[b4b71892] | 1 | <?xml version="1.0" encoding="ISO-8859-1"?>
|
---|
| 2 | <!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.3//EN"
|
---|
| 3 | "http://www.oasis-open.org/docbook/xml/4.3/docbookx.dtd" [
|
---|
| 4 | <!ENTITY % general-entities SYSTEM "../../general.ent">
|
---|
| 5 | %general-entities;
|
---|
| 6 | ]>
|
---|
| 7 |
|
---|
[dd362e5] | 8 | <sect1 id="fw-firewall" xreflabel="Firewalling">
|
---|
[a0f03b0] | 9 | <sect1info>
|
---|
[5cd0959d] | 10 | <othername>$LastChangedBy$</othername>
|
---|
| 11 | <date>$Date$</date>
|
---|
[a0f03b0] | 12 | </sect1info>
|
---|
[bae6e15] | 13 | <?dbhtml filename="firewall.html"?>
|
---|
[f45b1953] | 14 | <title>Setting up a network firewall</title>
|
---|
| 15 |
|
---|
[36ae4612] | 16 | <para>Before you read this part of the chapter, note that we assume that you
|
---|
| 17 | have already installed iptables as described in the previous section.</para>
|
---|
[f45b1953] | 18 |
|
---|
[dd362e5] | 19 | <sect2 id="fw-intro" xreflabel="Firewalling Introduction">
|
---|
[b4b71892] | 20 | <title>Introduction to Firewall Creation</title>
|
---|
| 21 |
|
---|
| 22 | <para>The general purpose of a firewall is to protect a network
|
---|
| 23 | against malicious access by using a single machine as a firewall.
|
---|
| 24 | This does imply that the firewall is to be considered a single point
|
---|
| 25 | of failure, but it can make the administrator's life a lot easier.</para>
|
---|
| 26 |
|
---|
| 27 | <para>In a perfect world where you knew that every daemon or service
|
---|
| 28 | on every machine was perfectly configured and was immune to, e.g.,
|
---|
| 29 | buffer-overflows and any other imaginable problem regarding its
|
---|
| 30 | security, and where you trusted every user accessing your services
|
---|
| 31 | to aim no harm, you wouldn't need to have a firewall!
|
---|
| 32 | In the real world however, daemons may be misconfigured,
|
---|
| 33 | exploits against essential services are freely available, you
|
---|
| 34 | may wish to choose which services are accessible by certain machines,
|
---|
| 35 | you may wish to limit which machines or applications are allowed
|
---|
[dd362e5] | 36 | to have Internet access, or you may simply not trust some of your
|
---|
| 37 | apps or users. In these situations you might benefit by using a
|
---|
| 38 | firewall.</para>
|
---|
[b4b71892] | 39 |
|
---|
| 40 | <para>Don't assume however, that having a firewall makes careful
|
---|
| 41 | configuration redundant, or that it makes any negligent
|
---|
| 42 | misconfiguration harmless. It also doesn't prevent anyone from exploiting a
|
---|
| 43 | service you intentionally offer but haven't recently updated or patched
|
---|
| 44 | after an exploit went public. Despite having a firewall, you need to
|
---|
| 45 | keep applications and daemons on your system well-configured and
|
---|
| 46 | up-to-date; a firewall is not a cure-all!</para>
|
---|
| 47 |
|
---|
| 48 | </sect2>
|
---|
| 49 |
|
---|
| 50 | <sect2>
|
---|
| 51 | <title>Meaning of the word firewall.</title>
|
---|
| 52 |
|
---|
| 53 | <para>The word firewall can have several different meanings.</para>
|
---|
| 54 |
|
---|
[dd362e5] | 55 | <sect3><title><xref linkend="fw-persFw"/></title>
|
---|
[b4b71892] | 56 |
|
---|
| 57 | <para>This is a setup or program, for Windows commercially sold by
|
---|
| 58 | companies such as Symantec, of which they claim or pretend that it
|
---|
| 59 | secures a home or desktop-pc with Internet access. This topic is
|
---|
| 60 | highly relevant for users who do not know the methods their computers
|
---|
| 61 | might be accessed via the Internet or how to disable them,
|
---|
| 62 | especially if they are always online and connected via
|
---|
| 63 | broadband links.</para></sect3>
|
---|
| 64 |
|
---|
[dd362e5] | 65 | <sect3><title><xref linkend="fw-masqRouter"/></title>
|
---|
[b4b71892] | 66 | <para>This is a box placed between the Internet and an intranet.
|
---|
| 67 | To minimize the risk of compromising the firewall itself it
|
---|
| 68 | should generally have only one role, that of protecting the intranet.
