Linux firewall, netfilter and iptables
1. Concepts
netfilter is a set of hooks inside the Linux kernel that allows kernel modules to register callback functions with the network stack. A registered callback function is then called back for every packet that traverses the respective hook within the network stack.
iptables is a userspace command line utility for configuring Linux kernel firewall implemented within the Netfilter project. The term iptables is also commonly used to refer to this kernel-level firewall. iptables is used for IPv4, and ip6tables is used for IPv6.
iptables is used to inspect, modify, redirect, and/or drop IPv4 packets. The code for filtering IPv4 packets is already built into the kernel and is organized into a collection of tables, each with a specific purpose. The tables are made up of a set of predefined chains, and the chains rules which are traversed in order. Each rule consists of a predicate of potential matches and a corresponding action (called a target) which is executed if the predicate is true; i.e. the conditions are matched.
iptables is the user utility which allows you to work with these chains/rules.
The key to understanding how iptables works is the chart above. The lowercase word on top is the table and the uppercase word below is the chain. Every IP packet that comes in on any network interface passes through this flow chart from top to bottom.
1.1. Tables
iptables contains five tables:
-
rawis used only for configuring packets so that they are exempt from connection tracking. -
filteris the default table, and is where all the action typically associated with a firwall take place. -
natis used for network address translation (e.g. port forwarding). -
mangleis used for specialized packet alterations. -
securityis used for Mandatory Access Control networking rules. (e.g. SELinux — see this article for more details).
In most common use cases you will only use two of these: filter and nat.
XXXXXXXXXXXXXXXXXX
XXX Network XXX
XXXXXXXXXXXXXXXXXX
+
|
v
+-------------+ +------------------+
|table: filter| <---+ | table: nat |
|chain: INPUT | | | chain: PREROUTING|
+-----+-------+ | +--------+---------+
| | |
v | v
[local process] | **************** +--------------+
| +---------+ Routing decision +------> |table: filter |
v **************** |chain: FORWARD|
**************** +------+-------+
Routing decision |
**************** |
| |
v **************** |
+-------------+ +------> Routing decision <---------------+
|table: nat | | ****************
|chain: OUTPUT| | +
+-----+-------+ | |
| | v
v | +-------------------+
+--------------+ | | table: nat |
|table: filter | +----+ | chain: POSTROUTING|
|chain: OUTPUT | +--------+----------+
+--------------+ |
v
XXXXXXXXXXXXXXXXXX
XXX Network XXX
XXXXXXXXXXXXXXXXXX1.2. Chains
Tables consist of chains, which are lists of rules which are followed in order.
The default table, filter, contains three bultin-chain: INPUT, OUTPUT and FORWARD which are actived at different points of the packet filtering process, as illustrated in the flow chart.
-
INPUT- This chain is used to control the behavior for incoming connections.For example, if a user attempts to SSH into your PC/Server, iptables will attempt to match the IP address and port to a rule in the input chain.
-
FORWARD- This chain is used for incoming connections that aren’t actually being delivered locally.Think of a router - data is always being sent to it but rarely actually destined for the router itself; the data is just forwarded to its target. Unless you’re doing some kind of routing, NATing, or something else your system that rquires forwarding, you won’t even use this chain.
-
OUTPUT- This chain is used for outgoing connections.For example, if you try to ping blog.codefarm.me, iptables will check output chain to see what the rules are regarding ping and blog.codefarm.me before making a decision to allow or deny the connection attempt.
The nat table incudes PREROUTING, POSTROUTING, and OUTPUT chains.
-
PREROUTING- Alters packets before routing. i.e Packet translation happens immediately after the packet comes to the system and before routing.This helps to translate the destination ip address of the packets to something that matches the routing on the local server. This is used for DNAT (destination NAT).
-
POSTROUTING- Alters packets after routing. i.e Packet translation happens when the packets are leaving the system.This helps to translate the source ip address of the packets to something that might match the routing on the desintation server. This is used for SNAT (source NAT).
-
OUTPUT- NAT for locally generated packets on the firewall.
1.3. Rules
Packet fitlering is based on rules, which are specified by multiple matches (condition the packet must satisfy so that the rule can be applied), and on target (action taken when the packet matches all conditions).
