source: postlfs/security/firewalling/busybox.xml@ 1ea79a1

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Last change on this file since 1ea79a1 was 1ea79a1, checked in by Bruce Dubbs <bdubbs@…>, 17 years ago

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1<sect3 id="postlfs-security-fw-busybox" xreflabel="BusyBox">
2<title>BusyBox</title>
3
4<para>This scenario isn't too different from (<xref linkend="postlfs-security-fw-masqRouter"/>),
5but in this case you want to offer some services to your intranet.
6Examples of this can be when you want to admin your box from another host
7on your intranet or use it as a proxy or a name server. Note: Outlining a true
8concept of how to protect a server that offers services on the Internet
9goes far beyond the scope of this document,
10see <xref linkend="postlfs-security-fw-disclaimer"/>.</para>
11
12<para>Be cautious. Every service you offer and have enabled makes your
13setup more complex and your box less secure. You induce the risks of
14misconfigured services or running a service with an exploitable bug. A firewall
15should generally not run any extra services. See the introduction to
16<xref linkend="postlfs-security-fw-masqRouter"/> for some more details.</para>
17
18<para>If the services you'd like to offer do not need to access the Internet
19themselves, like internal-only samba- or name-servers, it's quite
20simple and should still be acceptable from a security standpoint.
21Just add the following lines <emphasis>before</emphasis> the logging-rules
22into the script.</para>
23
24<screen>iptables -A INPUT -i ! ppp+ -j ACCEPT
25iptables -A OUTPUT -o ! ppp+ -j ACCEPT</screen>
26
27<para>If your daemons have to access the web themselves, like squid would need
28to, you could open OUTPUT generally and restrict INPUT.</para>
29
30<screen>iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
31iptables -A OUTPUT -j ACCEPT</screen>
32
33<para>However, it is generally not advisable to leave OUTPUT unrestricted. You lose
34any control over trojans who'd like to "call home", and a bit of redundancy in case
35you've (mis-)configured a service so that it does broadcast its existence to the
36world.</para>
37
38<para>If you prefer to have this protection, you may restrict INPUT and OUTPUT
39on all ports except those that it's absolutely necessary to have open.
40Which ports you have to open depends on your needs: mostly you will find them
41by looking for failed accesses in your log-files.</para>
42<itemizedlist spacing="compact">
43<!-- <orderedlist numeration="arabic" spacing="compact"> -->
44<title>Have a look at the following examples:</title>
45
46<listitem><para>Squid is caching the web:</para>
47<screen>iptables -A OUTPUT -p tcp --dport 80 -j ACCEPT
48iptables -A INPUT -p tcp --sport 80 -m state --state ESTABLISHED -j ACCEPT</screen></listitem>
49
50<listitem><para>Your caching name server (e.g., dnscache) does its
51lookups via udp:</para>
52<screen>iptables -A OUTPUT -p udp --dport 53 -j ACCEPT
53iptables -A INPUT -p udp --sport 53 -m state --state ESTABLISHED -j ACCEPT</screen></listitem>
54
55<listitem><para>Alternatively, if you want to be able to ping your box to ensure
56it's still alive:</para>
57<screen>iptables -A INPUT -p icmp -m icmp --icmp-type echo-request -j ACCEPT
58iptables -A OUTPUT -p icmp -m icmp --icmp-type echo-reply -j ACCEPT</screen></listitem>
59
60<listitem><para><anchor id='postlfs-security-fw-BB-4' xreflabel="example no. 4"/>If you are
61frequently accessing ftp-servers or enjoy chatting, you might notice certain
62delays because some implementations of these daemons have the feature of
63querying an identd on your box for logging usernames.
64Although there's really no harm in this, having an identd running is not
65recommended because some implementations are known to be vulnerable.</para>
66
67<para>To avoid these delays you could reject the requests
68with a 'tcp-reset':</para>
69
70<screen>iptables -A INPUT -p tcp --dport 113 -j REJECT --reject-with tcp-reset
71iptables -A OUTPUT -p tcp --sport 113 -m state --state RELATED -j ACCEPT</screen></listitem>
72
73<listitem><para>To log and drop invalid packets (harmless packets
74that came in after netfilter's timeout or some types of network scans):</para>
75
76<screen>iptables -I INPUT 1 -p tcp -m state --state INVALID -j LOG --log-prefix \
77"FIREWALL:INVALID"
78iptables -I INPUT 2 -p tcp -m state --state INVALID -j DROP</screen></listitem>
79
80<listitem><para>Anything coming from the outside should not have a
81private address, this is a common attack called IP-spoofing:</para>
82
83<screen>iptables -t nat -A PREROUTING -i ppp+ -s 10.0.0.0/8 -j DROP
84iptables -t nat -A PREROUTING -i ppp+ -s 172.16.0.0/12 -j DROP
85iptables -t nat -A PREROUTING -i ppp+ -s 192.168.0.0/16 -j DROP</screen></listitem>
86
87<listitem><para>To simplify debugging and be fair to anyone who'd like to
88access a service you have disabled, purposely or by mistake, you should REJECT
89those packets that are dropped.</para>
90
91<para>Obviously this must be done directly after logging as the very
92last lines before the packets are dropped by policy:</para>
93
94<screen>iptables -A INPUT -j REJECT
95iptables -A OUTPUT -p icmp --icmp-type 3 -j ACCEPT</screen></listitem>
96</itemizedlist>
97<!--</orderedlist>-->
98
99<para>These are only examples to show you some of the capabilities of the new
100firewall code in Linux-Kernel 2.4. Have a look at the man page of
101iptables.
102There you will find more of them. The port-numbers you'll need for this
103can be found in <filename>/etc/services</filename>, in case you didn't
104find them by trial and error in your log file.</para>
105
106<para>If you add any of your offered or accessed services such as the above,
107maybe even in FORWARD and for intranet-communication, and delete the
108general clauses, you get an old fashioned packet filter.</para>
109
110
111</sect3>
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