Basic iptables howto
Iptables is a firewall, installed by default on all official Ubuntu distributions (Ubuntu, Kubuntu, Xubuntu). When you install Ubuntu, iptables is there, but it allows all traffic by default. Ubuntu 8.04 Comes with ufw - a program for managing the iptables firewall easily.
There is a wealth of information available about iptables, but much of it is fairly complex, and if you want to do a few basic things, this How To is for you.
Basic Commands
Typing
# iptables -L
lists your current rules in iptables. If you have just set up your server, you will have no rules, and you should see
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
Basic Iptables Options
Here are explanations for some of the iptables options you will see in this tutorial. Don't worry about understanding everything here now, but remember to come back and look at this list as you encounter new options later on.
- -A - Append this rule to a rule chain. Valid chains for what we're doing are INPUT, FORWARD and OUTPUT, but we mostly deal with INPUT in this tutorial, which affects only incoming traffic.
- -L - List the current filter rules.
- -m conntrack - Allow filter rules to match based on connection state. Permits the use of the --ctstate option.
- --ctstate - Define the list of states for the rule to match on. Valid states are:
- NEW - The connection has not yet been seen.
- RELATED - The connection is new, but is related to another connection already permitted.
- ESTABLISHED - The connection is already established.
- INVALID - The traffic couldn't be identified for some reason.
- -m limit - Require the rule to match only a limited number of times. Allows the use of the --limit option. Useful for limiting logging rules.
- --limit - The maximum matching rate, given as a number followed by "/second", "/minute", "/hour", or "/day" depending on how often you want the rule to match. If this option is not used and -m limit is used, the default is "3/hour".
- -p - The connection protocol used.
- --dport - The destination port(s) required for this rule. A single port may be given, or a range may be given as start:end, which will match all ports from start to end, inclusive.
- -j - Jump to the specified target. By default, iptables allows four targets:
- ACCEPT - Accept the packet and stop processing rules in this chain.
- REJECT - Reject the packet and notify the sender that we did so, and stop processing rules in this chain.
- DROP - Silently ignore the packet, and stop processing rules in this chain.
- LOG - Log the packet, and continue processing more rules in this chain. Allows the use of the --log-prefix and --log-level options.
- --log-prefix - When logging, put this text before the log message. Use double quotes around the text to use.
- --log-level - Log using the specified syslog level. 7 is a good choice unless you specifically need something else.
- -i - Only match if the packet is coming in on the specified interface.
- -I - Inserts a rule. Takes two options, the chain to insert the rule into, and the rule number it should be.
- -I INPUT 5 would insert the rule into the INPUT chain and make it the 5th rule in the list.
- -v - Display more information in the output. Useful for if you have rules that look similar without using -v.
- -s --source - address[/mask] source specification
- -d --destination - address[/mask] destination specification
- -o --out-interface - output name[+] network interface name ([+] for wildcard)
Allowing Established Sessions
We can allow established sessions to receive traffic:
# iptables -A INPUT -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT
- The above rule has no spaces either side of the comma in ESTABLISHED,RELATED
If the line above doesn't work, you may be on a castrated VPS whose provider has not made available the extension, in which case an inferior version can be used as last resort:
# iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
Allowing Incoming 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 incoming traffic on the default SSH port (22), you could tell iptables to allow all TCP traffic on that port to come in.
# iptables -A INPUT -p tcp --dport ssh -j ACCEPT
Referring 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 incoming 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)
# iptables -L Chain INPUT (policy ACCEPT) target prot opt source destination ACCEPT all -- anywhere anywhere state RELATED,ESTABLISHED ACCEPT tcp -- anywhere anywhere tcp dpt:ssh
Now, let's allow all incoming web traffic
# iptables -A INPUT -p tcp --dport 80 -j ACCEPT
Checking our rules, we have
# iptables -L Chain INPUT (policy ACCEPT) target prot opt source destination ACCEPT all -- anywhere anywhere state RELATED,ESTABLISHED ACCEPT tcp -- anywhere anywhere tcp dpt:ssh ACCEPT tcp -- anywhere anywhere tcp dpt:www
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.
Blocking 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.
