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Cork Institute of Technology
Bachelor of Science in Software Development & Computer Networking - Award
Autumn 2006
Computer Security
(Time: 2 Hours)
Instructions:
Answer any three questions.
Each Question is worth 40 Marks.
Note that 120 Marks = 100%.
Examiners: Dr. J. Buckley Dr. A. Kinsella Mr. V. Ryan
Question 1.
a) Explain what is meant by the term NAT?
Use a suitable example to help illustrate your answer.
[8 Marks]
b) Write IPTABLES rules to do each of the following:
i) limit ICMP packets to 3 per minute;
ii) filter incoming packets by MAC-address, allowing only those packets from MAC-
address 01:A7:4E:35:23 ;
iii) limits the rate of writing log messages to 2 per second;
iv) drops all packets from IP_address 156.191.241.76.
[20 Marks]
c) Using the hitcount and recent facilities in IPTABLES, limit the number of login
attempts using SSH (port 22) from the same IP Address to 4 per minute. You will need two
IPTABLES rules to achieve this. In the rules that you use, explain the significance of every
item in the rule.
[12 Marks]
Question 2.
a) Outline the different types of malicious logic (virus, worm, trojan etc.) [8 Marks]
b) Explain how compression viruses typically operate. [5 Marks]
c) What signs might you look for to indicate that a computer is infected by viruses.
[9 Marks]
d) What do you understand by the term virus signature? [8 Marks]
e) Outline how the program that was used in the Morris Worm. [10 Marks]
Question 3.
a) What are rootkits?
[5 Marks]
b) How can we best protect against rootkits?
[6 Marks]
c) Outline the differences between a HIDS and a NIDS.
[7 Marks]
d) Explain how the UNIX password system works, both at password creation and login.
[10 Marks]
d) Explain in detail the purpose of the Spafford-Bloom Filter , and exactly how it works.
[12 Marks]
Question 4.
a) Explain what is meant by defence in depth.
[13 Marks]
b) List the 10 sections in ISO17799.
[7 Marks]
c) Under each of the the (three) headings of personnel security,
physical and environmental security and compliance , give examples of the types of
questions that an ISO17799 auditor might pose.
[16 Marks]
d) Why is it important that an organisation have a security policy?
[6 Marks]
e) Why might an organisation seek ISO17799 certification?
[8 Marks]
locally-generated packets before routing), and POSTROUTING (for altering packets as they are about to go out).
mangle This table is used for specialized packet alter- ation. Until kernel 2.4.17 it had two built-in chains: PREROUTING (for altering incoming packets before routing) and OUTPUT (for altering locally- generated packets before routing). Since kernel 2.4.18, three other built-in chains are also sup- ported: INPUT (for packets coming into the box itself), FORWARD (for altering packets being routed through the box), and POSTROUTING (for altering packets as they are about to go out).
OPTIONS
The options that are recognized by iptables can be divided into several different groups.
COMMANDS
These options specify the specific action to perform. Only one of them can be specified on the command line unless otherwise specified below. For all the long ver- sions of the command and option names, you need to use only enough letters to ensure that iptables can differen- tiate it from all other options.
-A, --append chain rule-specification Append one or more rules to the end of the selected chain. When the source and/or destination names resolve to more than one address, a rule will be added for each possible address combination.
-D, --delete chain rule-specification -D, --delete chain rulenum Delete one or more rules from the selected chain. There are two versions of this command: the rule can be specified as a number in the chain (starting at 1 for the first rule) or a rule to match.
-I, --insert chain [rulenum] rule-specification Insert one or more rules in the selected chain as the given rule number. So, if the rule number is 1, the rule or rules are inserted at the head of the chain. This is also the default if no rule number is specified.
-R, --replace chain rulenum rule-specification Replace a rule in the selected chain. If the source and/or destination names resolve to multiple addresses, the command will fail. Rules are num- bered starting at 1.
-L, --list [chain] List all rules in the selected chain. If no chain is selected, all chains are listed. As every other iptables command, it applies to the specified table (filter is the default), so NAT rules get listed by iptables -t nat -n -L
Please note that it is often used with the -n option, in order to avoid long reverse DNS lookups. It is legal to specify the -Z (zero) option as well, in which case the chain(s) will be atomically listed and zeroed. The exact output is affected by the other arguments given. The exact rules are sup- pressed until you use iptables -L -v
-F, --flush [chain] Flush the selected chain (all the chains in the table if none is given). This is equivalent to deleting all the rules one by one.
