Mysql Developing And Programming, Personal Web Hosting Blog

Chapter 6 . Securing Linux 221 # ping www.example.com PING www.example.com (192.0.34.166) from 10.0.0.11: 56(84) bytes of data 64 bytes from 192.0.34.166: icmp_seq=1 ttl=49 time=40.1 ms 64 bytes from 192.0.34.166: icmp_seq=2 ttl=49 time=42.5 ms 64 bytes from 192.0.34.166: icmp_seq=3 ttl=49 time=39.5 ms 64 bytes from 192.0.34.166: icmp_seq=4 ttl=49 time=38.4 ms 64 bytes from 192.0.34.166: icmp_seq=5 ttl=49 time=39.0 ms — www.example.com ping statistics — 5 packets transmitted, 5 received, 0% loss, time 4035ms rtt min/avg/max/mdev = 38.472/39.971/42.584/1.432 ms In the preceding example, the average time for a ping packet to make the roundtrip was about 39 thousandths of a second. A ping to a nearly saturated link looks like the following: # ping www.example.com PING www.example.com (192.0.34.166): from 10.0.0.11: 56(84)bytes of data 64 bytes from 192.0.34.166: icmp_seq=1 ttl=62 time=1252 ms 64 bytes from 192.0.34.166: icmp_seq=2 ttl=62 time=1218 ms 64 bytes from 192.0.34.166: icmp_seq=3 ttl=62 time=1290 ms 64 bytes from 192.0.34.166: icmp_seq=4 ttl=62 time=1288 ms 64 bytes from 192.0.34.166: icmp_seq=5 ttl=62 time=1241 ms — www.example.com ping statistics — 5 packets transmitted, 5 received, 0% loss, time 5032ms rtt min/avg/max/mdev = 1218.059/1258.384/1290.861/28.000 ms In this example, a ping packet took, on average, 1.3 seconds to make the roundtrip. From the first example to the second example, latency increased by a factor of 31! A data link that goes from working normally to slowing down by a factor of 31 is a clear sign that link utilization should be investigated. For a more accurate measure of data throughput, a tool such as ttcp can be used. To test your connection with ttcp you must have installed the ttcp package on machines inside and outside of your network. (The ttcp package is available with Fedora Core and other Linux systems.) If you are not sure whether the package is installed, simply type ttcp at a command prompt. You should see something like the following: # ttcp Usage: ttcp -t [-options] host [ < in ] ttcp -r [-options > out] Common options: -l ## length of bufs read from or written to network (default 8192) -u use UDP instead of TCP -p ## port number to send to or listen at (default 5001) -s -t: source a pattern to network -r: sink (discard) all data from network -A align the start of buffers to this modulus (default 16384) -O start buffers at this offset from the modulus (default 0)
Visit our web design programs services for an affordable and reliable webhost to suit all your needs.

220 Part II . Running the Show subnet instead of a single host. The return address is forged to be your machine s address instead of the actual sender. When the ICMP packet arrives at the unwitting relay s network, every host on that subnet replies to the ping! Furthermore, they reply to your computer instead of to the actual sender. If the relay s network has hundreds of computers, your Internet connection can be quickly flooded. The best fix is to contact the organization being used as a relay and inform them of the abuse. Usually, they need only to reconfigure their Internet router to stop any future attacks. If the organization is uncooperative, you can minimize the effect of the attack by blocking the ICMP protocol on your router. This will at least keep the traffic off your internal network. If you can convince your ISP to block ICMP packets aimed at your network, it will help even more. (Note that there is some debate about whether or not blocking ICMP packets is a good idea, since ICMP services can be useful for various administrative purposes.) Protecting Against Distributed DOS Attacks DDOS attacks are much harder to initiate and extremely difficult to stop. A DDOS attack begins with the penetration of hundreds or even thousands of weakly secured machines. These machines can then be directed to attack a single host based on the whims of the attacker. With the advent of DSL and cable modem, millions of people are enjoying Internet access with virtually no speed restrictions. In their rush to get online, many of those people neglect even the most basic security. Since the vast majority of these people run Microsoft operating systems, they tend to get hit with worms and viruses rather quickly. After the machine has been infiltrated, quite often the worm or virus installs a program on the victim s machine that instructs it to quietly call home and announce that it is now ready to do the master s bidding. At the whim of the master, the infected machines can now be used to focus a concentrated stream of garbage data at a selected host. In concert with thousands of other infected machines, a script kiddie now has the power to take down nearly any site on the Internet. Detecting a DDOS is similar to detecting a DOS attack. One or more of the following signs are likely to be present: . Sustained saturated data link . No reduction in link saturation during off-peak hours . Hundreds or even thousands of simultaneous network connections . Extremely slow system performance To determine if your data link is saturated, the act of pinging an outside host can tell much of the story. Much higher than usual latency is a dead giveaway. Normal ping latency (that is, the time it takes for a ping response to come back from a remote host) looks like the following:
You want to have a cheap webhost for your apache application, then check apache web hosting services.

