Most Linux distributions are MASQ-Ready but its always good to check before you try to set things up. You will need things like:
IPTABLES (2.4.x) / IPCHAINS (2.2.x) / IPFWADM (2.0.x)
Kernel support for:
IP forwarding
IP masquerading
IP Firewalling
etc.
and have most MASQ-related modules compiled (most modular kernels will already have all you need), then you will NOT need to re-compile the kernel. If you AREN'T SURE if your Linux distribution is MASQ ready, do the following:
Run the command "ls /proc/sys/net/ipv4" while logged into the Linux box
2.4.x kernels (look for most of the following entries out of the much longer list):
ip_dynaddr
ip_forward
2.2.x kernels (look for most of the following entries out of the much longer list):
ip_always_defrag
ip_dynaddr
ip_forward
ip_masq_debug
ip_masq_udp_dloose
Running "ls /proc/net"
ip_fwchains
ip_fwnames
ip_masquerade
Running "ls /proc/net/ip_masq"
app
icmp
icq
mfw
portfw
tcp
udp
2.0.x kernels (look for most of the following entries out of the much longer list):
ip_dynaddr
ip_forward
running "ls /proc/net"
ip_forward
ip_masq_app
ip_masquerade
ip_portfw
See if files such as "ip_forward", "ip_masq_debug", "ip_masq_udp_dloose"(optional), and "ip_always_defrag"(optional) exist.
Does most of the entries for your major kernel version exist? If so, thats good! If you cannot find any of the above files or if you aren't sure if your distribution supports IP Masquerading by default, ASSUME IT DOESN'T. If this is the case.. you'll need to compile a kernel but don't worry.. it isn't hard.
Regardless if your current kernel has MASQ support or not, reading the remainder of this section is still highly recommended as it contains other useful information.
First of all, you need the kernel source for 2.4.x (preferably the latest kernel version)
NOTE #1: As the 2.4.x train and IPTABLES archive develoment progresses, the compile configurion options will probably keep changing. As of this version, this section reflects the settings for IPTABLES 1.2.4 and the 2.4.14 kernel. If you are compiling against a previous version, the dialogs will look different. It is recommended that you update to the newest versions for added capability, performance, and stability of the kernel.
Next, you really need the IPTABLES archive to apply patches against the kernel. For example, the IPTABLES patches enable the masquerading of IRC and FTP traffic as well as other network applications. You can find this archive in the 2.4.x requirements chapter which is in Section 2.6.
If this is your first time compiling the kernel, don't be scared. In fact, it's rather easy and it's covered in several URLs found in Section 2.6. Please note that the instructions included here is just one way to do build a kernel. Please see the Kernel HOWTO for full details.
NOTE: Please notice that it isn't recommended to put the new kernel sources into /usr/src/linux. You should leave the original kernel sources that came with your Linux distribution in /usr/src/linux. For more details on this topic, please read the "README" file in the top level directory of your kernel sources.
For this HOWTO example, create a directory called /usr/src/kernel. Next, "cd" into this directory and download the newest 2.4.x kernel sources into it. Once downloaded, issue the following command (if the file ends in a .tar.gz): tar xvzf linux-2.4.x.tar.gz or (if the file ends in a .tar.bzip2): tar xyvf linux-2.4.x.tar.bz2. Please substitute the "x" in the 2.4.x filename with the Linux 2.4 kernel version you downloaded.
NOTE: Some Linux distributions use the "I" option instead of the "y" option to decompress bzip2 archives.
Once uncompressed, I recommend that you rename the directory from "linux" to "linux-2.4.x" for clarity. To do this, run the command mv linux linux-2.4.x. Next, make sure there is a directory or symbolic link pointing to /usr/src/kernel/linux ie. run the command: ln -s /usr/src/kernel/linux-2.4.x /usr/src/kernel/linux again subsituting the "x" for your proper kernel version.
Next, it is highly recommended that you apply any appropriate or optional patches to the kernel source code BEFORE you compile the kernel. As of 2.4.4, you can apply the IPTABLES patches to enable special protocol support for programs like IRC, FTP, etc. Ultimately, IP Masq does not require any specific patching in order for the system to work for NAT-friendly network applications. Please refer to Section 2.6 for URLs and the IP Masquerade Resources for up-to-date information and patch URLs.
Applying the IPTABLES kernel patches
Download the iptables package from the Section 2.6 and put it into a directory, say /usr/src/archive/netfilter. Next, go into this new netfilter directory and uncompress the iptables archive with the command:
tar xyvf iptables-1.2.x.tar.bz2 |
Now, go into the new iptables-1.2.x directory and run the command
make pending-patches KERNEL_DIR=/usr/src/kernel/linux |
NOTE: this assumes that your 2.4.x kernel sources are in the /usr/src/kernel/linux directory.
NOTE #2: If you append a "/" to the end of the command line, you will get an error stating: "make: *** [/usr/src/kernel/linux/include/asm/socket.h] Error 1". Remove the trailing "/" and try again.
Here is an example of the prompts you might receive for a 2.4.14 kernel. Please note that these prompts WILL CHANGE over time.
Making dependencies: please wait... ----------------------------------------------------------------- Welcome to Rusty's Patch-o-matic! Each patch is a new feature: many have minimal impact, some do not. Almost every one has bugs, so I don't recommend applying them all! ------------------------------------------------------- Testing... ipt_MIRROR-ttl.patch NOT APPLIED (3 rejects out of 3 hunks) The ipt_MIRROR-ttl patch: Author: Harald Welte Status: Compiles, yet untested This adds TTL decrementing (and checking/dropping) in case the MIRROR target is used in INPUT or PREROUTING chains/hooks. This is to avoid endless packet loops. Do you want to apply this patch [N/y/t/f/q/?] y ------------------------------------------------------- Already applied: ipt_MIRROR-ttl ipt_REJECT-checkentry Testing... ipt_LOG.patch NOT APPLIED (1 rejects out of 1 hunks) The ipt_LOG patch: Author: Jozsef Kadlecsik Status: Submitted for kernel inclusion The LOG target has a bug when attempting to print the inner ip packet in icmp error messages. This patch fixes the bug. Do you want to apply this patch [N/y/t/f/q/?] y ------------------------------------------------------- Already applied: ipt_MIRROR-ttl ipt_REJECT-checkentry ipt_LOG 2.4.1 tos-fix Already applied: tcp-MSS 2.4.4 ip6tables-export-symbols Testing... sackperm.patch ALREADY APPLIED (0 rejects out of 1 hunks). Excellent! Kernel is now ready for compilation. |
If everything patches fine, you should see something like the text
Excellent! Kernel is now ready for compilation.
towards the bottom of the screen.