The typical things a rule might match on are what interface the packet came in on (e.g eth0 or eth1), what type of packet it is (ICMP, TCP, or UDP), or the desitination port of the packet.
Targets are specified using the -j or --jump option. Targets can be either user-defined chains, (i.e. if these conditions are matched, jump to the following user-defined chain and continue processing there), one of the special built-in targets, or a target extension.
-
If the target is a built-in target, the fate of the packet is decided immediately and processing of the packet in current table is stopped.
-
If the target is a user-defined chain and the fate of the packet is not decided by this second chain, it will be filtered against the remaining rules of the original chain.
-
Target extensions can be either terminating (as built-in targets) or non-terminating (as user-defined chains).
1.4. Policy
To see what the policy chains on the default table filter are currently configured to do with unmatched traffic, run the iptables -L command.
$ sudo iptables -L | grep policy
Chain INPUT (policy ACCEPT)
Chain FORWARD (policy ACCEPT)
Chain OUTPUT (policy ACCEPT)
$ sudo iptables -t nat -L | grep policy
Chain PREROUTING (policy ACCEPT)
Chain INPUT (policy ACCEPT)
Chain OUTPUT (policy ACCEPT)
Chain POSTROUTING (policy ACCEPT)As you can see, we also used the grep command to give use cleaner output. In that screenshot, our chains are currently figured to accpet traffic.
More times than not, you’ll want your system to accept connections by default. Unless you’ve changed the policy chain rules previously, this setting should already be configured. Either way, here’s the command to accept connections by default:
$ sudo iptables -P INPUT ACCEPT
$ sudo iptables -P OUTPUT ACCEPT
$ sudo iptables -P FORWARD ACCEPTBy default to the ACCEPT rule, you can then use iptables to deny specific IP addresses or port numbers, while continuing to accept all other connections.
If you would rather deny all connections manually specify which ones you want to allow to connect, you should change the default policy of your chains to DROP. Doing this probably only be useful for servers that contain sensitive information and only ever have the same IP addresses connect to them.
$ sudo iptables --policy INPUT DROP
$ sudo iptables --policy OUTPUT DROP
$ sudo iptables --policy FORWARD DROP2. Usage
IPTABLES(8) iptables 1.8.9 IPTABLES(8)
NAME
iptables/ip6tables — administration tool for IPv4/IPv6 packet filtering and NAT
SYNOPSIS
iptables [-t table] {-A|-C|-D|-V} chain rule-specification
ip6tables [-t table] {-A|-C|-D|-V} chain rule-specification
iptables [-t table] -I chain [rulenum] rule-specification
iptables [-t table] -R chain rulenum rule-specification
iptables [-t table] -D chain rulenum
iptables [-t table] -S [chain [rulenum]]
iptables [-t table] {-F|-L|-Z} [chain [rulenum]] [options...]
iptables [-t table] -N chain
iptables [-t table] -X [chain]
iptables [-t table] -P chain target
iptables [-t table] -E old-chain-name new-chain-name
rule-specification = [matches...] [target]
match = -m matchname [per-match-options]
target = -j targetname [per-target-options]
DESCRIPTION
Iptables and ip6tables are used to set up, maintain, and inspect the tables of
IPv4 and IPv6 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 speci‐
fies 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.
TARGETS
A firewall rule specifies criteria for a packet and a target. If the packet does
not match, the next rule in the chain is 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, one of the targets described in iptables-extensions(8), or
one of the special values ACCEPT, DROP or RETURN.
ACCEPT means to let the packet through. DROP means to drop the packet on the
floor. 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 five 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 destined to local sockets), 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 four built-ins: PREROUTING (for altering
packets as soon as they come in), INPUT (for altering packets destined
for local sockets), OUTPUT (for altering locally-generated packets be‐
fore routing), and POSTROUTING (for altering packets as they are about
to go out). IPv6 NAT support is available since kernel 3.7.
mangle:
This table is used for specialized packet alteration. 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 supported: INPUT (for packets coming into the box it‐
self), FORWARD (for altering packets being routed through the box),
and POSTROUTING (for altering packets as they are about to go out).
raw:
This table is used mainly for configuring exemptions from connection
tracking in combination with the NOTRACK target. It registers at the
netfilter hooks with higher priority and is thus called before ip_con‐
ntrack, or any other IP tables. It provides the following built-in
chains: PREROUTING (for packets arriving via any network interface)
OUTPUT (for packets generated by local processes)
security:
This table is used for Mandatory Access Control (MAC) networking
rules, such as those enabled by the SECMARK and CONNSECMARK targets.
