see if it meets your requirements.

I've tried all the rules above, no problem.

It took me nearly 1 months to write this article. Look for information, do your own experiments, I hope to help you. If there is incomplete and imperfect place also please raise.

The Prairie mailbox: mr.jembai#gmail.com

Penguin Number: 478721372

Linux Server iptables firewall configuration detailed