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iptables -A INPUT -j DROP -i eth0 -s 192168045 iptables -A INPUT -j ACCEPT -i lo
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With IPtables, the FORWARD and INPUT chains are evaluated separately One does not feed into the other This means that if you want to completely block certain addresses from passing through your system, you will need to add both a FORWARD rule and an INPUT rule for them
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A common method for reducing repeated INPUT and FORWARD rules is to create a user chain that both the INPUT and FORWARD chains feed into You define a user chain with the -N option The next example shows the basic format for this arrangement A new chain is created called incoming (it can be any name you choose) The rules you define for your FORWARD and INPUT chains are now defined for the incoming chain The INPUT and FORWARD chains then use the incoming chain as a target, jumping directly to it and using its rules to process any packets they receive
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Firewalls often block certain Internet Control Message Protocol (ICMP) messages ICMP redirect messages, in particular, can take control of your routing tasks You need to enable some ICMP messages, however, such as those needed for ping, traceroute, and particularly destination-unreachable operations In most cases, you always need to make sure destinationunreachable packets are allowed; otherwise, domain name queries could hang Some of the more common ICMP packet types are listed in Table 20-6 You can enable an ICMP type of packet with the --icmp-type option, which takes as its argument a number or a name representing the message The following examples enable the use of echo-reply, echo-request, and destination-unreachable messages, which have the numbers 0, 8, and 3:
iptables -A INPUT -j ACCEPT -p icmp -i eth0 --icmp -type echo-reply -d 10001 iptables -A INPUT -j ACCEPT -p icmp -i eth0 --icmp-type echo-request -d 10001 iptables -A INPUT -j ACCEPT -p icmp -i eth0 --icmp-type destination-unreachable -d 10001
Their rule listing will look like this:
ACCEPT ACCEPT ACCEPT icmp -icmp -icmp -0000/0 0000/0 0000/0 10001 10001 10001 icmp type 0 icmp type 8 icmp type 3
Ping operations need to be further controlled to avoid the ping-of-death security threat You can do this several ways One way is to deny any ping fragments Ping packets are normally very small You can block ping-of-death attacks by denying any ICMP packet that is a fragment Use the -f option to indicate fragments
iptables -A INPUT -p icmp -j DROP -f
Another way is to limit the number of matches received for ping packets You use the limit module to control the number of matches on the ICMP ping operation Use -m limit to use the limit module, and --limit to specify the number of allowed matches 1/s will allow one match per second
PART V
iptables -A FORWARD -p icmp --icmp-type echo-request -m limit --limit 1/s -j ACCEPT
Number 0 3 5 8 11
Name echo-reply destination-unreachable redirect echo-request time-exceeded
Required By ping Any TCP/UDP traffic Routing, if not running routing daemon ping traceroute
TABLE 20-6 Common ICMP Packets
Part V:
Security
Controlling Port Access
If your system is hosting an Internet service, such as a web or FTP server, you can use IPtables to control access to it You can specify a particular service by using the source port (--sport) or destination port (--dport) options with the port that the service uses IPtables lets you use names for ports such as www for the web server port The names of services and the ports they use are listed in the /etc/services file, which maps ports to particular services For a domain name server, the port would be domain You can also use the port number if you want, preceding the number with a colon The following example accepts all messages to the web server located at 192168043:
iptables -A INPUT -d 192168043 --dport www -j ACCEPT
You can also use port references to protect certain services and deny others This approach is often used if you are designing a firewall that is much more open to the Internet, letting users make freer use of Internet connections Certain services you know can be harmful, such as Telnet and NTP, can be denied selectively For example, to deny any kind of Telnet operation on your firewall, you can drop all packets coming in on the Telnet port, 23 To protect NFS operations, you can deny access to the port used for the portmapper, 111 You can use either the port number or the port name
# deny outside access iptables -A arriving # deny outside access iptables -A arriving to -j to -j portmapper port on firewall DROP -p tcp -i eth0 --dport 111 telnet port on firewall DROP -p tcp -i eth0 --dport telnet
The rule listing will look like this:
DROP DROP tcp tcp --0000/0 0000/0 0000/0 0000/0 tcp dpt:111 tcp dpt:23
One port-related security problem is access to your X server on the XFree86 ports that range from 6000 to 6009 On a relatively open firewall, these ports could be used to illegally access your system through your X server A range of ports can be specified with a colon, as in 6000:6009 You can also use x11 for the first port, x11:6009 Sessions on the X server can be secured by using SSH, which normally accesses the X server on port 6010
iptables -A arriving -j DROP -p tcp -i eth0 --dport 6000:6009
Common ports checked and their labels are shown here:
Service Auth Finger F TP NTP Portmapper Telnet Web server XFree86 Port Number 113 79 21 123 111 23 80 6000:6009 Port Label auth finger ftp ntp sunrpc telnet www x11:6009
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