-Z, --zero [chain] Zero the packet and byte counters in all chains. It is legal to specify the -L, --list (list) option as well, to see the counters immediately before they are cleared. (See above.)
-N, --new-chain chain Create a new user-defined chain by the given name. There must be no target of that name already.
-X, --delete-chain [chain] Delete the optional user-defined chain specified. There must be no references to the chain. If there are, you must delete or replace the referring rules before the chain can be deleted. If no argument is given, it will attempt to delete every non-builtin chain in the table.
-P, --policy chain target Set the policy for the chain to the given target. See the section TARGETS for the legal targets. Only built-in (non-user-defined) chains can have policies, and neither built-in nor user-defined chains can be policy targets.
-E, --rename-chain old-chain new-chain Rename the user specified chain to the user sup- plied name. This is cosmetic, and has no effect on the structure of the table.
-h Help. Give a (currently very brief) description of the command syntax.
PARAMETERS
The following parameters make up a rule specification (as used in the add, delete, insert, replace and append com- mands).
-p, --protocol [!] protocol The protocol of the rule or of the packet to check. The specified protocol can be one of tcp, udp, icmp, or all, or it can be a numeric value, repre- senting one of these protocols or a different one. A protocol name from /etc/protocolsis also allowed. A "!" argument before the protocol inverts the test. The number zero is equivalent to all. Protocol all will match with all protocols
This enables the administrator to initialize the packet and byte counters of a rule (during INSERT, APPEND, REPLACE operations).
OTHER OPTIONS
The following additional options can be specified:
-v, --verbose Verbose output. This option makes the list command show the interface name, the rule options (if any), and the OS masks. The packet and byte counters are also listed, with the suffix 'K', 'M' or 'G' for 1000, 1,000,000 and 1,000,000,000 multipliers respectively (but see the -x flag to change this). For appending, insertion, deletion and replacement, this causes detailed information on the rule or rules to be printed.
-n, --numeric Numeric output. IP addresses and port numbers will be printed in numeric format. By default, the pro- gram will try to display them as host names, net- work names, or services (whenever applicable).
-x, --exact Expand numbers. Display the exact value of the packet and byte counters, instead of only the rounded number in K's (multiples of 1000) M's (mul- tiples of 1000K) or G's (multiples of 1000M). This option is only relevant for the -L command.
--line-numbers When listing rules, add line numbers to the begin- ning of each rule, corresponding to that rule's position in the chain.
--modprobe=command When adding or inserting rules into a chain, use command to load any necessary modules (targets, match extensions, etc).
MATCH EXTENSIONS
iptables can use extended packet matching modules. These are loaded in two ways: implicitly, when -p or --protocol is specified, or with the -m or --match options, followed by the matching module name; after these, various extra command line options become available, depending on the specific module. You can specify multiple extended match modules in one line, and you can use the -h or --help options after the module has been specified to receive help specific to that module.
The following are included in the base package, and most of these can be preceded by a! to invert the sense of the match.
ah
This module matches the SPIs in AH header of IPSec pack- ets.
--ahspi [!] spi[:spi]
conntrack This module, when combined with connection tracking, allows access to more connection tracking information than the "state" match. (this module is present only if ipta- bles was compiled under a kernel supporting this feature)
--ctstate state Where state is a comma separated list of the connection states to match. Possible states are INVALID meaning that the packet is associated with no known connection, ESTABLISHED meaning that the packet is associated with a connection which has seen packets in both directions, NEW meaning that the packet has started a new connection, or other- wise associated with a connection which has not seen packets in both directions, and RELATED mean- ing that the packet is starting a new connection, but is associated with an existing connection, such as an FTP data transfer, or an ICMP error. SNAT A virtual state, matching if the original source address differs from the reply destination. DNAT A virtual state, matching if the original destination differs from the reply source.
--ctproto proto Protocol to match (by number or name)
--ctorigsrc [!] address[/mask] Match against original source address
--ctorigdst [!] address[/mask] Match against original destination address
--ctreplsrc [!] address[/mask] Match against reply source address
--ctrepldst [!] address[/mask] Match against reply destination address
--ctstatus [NONE|EXPECTED|SEEN_REPLY|ASSURED][,...] Match against internal conntrack states
--ctexpire time[:time] Match remaining lifetime in seconds against given value or range of values (inclusive)
dscp This module matches the 6 bit DSCP field within the TOS field in the IP header. DSCP has superseded TOS within the IETF.
