Principles of Network and System Administration 2nd phần 8 pps

11.7. PREVENTING AND MINIMIZING FAILURE MODES 441 Traditionally backups have been made to tape, since tape is relatively cheap and mobile. This is still the case at many sites, particularly larger ones; but tapes usually need to be dealt with manually, by a human or by an expensive robot. This adds a price tag to tape-backup which smaller institutions can find difficult to manage. By way of contrast, the price of disks and networking has fallen dramatically. For an organization with few resources, a cheap solution to the backup problem is to mirror disks across a network [244], using well-known tools like rdump, rdist or cfengine. This solves the problems of redundancy and location; and, for what it costs to employ a human or tape robot, one can purchase quite a lot of disk space. Another change is the development of fast, reliable media like CD-ROM. In earlier times, it was normal to backup the operating system partitions of hosts to tape. Today that practice is largely unnecessary: the operating system is readily available on some straightforward medium (e.g. CD-ROM or DVD) which is at least as fast as a tape streamer and consumes a fraction of the space. It is only necessary to make backups of whatever special configuration files have been modified locally. Sites which use cfengine can simply allow cfengine to reconstruct local modifications after an OS installation. In any event, if we have followed the principle of separating the operating system from local modifications, this is no problem at all. Similar remarks can be made about other software. Commercial software is now sold on CD-ROM and is trivial to reinstall (remember to keep a backup of license keys). For freely available software, there are already many copies and mirrors at remote locations by virtue of the Internet. For convenience, a local source repository can also be kept, to speed up recovery in the case of an accident. In the unlikely event of every host being destroyed simultaneously, downloading the software again from the network is the least of your worries! Reconstructing a system from source rather than from backup has never been easier than now. Moreover, a policy of not backing up software which is easily accessible from source, can make a considerable saving in the volume of backup space required, at the price of more work in the event of accident. In the end this is a matter of policy. It should be clear that user-data must have maximum priority for backup. This is where local creativity manifests itself; these are the data which form your assets. 11.7.2 Loss of service Loss of service might be less permanent than the loss of data, but it can be just as debilitating. Downtime costs money for businesses and wastes valuable time in academia. The basic source of all computing power is electricity. Loss of electrical power can be protected against, to a limited extent, with an un-interruptible power supply (UPS). This is not an infallible security, but it helps to avoid problems due to short breaks in the power. UPS solutions use a battery backup to keep the power going for a few hours when power has failed. When the battery begins to run down, they can signal the host so as to take it down in a controlled fashion, thus minimizing 442 CHAPTER 11. PRINCIPLES OF SECURITY damage to disks and data. Investing in a UPS for an important server could be the best thing one ever does. Electrical spike protectors are another important accessory for anyone living in a region where lightning strikes are frequent, or where the power supply is of variable quality. No fuse will protect a computer from a surge of electricity: microelectronics burn out much quicker than any fuse. Service can also be interrupted by a breach of the network infrastructure: a failed router or broken cable, or even a blown fuse. It can be interrupted by cleaning staff, or carelessness. A backup or stand-by replacement is the only option for hardware failure. It helps to have the telephone number of those responsible for network hardware when physical breaches occur. Software can be abused in a denial of service attack. Denial of service attacks are usually initiated by sending information to a host which confuses it into inactivity. There are as many variations on this theme as there are vandals on the network. Some attacks exploit bugs, while others are simply spamming episodes, repeatedly sending a deluge of service requests to the host, so that it spends all of its resources on handling the attack. 11.7.3 Protocols What is the solution to uncertainty? An amount of uncertainty is inevitable in any complex system. Where humans are concerned, uncertainty is always significant. A strict mode of behavior is the usual way of counteracting this uncertainty. Protocols are ways of eliminating unnecessary uncertainty by reducing the freedom of the participants. Principle 61 (Protocols offer predictability). A well-designed protocol, either for human behavior or machine behavior, standardizes behavior and offers pre- dictability. 