# iptables -A INPUT -j DROP # iptables -L Chain INPUT (policy ACCEPT) target prot opt source destination ACCEPT all -- anywhere anywhere state RELATED,ESTABLISHED ACCEPT tcp -- anywhere anywhere tcp dpt:ssh ACCEPT tcp -- anywhere anywhere tcp dpt:www DROP all -- anywhere anywhere
Because we didn't specify an interface or a protocol, any traffic for any port on any interface is blocked, except for web and ssh.
Editing iptables
The only problem with our setup so far is that even the loopback port is blocked. We could have written the drop rule for just eth0 by specifying -i eth0, but we could also add a rule for the loopback. If we append this rule, it will come too late - after all the traffic has been dropped. We need to insert this rule before that. Since this is a lot of traffic, we'll insert it as the first rule so it's processed first.
# iptables -I INPUT 1 -i lo -j ACCEPT # iptables -L Chain INPUT (policy ACCEPT) target prot opt source destination ACCEPT all -- anywhere anywhere ACCEPT all -- anywhere anywhere state RELATED,ESTABLISHED ACCEPT tcp -- anywhere anywhere tcp dpt:ssh ACCEPT tcp -- anywhere anywhere tcp dpt:www DROP all -- anywhere anywhere
The first and last lines look nearly the same, so we will list iptables in greater detail.
# iptables -L -v
Chain INPUT (policy ACCEPT 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination
0 0 ACCEPT all -- lo any anywhere anywhere
0 0 ACCEPT all -- any any anywhere anywhere state RELATED,ESTABLISHED
0 0 ACCEPT tcp -- any any anywhere anywhere tcp dpt:ssh
0 0 ACCEPT tcp -- any any anywhere anywhere tcp dpt:www
0 0 DROP all -- any any anywhere anywhereYou can now see a lot more information. This rule is actually very important, since many programs use the loopback interface to communicate with each other. If you don't allow them to talk, you could break those programs!
Logging
In the above examples none of the traffic will be logged. If you would like to log dropped packets to syslog, this would be the quickest way:
# iptables -I INPUT 5 -m limit --limit 5/min -j LOG --log-prefix "iptables denied: " --log-level 7
See Tips section for more ideas on logging.
Saving iptables
If you were to reboot your machine right now, your iptables configuration would disappear. Rather than type this each time you reboot, however, you can save the configuration, and have it start up automatically. To save the configuration, you can use iptables-save and iptables-restore.
Configuration on startup
WARNING: Iptables and NetworkManager seem to have a conflict. However NetworkManager is still in Beta. If you are concerned enough about security to install a firewall you might not want to trust NetworkManager to manage it yet. Also note NetworkManager and iptables have opposite aims. Iptables aims to keep any questionable network traffic out. NetworkManager aims to keep you connected at all times. Therefore if you want security all the time, run iptables at boot time. If you want security some of the time then NetworkManager might be the right choice.
WARNING: If you use NetworkManager (installed by default on Feisty and later) these steps will leave you unable to use NetworkManager for the interfaces you modify. Please follow the steps in the next section instead.
NOTE: It appears on Hardy, NetworkManager has an issue with properly on saving and restoring the iptable rules when using the method in the next section. Using this first method appears to work. If you find otherwise, please update this note.
Save your firewall rules to a file
# sudo bash -c "iptables-save > /etc/iptables.rules"
At this point you have several options. You can make changes to /etc/network/interfaces or add scripts to /etc/network/if-pre-up.d/ and /etc/network/if-post-down.d/ to achieve similar ends. The script solution allows for slightly more flexibility.
Solution #1 - /etc/network/interfaces
(NB: I followed the directions in this section and this disabled most (all?) of my scripts in /etc/rc2.d/: polipo, dnsmasq, bluetooth, etc. I recommend using ufw/gufw instead. ~gasull)
Modify the /etc/network/interfaces configuration file to apply the rules automatically. You will need to know the interface that you are using in order to apply the rules - if you do not know, you are probably using the interface eth0, although you should check with the following command first to see if there are any wireless cards:
$ iwconfig
If you get output similar to the following, then you do not have any wireless cards at all and your best bet is probably eth0.