--dscp value Match against a numeric (decimal or hex) value [0-32].
--dscp-class DiffServ Class Match the DiffServ class. This value may be any of the BE, EF, AFxx or CSx classes. It will then be
mark This module matches the netfilter mark field associated with a packet (which can be set using the MARK target below).
--mark value[/mask] Matches packets with the given unsigned mark value (if a mask is specified, this is logically ANDed with the mask before the comparison).
multiport This module matches a set of source or destination ports. Up to 15 ports can be specified. It can only be used in conjunction with -p tcp or -p udp.
--source-ports port[,port[,port...]] Match if the source port is one of the given ports. The flag --sports is a convenient alias for this option.
--destination-ports port[,port[,port...]] Match if the destination port is one of the given ports. The flag --dports is a convenient alias for this option.
--ports port[,port[,port...]] Match if the both the source and destination ports are equal to each other and to one of the given ports.
owner This module attempts to match various characteristics of the packet creator, for locally-generated packets. It is only valid in the OUTPUT chain, and even this some packets (such as ICMP ping responses) may have no owner, and hence never match.
--uid-owner userid Matches if the packet was created by a process with the given effective user id.
--gid-owner groupid Matches if the packet was created by a process with the given effective group id.
--pid-owner processid Matches if the packet was created by a process with the given process id.
--sid-owner sessionid Matches if the packet was created by a process in the given session group.
--cmd-owner name Matches if the packet was created by a process with the given command name. (this option is present only if iptables was compiled under a kernel sup- porting this feature)
physdev This module matches on the bridge port input and output Devices enslaved to a bridge device. This is only useful if the input device or output device is a bridge device.
This module is a part of the infrastructure that enables a transparent bridging IP firewall and is only useful for kernel versions above version 2.5.44.
--physdev-in name Name of a bridge port via which a packet is received (only for packets entering the INPUT, FOR- WARD and PREROUTING chains). If the interface name ends in a "+", then any interface which begins with this name will match.
--physdev-out name Name of a bridge port via which a packet is going to be sent (for packets entering the FORWARD, OUT- PUT and POSTROUTING chains). If the interface name ends in a "+", then any interface which begins with this name will match. Note that in the nat and man- gle OUTPUT chains one cannot match on the bridge output port, however one can in the filter OUTPUT chain.
pkttype This module matches the link-layer packet type.
--pkt-type [unicast|broadcast|multicast]
state This module, when combined with connection tracking, allows access to the connection tracking state for this packet. --state state Where state is a comma separated list of the con- nection states to match. Possible states are INVALID meaning that the packet is associated with no known connection, ESTABLISHED meaning that the packet is associated with a connection which has seen packets in both directions, NEW meaning that the packet has started a new connection, or other- wise associated with a connection which has not seen packets in both directions, and RELATED mean- ing that the packet is starting a new connection, but is associated with an existing connection, such as an FTP data transfer, or an ICMP error. tcp These extensions are loaded if `--protocol tcp' is speci- fied. It provides the following options:
--source-port [!] port[:port] Source port or port range specification. This can either be a service name or a port number. An inclusive range can also be specified, using the format port:port. If the first port is omitted, "0" is assumed; if the last is omitted, "65535" is assumed. If the second port greater then the first they will be swapped. The flag --sport is a conve- nient alias for this option.
--destination-port [!] port[:port] Destination port or port range specification. The flag --dport is a convenient alias for this option.
--tcp-flags [!] mask comp Match when the TCP flags are as specified. The
TARGET EXTENSIONS
iptables can use extended target modules: the following are included in the standard distribution.
DNAT This target is only valid in the nat table, in the PRE- ROUTING and OUTPUT chains, and user-defined chains which are only called from those chains. It specifies that the destination address of the packet should be modified (and all future packets in this connection will also be man- gled), and rules should cease being examined. It takes one type of option:
--to-destination ipaddr[-ipaddr][:port-port] which can specify a single new destination IP address, an inclusive range of IP addresses, and optionally, a port range (which is only valid if the rule also specifies -p tcp or -p udp). If no port range is specified, then the destination port will never be modified.
You can add several --to-destination options. If you specify more than one destination address, either via an address range or multiple --to-destination options, a simple round-robin (one after another in cycle) load balancing takes place between these adresses.