11.7.4 Authentication In order to provide basic security for individuals, we need to keep track of the identity of users who make requests of the system. Authentication means determining whether the claim of identity is authentic. Usually we mean verifying somebody’s identity. There are two reasons for authenticating users: • User-based access control of files and programs requires users to be distin- guished by an identity. • Accountability: attaching actions to users for recording in logs. All authentication is based on the idea of comparing unique attributes of indi- viduals with some database. Often ownership of a shared secret is used for this purpose, such as a password or encryption key, known only to the individual and the authenticator. There is much confusion surrounding authentication. Much of this stems from the many claims made by cryptographic methods to provide secure methods for authenticating user identities. While this is not incorrect, it misses a crucial point. 11.7. PREVENTING AND MINIMIZING FAILURE MODES 443 Principle 62 (Identification requires trust). Establishing identity is ‘impossi- ble’. Identification requires an initial introduction, based on trust. Corollary to principle (Authentication is re-identification). Authentication is the confirmation of a previously trusted identity. The first time we meet a person or contact a host on a network, we know nothing about them. When a previously unknown person or host claims their identity we must accept this information on trust. No matter how many detailed measurements we make (DNA test, processor serial number, secure exchange of keys etc.), there is no basis for matching those identifying marks to the identity claimed – since we cannot mind-read, we simply have to trust it. Once an initial identity has been accepted as true, one can then use unique properties to identify the individual again in the future, in a variety of ways, some more secure than others. The special markers or unique properties can only confirm that a person or host is the same person or host as we met previously. If the original introduction was faked, the accuracy of recognition cannot detect this. Password login The provision of a username claims our identity and a password verifies that claim. If this authentication succeeds, we are granted access to the system, and all of our activities then occur within the scope of an identifier which represents that user. On Unix-like systems, the username is converted into a global unique user-id number (UID). On Windows systems, the username is converted into a security-id (SID) which is only unique on a local host. There are obvious problems with password authentication: passwords can be guessed and they can be leaked. Users with only weak passwords are vulnerable to dictionary and other brute-force attacks. This type of login is called unilateral authentication, that is, it identifies the user to the computer. It does not verify the identity of the computer to the user. Thus a malicious party could fake a login dialogue on a computer, using this to collect passwords and account information. Unix does not attempt to solve this problem, but NT and its successors provide a ‘secure attention sequence’. If the user types CTRL+ALT+DEL, they are guaranteed to be directed to the operating system, rather than any user programs which might be trying to look like the OS. Authentication types The OSI security architecture (ISO 7498-2) makes a distinction between different kinds of authentication: • Entity authentication: checking the identity of an individual or entity. • Origin authentication: checking the location of an individual or entity. • Unilateral authentication: verifying the entity to the authenticator. • Mutual authentication: verifying both parties to one another. 444 CHAPTER 11. PRINCIPLES OF SECURITY Authentication is usually performed at the start of a session between client and system. Once one stops checking, an attacker could subsequently sneak in and change places with an authenticated user. Thus to ensure security in an on-going conversation, we have to verify identity and then use some kind of secret key to ensure that the identity cannot be changed, e.g. by encrypting the conversation. The key is only known by the authenticated parties, such as a secret that has been exchanged. Challenge response protocols Consider two parties A and B, who need to open a dialogue and verify a previously trusted identity. || |M1 | | > | |M2 | A | < | B |M3 | | > | |M4 | | < | || A starts the protocol by sending a message to B, M1. B replies with