$ iwconfig lo no wireless extensions. eth0 no wireless extensions. $
When you have found out the interface you are using, please open your /etc/network/interfaces file depending on what editor you want and/or what distribution you have:
Command line:
# nano /etc/network/interfaces
For Ubuntu and Xubuntu: type ALT+F2, then in the window that pops up, type:
gksudo gedit /etc/network/interfaces
and press Enter.
For Kubuntu: type ALT+F2, then in the window that pops up, type:
kdesu kate /etc/network/interfaces
and press enter.
When in the file, search for the interface you found, and at the end of the network related lines for that interface, add the line:
pre-up iptables-restore < /etc/iptables.rules
You can also prepare a set of down rules, save them into second file /etc/iptables.downrules and apply it automatically using the above steps:
post-down iptables-restore < /etc/iptables.downrules
A fully working example using both from above:
auto eth0 iface eth0 inet dhcp pre-up iptables-restore < /etc/iptables.rules post-down iptables-restore < /etc/iptables.downrules
You may also want to keep information from byte and packet counters.
iptables-save -c > /etc/iptables.rules
The above command will save the whole rule-set to a file called /etc/iptables.rules with byte and packet counters still intact.
Solution #2 /etc/network/if-pre-up.d and ../if-post-down.d
NOTE: This solution uses iptables-save -c to save the counters. Just remove the -c to only save the rules.
Alternatively you could add the iptables-restore and iptables-save to the if-pre-up.d and if-post-down.d directories in the /etc/network directory instead of modifying /etc/network/interface directly.
The script /etc/network/if-pre-up.d/iptablesload will contain:
#!/bin/sh iptables-restore < /etc/iptables.rules exit 0
and /etc/network/if-post-down.d/iptablessave will contain:
#!/bin/sh iptables-save -c > /etc/iptables.rules if [ -f /etc/iptables.downrules ]; then iptables-restore < /etc/iptables.downrules fi exit 0
Then be sure to give both scripts execute permissions:
# chmod +x /etc/network/if-post-down.d/iptablessave # chmod +x /etc/network/if-pre-up.d/iptablesload
Solution #3 iptables-persistant
Install and use the iptables-persistent package.
Configuration on Startup for NetworkManager
NetworkManager includes the ability to run scripts when it activates or deactivates an interface. To save iptables rules on shutdown, and to restore them on startup, we are going to create such a script. To begin, press Alt+F2 and enter this command:
For Ubuntu:
$ gksudo gedit /etc/NetworkManager/dispatcher.d/01firewall
For Kubuntu:
kdesu kate /etc/NetworkManager/dispatcher.d/01firewall
Then, paste this script into your editor, save, and exit the editor.
if [ -x /usr/bin/logger ]; then
LOGGER="/usr/bin/logger -s -p daemon.info -t FirewallHandler"
else
LOGGER=echo
fi
case "$2" in
up)
if [ ! -r /etc/iptables.rules ]; then
${LOGGER} "No iptables rules exist to restore."
return
fi
if [ ! -x /sbin/iptables-restore ]; then
${LOGGER} "No program exists to restore iptables rules."
return
fi
${LOGGER} "Restoring iptables rules"
/sbin/iptables-restore -c < /etc/iptables.rules
;;
down)
if [ ! -x /sbin/iptables-save ]; then
${LOGGER} "No program exists to save iptables rules."
return
fi
${LOGGER} "Saving iptables rules."
/sbin/iptables-save -c > /etc/iptables.rules
;;
*)
;;
esacFinally, we need to make sure NetworkManager can execute this script. In a terminal window, enter this command:
# chmod +x /etc/NetworkManager/dispatcher.d/01firewall
Tips
If you manually edit iptables on a regular basis
The above steps go over how to setup your firewall rules and presume they will be relatively static (and for most people they should be). But if you do a lot of development work, you may want to have your iptables saved everytime you reboot. You could add a line like this one in /etc/network/interfaces:
pre-up iptables-restore < /etc/iptables.rules post-down iptables-save > /etc/iptables.rules
The line "post-down iptables-save > /etc/iptables.rules" will save the rules to be used on the next boot.
Using iptables-save/restore to test rules
If you edit your iptables beyond this tutorial, you may want to use the iptables-save and iptables-restore feature to edit and test your rules. To do this open the rules file in your favorite text editor (in this example gedit).
