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标 题: Linux IP Masquerade mini HOWTO
发信站: BBS 水木清华站 (Wed Apr 30 15:53:01 1997)
Linux IP Masquerade mini HOWTO
Ambrose Au, achau@wwonline.com
v0.80, August 17, 1996
This document describes how to enable IP masquerade feature on a Linux
host, allowing connected computers that do not have registered Inter*
net IP addresses to connect to the Internet through your Linux box.
1. Introduction
1.1. Introduction
This document describes how to enable IP masquerade feature on a Linux
host, allowing connected computers that do not have registered
Internet IP addresses to connect to the Internet through your Linux
box. It is possible to connect your machines to the Linux host with
ethernet, as well as other kinds of connection such as a dialup ppp
link. This document will emphasize on ethernet connection, since it
should be the most likely case.
This document is intended for users using kernel 2.x, which
should also apply to later 1.3.x kernels.
1.2. Foreword, Feedback & Credits
First of all, I would like to let you know that I am *NOT* a
knowledgeable nor an experienced user of IP masquerade.
I find it very confusing as a new user setting up IP masquerade on a
newer kernel, i.e. 2.x kernel. Although there is a FAQ and a mailing
list, there is no document dedicates on that; and there are some
requests on the mailing list for such a HOWTO. So, I decided to write
this up as a starting point for new users, and possibly a building
block for knowledgeable users to build on for documentation. If you
think I'm not doing a good job, feel free to tell me so that I can
make it better.
This document is heavily based on the original FAQ by Ken Eves , and
numerous helpful messages in the ip_masq mailing list. And a special
thanks to Mr. Matthew Driver whose mailing list message inspired me to
set up ip_masq and eventually writing this.
Please feel free to send any feedback or comments to
achau@wwonline.com if I'm mistaken on any information, or if any
information is missing. Your invaluable feedback will certainly be
influencing the future of this HOWTO!
If you have any technical questions on IP Masquerade, please join the
IP Masquerade Mailing List instead of sending email to me since I have
limited time, and the developers of IP_Masq are more capable of
answering your questions.
The latest version of this document can be found at the IP Masquerade
Resource, which also contains the HTML and postscript version:
* http://www.hwy401.com/achau/ipmasq/
* http://www.undergrad.math.uwaterloo.ca/~ach2au
1.3. Copyright & Disclaimer
This document is copyright(c) 1996 Ambrose Au, and it's a free
document. You can redistribute it under the terms of the GNU General
Public License.
The information and other contents in this document are to the best of
my knowledge. However, ip_masq is experimental, and there is chance
that I make mistakes as well; so you should determine if you want to
follow the information in this document.
Nobody is responsible for any damage on your computers and any other
losses by using the information on this document. i.e.
THE AUTHOR IS NOT RESPONSIBLE FOR ANY DAMAGES INCURRED DUE
TO ACTIONS TAKEN BASED ON THE INFORMATION IN THIS DOCUMENT.
2. Background Knowledge
2.1. What is IP Masquerade?
IP Masquerade is a developing networking function in Linux. If a Linux
host is connected to the Internet with IP Masquerade enabled, then
computers connecting to it (either on the same LAN or connected with
modems) can reach the Internet as well, even though they have no
official assigned IP addresses.
This allows a set of machines to invisibly access the Internet hidden
behind a gateway system, which appears to be the only system using the
Internet. Breaking the security of a well set-up masquerading system
should be considerably more difficult than breaking a good packet
filter based firewall (assuming there are no bugs in either).
2.2. Current Status
IP Masquerade is still at its experimental stages. However, kernels
since 1.3.x had built-in support already. Browsing web pages and
telnet are reported to work well over ip_masq. FTP, IRC and listening
to Real Audio are working with certain modules loaded. Other network
streaming audio such as True Speech and Internet Wave work too. Some
fellow users on the mailing list even tried video conferencing
software.
Ping is not fully working yet, it normally only works for TCP and UDP
(masquerading needs port numbers). It will work for ICMP packets that
contain information about an original TCP or UDP packet, masquerading
will not work for ICMP packets that do not contain port data. For
example, traceroute should work, ping will not.
Please refer to section 4.3 for a more complete listing of software
supported.