Mandatory Access Control is implemented by Linux Security Modules such
as SELinux. The security table is called after the filter table, al‐
lowing any Discretionary Access Control (DAC) rules in the filter ta‐
ble to take effect before MAC rules. This table provides the follow‐
ing built-in chains: INPUT (for packets coming into the box itself),
OUTPUT (for altering locally-generated packets before routing), and
FORWARD (for altering packets being routed through the box).
OPTIONS
The options that are recognized by iptables and ip6tables can be divided into
several different groups.
COMMANDS
These options specify the desired action to perform. Only one of them can be
specified on the command line unless otherwise stated below. For long versions of
the command and option names, you need to use only enough letters to ensure that
iptables can differentiate 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.
-C, --check chain rule-specification
Check whether a rule matching the specification does exist in the selected
chain. This command uses the same logic as -D to find a matching entry,
but does not alter the existing iptables configuration and uses its exit
code to indicate success or failure.
-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
numbered starting at 1.
-L, --list [chain]
List all rules in the selected chain. If no chain is selected, all chains
are listed. Like 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 suppressed until you use
iptables -L -v
or iptables-save(8).
-S, --list-rules [chain]
Print all rules in the selected chain. If no chain is selected, all
chains are printed like iptables-save. Like every other iptables command,
it applies to the specified table (filter is the default).
-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 [rulenum]]
Zero the packet and byte counters in all chains, or only the given chain,
or only the given rule in a chain. 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 tar‐
get of that name already.
-X, --delete-chain [chain]
Delete the 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. The chain must be empty, i.e. not contain any rules. If
no argument is given, it will delete all empty chains in the table. Empty
builtin chains can only be deleted with iptables-nft.
-P, --policy chain target
Set the policy for the built-in (non-user-defined) chain to the given tar‐
get. The policy target must be either ACCEPT or DROP.
-E, --rename-chain old-chain new-chain
Rename the user specified chain to the user supplied name. This is cos‐
metic, 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 commands).
-4, --ipv4
This option has no effect in iptables and iptables-restore. If a rule us‐
ing the -4 option is inserted with (and only with) ip6tables-restore, it
will be silently ignored. Any other uses will throw an error. This option
allows IPv4 and IPv6 rules in a single rule file for use with both ipta‐
bles-restore and ip6tables-restore.
-6, --ipv6
If a rule using the -6 option is inserted with (and only with) iptables-
restore, it will be silently ignored. Any other uses will throw an error.
This option allows IPv4 and IPv6 rules in a single rule file for use with
both iptables-restore and ip6tables-restore. This option has no effect in
ip6tables and ip6tables-restore.
[!] -p, --protocol protocol
The protocol of the rule or of the packet to check. The specified proto‐
col can be one of tcp, udp, udplite, icmp, icmpv6,esp, ah, sctp, mh or the
special keyword "all", or it can be a numeric value, representing one of
these protocols or a different one. A protocol name from /etc/protocols
is also allowed. A "!" argument before the protocol inverts the test.
The number zero is equivalent to all. "all" will match with all protocols
and is taken as default when this option is omitted. Note that, in ip6ta‐
bles, IPv6 extension headers except esp are not allowed. esp and
ipv6-nonext can be used with Kernel version 2.6.11 or later. The number
zero is equivalent to all, which means that you cannot test the protocol
field for the value 0 directly. To match on a HBH header, even if it were
the last, you cannot use -p 0, but always need -m hbh.
[!] -s, --source address[/mask][,...]
Source specification. Address can be either a network name, a hostname, a
network IP address (with /mask), or a plain IP address. Hostnames will be
resolved once only, before the rule is submitted to the kernel. Please
note that specifying any name to be resolved with a remote query such as
DNS is a really bad idea. The mask can be either an ipv4 network mask
(for iptables) or a plain number, specifying the number of 1's at the left
side of the network mask. Thus, an iptables 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.
