Download IPTables Rules for Firewall Configuration and more Exams Computer Security in PDF only on Docsity!
Cork Institute of Technology
Bachelor of Science (Honours) in Software Development and Computer
Networking – Award
(NFQ Level 8)
January 2007
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 stateful firewall?
Use a suitable example to help illustrate your answer. [9 Marks]
b) Write IPTABLES rules to do each of the following:
i) limit ICMP packets bound for the firewall to 3 per minute.
ii) allow tcp packets for host 192.168.10.10 and for destination port 80 through the
firewall.
iii) allows inbound connections to the telnet port on the firewall, but only if they
originate from a host with MAC address 00:C7:8D:A6:
iv) drops all packets which were started with the scp command, and also logs such
connection attempts with a suitable log message prefix. [16 Marks]
c) Explain, using an example, how iptables can be used to thwart SSH attacks.
[9 Marks]
d) Explain, using an example, how iptables can be used to implement
Source Network Address Translation. [6 Marks]
Question 2.
a) Outline the different types of malicious logic (virus, worm, trojan etc.) [8 Marks]
b) Detail the differences between a virus and a worm. [5 Marks]
c) Explain how polymorphic viruses operate. [9 Marks]
d) Explain what is meant by generic decryption. [7 Marks]
e) Outline how the Morris Worm operated. [11 Marks]
Question 3.
a) What are automated login scripts? [6 Marks]
b) What do you understand by the term rootkit? [8 Marks]
c) Discuss how anomalies can help us with intrusion detection. [9 Marks]
d) Compare Host intrusion detection with Network intrusion detection. [5 Marks]
e) Discuss the importance of strong passwords being used in an organisation, and how an
IT manager can help ensure that users use only strong passwords. [12 Marks]
Question 4.
a) Explain the differences between safety and security. [6 Marks]
b) Explain the objectives of ISO17799. [6 Marks]
c) Under each of the the (four) headings of P ersonnel Security, Communications and
Operations Management, Physical and Environmental Security and C ompliance , give four
examples of the types of audit questions that an ISO17799 auditor might pose.
[16 Marks]
d) Why is it important that an organisation have a security policy? [5 Marks]
e) Why might an organisation seek ISO17799 certification? [7 Marks]
(for packets being routed through the box), and OUTPUT (for locally-generated packets).
nat This table is consulted when a packet that creates a new connection is encountered. It consists of three built-ins: PREROUTING (for altering packets as soon as they come in), OUTPUT (for altering 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.
name will match.
-o, --out-interface [!] name Name of an interface via which a packet is going to be sent (for packets entering the FORWARD, OUTPUT and POSTROUTING chains). When the "!" argument is used before the interface name, the sense is inverted. If the interface name ends in a "+", then any interface which begins with this name will match. If this option is omitted, any interface name will match.
[!] -f, --fragment This means that the rule only refers to second and further fragments of fragmented packets. Since there is no way to tell the source or destination ports of such a packet (or ICMP type), such a packet will not match any rules which specify them. When the "!" argument precedes the "-f" flag, the rule will only match head fragments, or unfragmented packets.
-c, --set-counters PKTS BYTES 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)
--icmp-type [!] typename This allows specification of the ICMP type, which can be a numeric ICMP type, or one of the ICMP type names shown by the command iptables -p icmp -h
length This module matches the length of a packet against a spe- cific value or range of values.
--length length[:length]
limit This module matches at a limited rate using a token bucket filter. A rule using this extension will match until this limit is reached (unless the `!' flag is used). It can be used in combination with the LOG target to give limited logging, for example.
--limit rate Maximum average matching rate: specified as a num- ber, with an optional /second',/minute', /hour', or/day' suffix; the default is 3/hour.
--limit-burst number Maximum initial number of packets to match: this number gets recharged by one every time the limit specified above is not reached, up to this number; the default is 5.
mac --mac-source [!] address Match source MAC address. It must be of the form XX:XX:XX:XX:XX:XX. Note that this only makes sense for packets coming from an Ethernet device and entering the PREROUTING, FORWARD or INPUT chains.
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
Only match TCP packets with the SYN bit set and the ACK and RST bits cleared. Such packets are used to request TCP connection initiation; for example, blocking such packets coming in an interface will prevent incoming TCP connections, but outgoing TCP connections will be unaffected. It is equivalent to --tcp-flags SYN,RST,ACK SYN. If the"!" flag precedes the "--syn", the sense of the option is inverted.
--tcp-option [!] number Match if TCP option set.
--mss value[:value] Match TCP SYN or SYN/ACK packets with the specified MSS value (or range), which control the maximum packet size for that connection.
tos This module matches the 8 bits of Type of Service field in the IP header (ie. including the precedence bits).
--tos tos The argument is either a standard name, (use iptables -m tos -h to see the list), or a numeric value to match.
ttl This module matches the time to live field in the IP header.
--ttl ttl Matches the given TTL value.
udp These extensions are loaded if `--protocol udp' is speci- fied. It provides the following options:
--source-port [!] port[:port] Source port or port range specification. See the description of the --source-port option of the TCP extension for details.
--destination-port [!] port[:port] Destination port or port range specification. See the description of the --destination-port option of the TCP extension for details.
unclean This module takes no options, but attempts to match pack- ets which seem malformed or unusual. This is regarded as experimental.
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)).
address). It takes one option:
--to-ports port[-port] This specifies a destination port or range of ports to use: without this, the destination port is never altered. This is only valid if the rule also spec- ifies -p tcp or -p udp.
REJECT
This is used to send back an error packet in response to the matched packet: otherwise it is equivalent to DROP so it is a terminating TARGET, ending rule traversal. This target is only valid in the INPUT, FORWARD and OUTPUT chains, and user-defined chains which are only called from those chains. The following option controls the nature of the error packet returned:
--reject-with type The type given can be icmp-net-unreachable, icmp- host-unreachable, icmp-port-unreachable, icmp- proto-unreachable, icmp-net-prohibited or icmp- host-prohibited, which return the appropriate ICMP error message (port-unreachable is the default). The option tcp-reset can be used on rules which only match the TCP protocol: this causes a TCP RST packet to be sent back. This is mainly useful for blocking ident (113/tcp) probes which frequently occur when sending mail to broken mail hosts (which won't accept your mail otherwise). SNAT This target is only valid in the nat table, in the POSTROUTING chain. It specifies that the source address of the packet should be modified (and all future packets in this connection will also be mangled), and rules should cease being examined. It takes one type of option:
--to-source ipaddr[-ipaddr][:port-port] which can specify a single new source 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 source ports below 512 will be mapped to other ports below 512: those between 512 and 1023 inclusive will be mapped to ports below 1024, and other ports will be mapped to 1024 or above. Where possible, no port alteration will occur.
You can add several --to-source options. If you specify more than one source address, either via an address range or multiple --to-source options, a simple round-robin (one after another in cycle) takes place between these adresses.
TCPMSS This target allows to alter the MSS value of TCP SYN pack- ets, to control the maximum size for that connection (usu-
ally limiting it to your outgoing interface's MTU minus 40). Of course, it can only be used in conjunction with -p tcp. This target is used to overcome criminally braindead ISPs or servers which block ICMP Fragmentation Needed packets. The symptoms of this problem are that everything works fine from your Linux firewall/router, but machines behind it can never exchange large packets:
- Web browsers connect, then hang with no data received.
- 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