IP Masquerade works well with 'client machines' on several different
OS and platforms. There are successful cases with systems using Unix,
Windows 95, Windows NT, Windows for Workgroup(with TCP/IP package),
OS/2, Macintosh System's OS with Mac TCP, Mac Open Transport, DOS with
NCSA Telnet package, VAX, Alpha with Linux, and even Amiga with AmiTCP
or AS225-stack.
2.3. Who Can Benefit From IP Masquerade?
* If you have a Linux host connected to the Internet, and
* if you have some computers running TCP/IP connected to that Linux
box on a local subnet, and/or
* if your Linux host has more than one modem and acts as a PPP or
SLIP server connecting to others, which
* those OTHER machines do not have official assigned IP addresses.
(these machines are represented by OTHER machines hereby)
* And of course, if you want those OTHER machines to make it onto the
Internet without spending extra bucks :)
2.4. Who Doesn't Need IP Masquerade?
* If your machine is a stand-alone Linux host connected to the
Internet, then it is pointless to have ip_masq running, or
* if you already have assigned addresses for your OTHER machines,
then you don't need IP Masquerade to have a firewall,
* and of course, if you don't like the idea of a 'free ride'.
2.5. How IP Masquerade Works?
From IP Masquerade FAQ by Ken Eves:
Here is a drawing of the most simple setup:
SLIP/PPP +------------+ +-------------+
to provider | Linux | SLIP/PPP | Anybox |
<---------- modem1| |modem2 ----------- modem | |
111.222.333.444 | | 192.168.1.100 | |
+------------+ +-------------+
In the above drawing a Linux box with ip_masquerading installed and
running is connected to the Internet via SLIP/or/PPP using modem1. It has
an assigned IP address of 111.222.333.444. It is setup that modem2 allows
callers to login and start a SLIP/or/PPP connection.
The second system (which doesn't have to be running Linux) calls into the
Linux box and starts a SLIP/or/PPP connection. It does NOT have an assigned
IP address on the Internet so it uses 192.168.1.100. (see below)
With ip_masquerade and the routing configured properly the machine
Anybox can interact with the Internet as if it was really connected (with a
few exceptions).
Quoting Pauline Middelink:
Do not forget to mention the ANYBOX should have the Linux box
as its gateway (whether is be the default route or just a subnet
is no matter). If the ANYBOX can not do this, the Linux machine
should do a proxy arp for all routed address, but the setup of
proxy arp is beyond the scope of the document.
The following is an excerpt from a post on comp.os.linux.networking which
has been edited to match the names used in the above example:
o I tell machine ANYBOX that my slipped linux box is its gateway.
o When a packet comes into the linux box from ANYBOX, it will assign it
new source port number, and slap its own ip address in the packet
header, saving the originals. It will then send the modified packet
out over the SLIP/or/PPP interface to the Internet.
o When a packet comes from the Internet to the linux box, if the port
number is one of those assigned above, it will get the original
port and ip address, put them back in the packet header, and send the
packet to ANYBOX.
o The host that sent the packet will never know the difference.
An IP Masquerading Example
typical example is given in the diagram below:-
+----------+
| | Ethernet
| abox |::::::
| |2 :192.168.1.x
+----------+ :
: +----------+ PPP
+----------+ : 1| Linux | link
| | ::::| masq-gate|:::::::::// Internet
| bbox |:::::: | |
| |3 : +----------+
+----------+ :
:
+----------+ :
| | :
| cbox |::::::
| |4
+----------+
<-Internal Network->
In this example there are 4 computer systems that we are concerned
about (there is presumably also something on the far right that your
IP connection to the internet comes through, and there is something
(far off the page) on the internet that you are interested in exchang*
ing information with). The Linux system masq-gate is the masquerading
gateway for the internal network of machines abox, bbox and cbox to
get to the internet. The internal network uses one of the assigned
private network addresses, in this case the class C network
192.168.1.0, with the linux box having address 192.168.1.1 and the
other systems having addresses on that network.
The three machines abox, bbox and cbox (which can, by the way, be
running any operating system as long as they can speak IP - such as
Windows 95, Macintosh MacTCP or even another linux box) can connect to
other machines on the internet, however the masquerading system masq-
gate converts all of their connections so that they appear to
originate from masq-gate, and arranges that data coming back in to a
masqueraded connection is relayed back to the originating system - so
the systems on the internal network see a direct route to the internet
and are unaware that their data is being masqueraded.