|
---|
| 69 | Although not completely risk free, the tasks of doing the routing
|
---|
| 70 | and eventually IP masquerading (rewriting IP-headers
|
---|
| 71 | of the packets it routes from clients with private IP-addresses onto
|
---|
| 72 | the Internet so that they seem to come from the firewall
|
---|
| 73 | itself) are commonly considered harmless.</para></sect3>
|
---|
| 74 |
|
---|
[dd362e5] | 75 | <sect3><title><xref linkend="fw-busybox"/></title>
|
---|
[b4b71892] | 76 | <para>This is often an old box you may have retired and nearly forgotten,
|
---|
| 77 | performing masquerading or routing functions, but offering a bunch of
|
---|
| 78 | services, e.g., web-cache, mail, etc. This may be very commonly used
|
---|
| 79 | for home networks, but can definitely not be considered as secure
|
---|
| 80 | anymore because the combining of server and router on one machine raises
|
---|
| 81 | the complexity of the setup.</para></sect3>
|
---|
| 82 |
|
---|
| 83 | <sect3><title>Firewall with a demilitarized zone [not further described
|
---|
| 84 | here]</title>
|
---|
| 85 | <para>This box performs masquerading or routing, but grants public access to
|
---|
| 86 | some branch of your network which, because of public IP's and a physically
|
---|
| 87 | separated structure, is neither considered to be part of the inter- nor
|
---|
| 88 | intranet. These servers are those which must be easily accessible
|
---|
| 89 | from both the inter- and intranet. The firewall protects
|
---|
| 90 | them all.</para></sect3>
|
---|
| 91 |
|
---|
| 92 | <sect3><title>Packetfilter / partly accessible net [partly described
|
---|
[dd362e5] | 93 | here, see <xref linkend="fw-busybox"/>]</title>
|
---|
[b4b71892] | 94 | <para>Doing routing or masquerading, but permitting only selected
|
---|
| 95 | services to be accessible, sometimes only by selected internal users or boxes;
|
---|
| 96 | mostly used in highly secure business contexts, sometimes by distrusting
|
---|
| 97 | employers. This was the common configuration of a firewall at the time of
|
---|
| 98 | the Linux 2.2 kernel. It's still possible to configure a firewall this way,
|
---|
| 99 | but it makes the rules quite complex and lengthy.</para></sect3>
|
---|
| 100 |
|
---|
| 101 | </sect2>
|
---|
| 102 |
|
---|
| 103 | <sect2 id="postlfs-security-fw-disclaimer" xreflabel="Disclaimer">
|
---|
| 104 | <title>Disclaimer</title>
|
---|
| 105 |
|
---|
| 106 | <!-- <para><emphasis>NEITHER THE AUTHOR NOR ANY OF THE LINUXFROMSCRATCH TEAM
|
---|
| 107 | ARE RESPONSIBLE FOR ANY DAMAGES INCURRED DUE TO ACTIONS TAKEN BASED ON THIS
|
---|
| 108 | DOCUMENT.</emphasis></para> -->
|
---|
| 109 |
|
---|
| 110 | <para>This document is meant as an introduction to how to setup a firewall. It
|
---|
| 111 | is not a complete guide to securing systems. Firewalling is a complex issue
|
---|
| 112 | that requires careful configuration. The scripts quoted here are simply
|
---|
| 113 | intended to give examples as to how a firewall works, they are not intended to
|
---|
| 114 | fit into any imaginable configuration and may not prevent any imaginable
|
---|
| 115 | attack.</para>
|
---|
| 116 |
|
---|
| 117 | <para>The purpose of this text is simply to give you a hint on how to get
|
---|
| 118 | started with a firewall.</para>
|
---|
| 119 |
|
---|
| 120 | <para>Customization of these scripts for your specific situation will
|
---|
| 121 | be necessary for an optimal configuration, but you should make a serious
|
---|
[dd362e5] | 122 | study of the iptables documentation and creating firewalls in general before
|
---|
| 123 | hacking away. Have a look at the list of
|
---|
| 124 | <xref linkend="fw-library"/> at the end of this section for
|
---|
| 125 | more details. Here you will find a list of URLs that contain quite
|
---|
| 126 | comprehensive information about building your own firewall.</para>
|
---|
[b4b71892] | 127 |
|
---|
| 128 | </sect2>
|
---|
| 129 |
|
---|
[dd362e5] | 130 | <sect2 id="fw-kernel" xreflabel="getting a firewalling-enabled Kernel">
|
---|
[b4b71892] | 131 | <title>Getting a firewall enabled Kernel</title>
|
---|
| 132 |
|
---|
| 133 | <para>If you want your Linux-Box to have a firewall, you must first ensure
|
---|
| 134 | that your kernel has been compiled with the relevant options turned on.
|
---|
[dd362e5] | 135 | <!-- <footnote><para>If you needed assistance how to configure, compile and
|
---|
| 136 | install a new kernel, refer back to chapter VIII of the LinuxFromScratch book,
|
---|
| 137 | <ulink url="http://www.linuxfromscratch.org/view/3.1/chapter08/kernel.html">
|
---|
| 138 | Installing a kernel</ulink> and eventually
|
---|
| 139 | <ulink url="http://www.linuxfromscratch.org/view/3.1/chapter08/lilo.html">
|
---|
| 140 | Making the LFS system bootable</ulink>; note, that you'll need to reboot
|
---|
[b4b71892] | 141 | to actually run your new kernel.</para></footnote>-->
|
---|
| 142 | </para>
|
---|
| 143 |
|
---|
