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kitten# iptables -N ICMP_IN kitten# iptables -N ICMP_OUT Now let s create some rules in the INPUT and OUTPUT chains to refer the ICMP traffic to the newly created ICMP_IN and ICMP_OUT chains. You send traffic to the user-created chains by referring to them as a rule target using the -j flag. Listing 2-35 shows the two rules directing ICMP traffic to the user-created chains. Listing 2-35. Directing ICMP Traffic to the User-Created Chains kitten# iptables -A INPUT -p icmp -j ICMP_IN kitten# iptables -A OUTPUT -p icmp -j ICMP_OUT Now when ICMP traffic is received by the INPUT chain, it is directed to be filtered by the user-created chain ICMP_IN; and when it is received by the OUTPUT chain, it is handled by the ICMP_OUT chain. The iptables rules can target individual ICMP messages types by selecting only ICMP traffic with the -p icmp flag in combination with the --icmp-type flag to select the particular ICMP message type. The next line shows this selection in the rule: kitten# iptables -A ICMP_IN -p icmp --icmp-type echo-request -j DROP I have added this rule to the ICMP_IN chain, which I have specified will handle incoming ICMP traffic. I have selected only ICMP traffic using the -p flag. Then I selected the type of ICMP traffic using the --icmp-type flag. Within the ICMP traffic I have selected the message type of echo-request, which indicates an incoming ping request, and I have opted to drop this traffic. You could have also indicated the echo-request traffic with the type number of the ICMP message type. kitten# iptables -A ICMP_IN -p icmp --icmp-type 8 -j DROP You can now create the rules you need to address the required policy. Allow inbound echo reply, time exceeded, and destination unreachable messages to the host (see Listing 2-36). Listing 2-36. Incoming ICMP Traffic kitten# --state kitten# --state kitten# --state kitten# iptables -A ICMP_IN ESTABLISHED,RELATED iptables -A ICMP_IN ESTABLISHED,RELATED iptables -A ICMP_IN ESTABLISHED,RELATED iptables -A ICMP_IN -i -j -i -j -i -j -i eth0 -p ACCEPT eth0 -p ACCEPT eth0 -p ACCEPT eth0 -p icmp --icmp-type 0 -m state icmp --icmp-type 3 -m state icmp --icmp-type 11 -m state icmp -j LOG_DROP
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I have added these rules to the ICMP_IN incoming ICMP traffic chain and selected ICMP Types 0, 3, and 11 that are in an ESTABLISHED or RELATED state, which indicates that this traffic is in reply to a request generated on the bastion host. It does not allow NEW connections using ICMP to be made. This means attempts to ping this host will result in an error. Finally, I have added a last rule to ensure any other incoming ICMP traffic is logged and dropped. I have done this by specifying the target of the last rule as a user-created chain called
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LOG_DROP. This chain is going to direct the ICMP traffic to a set of iptables rules that will log the packets to be dropped and then drop the packets. First, create the LOG_DROP chain. kitten# iptables -N LOG_DROP Second, create a rule to log the incoming ICMP traffic. You will log the ICMP traffic to syslog adding a prefix of IPT_ICMP_IN (with a trailing space) to the log entries to allow you to identify them. kitten# iptables -A LOG_DROP -i eth0 -p icmp -j LOG --log-prefix "IPT_ICMP_IN " kitten# iptables -A LOG_DROP -i eth0 -p icmp -j DROP The last rule drops the traffic after it has been logged. This takes care of all the incoming ICMP traffic.
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s Caution Be careful about logging your ICMP traffic. Large amounts of logging traffic can be generated
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by ICMP traffic. You should ensure you have sufficient disk space and a suitable log rotation regime.
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Now you add the rules to take care of the outbound ICMP traffic. You can see these rules on the following lines: kitten# iptables -A ICMP_OUT -o eth0 -p icmp --icmp-type 8 -m state --state NEW -j ACCEPT kitten# iptables -A ICMP_OUT -o eth0 -p icmp -j LOG_DROP I have allowed outgoing echo messages so that I can ping remote hosts; then you added a rule to log and drop all other outgoing ICMP traffic. I will also add two more rules to the user-created chain LOG_DROP to handle logging and dropping the outgoing ICMP traffic. kitten# iptables -A LOG_DROP -o eth0 -p icmp -j LOG --log-prefix "IPT_ICMP_OUT " kitten# iptables -A LOG_DROP -o eth0 -p icmp -j DROP From this information and these rules, you should now be able to design and implement some rules to handle incoming and outgoing ICMP traffic in your environment.
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