Study with the several resources on Docsity
Earn points by helping other students or get them with a premium plan
Prepare for your exams
Study with the several resources on Docsity
Earn points to download
Earn points by helping other students or get them with a premium plan
Main points of this past exam are: Generic Decryption, Malicious Logic, Virus, Polymorphic Virus, Compression Operates, Heuristic-Based Generic Decryption, Morris Worm Operated, Connection Requests, Telnet Port, User-Defined Chains
Typology: Exams
1 / 19
This page cannot be seen from the preview
Don't miss anything!
Instructions:
Examiners: Dr. J. Buckley Dr. A. Kinsella Mr. V. Ryan
iptables - administration tool for IPv4 packet filtering and NAT
iptables [-t table] -[ADC] chain rule-specification [options] iptables [-t table] -I chain [rulenum] rule-specification [options] iptables [-t table] -R chain rulenum rule-specification [options] iptables [-t table] -D chain r ulenum [options] iptables [-t table] -[LFZ] [chain] [options] iptables [-t table] -N chain iptables [-t table] -X [chain] iptables [-t table] -P chain target [options] iptables [-t table] -E old-chain-name new-chain-name
Iptables is used to set up, maintain, and inspect the tables of IP packet filter rules in the Linux kernel. Several different tables may be defined. Each table contains a number of built-in chains and may also contain user-defined chains.
Each chain is a list of rules which can match a set of packets. Each rule specifies what to do with a packet that matches. This is called a `target', which may be a jump to a user-defined chain in the same table.
A firewall rule specifies criteria for a packet, and a target. If the packet does not match, the next rule in the chain is the examined; if it does match, then the next rule is specified by the value of the target, which can be the name of a user-defined chain or one of the special values ACCEPT, DROP, QUEUE, or RETURN.
ACCEPT means to let the packet through. DROP means to drop the packet on the floor. QUEUE means to pass the packet to userspace (if supported by the kernel). RETURN means stop traversing this chain and resume at the next rule in the previous (calling) chain. If the end of a built-in chain is reached or a rule in a built-in chain with target RETURN is matched, the target specified by the chain policy determines the fate of the packet.
TABLES There are currently three independent tables (which tables are present at any time depends on the kernel configuration 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).
The options that are recognized by iptables can be divided into several different groups.
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.
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 and is taken as default when this optionis omit- ted.
-s, --source [!] address[/mask] Source specification. Address can be either a net- work name, a hostname (please note that specifying any name to be resolved with a remote query such as DNS is a really bad idea), a network IP address (with /mask), or a plain IP address. The mask can be either a network mask or a plain number, speci- fying the number of 1's at the left side of the network mask. Thus, a mask of 24 is equivalent to 255.255.255.0. A "!" argument before the address specification inverts the sense of the address. The flag --src is an alias for this option.
-d, --destination [!] address[/mask] Destination specification. See the description of the -s (source) flag for a detailed description of the syntax. The flag --dst is an alias for this option.
-j, --jump target 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 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).
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
--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 converted into it's according numeric value.
esp This module matches the SPIs in ESP header of IPSec pack- 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...]]
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 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.
iptables can use extended target modules: the following are included in the standard distribution.
--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
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.
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.
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-
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).