Chapter 6 . Securing Linux 219 Spam Relaying Your e-mail services can also be abused is by having your system used as a spam relay. Spam refers to the unsolicited junk e-mail that has become a common occurrence on the Internet. Relay refers to the mail server feature that causes it to send mail it receives to another server. (Normally, only users with valid e-mail accounts on the server are allowed to use a mail server to relay messages in their behalf. A mail server configured as an open relay will allow anyone to forward e-mail messages through it and is, therefore, considered to be a very bad practice.) Spammers often deliver their annoying messages from a normal dial-up Internet account. They need some kind of high-capacity e-mail server to accept and buffer the payload of messages. They deliver the spam to the server all in one huge batch and then log off, letting the server do the work of delivering the messages to the many victims. Naturally, no self-respecting Internet service provider will cooperate with this action, so spammers resort to hijacking servers at another ISP to do the dirty work. Having your mailserver hijacked to act as a spam relay can have a devastating effect on your system and your reputation. Fortunately, open mail relaying is deactivated by default on Fedora and Red Hat Linux installations. Open mail relaying is one security issue that you will not have to worry about. You can allow specific hosts or domains to relay mail through your system by adding those senders to your /etc/mail/access file with keyword RELAY. By default, relaying is allowed from the local host only. One package you might consider using to filter out spam on your mail server is SpamAssassin. SpamAssassin examines the text of incoming mail messages and attempts to filter out messages that are determined to be spam. SpamAssassin is described in Chapter 25. Smurf Amplification Attack Smurfing refers to a particular type of denial of service attack aimed at flooding your Internet connection. It can be a difficult attack to defend against because it is not easy to trace the attack to the attacker. Here is how smurfing works. The attack makes use of the ICMP protocol, a service intended for checking the speed and availability of network connections. Using the ping command, you can send a network packet from your computer to another computer on the Internet. The remote computer will recognize the packet as an ICMP request and echo a reply packet to your computer. Your computer can then print a message revealing that the remote system is up and telling you how long it took to reply to the ping. A smurfing attack uses a malformed ICMP request to bury your computer in network traffic. The attacker does this by bouncing a ping request off an unwitting third party in such a way that the reply is duplicated dozens or even hundreds of times. An organization with a fast Internet connection and a large number of computers is used as the relay. The destination address of the ping is set to an entire Tip
From our experience, we can recommend PHP Web Hosting services, if you need affordable webhost to host and run your web application.

218 Part II . Running the Show the mailbomber s e-mail address or system name to the access file located in the /etc/mail directory. Each line of the access file contains an e-mail address, host name, domain, or IP address followed by a tab and then a keyword specifying what action to take when that entity sends you a message. Valid keywords are OK, RELAY, REJECT, DISCARD, and ERROR. Using the REJECT keyword will cause a sender s e-mail to be bounced back with an error message. The keyword DISCARD will cause the message to be silently dropped without sending an error back. You can even return a custom error message by using the ERROR keyword. Thus, an example /etc/mail/access file may look similar to this: # Check the /usr/share/doc/sendmail/README.cf file for a description # of the format of this file. (search for access_db in that file) # The /usr/share/doc/sendmail/README.cf is part of the sendmail-doc # package. # # by default we allow relaying from localhost… localhost.localdomain RELAY localhost RELAY 127.0.0.1 RELAY # # Senders we want to Block # evilmailer@yahoo.com REJECT stimpy.glaci.com REJECT cyberpromo.com DISCARD 199.170.176.99 ERROR: 550 Die Spammer Scum! 199.170.177 ERROR: 550 Email Refused As with most Linux configuration files, lines that begin with a pound (#) sign are comments. The list of blocked spammers is at the end of this example file. Note that the address to block can be a complete e-mail address, a full host name, a domain only, an IP address, or a subnet. To block a particular e-mail address or host from mailbombing you, log in to your system as root, edit the /etc/mail/access file, and add a line to DISCARD mail from the offending sender. After saving the file and exiting the editor, you must convert the access file into a hash-indexed database called access.db. The database is updated automatically the next time sendmail starts. On Fedora and other Red Hat systems, you can convert the database immediately, as follows: # cd /etc/mail # make Sendmail should now discard e-mail from the addresses you added.
We would like to recommend you tested and proved virtual web hosting services, which you will surely find to be of great quality.

