11 comment

  1. Blocking really _ALL_ traffic can cause undesired effects. In fact quite a few applications use the internal loopback interface for internal communication. So the following two rules should be added to allow this:

    iptables -A INPUT -i lo
    iptables -A OUTPUT -o lo

    which allow all traffic via the loopback interface. This should be perfectly safe, even in a hostile environment since all external traffic is still blocked.

  2. @Thomas, a “-j ACCEPT” at the end of the commands is needed.

    iptables -A INPUT -i lo -j ACCEPT
    iptables -A OUTPUT -o lo -j ACCEPT

  3. Can someone help me to figure out how to use iptables to block all traffic from an IP address, I am not sure what the command would be.

  4. Hi all,
    thanks for your input. Unfortunately I don’t know how to extend these commands for a slightly more subtle scenario: I want to prevent users to move server content to the internet (=> iptables -P OUTPUT DROP on that server) but I do want to enable them to move content to the server (e.g., read e-mail, download attachments, …)

    It seems that for general web browsing you do need output traffic (I guess just sending your request string??) but I was hoping that iptables could distinguish between control commands (e.g. sending a HTTP get from the machine to the internet) and actual file transfers from the machine to the internet…

    Is my hope in vain or is there a solution?

    Best regards and thanks in advance,

  5. Hi All,

    I am very new to iptables and new to linux even. I am trying to allow all connections to my eth1 (public interface) except for the traffic coming in and out from udp ports 5060 until 5080 which should be only allowed for specific IP addresses. Here’s my config below:

    iptables –flush
    iptables -P INPUT DROP
    iptables -P FORWARD DROP
    iptables -P OUTPUT DROP
    iptables -A INPUT -i lo -j ACCEPT
    iptables -A OUTPUT -o lo -j ACCEPT
    iptables -A INPUT -s -j ACCEPT
    iptables -A OUTPUT -d -j ACCEPT
    iptables -A OUTPUT -o eth1 -j ACCEPT

    ###DROP ALL 5060-5080 ports
    #iptables -A INPUT -p udp –dport 5050:5080 -j DROP
    #iptables -A INPUT -p udp –dport 5050:5080 -j DROP

    iptables -A INPUT -p udp -s A.B.C.D –dport 5050:5080 -j ACCEPT
    iptables -A INPUT -p udp -s E.F.G.H –dport 5050:5080 -j ACCEPT
    iptables -A INPUT -p udp -s I.J.K.L –dport 5050:5080 -j ACCEPT

    I tested it but I can still connect to that server using the 5061 port using other public ip which is not declared to be allowed.

    Help Please…..


  6. James,

    Not sure if you have got any further with this I’m looking for the same sort solution as my Trixbox was hacked even though I ‘thought’ it was quite secure. I would say that you ought to consider blocking all ports from all IP’s that you do not know, not just the SIP ones.


  7. iptables -A INPUT -s -j ACCEPT
    accepts all input from subnets,, and
    iptables -A OUTPUT -d -j ACCEPT
    accepts all output from subnets,, and
    you never reach your drop rules

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