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Thread: iptables rules

  1. #1
    Join Date
    Feb 2016
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    18

    iptables rules

    hello team:

    i need help to check my rules for iptables (converted from ipfw). what i intend to do is to specify specific rules first (drop or accept) then drop (and log) anything else:

    sudo iptables -A INPUT -i lo -j ACCEPT


    sudo iptables -A INPUT -d 127.0.0.0/8 -j DROP


    sudo iptables -A INPUT -s 127.0.0.0/8 -j DROP


    sudo iptables -A INPUT -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT


    sudo iptables -A INPUT -i em0 -p tcp -m state --state NEW --source 192.168.1.0/24 --dport 22 -j ACCEPT


    sudo iptables -A INPUT -i em0 -p tcp -m state --state NEW --source 100.100.100.0/24 --dport 22 -j ACCEPT


    sudo iptables -A INPUT -i em0 -p icmp -m state --state NEW --source 192.168.1.0/24 -j ACCEPT

    sudo iptables -P INPUT DROP


    sudo iptables -P FORWARD DROP

    thanks

    _dave

  2. #2
    Join Date
    Nov 2008
    Location
    Metro Boston
    Beans
    15,581
    Distro
    Kubuntu 20.04 Focal Fossa

    Re: iptables rules

    I don't understand the first three rules. You permit traffic to the lo interface, then block addresses in 127/8. I doubt the second and third rules are ever reached.

    You don't have a logging rule. Usually I have

    Code:
    iptables -A INPUT -j LOG
    at the bottom of my ruleset to log unmatched packets.
    If you ask for help, do not abandon your request. Please have the courtesy to check for responses and thank the people who helped you.

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  3. #3
    Join Date
    Feb 2016
    Beans
    18

    Re: iptables rules

    Quote Originally Posted by SeijiSensei View Post
    I don't understand the first three rules. You permit traffic to the lo interface, then block addresses in 127/8. I doubt the second and third rules are ever reached.

    You don't have a logging rule. Usually I have

    Code:
    iptables -A INPUT -j LOG
    at the bottom of my ruleset to log unmatched packets.
    thanks.

    those rule sets are to drop packets from and to 128.0.0.0/8. i borrowed them from ipfw rules.

    btw, what is the default action for "iptables -A INPUT -j LOG"? just log packets? so shall i put "iptables -P INPUT DROP" after the line "iptables -A INPUT -j LOG"?

  4. #4
    Join Date
    Nov 2008
    Location
    Metro Boston
    Beans
    15,581
    Distro
    Kubuntu 20.04 Focal Fossa

    Re: iptables rules

    Put the logging rule ahead of a reject one like this:
    Code:
    iptables -A INPUT -j LOG
    iptables -A INPUT -j REJECT --reject-with icmp-host-prohibited
    and put them both at the bottom of the INPUT chain.

    Or you can use DROP rather than REJECT to drop the packets on the floor. REJECT informs the source of the packets that they are being ignored.

    Do you already have these rules implemented somewhere? If so, run
    Code:
    sudo iptables -L -nv
    and see how many packets have hit the rules for 127.0.0.0/8. I bet the number is zero. There's literally no reason to block any traffic to the localhost interface, and doing so can interfere with the operation of some programs. For instance, Ubuntu since 18.04 with systemd-resolved accepts DNS queries at the address 127.0.0.53. You don't want to block that.
    Last edited by SeijiSensei; March 7th, 2021 at 12:05 AM.
    If you ask for help, do not abandon your request. Please have the courtesy to check for responses and thank the people who helped you.

    Blog · Linode System Administration Guides · Android Apps for Ubuntu Users

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