how to generate barcode in visual basic 2010 Ports 143 and 993, respectively in Font

Make DataMatrix in Font Ports 143 and 993, respectively

Ports 143 and 993, respectively
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CHAPTER 2 s FIREWALLING YOUR HOSTS
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In the last three rules you have also replaced the references to the numeric port numbers with the name of the services being filtered, http and https and imap and imaps. These services are defined in the file /etc/services. Listing 2-1 shows the service definitions for these protocols from this file. Listing 2-1. Service Definitions in the /etc/services File http imap https imaps 80/tcp 143/tcp 443/tcp 993/tcp www www-http imap imaps # # # # WorldWideWeb HTTP IMAP MCom IMAPS
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I recommend using the service name rather than the port for your source and destination ports, as it makes your rules easier to read and understand. Finally, you have again used the target of ACCEPT as defined by the -j flag to indicate that this traffic is allowed to leave the host. In combination, the four rules you have defined allow a Web server to receive and send HTTP and HTTPS traffic from a host. While not an ideal (or complete) configuration, this represents a limited-functioning iptables firewall. From this you will build more complicated firewall configurations, but first you will examine how to identify what to filter on and look at the iptables command and some of its options.
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THE /etc/services FILE
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It is important to secure the /etc/services file. It contains a list of network services and matching ports. Listing 2-2 shows a sample of this file. Listing 2-2. Sample /etc/services File ftp ftp ssh ssh telnet telnet 21/tcp 21/udp 22/tcp 22/udp 23/tcp 23/udp
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fsp fspd # SSH Remote Login Protocol # SSH Remote Login Protocol
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Although actually disabling services you do not use in this file can inconvenience attackers, it will not actively stop them using the service you have disabled. But I recommend not allowing anyone to edit this file and potentially add any services to your host. Use the following commands to secure the file: puppy# chown root:root /etc/services puppy# chmod 0644 /etc/services puppy# chattr +i /etc/services The chattr +i command makes the /etc/services immutable: it cannot be deleted, it cannot be renamed, and no link can be created to this file.
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Choosing Filtering Criteria
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Determining what an iptables rule is going to filter on is an important part of the configuration process, and you need to understand the basic structure of a TCP/IP transaction. As I have discussed, you can filter on source and destination IP addresses, source and destination ports, protocols, and a variety of other options. The best method of choosing how to filter your traffic is to make a map of your incoming and outgoing traffic. Table 2-1 provides an example of how you do this. Table 2-1. HTTP Traffic Flow Incoming
Interface
eth0
Source Address
Source Port
32768 to 61000
Protocol
Destination Address
Destination Port
For the example in Table 2-1. I have used incoming HTTP traffic and laid out all the information I know about the incoming traffic. First I have highlighted the incoming interface, eth0, that will be handling the traffic. Then I have identified the potential source addresses that will be the clients querying the Web server. The first question is now whether you can determine who the client is. Most Web servers will be open to traffic from all source addresses, but in some cases for example, for an Intranet Web server used only in a local network you may be able to use the local network source address as a filtering criteria. In the example in Table 2-1. I will be allowing traffic from any source address. The next item is the source port of the incoming traffic. The source and destination ports of a TCP connection are determined in one of two ways: the server end of a connection is generally assigned a predetermined port number for that particular service; for example, by default DNS servers use port 53 and SMTP server use port 25. The Internet Assigned Numbers Authority (IANA) assigns these numbers, and you can see the definitive list at http://www.iana.org/ assignments/port-numbers. At the client end, incoming requests from remote clients can come in from a range of random source ports called ephemeral ports. The remote client assigns each outgoing connection a port from this range. The exact range varies from operating system to operating system. On Linux systems to see what the range of your ephemeral ports is, you can review the contents of the file /proc/sys/net/ipv4/ip_local_port_range. For Red Hat Linux systems this range is generally 32768 to 61000. For Debian systems the range is 1024 to 4099. Unless you know the range of ephemeral ports being used by all your client systems I recommend not using this as a filter for rules. Next I have identified the protocol the traffic will be using, tcp, which is a filtering criteria you should be able use in most rules to filter traffic. Finally, I have identified the destination address and destination port; in this case for the incoming HTTP traffic is the IP address of the , local Web server and the HTTP port 80. Again, for incoming traffic, these are going to be commonly used to filter your traffic. You can list all your proposed incoming traffic this way (see Table 2-2).
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