Detailed analysis of Linux Firewall Configuration

If you do not know the basic knowledge of Linux firewall, You can first look at some basic things. Here we will introduce in detail some Linux firewall configurations.

Start Configuration

Let's configure a filter table firewall. (1) view the settings of IPTABLES on the local machine [root @ tp ~] # Iptables-L-n

Chain INPUT (policy ACCEPT)

Target prot opt source destination Chain FORWARD (policy ACCEPT)

Target prot opt source destination Chain OUTPUT (policy ACCEPT)

Target prot opt source destination Chain RH-Firewall-1-INPUT (0 references)

Target prot opt source destination

ACCEPT all -- 0.0.0.0/0 0.0.0.0/0

ACCEPT icmp -- 0.0.0.0/0 0.0.0.0/0 icmp type 255

ACCEPT esp -- 0.0.0.0/0 0.0.0.0/0

ACCEPT ah -- 0.0.0.0/0 0.0.0.0/0

ACCEPT udp -- 0.0.0.0/0 224.0.0.20.udp dpt: 5353

ACCEPT udp -- 0.0.0.0/0 0.0.0.0/0 uddpt: 631

ACCEPT all -- 0.0.0.0/0 0.0.0.0/0 state RELATED, ESTABLISHED

ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 state NEW tcp dpt: 22

ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 state NEW tcp dpt: 80

ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 state NEW tcp dpt: 25

REJECT all -- 0.0.0.0/0 0.0.0.0/0 reject-with icmp-host-prohibited

We can see that when I installed linux, I chose to have a firewall and opened ports 22, 80, and 25. if you do not choose to start the firewall when installing linux, this is the case [root @ tp ~] # Iptables-L-n

Chain INPUT (policy ACCEPT)

Target prot opt source destination Chain FORWARD (policy ACCEPT)

Target prot opt source destination Chain OUTPUT (policy ACCEPT)

Target prot opt source destination has no rules. (2) Clear the original rules. whether or not you have enabled the firewall when installing linux, if you want to configure your own firewall, clear all the filter rules. [root @ tp ~] # Iptables-F clear the rules of all rule chains in the filter of the preset table

[Root @ tp ~] # Iptables-X clear the rules in the User-Defined chain in the filter of the preset table. Let's take a look at [root @ tp ~]. # Iptables-L-n

Chain INPUT (policy ACCEPT)

Target prot opt source destination Chain FORWARD (policy ACCEPT)

Target prot opt source destination Chain OUTPUT (policy ACCEPT)

Target prot opt source destination
Nothing, just like we didn't start the firewall when installing linux. (say in advance, these configurations are the same as configuring IP addresses with commands, so restarting them will lose effect.) How to save them. [root @ tp ~] #/Etc/rc. d/init. d/iptables save to write it to the/etc/sysconfig/iptables file. after writing, remember to repeat the firewall to make it work. [root @ tp ~] # Service iptables restart: No configuration is available in the IPTABLES configuration table. Let's start our configuration. (3) set the default rule [root @ tp ~] # Iptables-p input drop [root @ tp ~] # Iptables-p output accept [root @ tp ~] # Iptables-p FORWARD DROP