Multiple addresses can be specified, but this will expand to multiple
rules (when adding with -A), or will cause multiple rules to be deleted
(with -D).
[!] -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.
-m, --match match
Specifies a match to use, that is, an extension module that tests for a
specific property. The set of matches make up the condition under which a
target is invoked. Matches are evaluated first to last as specified on the
command line and work in short-circuit fashion, i.e. if one extension
yields false, evaluation will stop.
-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 (and -g is not used), then matching the
rule will have no effect on the packet's fate, but the counters on the
rule will be incremented.
-g, --goto chain
This specifies that the processing should continue in a user specified
chain. Unlike the --jump option return will not continue processing in
this chain but instead in the chain that called us via --jump.
[!] -i, --in-interface name
Name of an interface via which a packet was received (only for packets en‐
tering the INPUT, FORWARD and PREROUTING chains). When the "!" argument
is used before the interface name, the sense is inverted. If the inter‐
face 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 "!" argu‐
ment is used before the interface name, the sense is inverted. If the in‐
terface 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 IPv4 fragments
of fragmented packets. Since there is no way to tell the source or desti‐
nation 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.
This option is IPv4 specific, it is not available in ip6tables.
-c, --set-counters packets 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. -v may be
specified multiple times to possibly emit more detailed debug statements:
Specified twice, iptables-legacy will dump table info and entries in li‐
biptc, iptables-nft dumps rules in netlink (VM code) presentation. Speci‐
fied three times, iptables-nft will also dump any netlink messages sent to
kernel.
-V, --version
Show program version and the kernel API used.
-w, --wait [seconds]
Wait for the xtables lock. To prevent multiple instances of the program
from running concurrently, an attempt will be made to obtain an exclusive
lock at launch. By default, the program will exit if the lock cannot be
obtained. This option will make the program wait (indefinitely or for op‐
tional seconds) until the exclusive lock can be obtained.
-n, --numeric
Numeric output. IP addresses and port numbers will be printed in numeric
format. By default, the program will try to display them as host names,
network 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 (multi‐
ples 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 beginning of each rule, corre‐
sponding to that rule's position in the chain.
--modprobe=command
When adding or inserting rules into a chain, use command to load any nec‐
essary modules (targets, match extensions, etc).
LOCK FILE
iptables uses the /run/xtables.lock file to take an exclusive lock at launch.
The XTABLES_LOCKFILE environment variable can be used to override the default
setting.
MATCH AND TARGET EXTENSIONS
iptables can use extended packet matching and target modules. A list of these is
available in the iptables-extensions(8) manpage.
DIAGNOSTICS
Various error messages are printed to standard error. The exit code is 0 for
correct functioning. Errors which appear to be caused by invalid or abused com‐
mand line parameters cause an exit code of 2, and other errors cause an exit code
of 1.
BUGS
Bugs? What's this? ;-) Well, you might want to have a look at
http://bugzilla.netfilter.org/ iptables will exit immediately with an error code
of 111 if it finds that it was called as a setuid-to-root program. iptables can‐
not be used safely in this manner because it trusts the shared libraries
(matches, targets) loaded at run time, the search path can be set using environ‐
ment variables.
COMPATIBILITY WITH IPCHAINS
This iptables is very similar to ipchains by Rusty Russell. The main difference
is that the chains INPUT and OUTPUT are only traversed for packets coming into
the local host and originating from the local host respectively. Hence every
packet only passes through one of the three chains (except loopback traffic,
which involves both INPUT and OUTPUT chains); previously a forwarded packet would
pass through all three.
The other main difference is that -i refers to the input interface; -o refers to
the output interface, and both are available for packets entering the FORWARD
chain.
The various forms of NAT have been separated out; iptables is a pure packet fil‐
ter when using the default `filter' table, with optional extension modules. This
should simplify much of the previous confusion over the combination of IP mas‐
querading and packet filtering seen previously. So the following options are
handled differently:
-j MASQ
-M -S
-M -L
There are several other changes in iptables.