Chapter 6 . Securing Linux 217 A tool called nmap is generally considered the best way to scan a host for services (note that nmap is a tool that can be used for good and evil). Once the attacker has a list of the available services running on his target, he needs to find a way to trick one of those services into letting him have privileged access to the system. Usually, this is done with a program called an exploit. While DOS attacks are disruptive, intrusion type attacks are the most damaging. The reasons are varied, but the result is always the same. An uninvited guest is now taking up residence on your machine and is using it in a way you have no control over. Protecting Against Denial of Service Attacks As explained earlier, a denial of service attack attempts to crash your computer or at least degrade its performance to an unusable level. There are a variety of denial of service exploits. Most try to overload some system resource, such as your available disk space or your Internet connection. Some common attacks and defenses are discussed in the following sections. Mailbombing Mailbombing is the practice of sending so much e-mail to a particular user or system that the computer s hard drive becomes full. There are several ways to protect yourself from mailbombing. You can use the Procmail e-mail-filtering tool or, if you are using sendmail as your mail transport agent, configure your sendmail daemon. Blocking Mail with Procmail The Procmail e-mail-filtering tool, installed by default with Fedora, RHEL, and many other Linux systems, is tightly integrated with the sendmail e-mail daemon; thus, it can be used to selectively block or filter out specific types of e-mail. You can learn more about Procmail at the Procmail Web site: www.procmail.org. To enable Procmail for your user account, create a .procmailrc file in your home directory. The file should be mode 0600 (readable by you but nobody else). Type the following, replacing evilmailer with the actual e-mail address that is mailbombing you. # Delete mail from evilmailer :0 * ^From.*evilmailer /dev/null The Procmail recipe looks for the From line at the start of each e-mail to see if it includes the string evilmailer. If it does, the message is sent to /dev/null (effectively throwing it away). Blocking Mail with sendmail The Procmail e-mail tool works quite well when only one user is being mailbombed. If, however, the mailbombing affects many users, you should probably configure your sendmail daemon to block all e-mail from the mailbomber. Do this by adding
Please visit Domain Name Hosting services for high quality webhost to host and run your jsp applications.

216 Part II . Running the Show a long list of subnets or domains, but better a few minutes spent on proper security measures than many hours recovering from a break-in. For Linux systems that use the xinetd service, you can further restrict access to services using various options within the /etc/xinetd.conf file, even to the point of limiting access to certain services to specific times of the day. Read the manual page for xinetd (by typing man xinetd at a command prompt) to learn more about these options. Understanding Attack Techniques Attacks on computing systems take on different forms, depending on the goal and resources of the attacker. Some attackers want to be disruptive, while others want to infiltrate your machines and utilize the resources for their own nefarious purposes. Still others are targeting your data for financial gain or blackmail. Here are three major categories of attacks: . Denial of Service (DOS) The easiest attacks to perpetrate are Denial of Service attacks. The primary purpose of these attacks is to disrupt the activities of a remote site by overloading it with irrelevant data. DOS attacks can be as simple as sending thousands of page requests per second at a Web site. These types of attacks are easy to perpetrate and easy to protect against. Once you have a handle on where the attack is coming from, a simple phone call to the perpetrator s ISP will get the problem solved. . Distributed Denial of Service (DDOS) More advanced DOS attacks are called distributed denial of service attacks. DDOS attacks are much harder to perpetrate and nearly impossible to stop. In this form of attack, an attacker takes control of hundreds or even thousands of weakly secured Internet connected computers. The attacker then directs them in unison to send a stream of irrelevant data to a single Internet host. The result is that the power of one attacker is magnified thousands of times. Instead of an attack coming from one direction, as is the case in a normal DOS, it comes from thousands of directions at once. The best defense against a DDOS attack is to contact your ISP to see if it can filter traffic at its border routers. Many people use the excuse, I have nothing on my machine anyone would want to avoid having to consider security. The problem with this argument is that attackers have a lot of reasons to use your machine. The attacker can turn your machine into an agent for later use in a DDOS attack. More than once, authorities have shown up at the door of a dumbfounded computer user asking questions about threats originating from their computer. By ignoring security, the owners have opened themselves up to a great deal of liability. . Intrusion attacks To remotely use the resources of a target machine, attackers must first look for an opening to exploit. In the absence of inside information such as passwords or encryption keys, they must scan the target machine to see what services are offered. Perhaps one of the services is weakly secured and the attacker can use some known exploit to finagle his or her way in. Tip
We recommend you use shared web hosting services, because many users agree that it is cheap, reliable and customer-satisfying webhost.