Ok, the kernel is ready to go but you should make sure that you also have the iptables program on your machine too. Run the command:
whereis iptables
Follow these simple steps:
make KERNEL_DIR=/usr/src/kernel/linux
make install KERNEL_DIR=/usr/src/kernel/linux
Now that the kernel is patched up, here is the MINIMUM kernel configuration options required to enable IP Masquerade functionality. Please understand that this HOWTO illustrates just ONE way to compile a kernel. The main difference from this method vs. a different one is some people wish to compile things either as modules OR monolithically right into the kernel. Basically, compiling things as modules gives you added flexibility to what is or isn't installed into the kernel (reduces unneeded memory use and allow for drop-in upgrades [no need to reboot]) BUT they add more complexity to your configuration. On the flip side, compiling things directly into the kernel makes things simpler BUT you loose a level of flexibility. The following example is a mixture of both built-in AND modules.
Side Note: It is assumed that you will also configure the kernel to use your other installed hardware such as network interfaces, optional SCSI controllers, etc as well. Please refer to the Linux Kernel HOWTO and the kernel source's README file and Documentation/ directory for detailed help on compiling a kernel.
Please note the YES or NO ANSWERS to the following. Not all options will be available without the proper kernel patches described later in this HOWTO.
Run the following commands to configure your kernel:
cd /usr/src/kernel/linux
make menuconfig
Please note the following kernel prompts reflect a 2.4.14 kernel:
[ Code maturity level options ]
* Prompt for development and/or incomplete code/drivers (CONFIG_EXPERIMENTAL) [Y/n/?]
- YES: though not required for IP MASQ, this option allows the kernel to create
the MASQ modules and enable the option for port forwarding
* Enable loadable module support (CONFIG_MODULES) [Y/n/?]
- YES: allows you to load kernel IP MASQ modules
* Set version information on all module symbols (CONFIG_MODVERSIONS) [Y/n/?]
- YES: allows newer kernels to load older modules if possible
* Kernel module loader (CONFIG_KMOD) [Y/n/?]
- OPTIONAL: Recommended : allows the kernel to load various kernel modules as it needs them
== Non-MASQ options skipped
== (CPU type, memory, SMP, FPU, specific stuff)
[ General setup ]
* Networking support (CONFIG_NET) [Y/n/?]
- YES: Enables the network subsystem
== Non-MASQ options skipped
== (specific hardware, PCI, kernel binaries, PCMCIA, etc.)
* Sysctl support (CONFIG_SYSCTL) [Y/n/?]
- YES: Enables the ability to enable disable options such as forwarding,
dynamic IPs, etc. via the /proc interface
[ Block devices ]
== Non-MASQ options skipped
== (kernel binaries, power management, PnP, RAID, etc.)
== Don't forget to compile in support for hardware that you might need:
== IDE controllers, HDs, CDROMs, etc.
[ Networking options ]
* Packet socket (CONFIG_PACKET) [Y/m/n/?]
- YES: Though this is OPTIONAL, this recommended feature will allow you
to use TCPDUMP to debug any problems with IP MASQ
* Packet socket: mmapped IO (CONFIG_PACKET_MMAP) [N/y/?] y
- YES: Speed up the packet protocol
* Kernel/User netlink socket (CONFIG_NETLINK) [Y/n/?]
- OPTIONAL: Recommended : this feature will allow the logging of
advanced firewall issues such as routing messages, etc
* Routing messages (CONFIG_RTNETLINK) [N/y/?] (NEW) y
- OPTIONAL: Allows for support of advanced kernel routing messages
if you enabled the CONFIG_NETLINK option
* Netlink device emulation (CONFIG_NETLINK_DEV) [N/y/m/?] (NEW)
- NO: This option does not have anything to do with packet firewall
logging
* Network packet filtering (replaces ipchains) (CONFIG_NETFILTER) [N/y/?] y
- YES: Enable this option to let IPTABLES configure the TCP/IP subsection
of the kernel. By enabling this, then you can turn on advanced
routing mechanisms like IP Masq, packet filtering, etc.
* Network packet filtering debugging (CONFIG_NETFILTER_DEBUG) [N/y/?] (NEW) n
- NO: Not required for Masquerading functionality though it may help
for troubleshooting. There might be a performance penalty when
enabling this.
* Socket Filtering (CONFIG_FILTER) [Y/n/?]
- OPTIONAL: Recommended : Though this doesn't have anything do with IPMASQ,
if you plan on implimenting a DHCP server on the internal network, you WILL
need to enable this option.
* Unix domain sockets (CONFIG_UNIX) [Y/m/n/?]
- YES: This enables the UNIX TCP/IP sockets mechanisms
* TCP/IP networking (CONFIG_INET) [Y/n/?]
- YES: Enables the TCP/IP protocol
* IP: multicasting (CONFIG_IP_MULTICAST) [N/y/?]
- OPTIONAL: You can enable this if you want to be able to receive
Multicast traffic. Please note that your ISP must
support Multicast as well for this all to work at all
* IP: advanced router (CONFIG_IP_ADVANCED_ROUTER) [Y/n/?]
- OPTIONAL: Though there is nothing in this section mandatory for
Masquerade, some specific options might be useful
== Non-MASQ options skipped
== ( autoconf, tunneling )
* IP: multicast routing (CONFIG_IP_MROUTE) [N/y/?] n
- OPTIONAL: Though not needed for IPMASQ, enabling this feature will
let you route multicast traffic through your Linux box.
Please note that this requires that your ISP be multicast
enabled as well.
== Non-MASQ options skipped
== (ARPd)
* IP: TCP Explicit Congestion Notification support (CONFIG_INET_ECN) [N/y/?] n
- NO: Though enabling this option would be great, there are many Internet
sites out there that will block this. Hit the "?" when configuring
the kernel to learn more about it but it is recommended to say NO for
now.