$ sudo iptables-save > /etc/iptables.rules $ gksudo gedit /etc/iptables.rules
You will have a file that appears similiar to (following the example above):
# Generated by iptables-save v1.3.1 on Sun Apr 23 06:19:53 2006 *filter :INPUT ACCEPT [368:102354] :FORWARD ACCEPT [0:0] :OUTPUT ACCEPT [92952:20764374] -A INPUT -i lo -j ACCEPT -A INPUT -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT -A INPUT -i eth0 -p tcp -m tcp --dport 22 -j ACCEPT -A INPUT -i eth0 -p tcp -m tcp --dport 80 -j ACCEPT -A INPUT -m limit --limit 5/min -j LOG --log-prefix "iptables denied: " --log-level 7 -A INPUT -j DROP COMMIT # Completed on Sun Apr 23 06:19:53 2006
Notice that these are iptables commands minus the iptable command. Feel free to edit this to file and save when complete. Then to test simply:
# iptables-restore < /etc/iptables.rules
NOTE: With iptables 1.4.1.1-1 and above, a script allow you to test your new rules without risking to brick your remote server. If you are applying the rules on a remote server, you should consider testing it with:
# iptables-apply /etc/iptables.rules
After testing, if you have not added the iptables-save command above to your /etc/network/interfaces remember not to lose your changes:
# iptables-save > /etc/iptables.rules
More detailed Logging
For further detail in your syslog you may want create an additional Chain. This will be a very brief example of my /etc/iptables.rules showing how I setup my iptables to log to syslog:
# Generated by iptables-save v1.3.1 on Sun Apr 23 05:32:09 2006 *filter :INPUT ACCEPT [273:55355] :FORWARD ACCEPT [0:0] :LOGNDROP - [0:0] :OUTPUT ACCEPT [92376:20668252] -A INPUT -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT -A INPUT -i eth0 -p tcp -m tcp --dport 22 -j ACCEPT -A INPUT -i eth0 -p tcp -m tcp --dport 80 -j ACCEPT -A INPUT -i lo -j ACCEPT -A INPUT -j LOGNDROP -A LOGNDROP -p tcp -m limit --limit 5/min -j LOG --log-prefix "Denied TCP: " --log-level 7 -A LOGNDROP -p udp -m limit --limit 5/min -j LOG --log-prefix "Denied UDP: " --log-level 7 -A LOGNDROP -p icmp -m limit --limit 5/min -j LOG --log-prefix "Denied ICMP: " --log-level 7 -A LOGNDROP -j DROP COMMIT # Completed on Sun Apr 23 05:32:09 2006
Note a new CHAIN called LOGNDROP at the top of the file. Also, the standard DROP at the bottom of the INPUT chain is replaced with LOGNDROP and add protocol descriptions so it makes sense looking at the log. Lastly we drop the traffic at the end of the LOGNDROP chain. The following gives some idea of what is happening:
- --limit sets the number of times to log the same rule to syslog
- --log-prefix "Denied..." adds a prefix to make finding in the syslog easier
- --log-level 7 sets the syslog level to informational (see man syslog for more detail, but you can probably leave this)
Disabling the firewall
If you need to disable the firewall temporarily, you can flush all the rules using
# iptables -F
or create a script using text editor such as nano
# nano -w /root/fw.stop
echo "Stopping firewall and allowing everyone..." iptables -F iptables -X iptables -t nat -F iptables -t nat -X iptables -t mangle -F iptables -t mangle -X iptables -P INPUT ACCEPT iptables -P FORWARD ACCEPT iptables -P OUTPUT ACCEPT
Make sure you can execute the script
$ chmod +x /root/fw.stop
You can run the script
$ /root/fw.stop
IPTables tu memiliki tiga macam daftar aturan bawaan dalam tabel
penyaringan, daftar tersebut dinamakan rantai firewall (firewall chain)
atau sering disebut chain saja. Ketiga chain tersebut adalah INPUT,
OUTPUT dan FORWARD.
Pada diagram tersebut, lingkaran
menggambarkan ketiga rantai atau chain. Pada saat sebuah paket sampai
pada sebuah lingkaran, maka disitulah terjadi proses penyaringan. Rantai
akan memutuskan nasib paket tersebut. Apabila keputusannnya adalah
DROP, maka paket tersebut akan di-drop. Tetapi jika rantai memutuskan
untuk ACCEPT, maka paket akan dilewatkan melalui diagram tersebut.