SEE ALSO
iptables-apply(8), iptables-save(8), iptables-restore(8), iptables-extensions(8),
The packet-filtering-HOWTO details iptables usage for packet filtering, the NAT-
HOWTO details NAT, the netfilter-extensions-HOWTO details the extensions that are
not in the standard distribution, and the netfilter-hacking-HOWTO details the
netfilter internals.
See http://www.netfilter.org/.
AUTHORS
Rusty Russell originally wrote iptables, in early consultation with Michael Neul‐
ing.
Marc Boucher made Rusty abandon ipnatctl by lobbying for a generic packet selec‐
tion framework in iptables, then wrote the mangle table, the owner match, the
mark stuff, and ran around doing cool stuff everywhere.
James Morris wrote the TOS target, and tos match.
Jozsef Kadlecsik wrote the REJECT target.
Harald Welte wrote the ULOG and NFQUEUE target, the new libiptc, as well as the
TTL, DSCP, ECN matches and targets.
The Netfilter Core Team is: Jozsef Kadlecsik, Pablo Neira Ayuso, Eric Leblond,
Florian Westphal and Arturo Borrero Gonzalez. Emeritus Core Team members are:
Marc Boucher, Martin Josefsson, Yasuyuki Kozakai, James Morris, Harald Welte and
Rusty Russell.
Man page originally written by Herve Eychenne <rv@wallfire.org>.
VERSION
This manual page applies to iptables/ip6tables 1.8.9.
iptables 1.8.9 IPTABLES(8)2.1. iptables [-t table] -L [chain [rulenum]] [options…]
-
List all rules in the selected chain. If no chain is selected, all chains are listed.
$ sudo iptables -L Chain INPUT (policy ACCEPT) target prot opt source destination KUBE-SERVICES all -- anywhere anywhere ctstate NEW /* kubernetes service portals */ KUBE-EXTERNAL-SERVICES all -- anywhere anywhere ctstate NEW /* kubernetes externally-visible service portals */ ... $ sudo iptables -L -n --line-numbers Chain INPUT (policy ACCEPT) num target prot opt source destination 1 KUBE-SERVICES all -- 0.0.0.0/0 0.0.0.0/0 ctstate NEW /* kubernetes service portals */ 2 KUBE-EXTERNAL-SERVICES all -- 0.0.0.0/0 0.0.0.0/0 ctstate NEW /* kubernetes externally-visible service portals */ ... $ sudo iptables -L INPUT 1 -n KUBE-SERVICES all -- 0.0.0.0/0 0.0.0.0/0 ctstate NEW /* kubernetes service portals */ -
Like every other
iptablescommand, it applies to the specified table (filteris the default), so NAT rules get listed by:$ sudo iptables -t nat -L -n Chain PREROUTING (policy ACCEPT) target prot opt source destination KUBE-SERVICES all -- 0.0.0.0/0 0.0.0.0/0 /* kubernetes service portals */ CNI-HOSTPORT-DNAT all -- 0.0.0.0/0 0.0.0.0/0 ADDRTYPE match dst-type LOCAL ... -
Please note that it is often used with the
-noption, in order to avoid long reverse DNS lookups.$ sudo iptables -L -n Chain INPUT (policy ACCEPT) target prot opt source destination KUBE-SERVICES all -- 0.0.0.0/0 0.0.0.0/0 ctstate NEW /* kubernetes service portals */ KUBE-EXTERNAL-SERVICES all -- 0.0.0.0/0 0.0.0.0/0 ctstate NEW /* kubernetes externally-visible service portals */ ...
2.2. iptables [-t table] -S [chain [rulenum]]
-
Print all rules in the selected chain. If no chain is selected, all chains are printed like
iptables-save.$ sudo iptables -t nat -S -P PREROUTING ACCEPT -P INPUT ACCEPT -P OUTPUT ACCEPT -P POSTROUTING ACCEPT -
Like every other
iptablescommand, it applies to the specified table (filteris the default).$ sudo iptables -t nat -S OUTPUT -P OUTPUT ACCEPT -A OUTPUT -m comment --comment "kubernetes service portals" -j KUBE-SERVICES -A OUTPUT -m comment --comment "portmap/canal-portmap" -j CANAL-DNAT -A OUTPUT -m addrtype --dst-type LOCAL -j DOCKER -A OUTPUT -m addrtype --dst-type LOCAL -j CNI-HOSTPORT-DNAT
2.3. iptables [-t table] -F [chain [rulenum]] [options…]
-
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.