* IP: TCP syncookie support (disabled per default) (CONFIG_SYN_COOKIES) [Y/n/?]
- YES: Recommended : for basic TCP/IP network security
[ Networking options --> IP: Netfilter Configuration ]
* Connection tracking (required for masq/NAT) (CONFIG_IP_NF_CONNTRACK) [N/y/m/?] (NEW) m
- YES: (Module) This enables the kernel to track various network connections.
This option is required for Masquerading support as well as to enable
Stateful tracking for various filewall mechanisms. Please note that
if you compile this directly into the kernel, you cannot enable
the legacy IPCHAINS or IPFWADM compatibility modules.
* FTP protocol support (CONFIG_IP_NF_FTP) [M/n/?] (NEW) m
- YES: (Module) This enables the proper Masquerading of FTP connections if
CONFIG_IP_NF_CONNTRACK was enabled above
* IRC protocol support (CONFIG_IP_NF_IRC) [M/n/?] (NEW) m
- YES: (Module) This enables the proper Masquerading of IRC connections if
CONFIG_IP_NF_CONNTRACK was enabled above
* Userspace queueing via NETLINK (EXPERIMENTAL) (CONFIG_IP_NF_QUEUE) [N/y/m/?] (NEW) m
- OPTIONAL: Though this is OPTIONAL, this feature will allow IPTABLES to
copy specific packets to UserSpace tools for additional checks
* IP tables support (required for filtering/masq/NAT) (CONFIG_IP_NF_IPTABLES) [N/y/m/?] (NEW) m
- YES: (Module) Enables IPTABLES support
* limit match support (CONFIG_IP_NF_MATCH_LIMIT) [N/y/m/?] (NEW) y
- OPTIONAL: (Module) Recommended : Though not required, this option can used to
enable rate limiting of both traffic and loggin messages help slow down denial
of service (DoS) attacks.
* MAC address match support (CONFIG_IP_NF_MATCH_MAC) [N/y/m/?] (NEW) m
- OPTIONAL: Though not required, the option can allow you to
filter traffic based upon the SOURCE Ethernet MAC address.
* netfilter MARK match support (CONFIG_IP_NF_MATCH_MARK) [N/y/m/?] (NEW) y
- YES: (Module) Recommended : This enables IPTABLES to take action upon marked packets.
This mechanism can allow for PORTFW functionality, TOS marking, etc.
* Multiple port match support (CONFIG_IP_NF_MATCH_MULTIPORT) [N/y/m/?] (NEW) y
- YES: (Module) Recommended : This enables IPTABLES to accept mutliple SRC/DST port
ranges (non-contiguous) instead of one port range per IPTABLES
statement.
* TOS match support (CONFIG_IP_NF_MATCH_TOS) [Y/m/n/?] n
- OPTIONAL: This allows IPTABLES to match packets based upon their
DIFFSERV settings.
* LENGTH match support (CONFIG_IP_NF_MATCH_LENGTH) [N/m/?] (NEW) n
- OPTIONAL: This allows IPTABLES to match packets based upon their
packet length.
* TTL match support (CONFIG_IP_NF_MATCH_TTL) [N/m/?] (NEW) ? n
- OPTIONAL: This allows IPTABLES to match packets based upon their
TTL settings.
* tcpmss match support (CONFIG_IP_NF_MATCH_TCPMSS) [N/y/m/?] m
- OPTIONAL: (Module) Recommended : This option allows users to examine the MSS value in
TCP SYN packets. This is an advanced knob but can be very valuable in
troubleshooting MTU problems.
* Connection state match support (CONFIG_IP_NF_MATCH_STATE) [M/n/?] m
- YES: (Module) Recommended : This option allows for Stateful tracking of network
connections.
* Unclean match support (EXPERIMENTAL) (CONFIG_IP_NF_MATCH_UNCLEAN) [N/y/m/?] y
- YES: (Module) Recommended : This option allows for connection tracking on odd packets.
It cal also help in the detection of possibly malicious packets.
This can be a valuable tool in tracking hostile people on the network.
* Owner match support (EXPERIMENTAL) (CONFIG_IP_NF_MATCH_OWNER) [N/y/m/?] n
- OPTIONAL: This option allows IPTABLES to match traffic based upon the
user login, group, etc. who created the traffic.
* Packet filtering (CONFIG_IP_NF_FILTER) [N/y/m/?] ? y
- YES: (Module) This option allows for the kernel to be able filter traffic at
the INPUT, FORWARDING, and OUTPUT traffic points.
* REJECT target support (CONFIG_IP_NF_TARGET_REJECT) [N/y/m/?] (NEW) y
- YES: (Module) With this option, a packet firewall can send an ICMP Reject packet
back to the originator when a packet is blocked.
* MIRROR target support (EXPERIMENTAL) (CONFIG_IP_NF_TARGET_MIRROR) [N/y/m/?] (NEW) n
- OPTIONAL: This option allows the packet firewall to mirror the exact same
network packet back to the originator when it is supposed to be
blocked. This is similar to the REJECT option above but it actually
sends the original packet back to the originator. i.e. a
hostile user could actually portscan themselves.
* Full NAT (CONFIG_IP_NF_NAT) [M/n/?] m
- YES: (Module) This option enables the future menus to enable Masquerading,
PORTFWing, Full (1:1) NAT, etc.
* MASQUERADE target support (CONFIG_IP_NF_TARGET_MASQUERADE) [M/n/?] (NEW) m
- YES: (Module) This option specifically enables Masquerade into the
kernel
* REDIRECT target support (CONFIG_IP_NF_TARGET_REDIRECT) [N/y/m/?] n
- OPTIONAL: Not needed for normal MASQ functionality though people who
want to do transparent proxy via Squid will want this.
* Basic SNMP-ALG support (EXPERIMENTAL) (CONFIG_IP_NF_NAT_SNMP_BASIC) [N/m/?] n
- OPTIONAL: This enables IPTABLES to properly NAT internal SNMP packets so
that machines with duplicate addressing ranges can be properly
managed.
* Packet mangling (CONFIG_IP_NF_MANGLE) [N/y/m/?] y
- YES: (Module) This option allows for advanced IPTABLES packet manipulation
options.