| 144 | <para>How to configure your kernel, with enabling the options to be
|
---|
| 145 | either compiled into the kernel or as modules, depends on your personal
|
---|
| 146 | preferences and experience. Note, that for the quoted scripts it is assumed
|
---|
| 147 | that the modules need to be loaded at first.</para>
|
---|
| 148 |
|
---|
| 149 | <screen>Network options menu
|
---|
| 150 | Network packet filtering: Y
|
---|
| 151 | Unix domain sockets: Y or M
|
---|
| 152 | TCP/IP networking: Y
|
---|
| 153 | IP: advanced router: Y
|
---|
| 154 | IP: verbose route monitoring: Y
|
---|
| 155 | IP: TCP Explicit Congestion Notification support: Y
|
---|
| 156 | IP: TCP syncookie support: Y
|
---|
| 157 | IP: Netfilter Configuration menu
|
---|
| 158 | Every option except: Y or M
|
---|
| 159 | ipchains (2.2-style) support N
|
---|
| 160 | ipfwadm (2.0-style) support N
|
---|
| 161 | Fast switching: N</screen>
|
---|
| 162 |
|
---|
| 163 | <!--
|
---|
| 164 | <table frame='none'>
|
---|
| 165 | <title>Essential config-options for a firewall enabled Kernel</title>
|
---|
| 166 |
|
---|
| 167 | <tgroup cols='5'>
|
---|
| 168 | <colspec colnum='1' colwidth='8*' align='center'/>
|
---|
| 169 | <colspec colnum='2' colwidth='19*' align='left'/>
|
---|
| 170 | <colspec colnum='3' colwidth='11*' align='center'/>
|
---|
| 171 | <colspec colnum='4' colwidth='1*' align='center'/>
|
---|
| 172 | <colspec colnum='5' colwidth='14*' align='left'/>
|
---|
| 173 |
|
---|
| 174 | <tbody>
|
---|
| 175 |
|
---|
| 176 | <row>
|
---|
| 177 | <entry><emphasis><userinput>Networking options:</userinput></emphasis></entry>
|
---|
| 178 | <entry><userinput>Network packet filtering</userinput></entry>
|
---|
| 179 | <entry></entry>
|
---|
| 180 | <entry>=</entry>
|
---|
| 181 | <entry>CONFIG_NETFILTER</entry>
|
---|
| 182 | </row>
|
---|
| 183 |
|
---|
| 184 | <row>
|
---|
| 185 | <entry></entry>
|
---|
| 186 | <entry><userinput>Unix domain sockets</userinput></entry>
|
---|
| 187 | <entry></entry>
|
---|
| 188 | <entry>=</entry>
|
---|
| 189 | <entry>CONFIG_UNIX</entry>
|
---|
| 190 | </row>
|
---|
| 191 |
|
---|
| 192 | <row>
|
---|
| 193 | <entry></entry>
|
---|
| 194 | <entry><userinput>IP: TCP/IP networking</userinput></entry>
|
---|
| 195 | <entry></entry>
|
---|
| 196 | <entry>=</entry>
|
---|
| 197 | <entry>CONFIG_INET</entry>
|
---|
| 198 | </row>
|
---|
| 199 |
|
---|
| 200 | <row>
|
---|
| 201 | <entry></entry>
|
---|
| 202 | <entry><userinput>IP: advanced router</userinput></entry>
|
---|
| 203 | <entry></entry>
|
---|
| 204 | <entry>=</entry>
|
---|
| 205 | <entry>CONFIG_IP_ADVANCED_ROUTER</entry>
|
---|
| 206 | </row>
|
---|
| 207 |
|
---|
| 208 | <row>
|
---|
| 209 | <entry></entry>
|
---|
| 210 | <entry><userinput>IP: verbose route monitoring</userinput></entry>
|
---|
| 211 | <entry></entry>
|
---|
| 212 | <entry>=</entry>
|
---|
| 213 | <entry>CONFIG_IP_ROUTE_VERBOSE</entry>
|
---|
| 214 | </row>
|
---|
| 215 |
|
---|
| 216 | <row>
|
---|
| 217 | <entry></entry>
|
---|
| 218 | <entry><userinput>IP: TCP Explicit Congestion Notification support</userinput></entry>
|
---|
| 219 | <entry></entry>
|
---|
| 220 | <entry>=</entry>
|
---|
| 221 | <entry>CONFIG_INET_ECN</entry>
|
---|
| 222 | </row>
|
---|
| 223 |
|
---|
| 224 | <row>
|
---|
| 225 | <entry></entry>
|
---|
| 226 | <entry><userinput>IP: TCP syncookie support</userinput></entry>
|
---|
| 227 | <entry></entry>
|
---|
| 228 | <entry>=</entry>
|
---|
| 229 | <entry>CONFIG_SYN_COOKIES</entry>
|
---|
| 230 | </row>
|
---|
| 231 |
|
---|
| 232 | <row>
|
---|
| 233 | <entry></entry>
|
---|
| 234 | <entry align='center'>
|
---|
| 235 | <emphasis><userinput>IP: Netfilter Configuration:</userinput></emphasis></entry>
|
---|
| 236 | <entry align='left'><userinput>every option</userinput></entry>
|
---|
| 237 | <entry>=</entry>
|
---|
| 238 | <entry>CONFIG_IP_NF_*</entry>
|
---|
| 239 | </row>
|
---|
| 240 |
|
---|
| 241 | <row>
|
---|
| 242 | <entry></entry>
|
---|
| 243 | <entry align='right'><emphasis>WITHOUT:</emphasis></entry>
|
---|
| 244 | <entry align='left'><literallayout><userinput>ipchains (2.2-style) support
|
---|
| 245 | ipfw-adm (2.0-style) support</userinput></literallayout></entry>
|
---|
| 246 | <entry>w\</entry>
|
---|
| 247 | <entry>CONFIG_IP_NF_COMPAT_*</entry>
|
---|
| 248 | </row>
|
---|
| 249 |
|
---|
| 250 | <row>
|
---|
| 251 | <entry></entry>
|
---|
| 252 | <entry><userinput>Fast switching</userinput></entry>
|
---|
| 253 | <entry>Make sure to disable it because it would setup a bypass around
|
---|
| 254 | your firewall rules.</entry>
|
---|
| 255 | <entry>w\</entry>
|
---|
| 256 | <entry>CONFIG_NET_FASTROUTE</entry>
|
---|
| 257 | </row>
|
---|
| 258 |
|
---|
| 259 | </tbody>
|
---|
| 260 |
|
---|
| 261 | </tgroup>
|
---|
| 262 |
|
---|
| 263 | </table> -->
|
---|
| 264 |
|
---|
| 265 | </sect2>
|
---|
| 266 |
|
---|
[dd362e5] | 267 | <sect2 id="fw-writing" xreflabel="writing the firewalling-setup-scripts">
|
---|
[b4b71892] | 268 | <title>Now you can start to build your Firewall</title>