DSCP This target allows to alter the value of the DSCP bits within the TOS header of the IPv4 packet. As this manipu- lates a packet, it can only be used in the mangle table.
--set-dscp value Set the DSCP field to a numerical value (can be decimal or hex)
--set-dscp-class class Set the DSCP field to a DiffServ class.
ECN This target allows to selectively work around known ECN blackholes. It can only be used in the mangle table.
--ecn-tcp-remove Remove all ECN bits from the TCP header. Of course, it can only be used in conjunction with -p tcp.
LOG Turn on kernel logging of matching packets. When this option is set for a rule, the Linux kernel will print some information on all matching packets (like most IP header fields) via the kernel log (where it can be read with dmesg or syslogd(8)). This is a "non-terminating target", i.e. rule traversal continues at the next rule. So if you want to LOG the packets you refuse, use two separate rules with the same matching criteria, first using target LOG then DROP (or REJECT).
--log-level level
Level of logging (numeric or see syslog.conf(5)).
--log-prefix prefix Prefix log messages with the specified prefix; up to 29 letters long, and useful for distinguishing messages in the logs.
--log-tcp-sequence Log TCP sequence numbers. This is a security risk if the log is readable by users.
--log-tcp-options Log options from the TCP packet header.
--log-ip-options Log options from the IP packet header.
MARK This is used to set the netfilter mark value associated with the packet. It is only valid in the mangle table. It can for example be used in conjunction with iproute2.
--set-mark mark
MASQUERADE This target is only valid in the nat table, in the POSTROUTING chain. It should only be used with dynami- cally assigned IP (dialup) connections: if you have a static IP address, you should use the SNAT target. Mas- querading is equivalent to specifying a mapping to the IP address of the interface the packet is going out, but also has the effect that connections are forgotten when the interface goes down. This is the correct behavior when the next dialup is unlikely to have the same interface address (and hence any established connections are lost anyway). It takes one option:
--to-ports port[-port] This specifies a range of source ports to use, overriding the default SNAT source port-selection heuristics (see above). This is only valid if the rule also specifies -p tcp or -p udp.
MIRROR This is an experimental demonstration target which inverts the source and destination fields in the IP header and retransmits the packet. It is only valid in the INPUT, FORWARD and PREROUTING chains, and user-defined chains which are only called from those chains. Note that the outgoing packets are NOT seen by any packet filtering chains, connection tracking or NAT, to avoid loops and other problems.
REDIRECT This target is only valid in the nat table, in the PRE- ROUTING and OUTPUT chains, and user-defined chains which are only called from those chains. It alters the destina- tion IP address to send the packet to the machine itself (locally-generated packets are mapped to the 127.0.0. address). It takes one option:
--to-ports port[-port] This specifies a destination port or range of ports
- Small mail works fine, but large emails hang.
- ssh works fine, but scp hangs after initial handshaking. Workaround: activate this option and add a rule to your firewall configuration like: iptables -A FORWARD -p tcp --tcp-flags SYN,RST SYN
-j TCPMSS --clamp-mss-to-pmtu
--set-mss value Explicitly set MSS option to specified value.
--clamp-mss-to-pmtu Automatically clamp MSS value to (path_MTU - 40).
These options are mutually exclusive.
TOS This is used to set the 8-bit Type of Service field in the IP header. It is only valid in the mangle table.
--set-tos tos You can use a numeric TOS values, or use iptables -j TOS -h to see the list of valid TOS names.
ULOG
This target provides userspace logging of matching pack- ets. When this target is set for a rule, the Linux kernel will multicast this packet through a netlink socket. One or more userspace processes may then subscribe to various multicast groups and receive the packets. Like LOG, this is a "non-terminating target", i.e. rule traversal contin- ues at the next rule.
--ulog-nlgroup nlgroup This specifies the netlink group (1-32) to which the packet is sent. Default value is 1.
--ulog-prefix prefix Prefix log messages with the specified prefix; up to 32 characters long, and useful for distinguish- ing messages in the logs.
--ulog-cprange size Number of bytes to be copied to userspace. A value of 0 always copies the entire packet, regardless of its size. Default is 0.
--ulog-qthreshold size Number of packet to queue inside kernel. Setting this value to, e.g. 10 accumulates ten packets inside the kernel and transmits them as one netlink multipart message to userspace. Default is 1 (for backwards compatibility).