# Flush your iptables all chains rules at filter table $ sudo iptables -F# Flush your iptables the specfic chain rules at filter table $ sudo iptables -F INPUT # Flush the INPUT chain only $ sudo iptables -F OUTPUT # Flush the OUTPUT chain only $ sudo iptables -F FORWARD # Flush the FORWARD chain only# Flush your iptables all chains rules at `nat` table $ sudo iptables -t nat -F
2.4. iptables [-t table] {-A|-C|-I|-R|-D} chain …
iptables [-t table] -A chain rule-specification
iptables [-t table] -C chain rule-specification
iptables [-t table] -I chain [rulenum] rule-specification
iptables [-t table] -R chain rulenum rule-specification
iptables [-t table] -D chain rulenumRules can be edited by appending -A a rule to a chain, inserting -I it at a specific position on the chain, replacing -R an existing rule, or delete -D it, and check exists with -C.
2.4.1. Allowing Incomming Traffic on Specific Ports
You could start by blocking traffic, but you might be working over SSH, where you would need to allow SSH before blocking everything else.
To allow incomming traffic on the default SSH port (22), you could tell iptables to allow all TCP traffic on that port to come in.
$ sudo iptables -C INPUT -p tcp --dport ssh -j ACCEPT
iptables: Bad rule (does a matching rule exist in that chain?).
$ sudo iptables -A INPUT -p tcp --dport ssh -j ACCEPT
$ sudo iptables -C INPUT -p tcp --dport ssh -j ACCEPTAlso, you can also specify the destination port with number.
$ sudo iptables -A INPUT -p tcp --dport 22 -j ACCEPTReferring back to the list above, you can see that this tells iptables:
-
append this rule to the input chain (
-A INPUT) so we look at incomming traffic. -
check to see if it is TCP (
-p tcp). -
if so, check to see if the input goes to the SSH port (
--dport ssh). -
if so, accept the input (
-j ACCEPT.
Lets check the rules: (only the first few lines shown, you will see more)
$ sudo iptables -L INPUT -n
Chain INPUT (policy ACCEPT)
target prot opt source destination
ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 tcp dpt:22Now, let’s allow all incomming web traffic
$ sudo iptables -A INPUT -p tcp --dport http -j ACCEPT
$ sudo iptables -A INPUT -p tcp --dport https -j ACCEPTOr
$ sudo iptables -A INPUT -p tcp -m multiport --dports http,https -j ACCEPTChecking our rules, we have
$ sudo iptables -L INPUT -n
Chain INPUT (policy ACCEPT)
target prot opt source destination
ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 tcp dpt:22
ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 tcp dpt:80
ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 tcp dpt:443
ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 multiport dports 80,443Allow default port range for Kubernetes NodePort servies.
$ sudo iptables -A INPUT -p tcp --dport 30000:32767 -j ACCEPT -m comment --comment "Allow default port range of kubernetes nodeport services"
$ sudo iptables -L INPUT -n
Chain INPUT (policy ACCEPT)
target prot opt source destination
ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 tcp dpt:22
ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 tcp dpt:80
ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 tcp dpt:443
ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 multiport dports 80,443
ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 tcp dpts:30000:32767 /* Allow default port range of kubernetes nodeport services */We have specifically allowed tcp traffic to the ssh and web ports, but as we have not blocked anything, all traffic can still come in.
2.4.2. Block Incomming Traffic
Once a decision is made to accept a packet, no more rules affect it. As our rules allowing ssh and web traffic come first, as long as our rule to block all traffic comes after them, we can still accept the traffic we want. All we need to do is put the rule to block all traffic at the end.
