Download IPTables Rule Configuration for Linux Kernel Packet Filtering and more Exams Cryptography and System Security in PDF only on Docsity!
Cork Institute of Technology
Bachelor of Science (Honours) in Computer Services
Management – Award
National Framework of Qualifications – Level 8
January 2005
Security and Cryptography
(Time: 3 Hours)
Answer any four questions. Examiners : Dr. D. Chambers
Mr. P. O' Connor Mr. T. Parslow Mr. V. Ryan
Question 1
a) Outline the UNIX find command, and explain its use from a security
viewpoint.
[2 Marks]
Explain why a security manager might issue the following command
find / -type d (-perm -g+w -o -perm -o+w ) -exec ls -l {}
[3 Marks]
b) Detail how the UNIX password system works.
[5 Marks]
c) List and describe the UNIX Shell feature attacks.
[5 Marks]
d) What is setuid?
Why is setuid of concern from a security viewpoint?
How should programmers structure code to prevent setuid attacks?
[6 Marks]
e) Why is Linux not as susceptible to viruses as Windows is?
Give details.
[4 Marks]
Question 2
a) Discuss the importance of patching.
[3 Marks]
Discuss some of the tools available for patching.
What tools would apply to a systems administrator and what tools would
apply to a user?
[4 Marks]
What do you understand by the term defence in depth?
[4 Marks]
b) Give a definition for a honeypot.
[3 Marks]
Compare honeypots with other types of Intrusion Detection systems.
[3 Marks]
Distinguish between High-interaction and Low-interaction honeypots.
[4 Marks]
What is a sticky honeypot? Give details.
[4 Marks]
Question 3
a) What is the difference between a virus and a worm?
Enhance your answer using an example.
[4 Marks]
b) Outline how the Morris Internet worm operated.
[6 Marks]
c) How does a typical polymorphic virus operate?
[5 Marks]
d) What is generic decryption?
[3 Marks]
How does it work?
[5 Marks]
What design decisions need to be made when setting up generic decryption?
Question 5
a) Using public-key cryptography, how does one sign a document?
[3 Marks]
Using public-key cryptography, how does one verify a signature?
[2 Marks]
b) It is said that public-key cryptography addresses the key distribution problem.
What is this problem, and how does public-key cryptography address it?
[3 Marks]
c) Outline the key-generation phase of RSA.
[3 Marks]
Explain why the security of RSA depends on the difficulty of factoring a large
number.
[4 Marks]
d) What is the main advantage of Elliptic Curve Cryptography (ECC) over RSA?
[2 Marks]
e) Detail the X.509 standard.
[5 Marks]
f) In a typical PKI, what is a Certificate Revocation List?
[3 Marks]
Question 6
a) Outline the main types of firewall, as proposed in the 2002 NIST document.
[5 Marks]
b) Explain in detail the advantage of a stateful inspection firewall over a
packet filter. Use an example to enhance your explanation.
[5 Marks]
c) Explain what NAT is and its significance from a security viewpoint.
[3 Marks]
d) IpTables has 3 default tables. List these, and outline the purpose of each.
[3 Marks]
e) Write an IpTables rule which will allow all tcp packets from the device
which has MAC address 00:C7:9A:72:
[3 Marks]
f) Write an IpTables rule which will allow all TCP packets with the SYN
and ACK bits set.
[3 Marks]
g) Write an IpTables rule which will allow all UDP traffic destined for port
number 68.
[3 Marks]
are present at any time depends on the kernel configura- tion options and which modules are present).
-t, --table table This option specifies the packet matching table which the command should operate on. If the kernel is configured with automatic module loading, an attempt will be made to load the appropriate module for that table if it is not already there.
The tables are as follows:
filter This is the default table (if no -t option is passed). It contains the built-in chains INPUT (for packets coming into the box itself), FORWARD (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
This specifies the target of the rule; i.e., what to do if the packet matches it. The target can be a user-defined chain (other than the one this rule is in), one of the special builtin targets which decide the fate of the packet immediately, or an extension (see EXTENSIONS below). If this option is omitted in a rule, then matching the rule will have no effect on the packet's fate, but the coun- ters on the rule will be incremented.
-i, --in-interface [!] name Name of an interface via which a packet is going to be received (only for packets entering the INPUT, FORWARD and PREROUTING chains). When the "!" argu- ment 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.
-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 unfrag- mented 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 TOS 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
ets.
--espspi [!] spi[:spi]
helper This module matches packets related to a specific con- ntrack-helper.
--helper string Matches packets related to the specified conntrack- helper.
string can be "ftp" for packets related to a ftp-session on default port. For other ports append -portnr to the value, ie. "ftp-2121". Same rules apply for other conntrack-helpers.
icmp This extension is loaded if `--protocol icmp' is speci- fied. It provides the following option:
--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
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 first argument is the flags which we should exam- ine, written as a comma-separated list, and the second argument is a comma-separated list of flags which must be set. Flags are: SYN ACK FIN RST URG PSH ALL NONE. Hence the command iptables -A FORWARD -p tcp --tcp-flags SYN,ACK,FIN,RST SYN will only match packets with the SYN flag set, and the ACK, FIN and RST flags unset.
[!] --syn 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
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 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.