* TOS target support (CONFIG_IP_NF_TARGET_TOS) [N/y/m/?] (NEW) n
- OPTIONAL: Enables the kernel to modify the TOS field in a packet
before routing it on
* MARK target support (CONFIG_IP_NF_TARGET_MARK) [N/y/m/?] (NEW) m
- OPTIONAL: (Module) Recommended : This enables the kernel to manipulate
packets based upon the MARK field. This can be used for PORTFW
as well as many other things.
* LOG target support (CONFIG_IP_NF_TARGET_LOG) [N/y/m/?] m
- YES: (Module) This allows for the logging of packets before they are accepted,
denied, rejected, etc.
* TCPMSS target support (CONFIG_IP_NF_TARGET_TCPMSS) [N/y/m/?] ? m
- YES: (Module) This option help some people with MTU problems. Typically,
most users have to set their Internet connection's MTU to
1500 as well as ALL internal machines to 1500. With this
option, this whole MTU issue might be finally solved.
* ipchains (2.2-style) support (CONFIG_IP_NF_COMPAT_IPCHAINS) [N/y/m/?] m
- OPTIONAL: (Module) Recommended : If you have an existing IPCHAINS ruleset
(2.2.x kernels) and enable this option, you can continue to use the
IPCHAINS program and the majority of your old ruleset except for the
use of any 2.2.x kernel-specific modules. Please note that if this
IPCHAINS module is loaded, ALL IPTABLES modules will be non-
operational. This is an either/or deal only intended for legacy
rulesets.
* ipfwadm (2.0-style) support (CONFIG_IP_NF_COMPAT_IPFWADM) [N/y/m/?] n
- OPTIONAL: If you have an existing IPFWADM ruleset (2.0.x kernels) and
enable this option, you can continue to use the IPFWADM program and
the majority of your old ruleset except for the use of any 2.0.x
kernel-specific modules. Please note that if this IPFWADM module
is loaded, ALL IPTABLES modules will be non operational. This is
an either/or deal only intended to support legacy rulesets.
== Non-MASQ options skipped
== (IPv6, khttpd, ATM, IPX, AppleTalk, etc.) --
* Fast switching (read help!) (CONFIG_NET_FASTROUTE) [N/y/?] n
- NO: This performance optimization is NOT compatible with IP MASQ and/or
packet filtering
== Non-MASQ options skipped
== (QoS, Telephony, IDE, SCSI, 1394FW, I2O, etc)
== Don't forget to compile in support for hardware that you might need:
== IDE: HDs, CDROMs, etc.
== SCSI: HDs, CDROMs, etc.
[ Network device support ]
* Network device support (CONFIG_NETDEVICES) [Y/n/?]
- YES: Enables the Linux Network device sublayer
== Non-MASQ options skipped
== (Arcnet)
* Dummy net driver support (CONFIG_DUMMY) [M/n/y/?]
- YES: Though OPTIONAL, this option can help when debugging problems
== Non-MASQ options skipped
== (EQL, etc..)
== Don't forget to compile in support for hardware that you might need:
== NICs: eth, tr, etc.
== MODEMs: ppp (ppp async) and/or slip
== WANs: T1, T3, ISDN, etc.
== ISDN: for internal ISDN modems
== Non-MASQ options skipped
== (Amateur Radio, IrDA, ISDN, USB, etc.)
[ Character devices ]
== Don't forget to compile in serial port support if you are a modem user
== Don't forget to compile in mouse support
== Non-MASQ options skipped
== (I2C, Watchdog cards, Ftape, Video for Linux, etc. )
[ File systems ]
== Non-MASQ options skipped
== (Quota, ISO9660, NTFS, etc )
* /proc filesystem support (CONFIG_PROC_FS) [Y/n/?]
- YES: Required to dynamically configure the Linux forwarding
and NATing systems
== Non-MASQ options skipped
== (Console drivers, Sound, USB, Kernel Hacking)
|
NOTE: These are just the components you need for IP Masquerade. You will need to select whatever other options needed for your specific setup. If you want more information on what each one of these kernel modules does, please see the FAQ section of this HOWTO for details.
Now compile the kernel (make dep; make clean; make bzImage; make modules; make modules_install) , etc. Again, it is beyond the scope of this HOWTO if you have problems compiling your kernel. Please see Section 2.6 for URLs to the KERNEL howto, etc.
You will then have move over the kernel binary, update your bootloader (LILO, Grub, etc.), and reboot. If you have questions about kernel compiling, I highly recommend to consult some of the URLs above in this section.
Then you should add a few lines towards the bottom of your /etc/rc.d/rc.local file to load the IP Masquerade ruleset automatically after each reboot:
.
.
.
#rc.firewall - Start IPMASQ and the firewall
# This specific file will be discussed in the next section
#
/etc/rc.d/rc.firewall
.
.
.
|
Please see Section 2.7 for any required software, patches, etc.
First of all, you need the kernel source for 2.2.x (preferably the latest kernel version)
NOTE #1: --- UPDATE YOUR KERNEL --- Linux 2.2.x kernels less than version 2.2.20 contain several different security vunerabilities (some were MASQ specific). Kernels less than 2.2.20 have a few local vunerabilities. Kernel versions less than 2.2.16 have a TCP root exploit vulnerability and versions less than 2.2.11 have a IPCHAINS fragmentation bug. Because of these issues, users running a firewall with strong IPCHAINS rulesets are open to possible instrusion. Please upgrade your kernel to a fixed version.
NOTE #2: As the 2.2.x train progressed, the compile-time options keep on changing. As of this version, this section reflects the settings for a 2.2.20 kernel.
If you are running either a newer or older kernel version, the dialogs will look different. It is recommended that you update to the newest kernel for added capability and stability of the system.
If this is your first time compiling the kernel, don't be scared. In fact, it's rather easy and it's covered in several URLs found in Section 2.7. Please note that the instructions included here is just one way to do build a kernel. Please see the Kernel HOWTO for full details.
NOTE: Please notice that it isn't recommended to put the new kernel sources into /usr/src/linux. You should leave the original kernel sources that came with your Linux distribution in /usr/src/linux. For more details on this topic, please read the "README" file in the top level directory of your kernel sources.