Chapter 6 . Securing Linux 215 . A string that begins with an at (@) sign is treated as an NIS netgroup name. A host name is matched if it is a host member of the specified netgroup. Netgroup matches are not supported for daemon process names or for client user names. . An expression of the form n.n.n.n/m.m.m.m is interpreted as a net/mask pair. A host address is matched if net is equal to the bitwise and of the address and the mask. For example, the net/mask pattern 131.155.72.0/255.255.254.0 matches every address in the range 131.155.72.0 through 131.155.73.255. The example host.allow contains the first two types of client specification. The entry 199.170.177. will match any IP address that begins with that string, such as 199.170.177.25. The client entry .linuxtoys.net will match host names such as jukebox.linuxtoys.net or picframe.linuxtoys.net. Let s examine what happens when a host named jukebox.linuxtoys.net (with IP address 199.170.179.18) connects to your Linux system using the Telnet protocol. In this case, the Linux system is Fedora, which uses the xinetd daemon to listen for service requests associated with TCP wrappers: 1. xinetd receives the connection request. 2. xinetd begins comparing the address and name of jukebox.linuxtoys.net to the rules listed in /etc/hosts.allow. It starts at the top of the file and works its way down the file until finding a match. Both the daemon (the program handling the network service on your Fedora box) and the connecting client s IP address or name must match the information in the hosts.allow file. In this case, the second rule that is encountered matches the request: in.telnetd: 199.170.177., .linuxtoys.net 3. The jukebox host is not in the 199.170.177 subnet, but it is in the linuxtoys.net domain. xinetd stops searching the file as soon as it finds this match. How about if jukebox connects to your box using the CUPS-lpd protocol? In this case, it matches none of the rules in hosts.allow; the only line that refers to the lpd daemon does not refer to the 199.170.179 subnet or to the linuxtoys.net domain. xinetd continues on to the hosts.deny file. The entry ALL: ALL matches anything, so tcpd denies the connection. The ALL wildcard was also used in the hosts.allow file. In this case, we are telling xinetd to permit absolutely any host to connect to the FTP service on the Linux box. This is appropriate for running an anonymous FTP server that anyone on the Internet can access. If you are not running an anonymous FTP site, you probably should not use the ALL flag. A good rule of thumb is to make your hosts.allow and hosts.deny files as restrictive as possible and then explicitly enable only those services that you really need. Also, grant access only to those systems that really need access. Using the ALL flag to grant universal access to a particular service may be easier than typing
Check Tomcat Web Hosting services for best quality webspace to host your web application.

214 Part II . Running the Show It is not necessary (or even possible) to list every single address that may try to connect to your computer. The hosts.allow and hosts.deny files enable you to specify entire subnets and groups of addresses. You can even use the keyword ALL to specify all possible addresses. You can also restrict specific entries in these files so they apply only to specific network services. Look at an example of a typical pair of hosts.allow and hosts.deny files. Here s the /etc/hosts.allow file: # # hosts.allow This file describes the names of the hosts are # allowed to use the local INET services, as decided # by the /usr/sbin/tcpd server. # cups-lpd: 199.170.177. in.telnetd: 199.170.177., .linuxtoys.net vsftpd: ALL Here s the /etc/hosts.deny file: # # hosts.deny This file describes names of the hosts which are # *not* allowed to use the local INET services, as # decided by the /usr/sbin/tcpd server. # ALL: ALL The preceding example is a rather restrictive configuration. It allows connections to the cups-lpd and telnet services from certain hosts, but then denies all other connections. It also allows connections to the FTP service (vsftp) to all hosts. Let s examine the files in detail. As usual, lines beginning with a # character are comments and are ignored by xinetd or inetd when it parses the file. Each noncomment line consists of a commaseparated list of daemons followed by a colon (:) character and then a comma-separated list of client addresses to check. In this context, a client is any computer that attempts to access a network service on your system. A client entry can be a numeric IP address (such as 199.170.177.25) or a host name (such as jukebox.linuxtoys.net), but is more often a wildcard variation that specifies an entire range of addresses. A client entry can take four different forms. The online manual page for the hosts.allow file describes them as follows: . A string that begins with a dot (.) character. A host name is matched if the last components of its name match the specified pattern. For example, the pattern .tue.nl matches the host name wzv.win.tue.nl. . A string that ends with a dot (.) character. A host address is matched if its first numeric fields match the given string. For example, the pattern 131.155. matches the address of (almost) every host on the Eindhoven University network (131.155.x.x).
Searching for affordable and reliable webhost to host and run your web applications? Go to our java web server services and you will be pleased.