M2, etc. We assume that message N + 1 is not sent until message N has been received and understood. During or after the exchange of the messages we need to be sure of the following: • That the messages were received (unaltered) from the hosts which were supposed to send them. • That the messages are fresh, i.e. not replays of old messages. • That message N + 1 is a correct reply to message N , not a misleading reply to a different question. The first of these assurances can be made by using cryptographic checksums (message digests such as MD5 or SHA-1) or Message Authentication Code (MAC) that verifies both the identity of the sender and the integrity of the message, using a cryptographic key. The second could be assured by the use of a time-stamp, though this would be vulnerable to errors of clock synchronization. A better approach is to use a random challenge or nonce (from the medieval English for ‘once only’). A nonce is usually a long random number that is encrypted with a key that can only be decrypted by the receiver. The receiver then replies to the sender of the nonce by decrypting it and sending it back. Only the keeper of the secret could do this, and thus this confirms the identity of the receiver as well as the freshness of the reply. To achieve a mutual authentication, both parties send challenges to one another. 11.8. SOME WELL-KNOWN ATTACKS 445 11.7.5 Integrity Trust is the pernicious problem of security. How are we able to trust files and data which others send? Programs that we download could contain viruses or Trojan horses. Assuming that we trust the person who wrote the program, how can we be sure that no one else has tampered with it in between? There are some things we can do to increase our confidence in data we receive from a foreign source. One is to compare message digests. Message digests or hashes are cryptographic checksums which quickly sum- marize the contents of a file. The idea is to create an algorithm which digests the contents of a file and produces a single value which uniquely summarizes its contents. If we change one bit of a file, then the value of the message digest also changes. Popular algorithms include: MD4 MD5 (Stronger than md4) SHA1 host$ md5 .cshrc MD5 (.cshrc) = 519ab7d30dba4a2d16b86328e025ec72 MD5 signatures are often quoted at security software repositories so that it is possible to verify the authenticity of software (assuming the MD5 signature is authentic!). 11.8 Some well-known attacks There are many ways to attack a networked computer in order to gain access to it, or simply disable it. Some well-known examples are listed below. The actual attack mechanisms used by attackers are often intricate and ingenious, but the common theme in all of them is to exploit naive limitations in the way network services are implemented. Time and again one sees crackers make use of software systems which were written in good faith, by forcing them into unnatural situations where the software fails through inadequate checking. 11.8.1 Ping attacks The RFC 791 specifies that Internet datagrams shall not exceed 64kB. Some implementations of the protocol can send packets which are larger than this, but not all implementations can receive them. ping -s 65510 targethost Some older network interfaces can be made to crash certain operating systems by sending them a ‘ping’ request like this with a very large packet size. Most modern operating systems are now immune to this problem (e.g. NT 3.51 is vulnerable, but NT 4 is not). If not, it can be combatted with a packet filtering router. See http://www.sophist.demon.co.uk/ping/. 446 CHAPTER 11. PRINCIPLES OF SECURITY 11.8.2 Denial of service (DoS) attacks Another type of attack is to overload a system with so many service requests that it grinds to a halt. One example is mail spamming, 2 in which an attacker sends large numbers of repetitive E-mail messages, filling up the server’s disk and causing the sendmail daemon to spawn rapidly and slow the system to a standstill. Denial of service attacks are almost impossible to protect against. It is the responsibility of local administrators to prevent their users from initiating such attacks wherever possible. 