For this HOWTO example, create a directory called /usr/src/kernel. Next, "cd" into this directory and download the newest 2.2.x kernel sources into it. Once downloaded, issue the following command (if the file ends in a .tar.gz): tar xvzf linux-2.2.x.tar.gz or (if the file ends in a .tar.bzip2): tar xyvf linux-2.2.x.tar.bz2. Please substitute the "x" in the 2.2.x filename with the Linux 2.2 kernel version you downloaded.
NOTE: Some Linux distributions use the "I" option instead of the "y" option to decompress bzip2 archives.
Once uncompressed, I recommend that you rename the directory from "linux" to "linux-2.2.x" for clarity. To do this, run the command mv linux linux-2.2.x. Next, make sure there is a directory or symbolic link pointing to /usr/src/kernel/linux ie. run the command: ln -s /usr/src/kernel/linux-2.2.x /usr/src/kernel/linuxo again subsituting the "x" for your proper kernel version.
Apply any appropriate or optional patches to the kernel source code. By default, stock Linux kernels do not require any specific patching in order for the system to work. Features like PPTP/IPSEC masqurading are already built-in in the newest kernels but other tools like Xwindows forwarders are optional. Please refer to Section 2.7 for URLs and the IP Masquerade Resources for up-to-date information and patch URLs.
Now that the kernel is patched up (if required), here are the MINIMUM kernel configuration options required to enable IP Masquerade functionality. Please understand that this HOWTO illustrates just ONE way to compile a kernel. The main difference from this method vs. a different one is some people wish to compile things either as modules OR monolithically right into the kernel. Basically, compiling things as modules gives you added flexibility to what is or isn't installed into the kernel (reduces unneeded memory use and allow for drop-in upgrades [no need to reboot]) BUT they add more complexity to your configuration. On the flip side, compiling things directly into the kernel makes things simpler BUT you loose a level of flexibility. The following example is a mixture of both built-in AND modules.
Side Note: It is assumed that you will also configure the kernel to use your other installed hardware such as network interfaces, optional SCSI controllers, etc. as well. Please refer to the Linux Kernel HOWTO and the kernel source's README file and Documentation/ directory for detailed help on compiling a kernel.
Please note the YES or NO ANSWERS to the following. Not all options will be available without the proper kernel patches described later in this HOWTO.
Run the following commands to configure your kernel:
cd /usr/src/kernel/linux
make menuconfig
The following kernel prompts reflect a 2.2.20 kernel:
[ Code maturity level options ]
* Prompt for development and/or incomplete code/drivers (CONFIG_EXPERIMENTAL) [Y/n/?]
- YES: though not entirely required for IP MASQ, this option allows the kernel
to create possible additional MASQ modules such as PORTFW, etc.
== Non-MASQ options skipped
== (CPU, memory, MTRR, SMP, etc.)
[ Loadable module support ]
* Enable loadable module support (CONFIG_MODULES) [Y/n/?] y
- YES: allows you to load kernel IP MASQ modules
* Set version information on all symbols for modules (CONFIG_MODVERSIONS) [N/y/?] y
- YES: allows newer kernels to load older modules if possible
* Kernel module loader (CONFIG_KMOD) [Y/n/?] y
- OPTIONAL: Recommended : allows the kernel to load various kernel modules as
it needs them
[ General setup ]
* Networking support (CONFIG_NET) [Y/n/?]
- YES: This enables the network subsystem
== Non-MASQ options skipped
== (PCI, kernel binaries, specific hardware options, etc.)
* Sysctl support (CONFIG_SYSCTL) [Y/n/?]
- YES: Enables the ability to enable disable options such as forwarding,
dynamic IPs, etc. via the /proc interface
[ Block devices ]
== Non-MASQ options skipped
== (kernel binaries, power management, PnP, IDE, SCSI, etc.)
== Don't forget to compile in support for hardware that you might need:
== IDE controllers, HDs, CDROMs, etc.
[ Networking options ]
* Packet socket (CONFIG_PACKET) [Y/m/n/?] y
- YES: Though this is OPTIONAL, this recommended feature will allow you
to use TCPDUMP to debug any problems with IP MASQ
* Kernel/User netlink socket (CONFIG_NETLINK) [Y/n/?] y
- OPTIONAL: Recommended : This feature will allow the logging of
advanced firewall issues such as routing messages, etc
* Routing messages (CONFIG_RTNETLINK) [Y/n/?] y
- OPTIONAL: If you enabled the CONFIG_NETLINK option above, this option
will send routing messages and other information to SYSLOG.
* Netlink device emulation (CONFIG_NETLINK_DEV) [N/y/m/?] (NEW) n
- NO: This option does not have anything to do with packet firewall
logging
* Network firewalls (CONFIG_FIREWALL) [Y/n/?] y
- YES: Enables the kernel to be comfigured by the IPCHAINS firewall tool
* Socket Filtering (CONFIG_FILTER) [Y/n/?] y
- OPTIONAL: Though this doesn't have anything do with IPMASQ, if you
plan on implimenting a DHCP server on the internal network, you
WILL need this option.
* Unix domain sockets (CONFIG_UNIX) [Y/m/n/?] y
- YES: This enables the UNIX TCP/IP sockets mechanisms
* TCP/IP networking (CONFIG_INET) [Y/n/?] y
- YES: Enables the TCP/IP protocol
* IP: multicasting (CONFIG_IP_MULTICAST) [N/y/?] y
- OPTIONAL: You can enable this if you want to be able to receive
Multicast traffic. Please note that your ISP must
support Multicast as well for this all to work
* IP: advanced router (CONFIG_IP_ADVANCED_ROUTER) [Y/n/?] n
- OPTIONAL: Though there is nothing in this section mandatory for
Masquerade, some specific options might be useful
* IP: kernel level autoconfiguration (CONFIG_IP_PNP) [N/y/?] ?
- NO: Not needed for normal MASQ functionality
* IP: firewalling (CONFIG_IP_FIREWALL) [Y/n/?] y
- YES: This enables the kernel to support packet filtering, NAT, etc.
* IP: firewall packet netlink device (CONFIG_IP_FIREWALL_NETLINK) [Y/n/?] n
- OPTIONAL: Though this is OPTIONAL, this feature will allow IPCHAINS to
copy some packets to UserSpace tools for additional checks
* IP: transparent proxy support (CONFIG_IP_TRANSPARENT_PROXY) [N/y/?] n
- OPTIONAL: Not needed for normal MASQ functionality though people who
want to do transparent proxy via Squid will want this. Please note
that there is a PERFORMANCE PENALTY enabling this feature.