2.6. Requirements for Using IP Masquerade on Linux 2.x
* Kernel 2.x source available from
ftp://ftp.funet.fi/pub/Linux/kernel/src/v2.0/
(Yes, you'll have to compile your kernel with certain supports....)
* Loadable kernel modules, preferably 2.0.0 available from
http://www.pi.se/blox/modules/modules-2.0.0.tar.gz
(modules-1.3.57 is the minimal requirement)
* A well set up TCP/IP network
covered in Linux NET-2 HOWTO
<http://www.caldera.com/LDP/HOWTO/NET-2-HOWTO.html> and the Network
Administrator's Guide <http://linuxwww.db.erau.edu/NAG/>
* Connectivity to Internet for your Linux host
covered in Linux ISP Hookup HOWTO
<http://www.caldera.com/LDP/HOWTO/ISP-Hookup-HOWTO.html> and Linux
PPP-over-ISDN mini-HOWTO
<http://www.caldera.com/LDP/HOWTO/mini/PPP-over-ISDN>
* Ipfwadm 2.3 available from
ftp://ftp.xos.nl/pub/linux/ipfwadm/ipfwadm-2.3.tar.gz
more information on version requirement is on the Linux Ipfwadm
page <http://www.xos.nl/linux/ipfwadm/>
* IP Masquerade ftp patch for kernel 2.0.0
<http://www.hwy401.com/achau/ipmasq/ip_masq_app.c.gz> writen by
Miquel van Smoorenburg (NOT necessary for kernel 2.0.1 and above)
* Keep Alive Support patch
<http://www.hwy401.com/achau/ipmasq/ip_masq_ftpalive.patch.gz>
writen by Keith Owens (this patch will not be included in the
kernel distribution till 2.1.x development kernels)
3. Setting Up IP Masquerade
If your private network contains any vital information,
think twice before using IP Masquerade. This may be a GATE*
WAY for you to get to the Internet, and vice versa for some*
one on the other side of the world to get into your network.
3.1. Compiling the Kernel for IP Masquerade Support
* First of all, you need the kernel source (preferably stable kernel
version 2.0.0 or above)
* If this is your first time compiling the kernel, don't be scared.
In fact, it's rather easy and it's covered in Linux Kernel HOWTO
<http://www.caldera.com/LDP/HOWTO/Kernel-HOWTO.html>.
* Unpack the kernel source to /usr/src/ with a command: tar xvzf
linux-2.0.x.tar.gz -C /usr/src, where x is the patch level beyond
2.0
(make sure there is a directory or symbolic link called linux )
* Apply the ftp patch for kernel 2.0.0
<http://www.hwy401.com/achau/ipmasq/ip_masq_app.c.gz>:
* You don't need this patch with kernel 2.0.1 and above
* this is not an usual diff patch. You should copy this
file to yourLinuxSourceDirectory/net/ipv4/ip_masq_app.c
* Apply the Keep Alive Support patch
<http://www.hwy401.com/achau/ipmasq/ip_masq_ftpalive.patch.gz> (not
in kernel 2.0.4 yet...):
* copy the patch file to /usr/src/, then gunzip
ip_masq_ftpalive.patch.gz
* apply the patch by typing patch < ip_masq_ftpalive.patch
* CU-Seeme user may also want to apply the CU-Seeme patch
<http://www.hwy401.com/achau/ipmasq/ip_masq_cuseeme.c.gz> :
* In order to get ip_masq_cuseeme to compile with "make
modules", you have to add ip_masq_cuseeme.o to the M_OBJS
line in the net/ipv4/Makefile that also has
ip_masq_ftp.o, ip_masq_irc.o, etc.
* Refer to the Kernel HOWTO and the README file in the kernel source
directory for further instructions on compiling a kernel
* Here are the options that you need to compile in:
Say YES to the following,
* Prompt for development and/or incomplete code/drivers
CONFIG_EXPERIMENTAL
- this will allow you to select experimental ip_masq code compiled
into the kernel
* Enable loadable module support
CONFIG_MODULES
- allows you to load modules
* Networking support
CONFIG_NET
* Network firewalls
CONFIG_FIREWALL
* TCP/IP networking
CONFIG_INET
* IP: forwarding/gatewaying
CONFIG_IP_FORWARD
* IP: firewalling
CONFIG_IP_FIREWALL
* IP: masquerading (EXPERIMENTAL)
CONFIG_IP_MASQUERADE
- although it is experimental, it is a *MUST*
* IP: always defragment
CONFIG_IP_ALWAYS_DEFRAG
- highly recommended
* Dummy net driver support
CONFIG_DUMMY
- recommended
NOTE: These are just the component you need for ip_masq, select
whatever other options you need for your specific setup.