11.8.3 TCP/IP spoofing Most network resources are protected on the basis of the host IP addresses of those resources. Access is granted by a server to a client if the IP address is contained in an access control list (ACL). Since the operating system kernel itself declares its own identity when packets are sent, it has not been common to verify whether packets actually do arrive from the hosts which they claim to arrive from. Ordinary users have not traditionally had access to privileges which allow them to alter network protocols. Today everyone can run a PC with privileged access to the networking hardware. Normally an IP datagram passing from host A to host B has a destination address ‘host B’ and source address ‘host A’ (see figure 11.4). IP spoofing is the act of forging IP datagrams in such a way that they appear to come from a third party host, i.e. an attacker at host A creates a packet with destination address ‘host B’ and source address ‘host C’. The reasons for this are various. Sometimes an attacker wants to appear to be host C in order to gain access to a special resource which host C has privileged access to. Another reason might be to attack host C, as part of a more elaborate attack. Usually it is not quite this simple however, since the forgery is quickly detected. The TCP handshake is such that host A sends a packet to host B and then replies to the source address with a sequence number which has to match the next number of an agreed sequence. If another packet is not received with an agreed sequence number the connection will be reset and abandoned. Indeed, if host C received the confirmation reply for a message which it never sent, it would send a reset signal back immediately, saying effectively ‘I know nothing about this’. To prevent this from happening it is common to take out host C first by attacking it with some kind of Denial of Service method, or simply choosing an address which is not used by any host. This prevents it from sending a reset message. The advantage of choosing a real host C is that the blame for the attack is placed on host C. 11.8.4 SYN flooding IP spoofing can also be used as a denial of service attack. By choosing an address for host C which is not in use so that it cannot reply with a reset, host A can send SYN packets (new connections) on the same and other ports repeatedly. The 2 From the Monty Python song ‘Spam spam spam spam ’. 11.8. SOME WELL-KNOWN ATTACKS 447 host C host A host B Figure 11.4: IP spoofing. A third party host C assumes the role of host A. RECV queue quickly fills up and cannot be emptied since the connections cannot be completed. Because the queues are filled the services are effectively cut off. These attacks could be prevented if routers can be configured so as to disallow packets with forged source addresses. 11.8.5 TCP sequence guessing This attack allows an attacker to make a TCP connection to a host by guessing the initial TCP sequence number used by the other end of the connection. This is a form of IP spoofing by a man in the middle. The attack was made famous by the break in to Tsutomo Shinomura’s computers which led to the arrest of Kevin Mitnick. This attack is used to impersonate other hosts for trusted access [29, 220]. This approach can now be combatted by using random initial sequence numbers (using the strategy expounded in section 7.7.5), though many operating systems require special configuration to enable such measures. 11.8.6 IP/UDP fragmentation (Teardrop) A Teardrop attack was responsible for the now famous twelve-hour attack which ‘blue-screened’ thousands of NT machines all over the world. This attack uses the idea of datagram fragmentation. Fragmentation is something which happens as a datagram passes through a router from one network to another network where the Minimum Transfer Unit (MTU) is lower. Large packets can be split up into smaller packets for more efficient network performance. In a Teardrop attack, the attacker forges two UDP datagrams which appear to be fragments of a larger packet, but with data offsets which overlap. When fragmentation occurs it is always the end host which reassembles the packets. In order to allocate memory for the data, the kernel calculates the difference between the end of the datagram and the offset at which the datagram fragment started. In a normal situation that would look like that in figure 11.5. In a Teardrop attack the packets are forged so that they overlap, as shown in figure 11.6. The assumption that the next fragment would follow on from the 448 CHAPTER 11. PRINCIPLES OF SECURITY 0 100 200 size = 100 − 0 size = 200 − 100 UDP frag#1 UDP frag#2 Figure 11.5: Normal UDP fragmentation. 0 100 200 UDP frag#1 UDP frag#2 size = 120 − 0 size = 90 − 120 Figure 11.6: Spoofed UDP fragmentation, generates a negative size. previous one leads to a negative number for the size of the fragment. As the kernel tries to allocate memory for this it calls malloc( size ) where the size is now a negative number. The kernel panics and the system crashes on implementations which did not properly check the bounds. 11.8.7 ICMP flooding (Smurf) ICMP flooding is another denial of service attack. The ICMP protocol is the part of TCP/IP which is used to transmit error messages and control information between hosts. Well-known services like ping and echo use ICMP. Normally all hosts respond to ping and echo requests without question, since they are useful for debugging. In an ICMP flooding attack, the attacker sends a spoofed ICMP packet to the broadcast address of a large network. The source address of the packet is forged so that it appears to come from the host which the attacker wishes to attack. Every host on the large network receives the ping/echo request and replies to the same host simultaneously. The host is then flooded with requests. The requests consume all the system resources. 