Chapter 6 . Securing Linux 213 It is important to note that once you have this working, it will work regardless of how many times the IP address changes on your local computer. The IP address has nothing to do with this form of authentication. Securing Linux Servers Opening up your Linux system as a server on a public network creates a whole new set of challenges when it comes to security. Instead of just turning away nearly all incoming requests, your computer will be expected to respond to requests for supported services (such as Web, FTP, or mail service) by supplying information or possibly running scripts that take in data. Entire books have been filled with information on how to go about securing your servers. Many businesses that rely on Internet servers assign full-time administrators to watch over the security of their servers. So, think of this section as an overview of some of the kinds of attacks to look out for and some tools available to secure your Linux server. Controlling Access to Services with TCP Wrappers Completely disabling an unused service is fine, but what about the services that you really need? How can you selectively grant and deny access to these services? For Linux systems that incorporate TCP wrapper support, the /etc/hosts.allow and /etc/hosts.deny files determine when a particular connection should be granted or refused for services such as rlogin, rsh, telnet, finger, and talk. Most Linux systems that implement TCP wrappers do so for a set of services that are monitored by a single listening process called the Internet super server. For Red Hat systems, that server is the xinetd daemon, while in other systems (such as Debian) the inetd daemon is used. When a service that relies on TCP wrappers is requested from the server process, the hosts.allow and hosts.deny files are scanned and checked for an entry that matches the IP address of the connecting machine. These checks are made when connection attempts occur: . If the address is listed in the hosts.allow file, the connection is allowed and hosts.deny is not checked. . If the address is in hosts.deny, the connection is denied. . If the address is in neither file, the connection is allowed. Keep in mind that the order in which hosts are evaluated is important. For example, you cannot deny access to a host in the hosts.deny file that has already been given access in the hosts.allow file.
Visit our web design programs services for an affordable and reliable webhost to suit all your needs.

212 Part II . Running the Show Using ssh, scp, and sftp Without Passwords For machines that you use a great deal (particularly machines behind a firewall on your LAN), it is often helpful to set them up so that you do not have to use a password to log in. The following procedure shows you how to do that. These steps take you through setting up password-less authentication from one machine to another. In this example, the local user is named chester on a computer named host1. The remote user is also chester on a computer named host2. 1. Log in to the local computer (in this example, I log in as chester to host1). Run Step 2 only once as local user on your local workstation. Do not run it again unless you lose your ssh keys. When configuring subsequent remote servers, skip right to Step 3. 2. Type the following to generate the ssh key: $ ssh-keygen -t dsa 3. Accept the defaults by pressing Enter at each request. 4. Type the following to copy the key to the remote server (replace chester with the remote username and host2 with the remote host name): $ cd ~/.ssh $ scp id_dsa.pub chester@host2:/tmp chester@host2 s password: ******* 5. Type the following to add the ssh key to the remote user s authorization keys (the code should be on one line, not wrapped): $ ssh chester@host2 cat /tmp/id_dsa.pub >> /home/chester/.ssh/authorized_keys2 In the previous two steps you are asked for passwords. This is okay. For the sshd daemon to accept the authorized_keys2 file you created, your home directories and that file itself must have secure permissions. To secure that file and those directories, type the following: $ ssh chester@host2 chmod go-w $HOME $HOME/.ssh $ ssh chester@host2 chmod 600 $HOME/.ssh/authorized_keys2 6. Type the following to remove the key from the temporary directory: $ ssh chester@host2 rm /tmp/id_dsa.pub Note You should not be asked for a password in the previous step. Note Note
From our experience, we are can tell you that you can find a reliable and cheap webhost service at Java Web Hosting services.