Sebuah
rantai adalah aturan-aturan yang telah ditentukan. Setiap aturan
menyatakan “jika paket memiliki informasi awal (header) seperti ini,
maka inilah yang harus dilakukan terhadap paket�. Jika aturan tersebut
tidak sesuai dengan paket, maka aturan berikutnya akan memproses paket
tersebut. Apabila sampai aturan terakhir yang ada, paket tersebut belum
memenuhi salah satu aturan, maka kernel akan melihat kebijakan bawaan
(default) untuk memutuskan apa yang harus dilakukan kepada paket
tersebut. Ada dua kebijakan bawaan yaitu default DROP dan default
ACCEPT.
Jalannya sebuah paket melalui diagram tersebut bisa dicontohkan sebagai berikut:
Perjalanan paket yang diforward ke host yang lain
1. Paket berada pada jaringan fisik, contoh internet.
2. Paket masuk ke interface jaringan, contoh eth0.
3.
Paket masuk ke chain PREROUTING pada table Mangle. Chain ini berfungsi
untuk me-mangle (menghaluskan) paket, seperti merubah TOS, TTL dan
lain-lain.
4. Paket masuk ke chain PREROUTING pada tabel nat. Chain
ini berfungsi utamanya untuk melakukan DNAT (Destination Network Address
Translation).
5. Paket mengalami keputusan routing, apakah akan diproses oleh host lokal atau diteruskan ke host lain.
6. Paket masuk ke chain FORWARD pada tabel filter. Disinlah proses pemfilteran yang utama terjadi.
7.
Paket masuk ke chain POSTROUTING pada tabel nat. Chain ini berfungsi
utamanya untuk melakukan SNAT (Source Network Address Translation).
8. Paket keluar menuju interface jaringan, contoh eth1.
9. Paket kembali berada pada jaringan fisik, contoh LAN.
Perjalanan paket yang ditujukan bagi host lokal
1. Paket berada dalam jaringan fisik, contoh internet.
2. Paket masuk ke interface jaringan, contoh eth0.
3. Paket masuk ke chain PREROUTING pada tabel mangle.
4. Paket masuk ke chain PREROUTING pada tabel nat.
5. Paket mengalami keputusan routing.
6. Paket masuk ke chain INPUT pada tabel filter untuk mengalami proses penyaringan.
7. Paket akan diterima oleh aplikasi lokal.
Perjalanan paket yang berasal dari host lokal
1. Aplikasi lokal menghasilkan paket data yang akan dikirimkan melalui jaringan.
2. Paket memasuki chain OUTPUT pada tabel mangle.
3. Paket memasuki chain OUTPUT pada tabel nat.
4. Paket memasuki chain OUTPUT pada tabel filter.
5. Paket mengalami keputusan routing, seperti ke mana paket harus pergi dan melalui interface mana.
6. Paket masuk ke chain POSTROUTING pada tabel NAT.
7. Paket masuk ke interface jaringan, contoh eth0.
8. Paket berada pada jaringan fisik, contoh internet.
3. Sintaks IPTables
iptables [-t table] command [match] [target/jump]
dan terakhir informasi bt u
IPTables
juga memiliki 3 buah tabel, yaitu NAT, MANGLE dan FILTER. Penggunannya
disesuaikan dengan sifat dan karakteristik masing-masing. Fungsi dari
masing-masing tabel adalah :
NAT : Secara umum digunakan untuk
melakukan Network Address Translation. NAT adalah penggantian field
alamat asal atau alamat tujuan dari sebuah paket.
MANGLE : Digunakan untuk melakukan penghalusan (mangle) paket, seperti TTL, TOS dan MARK.
FILTER
: Secara umum, inilah pemfilteran paket yang sesungguhnya.. Di sini
bisa dintukan apakah paket akan di-DROP, LOG, ACCEPT atau REJECT
Command
pada baris perintah IPTables akan memberitahu apa yang harus dilakukan
terhadap lanjutan sintaks perintah. Umumnya dilakukan penambahan atau
penghapusan sesuatu dari tabel atau yang lain.