* IP: masquerading (CONFIG_IP_MASQUERADE) [Y/n/?] y
- YES: Enable IP Masquerade to re-address specific internal to external
TCP/IP packets
* IP: ICMP masquerading (CONFIG_IP_MASQUERADE_ICMP) [Y/n/?] y
- YES: Enable support for masquerading ICMP ping packets (ICMP error
codes will be MASQed regardless). This is an important feature
for troubleshooting connections.
* IP: masquerading special modules support (CONFIG_IP_MASQUERADE_MOD) [Y/n/?] y
- YES: Though OPTIONAL, this enables the option to later enable other
modules like the PORTFW to give external computers a directly
connection to specified internal MASQed machines.
* IP: ipautofw masq support (EXPERIMENTAL) (CONFIG_IP_MASQUERADE_IPAUTOFW) [N/y/m/?] n
- NO: NOT recommended : IPautofw is a legacy method of port forwarding. It
is mainly old code and has been found to have some issues.
* IP: ipportfw masq support (EXPERIMENTAL) (CONFIG_IP_MASQUERADE_IPPORTFW) [Y/m/n/?] y
- OPTIONAL: Recommended : This enables PORTFW which allows external computers
on the Internet to directly communicate to specified internal MASQed
machines. This feature is typically used to allow access to internal
SMTP, TELNET, and WWW servers. Please note that FTP port forwarding
needs an additional patch, as described in the FAQ section of the MASQ
HOWTO. Please see the this FAQ section in the HOWTO for additional
information.
* IP: ip fwmark masq-forwarding support (EXPERIMENTAL) (CONFIG_IP_MASQUERADE_MFW) [Y/m/n/?] y
- OPTIONAL: This is a NEW method of performing PORTFW-like functionality which is
similar to how the new 2.4.x kernels do things. With this option, IPCHAINS
can mark packets that should have additional work done upon it. Using a
UserSpace tool, much like IPMASQADM or IPPORFW, IPCHAINS would then
do things like re-address the packets, change their TOS value, etc.
Currently, this code is less tested than PORTFW but it looks promising.
For now, this HOWTO recommends to use IPMASQADM and IPPORTFW. If you
have specific thoughts or comments on MFW, please email dranch.
* IP: optimize as a router not host (CONFIG_IP_ROUTER) [Y/n/?] y
- YES: This optimizes the kernel for the network subsystem, though it
isn't well known if this makes a siginificant performance difference
or not.
== Non-MASQ options skipped
== ( autoconf, tunneling, GRE )
* IP: multicast routing (CONFIG_IP_MROUTE) [N/y/?] n
- OPTIONAL: Though not needed for IPMASQ, enabling this feature will
let you route multicast traffic through your Linux box.
Please note that this requires that your ISP be multicast
enabled as well.
== Non-MASQ options skipped
== (Aliasing, ARPd)
* IP: TCP syncookie support (disabled per default) (CONFIG_SYN_COOKIES) [Y/n/?]
- YES: Recommended : for basic TCP/IP network security
* IP: GRE tunnels over IP (CONFIG_NET_IPGRE) [N/y/m/?]
- NO: This OPTIONAL selection is to enable PPTP and GRE tunnels through
the IP MASQ box
== Non-MASQ options skipped
== (aliasing, ARPd)
* IP: TCP syncookie support (not enabled per default) (CONFIG_SYN_COOKIES) [Y/n/?]
- YES: HIGHLY recommended for basic TCP/IP network security
== Non-MASQ options skipped
== (RARP)
* IP: Allow large windows (not recommended if <16Mb of memory) * (CONFIG_SKB_LARGE) [Y/n/?]
- YES: This is recommended to optimize Linux's TCP window
== Non-MASQ options skipped
== (IPv6, IPX, WAN router, etc.)
* Fast switching (read help!) (CONFIG_NET_FASTROUTE) [N/y/?] n
- NO: This performance optimization is NOT compatible with IP MASQ and/or
packet filtering
== Non-MASQ options skipped
== (Slow CPU, Telephony, SCSI, I2O, etc. )
== Don't forget to compile in support for hardware that you might need:
== SCSI: HDs, CDROMs, etc.
[ Network device support ]
* Network device support (CONFIG_NETDEVICES) [Y/n/?]
- YES: Enables the Linux Network device sublayer
== Non-MASQ options skipped
== (Arcnet)
* Dummy net driver support (CONFIG_DUMMY) [M/n/y/?]
- YES: Though OPTIONAL, this option can help when debugging problems
== Non-MASQ options skipped
== (EQL, NICs, Wireless, IrDA, ISDN, etc..)
== Don't forget to compile in support for hardware that you might need:
== NICs: eth, tr, etc.
== MODEMs: ppp and/or slip
== WANs: T1, T3, ISDN, etc.
== ISDN: for internal ISDN modems
[ Character devices ]
== Don't forget to compile in serial port support for modem users
== Don't forget to compile in mouse support
== Non-MASQ options skipped
== (I2C, Watchdog cards, Ftape, Video for Linux, USB, etc. )
[ File systems ]
== Non-MASQ options skipped
== (Quota, ISO9660, NTFS, etc )
* /proc filesystem support (CONFIG_PROC_FS) [Y/n/?]
- YES: Required to dynamically configure the Linux forwarding
and NATing systems
== Non-MASQ options skipped
== (network fs, NLS, video section, sound, kernel hacking)
|
NOTE: These are just the components you need for IP Masquerade. You will need to select whatever other options needed for your specific setup.
Now compile the kernel (make dep; make clean; make bzImage; make modules; make modules_install) , etc. Again, it is beyond the scope of this HOWTO if you have problems compiling your kernel. Please see Section 2.7 for URLs to the KERNEL howto, etc.
You will then have move over the kernel binary, update your bootloader (LILO, Grub, etc.), and reboot. If you have questions about kernel compiling, I highly recommend to consult some of the URLs above in this section.
Then you should add a few lines towards the bottom of your /etc/rc.d/rc.local file to load the IP Masquerade ruleset automatically after each reboot:
.
.
.