* After compiling the kernel, you should compile and install the
modules:
make modules; make modules_install
* Then you should add a few lines into your /etc/rc.d/rc.local file
(or any file you think is appropriate) to load the required modules
reside in /lib/modules/2.0.x/ipv4/ automatically during each
reboot:
.
.
.
/sbin/depmod -a
/sbin/modprobe ip_masq_ftp.o
/sbin/modprobe ip_masq_raudio.o
/sbin/modprobe ip_masq_irc.o
.
.
.
Note: You can also load it manually before using ip_masq, but DON'T
use kerneld for this!
If your modules are not loading, you may want to take out the '.o'
extension
3.2. Assigning Private Network IP Address
Since all OTHER machines do not have official assigned addressees,
there must be a right way to allocate address to those machines.
From IP Masquerade FAQ:
There is an RFC (#1597) on which IP addresses are to be used on a non-
connected network. There are 3 blocks of numbers set aside
specifically for this purpose. One which I use is 255 Class-C subnets
at 192.168.1.n to 192.168.255.n .
From RCF 1597:
Section 3: Private Address Space
The Internet Assigned Numbers Authority (IANA) has reserved the
following three blocks of the IP address space for private networks:
10.0.0.0 - 10.255.255.255
172.16.0.0 - 172.31.255.255
192.168.0.0 - 192.168.255.255
We will refer to the first block as "24-bit block", the second as
"20-bit block", and to the third as "16-bit" block". Note that the
first block is nothing but a single class A network number, while the
second block is a set of 16 contiguous class B network numbers, and
third block is a set of 255 contiguous class C network numbers.
So, if you're using a class C network, you should name your machines
as 192.168.1.1, 1.92.168.1.2, 1.92.168.1.3, ..., 192.168.1.x
192.168.1.1 is usually the gateway machine, which is your Linux host
connecting to the Internet. Notice that 192.168.1.0 and 192.168.1.255
are the Network and Broadcast address respectively, which are
reserved. Avoid using these addresses on your machines.
3.3. Configuring the OTHER machines
Besides setting the appropriate IP address for each machine, you
should also set the appropriate gateway. In general, it is rather
straight forward. You simply enter the address of your Linux host
(usually 192.168.1.1) as the gateway address.
For the Domain Name Service, you can add in any DNS available. The
most apparent one should be the one that your Linux is using. You can
optionally add any domain search suffix as well.
After you have reconfigured those IP addresses, remember to restart
the appropriate services or reboot your systems.
The following configuration instructions assume that you are using a
Class C network with 192.168.1.1 as your Linux host's address. Please
note that 192.168.1.0 and 192.168.1.255 are reserved.
3.3.1. Configuring Windows 95
1. If you haven't installed your network card and adapter driver, do
so now.
2. Go to 'Control Panel'/'Network'.
3. Add 'TCP/IP protocol' if you don't already have it.
4. In 'TCP/IP properties', goto 'IP Address' and set IP Address to
192.168.1.x, (1 < x < 255), and then set Subnet Mask to
255.255.255.0
5. Add 192.168.1.1 as your gateway under 'Gateway'.
6. Under 'DNS Configuration'/'DNS Server search order' add your the
DNS that your Linux host uses (usually find in /etc/resolv.conf).
Optionally, you can add the appropriate domain search suffix.
7. Leave all the other settings as they are unless you know what
you're doing.
8. Click 'OK' on all dialog boxes and restart system.
9. Ping the linux box to test the network connection: 'Start/Run',
type: ping 192.168.1.1
(This is only a LAN connection testing, you can't ping the outside
world yet.)
10.
You can optionally create a HOSTS file in the windows directory so
that you can use hostname of the machines on your LAN. There is an
example called HOSTS.SAM in the windows directory.
3.3.2. Configuring Windows for Workgroup 3.11
1. If you haven't installed your network card and adapter driver, do
so now.
2. Install the TCP/IP 32b package if you don't have it already.
3. In 'Main'/'Windows Setup'/'Network Setup', click on 'Drivers'.
4. Highlight 'Microsoft TCP/IP-32 3.11b' in the 'Network Drivers'
section, click 'Setup'.