11.8.8 DNS cache poisoning This attack is an example of the exploitation of a trusted service in order to gain access to a foreign host. Again it uses a common theme, that of forging a network service request. This time, however, the idea is to ask a server to cache some information which is incorrect so that future look-ups will result in incorrect information being given instead of the correct information [29]. DNS is a hierarchical service which attempts to answer queries about IP names and addresses locally. If a local server does not have the information requested it EXERCISES 449 asks an authoritative server for that information. Having received the information from the authoritative server it caches it locally to avoid having to contact the other server again; after all, since the information was required once, it is likely that the same information will be required again soon. The information is thus placed in the cache for a period of time called the TTL (Time To Live). After that time has expired it has to be obtained again from the authoritative server. In a cache poisoning attack, the aim is to insert incorrect information into the cache of a server. Once it is there it will be there for the TTL period. In order to arrange this an attacker does the following. 1. The attacker launches his/her attack from the authoritative nameserver for his/her network. This gives him/her the chance to send information to another nameserver which will be trusted. 2. The attacker sends a query for the IP address of the victim host to the victim’s default DNS server in order to obtain a DNS query ID. This provides a point of reference for guessing, i.e. forging, the next few query IDs from that server. 3. The attacker then sends a query asking for the address of a host which the victim machine trusts, i.e. the host which the attacker would like to impersonate. 4. The attacker hopes that the victim host will soon need to look up the IP address of the host it trusts; he/she sends a fake ‘reply’ to such a DNS lookup request, forged with the query ID to look as though it comes from a lookup of the trusted host’s address. The answer for the IP address of the trusted host is altered so that it is the IP address of the attacker’s host. 5. Later when the victim host actually sends such a DNS request it finds that it has already received a UDP reply to that request (this is the nature of UDP) and it ignores the real reply because it arrives later. Now the victim’s DNS cache has been poisoned. 6. The attacker now attempts to connect directly to the victim host, posing as the trusted host. The victim host tries to verify the IP address of the host by looking up the address in its DNS server. This now responds from its cache with the forged address. 7. The attacker’s system is accepted. This kind of attack requires the notion of external login based on trust, e.g. with Unix .rhosts files. This doesn’t help with NT because NT doesn’t have trusted hosts in the same way. On the other hand, NT is much easier to gain access to through NULL sessions. Exercises Self-test objectives 1. Describe the nature of possible threats to the security of a human–computer system. 450 CHAPTER 11. PRINCIPLES OF SECURITY 2. What is meant by ‘security is a property of systems’? 3. What are the four main themes in computer security? 4. What role does trust play in setting the ground rules for security? 5. Explain how security relates to risk assessment. 6. What are the main threats to human–computer security? 7. Who present the main threats to human–computer security? 8. What is ISO17799? 9. What is RFC 2196? 10. What is meant by social engineering? 11. List some ways of countering social engineering. 12. What is meant by a honey pot? 13. What is meant by a sacrificial lamb? 14. What are the pros and cons of system homogeneity in security? 15. Explain how laptops and mobile devices can compromise security. 16. What are the problems with the security of the Internet Protocol? 17. State the ways of minimizing the likelihood of a serious security breach. 18. How does economy play a role in security? 19. What is the point of strict protocols in human–computer systems? 20. Explain why it is not possible to ever really identify someone – only to re- identify someone whose identity we have already trusted. 21. What is mutual authentication? 22. What is a challenge–response system? 23. What is meant by a nonce? 24. What is a cryptographic hash or checksum? 