The above means that when two chain rules (INPUT and FORWARD) in the filter table in IPTABLES are exceeded, how can we process data packets not in these two rules, that is, DROP (discard ). it should be said that the configuration is safe. we need to control inbound data packets, but we do not need to impose too many restrictions on the OUTPUT chain, that is, the outgoing packet, but adopt ACCEPT. That is to say, what should we do if the packet is not in a rule, that is through. we can see what packets are allowed to pass through the INPUT and FORWARD chains, and what packets are not allowed to pass through the OUTPUT chain. this setting is quite reasonable. Of course you can also DROP all three links, but I don't think it is necessary to do so, and the rules to be written will increase. but if you only want a limited number of rules, for example, only WEB servers. we recommend that all three links be DROP. note: If you log on remotely through SSH, you should drop it when you enter the first command and press Enter. because you have not set any rules. what should I do? Go to the local machine to operate it! (4) Add a rule. first, add the INPUT chain. The default rule of the INPUT chain is DROP, so we will write the chain requiring ACCETP (VIA). In order to enable remote SSH Login, We need to enable port 22. [root @ tp ~] # Iptables-a input-p tcp -- dport 22-j ACCEPT [root @ tp ~] # Iptables-a output-p tcp -- sport 22-j ACCEPT (Note: If you set OUTPUT to DROP, write this rule, many people are eager to write this rule, and SSH is always unavailable. remotely, is it okay. the same applies to other ports. If the web server is enabled and the OUTPUT is set to DROP, a chain should also be added: [root @ tp ~] # Iptables-a output-p tcp -- sport 80-j ACCEPT, similarly.) If the WEB server is configured, enable port 80. [root @ tp ~] # Iptables-a input-p tcp -- dport 80-j ACCEPT

If the email server is configured, enable port 25,110. [root @ tp ~] # Iptables-a input-p tcp -- dport 110-j ACCEPT

[Root @ tp ~] # Iptables-a input-p tcp -- dport 25-j ACCEPT

If the FTP server is configured, enable port 21 [root @ tp ~]. # Iptables-a input-p tcp -- dport 21-j ACCEPT [root @ tp ~] # Iptables-a input-p tcp -- dport 20-j ACCEPT if the DNS server is configured, enable port 53 [root @ tp ~] # Iptables-a input-p tcp -- dport 53-j ACCEPT. the above mainly writes the INPUT chain. For anything not in the above rules, DROP allows icmp packets to pass through, that is, allow ping, [root @ tp ~] # Iptables-a output-p icmp-j ACCEPT (if OUTPUT is set to DROP) [root @ tp ~] # Iptables-a input-p icmp-j ACCEPT (if INPUT is set to DROP) Allow loopback! (Otherwise, DNS may fail to be shut down normally.) IPTABLES-a input-I lo-p all-j ACCEPT (if it is INPUT DROP)

IPTABLES-a output-o lo-p all-j ACCEPT (if it is output drop)

The OUTPUT chain is written below. The default rule of the OUTPUT chain is ACCEPT, so we will write the chain that requires DROP (discard). Reduce insecure port connections [root @ tp ~] # Iptables-a output-p tcp -- sport 31337-j DROP [root @ tp ~] # Iptables-a output-p tcp -- dport 31337-j DROP some Trojans scan services on ports 31337 to 31340 (elite ports in hacking languages. Since legal services do not use these non-standard ports for communication, blocking these ports can effectively reduce the chances of independent communication between infected machines on your network and their remote master servers. There are also other ports, such: 31335, 27444, 27665, 20034 NetBus, 9704, 137-139smb), 2049 (NFS) port should also be disabled, I did not write all here, if you are interested, check related information. of course, you can set the OUTPUT chain to DROP for more secure access, so you can add more rules, just like adding the above to allow SSH login. just write it. the following is a more detailed rule that limits access to a machine. For example, we only allow hosts with 192.168.0.3 to perform SSH connections [root @ tp ~]. # Iptables-a input-s 192.168.0.3-p tcp -- dport 22-j ACCEPT if you want to allow, you can also restrict the number of subnet masks for a certain IP address. The value 192.168.0.0/24 indicates all IP addresses in the range of 192.168.0.1-255. but remember to delete this line in/etc/sysconfig/iptables. -a input-p tcp-m tcp -- dport 22-j ACCEPT because it indicates that all addresses can log on. or use the command: [root @ tp ~] # Iptables-d input-p tcp -- dport 22-j ACCEPT and save it. I'll talk about it again. Instead, it uses the command method and takes effect only at the time. If you want to restart it, it will also take effect, save it. write to the/etc/sysconfig/iptables file. [root @ tp ~] #/Etc/rc. d/init. d/iptables save! 192.168.0.3 indicates that other rule connections except the IP address 192.168.0.3 are also set. the following is the FORWARD chain. The default rule of the FORWARD chain is DROP, so we will write the chain that requires ACCETP (VIA) to monitor the ongoing forwarding chain. enable the forwarding function (required when the default FORWARD rule is DROP when performing NAT) [root @ tp ~] # Iptables-a forward-I eth0-o eth1-m state -- state RELATED, ESTABLISHED-j ACCEPT [root @ tp ~] # Iptables-a forward-I eth1-o eh0-j ACCEPT discard bad TCP Packets [root @ tp ~] # Iptables-a forward-p TCP! -- Syn-m state -- state NEW-j DROP: number of IP fragments processed to prevent attacks. 100 [root @ tp ~] # Iptables-a forward-f-m limit -- limit 100/s -- limit-burst 100-j ACCEPT: sets ICMP packet filtering and allows one packet per second, the trigger condition is 10 packets. [root @ tp ~] # Iptables-a forward-p icmp-m limit -- limit 1/s -- limit-burst 10-j ACCEPT, this is because I have restrictions here. 2. configure a NAT table to go to firewall 1 and view local NAT settings [root @ tp rc. d] # iptables-t nat-L