#rc.firewall - Start IPMASQ and the firewall
# This specific file will be discussed in the next section
#
/etc/rc.d/rc.firewall
.
.
.
|
Please see Section 2.8 for any required software, patches, etc.
First of all, you need the kernel source for 2.0.x (preferably the latest kernel version)
As the 2.0.x train progress, the compile-time options keep on changing. As of this version, this section reflects the settings for a 2.0.39 kernel.
If this is your first time compiling the kernel, don't be scared. In fact, it's rather easy and it's covered in several URLs found in Section 2.8. Please note that the instructions included here is just one way to do build a kernel. Please see the Kernel HOWTO for full details.
NOTE: Please notice that it isn't recommended to put the new kernel sources into /usr/src/linux. You should leave the original kernel sources that came with your Linux distribution in /usr/src/linux. For more details on this topic, please read the "README" file in the top level directory of your kernel sources.
For this HOWTO example, create a directory called /usr/src/kernel. Next, "cd" into this directory and download the newest 2.0.x kernel sources into it. Once downloaded, issue the following command: tar xvzf linux-2.0.x.tar.gz . Please substitute the "x" in the 2.0.x filename with the Linux 2.0 kernel version you downloaded.
Once uncompressed, I recommend that you rename the directory from "linux" to "linux-2.0.x" for clarity. To do this, run the command mv linux linux-2.0.x. Next, make sure there is a directory or symbolic link pointing to /usr/src/kernel/linux ie. run the command: ln -s /usr/src/kernel/linux-2.0.x /usr/src/kernel/linuxo again subsituting the "x" for your proper kernel version.
Apply any appropriate or optional patches to the kernel source code. By default, stock Linux kernels do not require any specific patching in order for the system to work. Features like IPPORTFW, PPTP, and Xwindows forwarders are optional but very useful. Please refer to Section 2.8 for URLs and the IP Masquerade Resources for up-to-date information and patch URLs.
Now that the kernel is patched up (if required), here are the MINIMUM kernel configuration options required to enable IP Masquerade functionality. Please understand that this HOWTO illustrates just ONE way to compile a kernel. The main difference from this method vs. a different one is some people wish to compile things either as modules OR monolithically right into the kernel. Basically, compiling things as modules gives you added flexibility to what is or isn't installed into the kernel (reduces unneeded memory use and allow for drop-in upgrades [no need to reboot]) BUT they add more complexity to your configuration. On the flip side, compiling things directly into the kernel makes things simpler BUT you loose a level of flexibility. The following example is a mixture of both built-in AND modules.
Side Note: It is assumed that you will also configure the kernel to use your other installed hardware such as network interfaces, optional SCSI controllers, etc. as well. Please refer to the Linux Kernel HOWTO and the kernel source's README file and Documentation/ directory for detailed help on compiling a kernel.
Please note the YES or NO ANSWERS to the following options. Not all options will be available without the proper kernel patches described later in this HOWTO:
Run the following commands to configure your kernel:
cd /usr/src/kernel/linux
make menuconfig
The following kernel prompts reflect a 2.0.39 kernel:
[ Code maturity level options ]
* Prompt for development and/or incomplete code/drivers (CONFIG_EXPERIMENTAL) [Y/n/?]
- YES: this will allow you to later select the IP Masquerade feature code
[ Loadable module support ]
* Enable loadable module support (CONFIG_MODULES) [Y/n/?] y
- YES: allows you to load kernel IP MASQ modules
* Set version information on all module symbols (CONFIG_MODVERSIONS) [N/y/?] y
- YES: allows newer kernels to load older modules if possible
* Kernel daemon support (e.g. autoload of modules) (CONFIG_KERNELD) [N/y/?] y
- OPTIONAL: Recommended : allows the kernel to load various kernel modules as
it needs them
[ General setup ]
== Non-MASQ options skipped
== (FPU, memory)
* Networking support (CONFIG_NET) [Y/n/?] y
- YES: Enables the network subsystem
== Non-MASQ options skipped
== (memory, PCI, binary format, APM, etc.)
== Don't forget to compile in support for hardware that you might need:
== IDE controllers, HDs, CDROMs, etc.
[ Networking options ]
* Network firewalls (CONFIG_FIREWALL) [Y/n/?] y
- YES: Enables the IPFWADM firewall tool
== Non-MASQ options skipped
== (Aliasing)
* TCP/IP networking (CONFIG_INET) [Y/n/?] y
- YES: Enables the TCP/IP protocol
* IP: forwarding/gatewaying (CONFIG_IP_FORWARD) [N/y/?] y
- YES: Enables Linux network packet forwarding and routing
- Controlled by IPFWADM
* IP: multicasting (CONFIG_IP_MULTICAST) [N/y/?] y
- OPTIONAL: You can enable this if you want to be able to receive
Multicast traffic. Please note that your ISP must
support Multicast as well for this all to work
* IP: syn cookies (CONFIG_SYN_COOKIES) [Y/n/?] y
- YES: HIGHLY recommended for basic network security
* IP: firewalling (CONFIG_IP_FIREWALL) [Y/n/?] y
- YES: Enable the packet firewall features
* IP: firewall packet logging (CONFIG_IP_FIREWALL_VERBOSE) [Y/n/?] y
- YES: Allows the kernel to report back on various packets traversing
the firewall.
* IP: masquerading (CONFIG_IP_MASQUERADE [Y/n/?] y
- YES: Enable the kernel to perform IP MASQ NAT functionality
* IP: ipautofw masquerade support (EXPERIMENTAL) (CONFIG_IP_MASQUERADE_IPAUTOFW) [Y/n/?] n
- NO: NOT Recommended : IPautofw is a legacy method of TCP/IP port forwarding.
Though IPautofw works, IPPORTFW is a better choice.
* IP: ipportfw masq support (EXPERIMENTAL) (CONFIG_IP_MASQUERADE_IPPORTFW) [Y/n/?] y
- YES: This option is ONLY AVAILABLE VIA A PATCH for the 2.0.x kernels.
With this option, external computers on the Internet can directly
communicate to specified internal MASQed machines. This feature is
typically used to access internal SMTP, TELNET, and WWW servers.
FTP port forwarding sometimes might require an additional patch as
described in the FAQ section. Additional information on port
forwarding is available in the Forwards section of this HOWTO.