|
---|
| 269 |
|
---|
[dd362e5] | 270 | <sect3 id="fw-persFw" xreflabel="Personal Firewall">
|
---|
[b4b71892] | 271 | <title>Personal Firewall</title>
|
---|
| 272 |
|
---|
| 273 | <para>A Personal Firewall is supposed to let you access all the services
|
---|
| 274 | offered on the Internet, but keep your box secure and your data private.</para>
|
---|
| 275 |
|
---|
| 276 | <para>Below is a slightly modified version of Rusty Russell's recommendation
|
---|
[dd362e5] | 277 | from the <ulink
|
---|
| 278 | url="http://www.netfilter.org/documentation/HOWTO/packet-filtering-HOWTO.html">
|
---|
| 279 | Linux 2.4 Packet Filtering HOWTO</ulink>:</para>
|
---|
[b4b71892] | 280 |
|
---|
| 281 | <screen><userinput><command>cat > /etc/rc.d/init.d/firewall << "EOF"</command>
|
---|
| 282 | #!/bin/sh
|
---|
| 283 |
|
---|
| 284 | # Begin $rc_base/init.d/firewall
|
---|
| 285 |
|
---|
[dd362e5] | 286 | # Insert connection-tracking modules
|
---|
| 287 | # (not needed if built into the kernel)
|
---|
[b4b71892] | 288 | modprobe ip_tables
|
---|
| 289 | modprobe iptable_filter
|
---|
| 290 | modprobe ip_conntrack
|
---|
| 291 | modprobe ip_conntrack_ftp
|
---|
| 292 | modprobe ipt_state
|
---|
| 293 | modprobe ipt_LOG
|
---|
| 294 |
|
---|
| 295 | # allow local-only connections
|
---|
| 296 | iptables -A INPUT -i lo -j ACCEPT
|
---|
[dd362e5] | 297 |
|
---|
| 298 | # free output on any interface to any ip for any service
|
---|
| 299 | # (equal to -P ACCEPT)
|
---|
[b4b71892] | 300 | iptables -A OUTPUT -j ACCEPT
|
---|
| 301 |
|
---|
| 302 | # permit answers on already established connections
|
---|
[dd362e5] | 303 | # and permit new connections related to established ones
|
---|
| 304 | # (eg active-ftp)
|
---|
[b4b71892] | 305 | iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
|
---|
| 306 |
|
---|
| 307 | # Log everything else: What's Windows' latest exploitable vulnerability?
|
---|
| 308 | iptables -A INPUT -j LOG --log-prefix "FIREWALL:INPUT "
|
---|
| 309 |
|
---|
| 310 | # set a sane policy: everything not accepted > /dev/null
|
---|
| 311 | iptables -P INPUT DROP
|
---|
| 312 | iptables -P FORWARD DROP
|
---|
| 313 | iptables -P OUTPUT DROP
|
---|
| 314 |
|
---|
[dd362e5] | 315 | # be verbose on dynamic ip-addresses (not needed in case of static IP)
|
---|
[b4b71892] | 316 | echo 2 > /proc/sys/net/ipv4/ip_dynaddr
|
---|
| 317 |
|
---|
[dd362e5] | 318 | # disable ExplicitCongestionNotification
|
---|
| 319 | # too many routers are still ignorant
|
---|
[b4b71892] | 320 | echo 0 > /proc/sys/net/ipv4/tcp_ecn
|
---|
| 321 |
|
---|
| 322 | # End $rc_base/init.d/firewall
|
---|
| 323 | <command>EOF</command></userinput></screen>
|
---|
| 324 |
|
---|
| 325 | <para>His script is quite simple, it drops all traffic coming in into your
|
---|
| 326 | computer that wasn't initiated from your box, but as long as you are simply
|
---|
| 327 | surfing the Internet you are unlikely to exceed its limits.</para>
|
---|
| 328 |
|
---|
| 329 | <para>If you frequently encounter certain delays at accessing ftp-servers,
|
---|
[dd362e5] | 330 | please have a look at <xref linkend="fw-busybox"/> -
|
---|
| 331 | <xref linkend="fw-BB-4"/>.</para>
|
---|
[b4b71892] | 332 |
|
---|
| 333 | <para>Even if you have daemons or services running on your box, these
|
---|
| 334 | should be inaccessible everywhere but from your box itself.
|
---|
[dd362e5] | 335 | If you want to allow access to services on your machine, such as ssh or
|
---|
| 336 | pinging, take a look at <xref linkend="fw-busybox"/>.</para>
|
---|
[b4b71892] | 337 |
|
---|
| 338 | </sect3>
|
---|
| 339 |
|
---|
| 340 |
|
---|
[dd362e5] | 341 | <sect3 id="fw-masqRouter" xreflabel="Masquerading Router">
|
---|
[b4b71892] | 342 | <title>Masquerading Router</title>
|
---|
| 343 |
|
---|
| 344 | <para>A true Firewall has two interfaces, one connected to an intranet,
|
---|
| 345 | in this example, <emphasis role="strong">eth0</emphasis>, and one
|
---|
| 346 | connected to the Internet, here, <emphasis role="strong">ppp0</emphasis>.
|
---|
| 347 | To provide the maximum security against the box itself being broken into,
|
---|
| 348 | make sure that there are no servers running on it, especially not
|
---|
| 349 | <application>X11</application> et
|
---|
[dd362e5] | 350 | al. And, as a general principle, the box itself should not access any
|
---|
| 351 | untrusted service (Think of a name server giving answers that make your
|
---|
[b4b71892] | 352 | bind crash, or, even worse, that implement a worm via a
|
---|
| 353 | buffer-overflow).</para>
|
---|
| 354 |
|
---|
| 355 | <screen><userinput><command>cat > /etc/rc.d/init.d/firewall << "EOF"</command>
|
---|
| 356 | #!/bin/sh
|
---|
| 357 |
|
---|
| 358 | # Begin $rc_base/init.d/firewall
|
---|
| 359 |
|
---|
| 360 | echo
|
---|
| 361 | echo "You're using the example-config for a setup of a firewall"
|
---|
| 362 | echo "from the firewalling-hint written for LinuxFromScratch."
