Ubuntu: Firewall, iptables

Basic Commands

Typing

sudo 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-leveloptions.
  • --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:

    sudo 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:

    sudo 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.

    sudo 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)

    sudo 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

    sudo iptables -A INPUT -p tcp --dport 80 -j ACCEPT

    Checking our rules, we have

    sudo 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.

    sudo iptables -A INPUT -j DROP
    sudo 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.

    sudo iptables -I INPUT 1 -i lo -j ACCEPT
    sudo 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.

    sudo 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             anywhere

    You 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:

    sudo 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.

    https://help.ubuntu.com/community/IptablesHowTo

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About zephyr9000

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