$ sudo iptables -A INPUT -j DROP
$ sudo iptables -L
Chain INPUT (policy ACCEPT)
target prot opt source destination
ACCEPT tcp -- anywhere anywhere tcp dpt:ssh
ACCEPT tcp -- anywhere anywhere tcp dpt:http
ACCEPT tcp -- anywhere anywhere tcp dpt:https
DROP all -- anywhere anywhere2.4.3. Allow Incomming Traffic on Specific IP Addresses
-
Here
-s 0/0stand for any incomming source with any IP addresses.$ sudo iptables -A INPUT -p tcp -s 0/0 --dport 22 -j ACCEPT $ sudo iptables -L Chain INPUT (policy ACCEPT) target prot opt source destination ACCEPT tcp -- anywhere anywhere tcp dpt:ssh -
-s 192.168.66.128/24using CIDR values, it stands for IP starting from 192.168.66.1 to 192.168.66.255.$ sudo iptables -A INPUT -p tcp -s 192.168.66.128/24 --dport 22 -j ACCEPT $ sudo iptables -L Chain INPUT (policy ACCEPT) target prot opt source destination ACCEPT tcp -- 192.168.66.0/24 anywhere tcp dpt:ssh$ sudo iptables -A INPUT -p tcp -s 192.168.66.128/32 --dport 22 -j ACCEPT $ sudo iptables -L Chain INPUT (policy ACCEPT) target prot opt source destination ACCEPT tcp -- 192.168.66.128 anywhere tcp dpt:ssh$ sudo iptables -A INPUT -p tcp -s 192.168.66.128 --dport 22 -j ACCEPT $ sudo iptables -L Chain INPUT (policy ACCEPT) target prot opt source destination ACCEPT tcp -- 192.168.66.128 anywhere tcp dpt:ssh
2.4.4. Blocking ICMP
$ sudo iptables -A OUTPUT -p icmp --icmp-type 8 -j DROP
$ sudo iptables -L
Chain OUTPUT (policy ACCEPT)
target prot opt source destination
DROP icmp -- anywhere anywhere icmp echo-request
$ ping blog.codefarm.me
PING blog.codefarm.me (104.27.162.235) 56(84) bytes of data.
ping: sendmsg: Operation not permitted2.4.5. Blocking MongoDB from outside attach
$ sudo iptables -A INPUT -p tcp -s 192.168.66.0/24 --dport 27017 -j ACCEPT
$ sudo iptables -L
Chain INPUT (policy ACCEPT)
target prot opt source destination
ACCEPT tcp -- 192.168.66.0/24 anywhere tcp dpt:270172.4.6. Blocking DDOS
$ sudo iptables -A INPUT -p tcp --dport 80 -m limit --limit 20/minute --limit-burst 100 -j ACCEPT
$ sudo iptables -L
Chain INPUT (policy ACCEPT)
target prot opt source destination
ACCEPT tcp -- anywhere anywhere tcp dpt:http limit: avg 20/min burst 1002.4.7. Insert a New Rule / Replace an Old Rule
$ sudo iptables -A INPUT -p tcp --dport 80 -j ACCEPT
$ sudo iptables -L
Chain INPUT (policy ACCEPT)
target prot opt source destination
ACCEPT tcp -- anywhere anywhere tcp dpt:http
$ sudo iptables -I INPUT 1 -p tcp --dport 22 -j ACCEPT
$ sudo iptables -L
Chain INPUT (policy ACCEPT)
target prot opt source destination
ACCEPT tcp -- anywhere anywhere tcp dpt:ssh
ACCEPT tcp -- anywhere anywhere tcp dpt:http
$ sudo iptables -R INPUT 1 -p tcp --dport 443 -j ACCEPT
$ sudo iptables -L
Chain INPUT (policy ACCEPT)
target prot opt source destination
ACCEPT tcp -- anywhere anywhere tcp dpt:https
ACCEPT tcp -- anywhere anywhere tcp dpt:http2.4.8. Create User Defined Chain / Target
$ sudo iptables -N CODE_FARM
$ sudo iptables -L | grep 'Chain'
Chain INPUT (policy ACCEPT)
Chain FORWARD (policy ACCEPT)
Chain OUTPUT (policy ACCEPT)
Chain CODE_FARM (0 references)
$ sudo iptables -A INPUT -p tcp --dport 22 -j CODE_FARM
$ sudo iptables -L
Chain INPUT (policy ACCEPT)
target prot opt source destination
CODE_FARM tcp -- anywhere anywhere tcp dpt:ssh
Chain CODE_FARM (1 references)
target prot opt source destination
$ sudo iptables -A CODE_FARM -p tcp -j ACCEPT
$ sudo iptables -L
Chain INPUT (policy ACCEPT)
target prot opt source destination
CODE_FARM tcp -- anywhere anywhere tcp dpt:ssh
Chain CODE_FARM (1 references)
target prot opt source destination
ACCEPT tcp -- anywhere anywhere
$ sudo iptables -P INPUT DROP
$ sudo iptables -L
Chain INPUT (policy DROP)
target prot opt source destination
CODE_FARM tcp -- anywhere anywhere tcp dpt:ssh
Chain CODE_FARM (1 references)
target prot opt source destination
ACCEPT tcp -- anywhere anywhere2.5. iptables-save & iptables-restore
Changes to iptables are transitory; if the system is rebooted or if the iptables service is restarted, the rules are automatically flushed and reset. To save the rules so that they are loaded when the iptables service is started, use the following command:
$ sudo service iptables saveThe rules are stored in the file /etc/sysconfig/iptables and are applied whenever the service is started or the machine is rebooted.