|
---|
| 363 | echo "This example is far from being complete, it is only meant"
|
---|
| 364 | echo "to be a reference."
|
---|
| 365 | echo "Firewall security is a complex issue, that exceeds the scope"
|
---|
| 366 | echo "of the quoted configuration rules."
|
---|
| 367 | echo "You can find some quite comprehensive information"
|
---|
| 368 | echo "about firewalls in Chapter 4 of the BLFS book."
|
---|
| 369 | echo "http://www.linuxfromscratch.org/blfs"
|
---|
| 370 | echo
|
---|
| 371 |
|
---|
| 372 | # Insert iptables modules (not needed if built into the kernel).
|
---|
| 373 |
|
---|
| 374 | modprobe ip_tables
|
---|
| 375 | modprobe iptable_filter
|
---|
| 376 | modprobe ip_conntrack
|
---|
| 377 | modprobe ip_conntrack_ftp
|
---|
| 378 | modprobe ipt_state
|
---|
| 379 | modprobe iptable_nat
|
---|
| 380 | modprobe ip_nat_ftp
|
---|
| 381 | modprobe ipt_MASQUERADE
|
---|
| 382 | modprobe ipt_LOG
|
---|
| 383 | modprobe ipt_REJECT
|
---|
| 384 |
|
---|
| 385 | # allow local-only connections
|
---|
| 386 | iptables -A INPUT -i lo -j ACCEPT
|
---|
| 387 | iptables -A OUTPUT -o lo -j ACCEPT
|
---|
| 388 |
|
---|
| 389 | # allow forwarding
|
---|
| 390 | iptables -A FORWARD -m state --state ESTABLISHED,RELATED -j ACCEPT
|
---|
| 391 | iptables -A FORWARD -m state --state NEW -i ! ppp+ -j ACCEPT
|
---|
| 392 |
|
---|
[dd362e5] | 393 | # do masquerading
|
---|
| 394 | # (not needed if intranet is not using private ip-addresses)
|
---|
[b4b71892] | 395 | iptables -t nat -A POSTROUTING -o ppp+ -j MASQUERADE
|
---|
| 396 |
|
---|
[dd362e5] | 397 | # Log everything for debugging
|
---|
| 398 | # (last of all rules, but before DROP/REJECT)
|
---|
[b4b71892] | 399 | iptables -A INPUT -j LOG --log-prefix "FIREWALL:INPUT "
|
---|
| 400 | iptables -A FORWARD -j LOG --log-prefix "FIREWALL:FORWARD"
|
---|
| 401 | iptables -A OUTPUT -j LOG --log-prefix "FIREWALL:OUTPUT "
|
---|
| 402 |
|
---|
| 403 | # set a sane policy
|
---|
| 404 | iptables -P INPUT DROP
|
---|
| 405 | iptables -P FORWARD DROP
|
---|
| 406 | iptables -P OUTPUT DROP
|
---|
| 407 |
|
---|
[dd362e5] | 408 | # be verbose on dynamic ip-addresses
|
---|
| 409 | # (not needed in case of static IP)
|
---|
[b4b71892] | 410 | echo 2 > /proc/sys/net/ipv4/ip_dynaddr
|
---|
| 411 |
|
---|
| 412 | # disable ExplicitCongestionNotification
|
---|
| 413 | echo 0 > /proc/sys/net/ipv4/tcp_ecn
|
---|
| 414 |
|
---|
| 415 | # activate TCPsyncookies
|
---|
| 416 | echo 1 > /proc/sys/net/ipv4/tcp_syncookies
|
---|
| 417 |
|
---|
| 418 | # activate Route-Verification = IP-Spoofing_protection
|
---|
| 419 | for f in /proc/sys/net/ipv4/conf/*/rp_filter; do
|
---|
| 420 | echo 1 > $f
|
---|
| 421 | done
|
---|
| 422 |
|
---|
| 423 | # activate IP-Forwarding
|
---|
| 424 | echo 1 > /proc/sys/net/ipv4/ip_forward
|
---|
| 425 | <command>EOF</command></userinput></screen>
|
---|
| 426 |
|
---|
| 427 | <para>With this script your intranet should be sufficiently secure against
|
---|
| 428 | external attacks. No one should be able to setup a new connection to any
|
---|
| 429 | internal service and, if it's masqueraded, it's even invisible. Furthermore,
|
---|
| 430 | your firewall should be nearly immune because there are no services running
|
---|
| 431 | that a cracker could attack.</para>
|
---|
| 432 |
|
---|
| 433 | <para>Note: if the interface you're connecting to the Internet
|
---|
| 434 | doesn't connect via ppp, you will need to change
|
---|
| 435 | <replaceable>ppp+</replaceable> to the name of the interface which you are
|
---|
| 436 | using. If you are using the same interface type to connect to both your
|
---|
| 437 | intranet and the Internet, you need to use the actual name of the
|
---|
| 438 | interface such as <emphasis role="strong">eth0</emphasis>,
|
---|
| 439 | on both interfaces.</para>
|
---|
| 440 |
|
---|
| 441 | <para>If you need stronger security (e.g., against DOS, connection
|
---|
| 442 | highjacking, spoofing, etc.), have a look at the list of
|
---|
[dd362e5] | 443 | <xref linkend="fw-library"/> at the end of this section.</para>
|
---|
[b4b71892] | 444 |
|
---|
| 445 | </sect3>
|
---|
| 446 |
|
---|
[dd362e5] | 447 | <sect3 id="fw-busybox" xreflabel="BusyBox">
|
---|
[b4b71892] | 448 | <title>BusyBox</title>
|
---|
| 449 |
|
---|
[dd362e5] | 450 | <para>This scenario isn't too different from (<xref linkend="fw-masqRouter"/>),
|
---|
[b4b71892] | 451 | but in this case you want to offer some services to your intranet.