* IP: MS PPTP masq support (EXPERIMENTAL) (CONFIG_IP_MASQUERADE_PPTP) [N/y/?] (NEW) n
- OPTIONAL: Enabling this feature will allow internal MASQ clients to
properly connect to PPTP servers on the Internet.
* IP: MS PPTP Call ID masq support (CONFIG_IP_MASQUERADE_PPTP_MULTICLIENT) [N/y/?] (NEW) n
- OPTIONAL: If you enabled the CONFIG_IP_MASQUERADE_PPTP above, this
option will allow for multiple internal PPTP clients behind the MASQ
server to communicate to the same PPTP server.
* IP: MS PPTP masq debugging (DEBUG_IP_MASQUERADE_PPTP) [N/y/?] n
- OPTIONAL: NOT recommended : This is not required for IP MASQ or MASQing PPTP
connections unless you need additional troubleshooting help. If enabled,
this can fill up your logs quickly.
* IP: MS PPTP masq verbose debugging (DEBUG_IP_MASQUERADE_PPTP_VERBOSE) [N/y/?] (NEW) n
- OPTIONAL: NOT Recommended : If you enabled the DEBUG_IP_MASQUERADE_PPTP
option above, this will make the logging even more verbose.
* IP: IPSEC ESP & ISAKMP masq support (EXPERIMENTAL) * (CONFIG_IP_MASQUERADE_IPSEC) [N/y/?] m
- OPTIONAL: This option allows for some forms of IPSEC tunnels to be
masquraded
* IP: IPSEC masq table lifetime (minutes) (CONFIG_IP_MASQUERADE_IPSEC_EXPIRE) * [30] (NEW)
- OPTIONAL: This feature allows to change the MASQ table timeouts so that
idle IPSEC tunnels won't be prematurely disconnected.
* IP: Disable inbound ESP destination guessing * (CONFIG_IP_MASQUERADE_IPSEC_NOGUESS) [N/y/?] n
- OPTIONAL: This feature allows the kernel to guess where the fully encrypted IPSEC VPN
might be going and add it to the MASQ table.
* IP: IPSEC masq debugging (DEBUG_IP_MASQUERADE_IPSEC) [N/y/?] ? n
- OPTIONAL: NOT recommended : This is not required for IP MASQ or MASQing IPSEC
connections unless you need additional troubleshooting help. If enabled,
this can fill up your logs quickly.
* IP: IPSEC masq verbose debugging (DEBUG_IP_MASQUERADE_IPSEC_VERBOSE) [N/y/?] (NEW) n
- OPTIONAL: NOT Recommended : If you enabled the DEBUG_IP_MASQUERADE_IPSEC
option above, this will make the logging even more verbose.
* IP: ICMP masquerading (CONFIG_IP_MASQUERADE_ICMP) [Y/n/?]
- YES: Enable support for masquerading ICMP packets. Though thought of as
optional, many programs will NOT function properly with out ICMP
support.
* IP: transparent proxy support (EXPERIMENTAL) (CONFIG_IP_TRANSPARENT_PROXY) [N/y/?] n
- OPTIONAL: Not needed for normal MASQ functionality though people who
want to do transparent proxy via Squid will want this. Please note
that there is a PERFORMANCE PENALTY enabling this feature.
* IP: loose UDP port managing (EXPERIMENTAL) (CONFIG_IP_MASQ_LOOSE_UDP) [Y/n/?]
- YES: This option is ONLY AVAILABLE VIA A PATCH for the 2.0.x kernels.
With this option, internally masqueraded computers can play
NAT-friendly games over the Internet. Explicit details are given
in the FAQ section of this HOWTO.
* IP: always defragment (CONFIG_IP_ALWAYS_DEFRAG) [Y/n/?]
- YES: This feature optimizes IP MASQ connections
== Non-MASQ options skipped
== (Accounting)
* IP: optimize as router not host (CONFIG_IP_ROUTER) [Y/n/?]
- YES: This optimizes the kernel for the network subsystem
== Non-MASQ options skipped
== (Tunneling, Mcast routing, RARP, PMTU, etc.)
* IP: Drop source routed frames (CONFIG_IP_NOSR) [Y/n/?]
- YES: HIGHLY recommended for basic network security
== Non-MASQ options skipped
== (IPX, Bridging, SCSI, etc.)
== Don't forget to compile in support for hardware that you might need:
== SCSI controllers, HDs, CDROMs, etc.
[ Network device support ]
* Network device support (CONFIG_NETDEVICES) [Y/n/?]
- YES: Enables the Linux Network device sublayer
== Non-MASQ options skipped
== (Dummy, EQL, PPP, SLIP, NICs, Wireless, etc.)
== Don't forget to compile in support for hardware that you might need:
== NICs: eth, tr, etc.
== MODEMs: ppp and/or slip
== WANs: T1, T3, ISDN, etc.
== ISDN: for internal ISDN modems
[ File systems ]
== Non-MASQ options skipped
== (Quota, ISO9660, Codepages, NTFS, etc )
* /proc filesystem support (CONFIG_PROC_FS) [Y/n/?]
- YES: Required to dynamically configure the Linux forwarding
and NATing systems
[ Character devices ]
== Non-MASQ options skipped
== (multi-port serial, parallel, mice, Ftape, Sound, etc. )
== Don't forget to compile in serial port support for modem users
== Don't forget to compile in mouse support
|
So go ahead and "exit" and you should be prompted to save your config.
NOTE: These are only components for IP Masquerade functionality. You may need to also select additional options to match your specific network and hardware setup.
Now compile the kernel (make dep; make clean; make bzImage; make modules; make modules_install) , etc. Again, it is beyond the scope of this HOWTO if you have problems compiling your kernel. Please see Section 2.8 for URLs to the KERNEL howto, etc.
You will then have move over the kernel binary, update your bootloader (LILO, Grub, etc.), and reboot. If you have questions about kernel compiling, I highly recommend to consult some of the URLs above in this section.
Then you should add a few lines towards the bottom of your /etc/rc.d/rc.local file to load the IP Masquerade ruleset automatically after each reboot:
.
.
.
#rc.firewall script - Start IPMASQ and the firewall
/etc/rc.d/rc.firewall
.
.
.
|