25. What is a message authentication code? 26. What is meant by a Denial of Service (DoS) attack? 27. What is meant by cache poisoning? [...]... security in the context of a computer system Now we apply the basic principles and consider what practical steps can be taken to provide a basic level of security 12.1 System design and normalization Security is a property of systems; to address security, we must speak of the system as a whole: • Identify what assets we are trying to protect • Evaluate the main sources of risk and where trust is placed... Output Each of these must be addressed and poses different threats Input exposes a part of the system to its outside environment Output exposes the environment to the system part The rules determine whether the part fulfills its role in the system Security of a system requires the safe and predictable functioning of all 454 CHAPTER 12 SECURITY IMPLEMENTATION parts within a system, and the safe and predictable... found in refs [310, 1 58, 169, 241, 1 48, 234, 335, 2 18, 257, 213] A forwardlooking backup scheme with a broad generality in its ability to use different services and devices for remote backups is described in ref [ 284 ] and backup to optical disks is discussed in refs [65, 320] Automated tape backup and restore is discussed in ref [ 184 ] and in the Amanda system [ 283 ]; the AFS backup system is discussed... Who is 1 28. 39 .89 .230, takpeh ? SMTP R port=4552 ETHER Type=0000 (LLC /80 2.3), size = 86 bytes ETHER Type=0000 (LLC /80 2.3), size = 1 28 bytes ETHER Type=0000 (LLC /80 2.3), size = 80 bytes One way to avoid the problem of password sniffing is to use fully encrypted links such as ssh [332] and SSL (Secure Socket Layer) enabled services which replace the standard services Another is to use a system of one-time... Counter-measures can be both preventative and reactive They consist of: • Rules • Codified responses The foundation of security is policy We must agree on what is valuable and acceptable in the system Without such an assessment, we cannot speak of the risk to those assets, and determine what level of risk is acceptable Policy is decided by social groups A system consists of an assembly of parts that exhibit three... not an assurance of security 12.4.6 Kerberos Another protocol for establishing identity and exchanging a session key was devised in 19 78 by R Needham and M Schroeder It uses the idea of a trusted third party or key-broker and uses symmetric encryption keys to pass messages, and forms the backbone of the Kerberos system In practice, the Needham–Schroeder protocol simulates the idea of public keys by... computer 12.3 Data integrity and protection As part of any infrastructure plan, we need to apply the principles of redundancy and protection to the system s data Although backup copies will not protect us against loss, they do provide minimal insurance against accidents, intentional 12.3 DATA INTEGRITY AND PROTECTION 455 damage and natural disasters, and make the business of recovery less painful There... On Unix-like systems there is a variety of tools which can be used to copy files and filesystems Backup Restore cp -ar tar cf GNU tar zcf dd cpio dump ufsdump rdump NTBackup cp -ar tar xpf tar zxpf dd cpio restore restore rrestore NTBackup Of course, commercial backup solutions exist for all operating systems, but they are often costly On both Unix and Windows, it is possible to backup filesystems either... time and attention Robotic automation can help here The level of redundancy which is appropriate for a given site has to be a question of economics based on four factors: 1 The cost of the backup (in time and media) 2 The expected rate of loss 3 The rate of data production 4 Media reliability These factors vary for different kinds of data, so the calculation needs to be thought out for each filesystem... breach or some other form of vandalism, then it is prudent to consider other security measures at the same time as we reconstruct the system from the pieces 12.3.4 Checksum or hash verification Every time we use the privileged system account, we are at risk of installing a virus or a Trojan horse, or of editing the contents of important files which define system security The list of ingenious ploys for tricking . the system. Security of a system requires the safe and predictable functioning of all 454 CHAPTER 12. SECURITY IMPLEMENTATION parts within a system, and the safe and predictable functioning of. in ref. [ 284 ] and backup to optical disks is discussed in refs. [65, 320]. Automated tape backup and restore is discussed in ref. [ 184 ] and in the Amanda system [ 283 ]; the AFS backup system is. in the context of a computer system. Now we apply the basic principles and consider what practical steps can be taken to provide a basic level of security. 12.1 System design and normalization Security