|
---|
| 452 | Examples of this can be when you want to admin your box from another host
|
---|
| 453 | on your intranet or use it as a proxy or a name server. Note: Outlining a true
|
---|
| 454 | concept of how to protect a server that offers services on the Internet
|
---|
| 455 | goes far beyond the scope of this document,
|
---|
| 456 | see <xref linkend="postlfs-security-fw-disclaimer"/>.</para>
|
---|
| 457 |
|
---|
| 458 | <para>Be cautious. Every service you offer and have enabled makes your
|
---|
| 459 | setup more complex and your box less secure. You induce the risks of
|
---|
[dd362e5] | 460 | misconfigured services or running a service with an exploitable bug. A
|
---|
| 461 | firewall should generally not run any extra services. See the introduction to
|
---|
| 462 | <xref linkend="fw-masqRouter"/> for some more details.</para>
|
---|
[b4b71892] | 463 |
|
---|
| 464 | <para>If the services you'd like to offer do not need to access the Internet
|
---|
| 465 | themselves, like internal-only samba- or name-servers, it's quite
|
---|
| 466 | simple and should still be acceptable from a security standpoint.
|
---|
| 467 | Just add the following lines <emphasis>before</emphasis> the logging-rules
|
---|
| 468 | into the script.</para>
|
---|
| 469 |
|
---|
| 470 | <screen>iptables -A INPUT -i ! ppp+ -j ACCEPT
|
---|
| 471 | iptables -A OUTPUT -o ! ppp+ -j ACCEPT</screen>
|
---|
| 472 |
|
---|
| 473 | <para>If your daemons have to access the web themselves, like squid would need
|
---|
| 474 | to, you could open OUTPUT generally and restrict INPUT.</para>
|
---|
| 475 |
|
---|
| 476 | <screen>iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
|
---|
[dd362e5] | 477 | iptables -A OUTPUT -j ACCEPT</screen>
|
---|
[b4b71892] | 478 |
|
---|
[dd362e5] | 479 | <para>However, it is generally not advisable to leave OUTPUT unrestricted. You
|
---|
| 480 | lose any control over trojans who'd like to "call home", and a bit of
|
---|
| 481 | redundancy in case you've (mis-)configured a service so that it does broadcast
|
---|
| 482 | its existence to the world.</para>
|
---|
[b4b71892] | 483 |
|
---|
| 484 | <para>If you prefer to have this protection, you may restrict INPUT and OUTPUT
|
---|
| 485 | on all ports except those that it's absolutely necessary to have open.
|
---|
| 486 | Which ports you have to open depends on your needs: mostly you will find them
|
---|
| 487 | by looking for failed accesses in your log-files.</para>
|
---|
| 488 | <itemizedlist spacing="compact">
|
---|
| 489 | <!-- <orderedlist numeration="arabic" spacing="compact"> -->
|
---|
| 490 | <title>Have a look at the following examples:</title>
|
---|
| 491 |
|
---|
| 492 | <listitem><para>Squid is caching the web:</para>
|
---|
[dd362e5] | 493 | <screen>iptables -A OUTPUT -p tcp --dport 80 -j ACCEPT
|
---|
| 494 | iptables -A INPUT -p tcp --sport 80 -m state --state ESTABLISHED \
|
---|
| 495 | -j ACCEPT</screen>
|
---|
| 496 | </listitem>
|
---|
[b4b71892] | 497 |
|
---|
| 498 | <listitem><para>Your caching name server (e.g., dnscache) does its
|
---|
| 499 | lookups via udp:</para>
|
---|
[dd362e5] | 500 | <screen>iptables -A OUTPUT -p udp --dport 53 -j ACCEPT
|
---|
| 501 | iptables -A INPUT -p udp --sport 53 -m state --state ESTABLISHED \
|
---|
| 502 | -j ACCEPT</screen>
|
---|
| 503 | </listitem>
|
---|
| 504 |
|
---|
| 505 | <listitem><para>Alternatively, if you want to be able to ping your box to
|
---|
| 506 | ensure it's still alive:</para>
|
---|
[b4b71892] | 507 |
|
---|
| 508 | <screen>iptables -A INPUT -p icmp -m icmp --icmp-type echo-request -j ACCEPT
|
---|
[dd362e5] | 509 | iptables -A OUTPUT -p icmp -m icmp --icmp-type echo-reply -j ACCEPT</screen>
|
---|
| 510 | </listitem>
|
---|
[b4b71892] | 511 |
|
---|
[dd362e5] | 512 | <listitem><para><anchor id='fw-BB-4' xreflabel="example no. 4"/>If you are
|
---|
[b4b71892] | 513 | frequently accessing ftp-servers or enjoy chatting, you might notice certain
|
---|
| 514 | delays because some implementations of these daemons have the feature of
|
---|
| 515 | querying an identd on your box for logging usernames.