Chain PREROUTING (policy ACCEPT)

Target prot opt source destination Chain POSTROUTING (policy ACCEPT)

Target prot opt source destination

SNAT all -- 192.168.0.0/24 anywhere to: 211.101.46.235Chain OUTPUT (policy ACCEPT)

Target prot opt source destination my NAT has been configured (only the simplest proxy Internet access function is provided, and no firewall rules have been added ). for more information about how to configure NAT, see my other article. If you have not configured NAT, you do not need to clear the rules, because NAT does not have anything by default. If you want to clear it, run the [root @ tp ~] command. # Iptables-F-t nat [root @ tp ~] # Iptables-X-t nat [root @ tp ~] # Iptables-Z-t nat2: add rules to add basic NAT address translation (see my other article on how to configure NAT). To add rules, we only add DROP links. because the default chain is all ACCEPT. prevent Internet spoofing by using an intranet IP Address [root @ tp sysconfig] # iptables-t nat-a prerouting-I eth0-s 10.0.0.0/8-j DROP

[Root @ tp sysconfig] # iptables-t nat-a prerouting-I eth0-s 172.16.0.0/12-j DROP

[Root @ tp sysconfig] # iptables-t nat-a prerouting-I eth0-s 192.168.0.0/16-j DROP

If we want to, for example, block MSN, QQ, BT, etc., we need to find the port or IP address they use (I personally think it is not necessary) for example: disable all connections to 211.101.46.253 [root @ tp ~] # Iptables-t nat-a prerouting-d 211.101.46.253-j DROP disable FTP (21) Port [root @ tp ~] # Iptables-t nat-a prerouting-p tcp -- dport 21-j DROP. If the write range is too large, we can define it more accurately. [root @ tp ~] # Iptables-t nat-a prerouting-p tcp -- dport 21-d 211.101.46.253-j DROP to disable only the FTP connection with the 211.101.46.253 address. Other connections are also allowed. for example, web (port 80) connection. according to what I wrote, you only need to find the IP addresses, ports, and protocols of other software such as QQ and MSN. finally: drop illegal connection

[Root @ tp ~] # Iptables-a input-m state -- state INVALID-j DROP

[Root @ tp ~] # Iptables-a output-m state -- state INVALID-j DROP

[Root @ tp ~] # Iptables-a forward-m state -- state INVALID-j DROP

Allow all established and related connections

[Root @ tp ~] # Iptables-a input-m state -- state ESTABLISHED, RELATED-j ACCEPT

[Root @ tp ~] # Iptables-a output-m state -- state ESTABLISHED, RELATED-j ACCEPT

[Root @ tp ~] #/Etc/rc. d/init. d/iptables save

In this way, you can write it to the/etc/sysconfig/iptables file. Remember to repeat the firewall after writing it to make it take effect.

[Root @ tp ~] # Service iptables restart

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