You can also save the current iptables into a file and restore it.
$ sudo iptables -L
Chain INPUT (policy DROP)
target prot opt source destination
ACCEPT tcp -- 192.168.66.0/24 anywhere tcp dpt:ssh
ACCEPT tcp -- anywhere anywhere multiport dports http,https
DROP all -- anywhere anywhere
Chain FORWARD (policy ACCEPT)
target prot opt source destination
Chain OUTPUT (policy ACCEPT)
target prot opt source destination# save current iptables into iptables.rules and print to standard output
sudo iptables-save | tee iptables.rules$ sudo Generated by iptables-save v1.6.0 on Fri Jan 18 16:43:19 2019
*filter
:INPUT DROP [0:0]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [278:30254]
-A INPUT -s 192.168.66.0/24 -p tcp -m tcp --dport 22 -j ACCEPT
-A INPUT -p tcp -m multiport --dports 80,443 -j ACCEPT
-A INPUT -j DROP
COMMIT
$ sudo Completed on Fri Jan 18 16:43:19 2019$ sudo iptables -P INPUT ACCEPT # allow any incomming traffic before delete all rules
$ sudo iptables -F # delete all rules
$ sudo iptables -L
Chain INPUT (policy ACCEPT)
target prot opt source destination
Chain FORWARD (policy ACCEPT)
target prot opt source destination
Chain OUTPUT (policy ACCEPT)
target prot opt source destination# restore iptables from iptables.rules
sudo iptables-restore iptables.rules$ sudo iptables -L
Chain INPUT (policy DROP)
target prot opt source destination
ACCEPT tcp -- 192.168.66.0/24 anywhere tcp dpt:ssh
ACCEPT tcp -- anywhere anywhere multiport dports http,https
DROP all -- anywhere anywhere
Chain FORWARD (policy ACCEPT)
target prot opt source destination
Chain OUTPUT (policy ACCEPT)
target prot opt source destination3. References
-
The netfilter.org project, https://netfilter.org/index.html
-
iptables - ArchWiki, https://wiki.archlinux.org/index.php/iptables
-
IPTABLES VS FIREWALLD | Unixmen, https://www.unixmen.com/iptables-vs-firewalld/
-
The Beginner’s Guide to iptables, the Linux Firewall, https://www.howtogeek.com/177621/the-beginners-guide-to-iptables-the-linux-firewall/
-
IptablesHowTo - Community Help Wiki, https://help.ubuntu.com/community/IptablesHowTo
-
HowTos/Network/IPTables - CentOS Wiki, https://wiki.centos.org/HowTos/Network/IPTables
-
RETURN target, https://www.frozentux.net/iptables-tutorial/chunkyhtml/x4625.html
-
Linux Firewall Tutorial: IPTables Tables, Chains, Rules Fundamentals, https://www.thegeekstuff.com/2011/01/iptables-fundamentals/
-
Saving Iptables Firewall Rules Permanently, https://www.thomas-krenn.com/en/wiki/Saving_Iptables_Firewall_Rules_Permanently