|
---|
| 516 | Although there's really no harm in this, having an identd running is not
|
---|
| 517 | recommended because some implementations are known to be vulnerable.</para>
|
---|
| 518 |
|
---|
| 519 | <para>To avoid these delays you could reject the requests
|
---|
| 520 | with a 'tcp-reset':</para>
|
---|
| 521 |
|
---|
| 522 | <screen>iptables -A INPUT -p tcp --dport 113 -j REJECT --reject-with tcp-reset
|
---|
[dd362e5] | 523 | iptables -A OUTPUT -p tcp --sport 113 -m state --state RELATED -j ACCEPT</screen>
|
---|
| 524 | </listitem>
|
---|
[b4b71892] | 525 |
|
---|
| 526 | <listitem><para>To log and drop invalid packets (harmless packets
|
---|
| 527 | that came in after netfilter's timeout or some types of network scans):</para>
|
---|
| 528 |
|
---|
[dd362e5] | 529 | <screen>iptables -I INPUT 1 -p tcp -m state --state INVALID -j LOG \
|
---|
| 530 | --log-prefix "FIREWALL:INVALID"
|
---|
[b4b71892] | 531 | iptables -I INPUT 2 -p tcp -m state --state INVALID -j DROP</screen></listitem>
|
---|
| 532 |
|
---|
| 533 | <listitem><para>Anything coming from the outside should not have a
|
---|
| 534 | private address, this is a common attack called IP-spoofing:</para>
|
---|
| 535 |
|
---|
| 536 | <screen>iptables -t nat -A PREROUTING -i ppp+ -s 10.0.0.0/8 -j DROP
|
---|
| 537 | iptables -t nat -A PREROUTING -i ppp+ -s 172.16.0.0/12 -j DROP
|
---|
[dd362e5] | 538 | iptables -t nat -A PREROUTING -i ppp+ -s 192.168.0.0/16 -j DROP</screen>
|
---|
| 539 | </listitem>
|
---|
[b4b71892] | 540 |
|
---|
| 541 | <listitem><para>To simplify debugging and be fair to anyone who'd like to
|
---|
| 542 | access a service you have disabled, purposely or by mistake, you should REJECT
|
---|
| 543 | those packets that are dropped.</para>
|
---|
| 544 |
|
---|
| 545 | <para>Obviously this must be done directly after logging as the very
|
---|
| 546 | last lines before the packets are dropped by policy:</para>
|
---|
| 547 |
|
---|
| 548 | <screen>iptables -A INPUT -j REJECT
|
---|
| 549 | iptables -A OUTPUT -p icmp --icmp-type 3 -j ACCEPT</screen></listitem>
|
---|
| 550 | </itemizedlist>
|
---|
| 551 | <!--</orderedlist>-->
|
---|
| 552 |
|
---|
| 553 | <para>These are only examples to show you some of the capabilities of the new
|
---|
| 554 | firewall code in Linux-Kernel 2.4. Have a look at the man page of
|
---|
| 555 | iptables.
|
---|
| 556 | There you will find more of them. The port-numbers you'll need for this
|
---|
| 557 | can be found in <filename>/etc/services</filename>, in case you didn't
|
---|
| 558 | find them by trial and error in your log file.</para>
|
---|
| 559 |
|
---|
| 560 | <para>If you add any of your offered or accessed services such as the above,
|
---|
| 561 | maybe even in FORWARD and for intranet-communication, and delete the
|
---|
| 562 | general clauses, you get an old fashioned packet filter.</para>
|
---|
| 563 | </sect3>
|
---|
| 564 |
|
---|
| 565 | </sect2>
|
---|
| 566 |
|
---|
[dd362e5] | 567 | <sect2 id="fw-finale" xreflabel="Conclusion">
|
---|
[b4b71892] | 568 | <title>Conclusion</title>
|
---|
| 569 |
|
---|
| 570 | <para>Finally, I'd like to remind you of one fact we must not forget:
|
---|
| 571 | The effort spent attacking a system corresponds to the value the cracker
|
---|
| 572 | expects to gain from it.
|
---|
| 573 | If you are responsible for such valuable assets that you expect great
|
---|
| 574 | effort to be made by potential crackers, you hopefully won't be in the
|
---|
| 575 | need of this hint!</para>
|
---|
| 576 |
|
---|
| 577 | <!-- <para><literallayout>Be cautious!
|
---|
| 578 |
|
---|
| 579 | Henning Rohde
|
---|
| 580 | <email>Henning.Rohde@uni-bayreuth.de</email></literallayout></para>
|
---|
| 581 |
|
---|
| 582 | <para>PS: And always do remember:
|
---|
| 583 | SecureIT is not a matter of a status-quo but one of never stopping
|
---|
| 584 | to take care!</para>
|
---|
| 585 |
|
---|
| 586 | <para>PPS: If any of these scripts fail, please tell me. I will try to trace
|
---|
| 587 | any faults.</para> -->
|
---|
| 588 |
|
---|
| 589 | </sect2>
|
---|
| 590 |
|
---|
| 591 | <sect2 id="postlfs-security-fw-extra" xreflabel="Extra Information">
|
---|
| 592 | <title>Extra Information</title>
|
---|
| 593 |
|
---|
[dd362e5] | 594 | <sect3 id="fw-library" xreflabel="Links for further reading">
|
---|
[b4b71892] | 595 | <title>Where to start with further reading on firewalls.</title>
|
---|
| 596 |
|
---|
| 597 | <para><blockquote><literallayout>
|
---|
| 598 | <ulink url="http://www.netfilter.org/">www.netfilter.org - Homepage of the netfilter/iptables project</ulink>
|
---|
| 599 | <ulink url="http://www.netfilter.org/documentation/FAQ/netfilter-faq.html">Netfilter related FAQ</ulink>
|
---|
| 600 | <ulink url="http://www.netfilter.org/documentation/index.html#HOWTO">Netfilter related HOWTO's</ulink>
|
---|
| 601 | <ulink url="http://en.tldp.org/LDP/nag2/x-087-2-firewall.html">en.tldp.org/LDP/nag2/x-087-2-firewall.html</ulink>
|
---|
| 602 | <ulink url="http://en.tldp.org/HOWTO/Security-HOWTO.html">en.tldp.org/HOWTO/Security-HOWTO.html</ulink>
|
---|
| 603 | <ulink url="http://en.tldp.org/HOWTO/Firewall-HOWTO.html">en.tldp.org/HOWTO/Firewall-HOWTO.html</ulink>
|
---|
| 604 | <ulink url="http://www-106.ibm.com/developerworks/security/library/s-fire.html">www.ibm.com/developerworks/security/library/s-fire.html</ulink>
|
---|
| 605 | <ulink url="http://www-106.ibm.com/developerworks/security/library/s-fire2.html">www.ibm.com/developerworks/security/library/s-fire2.html</ulink>
|
---|
| 606 | <ulink url="http://www.interhack.net/pubs/fw-faq/">www.interhack.net/pubs/fw-faq/</ulink>
|
---|
| 607 | <ulink url="http://www.linuxsecurity.com/docs/">www.linuxsecurity.com/docs/</ulink>
|
---|
| 608 | <ulink url="http://www.little-idiot.de/firewall">www.little-idiot.de/firewall (German & outdated, but very comprehensive)</ulink>
|
---|
| 609 | <ulink url="http://www.linuxgazette.com/issue65/stumpel.html">www.linuxgazette.com/issue65/stumpel.html</ulink>
|
---|
| 610 | <ulink url="http://linux.oreillynet.com/pub/a/linux/2000/03/10/netadmin/ddos.html">linux.oreillynet.com/pub/a/linux/2000/03/10/netadmin/ddos.html</ulink>
|
---|
| 611 | <ulink url="http://staff.washington.edu/dittrich/misc/ddos">staff.washington.edu/dittrich/misc/ddos</ulink>
|
---|
| 612 | <ulink url="http://www.e-infomax.com/ipmasq">www.e-infomax.com/ipmasq</ulink>
|
---|
| 613 | <ulink url="http://www.circlemud.org/~jelson/writings/security/index.htm">www.circlemud.org/~jelson/writings/security/index.htm</ulink>
|
---|
| 614 | <ulink url="http://www.securityfocus.com">www.securityfocus.com</ulink>
|
---|
| 615 | <ulink url="http://www.cert.org/tech_tips/">www.cert.org - tech_tips</ulink>
|
---|
| 616 | <ulink url="http://security.ittoolbox.com/">security.ittoolbox.com</ulink>
|
---|
| 617 | <ulink url="http://www.linux-firewall-tools.com/linux/">www.linux-firewall-tools.com/linux/</ulink>
|
---|
| 618 | <ulink url="http://logi.cc/linux/athome-firewall.php3">logi.cc/linux/athome-firewall.php3</ulink>
|
---|
| 619 | <ulink url="http://www.insecure.org/reading.html">www.insecure.org/reading.html</ulink>
|
---|
| 620 | <ulink url="http://www.robertgraham.com/pubs/firewall-seen.html">www.robertgraham.com/pubs/firewall-seen.html</ulink>
|
---|
| 621 | </literallayout></blockquote></para>
|
---|
| 622 | </sect3>
|
---|
| 623 |
|
---|
[dd362e5] | 624 | <sect3 id="fw-status" xreflabel="/etc/rc.d/init.d/firewall.status">
|
---|
[b4b71892] | 625 | <title>firewall.status</title>
|
---|
| 626 |
|
---|
| 627 | <para>If you'd like to have a look at the chains your firewall consists of and
|
---|
| 628 | the order in which the rules take effect:</para>
|
---|
| 629 |
|
---|
| 630 | <screen><userinput><command>cat > /etc/rc.d/init.d/firewall.status << "EOF"</command>
|
---|
| 631 | #!/bin/sh
|
---|
| 632 |
|
---|
| 633 | # Begin $rc_base/init.d/firewall.status
|
---|
| 634 |
|
---|
| 635 | echo "iptables.mangling:"
|
---|
| 636 | iptables -t mangle -v -L -n --line-numbers
|
---|
| 637 |
|
---|
| 638 | echo
|
---|
| 639 | echo "iptables.nat:"
|
---|
| 640 | iptables -t nat -v -L -n --line-numbers
|
---|
| 641 |
|
---|
| 642 | echo
|
---|
| 643 | echo "iptables.filter:"
|
---|
| 644 | iptables -v -L -n --line-numbers
|
---|
| 645 | <command>EOF</command></userinput></screen>
|
---|
| 646 | </sect3>
|
---|
| 647 |
|
---|
| 648 | <sect3 id="postlfs-security-fw-stop" xreflabel="/etc/rc.d/init.d/firewall.stop">
|
---|
| 649 | <title>firewall.stop</title>
|
---|
| 650 |
|
---|
| 651 | <para>If you need to turn the firewall off, this script will do it:</para>
|
---|
| 652 |
|
---|
| 653 | <screen><userinput><command>cat > /etc/rc.d/init.d/firewall.stop << "EOF"</command>
|
---|
| 654 | #!/bin/sh
|
---|
| 655 |
|
---|
| 656 | # Being $rc_base/init.d/firewall.stop
|
---|
| 657 |
|
---|
| 658 | # deactivate IP-Forwarding
|
---|
| 659 | echo 0 > /proc/sys/net/ipv4/ip_forward
|
---|
| 660 |
|
---|
| 661 | iptables -Z
|
---|
| 662 | iptables -F
|
---|
| 663 | iptables -t nat -F PREROUTING
|
---|
| 664 | iptables -t nat -F OUTPUT
|
---|
| 665 | iptables -t nat -F POSTROUTING
|
---|
| 666 | iptables -t mangle -F PREROUTING
|
---|
| 667 | iptables -t mangle -F OUTPUT
|
---|
| 668 | iptables -X
|
---|
| 669 | iptables -P INPUT ACCEPT
|
---|
| 670 | iptables -P FORWARD ACCEPT
|
---|
| 671 | iptables -P OUTPUT ACCEPT
|
---|
| 672 | <command>EOF</command></userinput></screen>
|
---|
| 673 | </sect3>
|
---|
| 674 |
|
---|
| 675 | </sect2>
|
---|
[dd362e5] | 676 |
|
---|
[f45b1953] | 677 | </sect1>
|
---|
| 678 |
|
---|