5. Set IP Address to 192.168.1.x (1 < x < 255), then set Subnet Mask
to 255.255.255.0 and Default Gateway to 192.168.1.1
6. Do not enable 'Automatic DHCP Configuration' and put anything in
those 'WINS Server' input areas unless you're in a Windows NT
domain and you know what you're doing.
7. Click 'DNS', fill in the appropriate information mentioned in STEP
6 of section 3.3.1, then click 'OK' when you're done with it.
8. Click 'Advanced', check 'Enable DNS for Windows Name Resolution'
and 'Enable LMHOSTS lookup' if you're using a look up host file,
similar to the one mentioned in STEP 10 of section 3.3.1
9. Click 'OK' on all dialog boxes and restart system.
10.
Ping the linux box to test the network connection: 'File/Run',
type: ping 192.168.1.1
(This is only a LAN connection testing, you can't ping the outside
world yet.)
3.3.3. Configuring Windows NT 3.51
1. If you haven't installed your network card and adapter driver, do
so now.
2. Go to 'Main'/'Control Panel'/'Network'
3. Add the TCP/IP Protocol and Related Component from the 'Add
Software' menu if you don't have TCP/IP service installed already.
4. Under 'Network Software and Adapter Cards' section, highlight
'TCP/IP Protocol' in the 'Installed Network Software' selection
box.
5. In 'TCP/IP Configuration', select the appropriate adapter, e.g.
[1]Novell NE2000 Adapter. Then set the IP Address to 192.168.1.x
(1 < x < 255), then set Subnet Mask to 255.255.255.0 and Default
Gateway to 192.168.1.1
6. Do not enable 'Automatic DHCP Configuration' and put anything in
those 'WINS Server' input areas unless you're in a Windows NT
domain and you know what you're doing.
7. Click 'DNS', fill in the appropriate information mentioned in STEP
6 of section 3.3.1, then click 'OK' when you're done with it.
8. Click 'Advanced', check 'Enable DNS for Windows Name Resolution'
and 'Enable LMHOSTS lookup' if you're using a look up host file,
similar to the one mentioned in STEP 10 of section 3.3.1
9. Click 'OK' on all dialog boxes and restart system.
10.
Ping the linux box to test the network connection: 'File/Run',
type: ping 192.168.1.1
(This is only a LAN connection testing, you can't ping the outside
world yet.)
3.3.4. Configuring UNIX Based Systems
1. If you haven't installed your network card and recompile your
kernel with the appropriate adapter driver, do so now.
2. Install TCP/IP networking, such as the nettools package, if you
don't have it already.
3. Set IPADDR to 192.168.1.x (1 < x < 255), then set NETMASK to
255.255.255.0, GATEWAY to 192.168.1.1, and BROADCAST to
192.168.1.255
For example, you can edit the /etc/sysconfig/network-scripts/ifcfg-
eth0 file on a Red Hat Linux system, or simply do it through the
Control Panel.
(it's different in SunOS, BSDi, Slackware Linux, etc...)
4. Add your domain name service (DNS) and domain search suffix in
/etc/resolv.conf
5. You may want to update your /etc/networks file depending on your
settings.
6. Restart the appropriate services, or simply restart your system.
7. Issue a ping command: ping 192.168.1.1 to test the connection to
your gateway machine.
(This is only a LAN connection testing, you can't ping the outside
world yet.)
3.3.5. Configuring DOS using NCSA Telnet package
1. If you haven't installed your network card, do so now.
2. Load the appropriate packet driver. For an NE2000 card, issue nwpd
0x60 10 0x300, with your network card set to IRQ 10 and hardware
address at 0x300
3. Make a new directory, and then unpack the NCSA Telnet package:
pkunzip tel2308b.zip
4. Use a text editor to open the config.tel file
5. Set myip=192.168.1.x (1 < x < 255), and netmask=255.255.255.0
6. In this example, you should set hardware=packet, interrupt=10,
ioaddr=60
7. You should have at least one individual machine specification set
as the gateway, i.e. the Linux host:
name=default
host=yourlinuxhostname
hostip=192.168.1.1
gateway=1
8. Have another specification for a domain name service:
name=dns.domain.com ; hostip=123.123.123.123; nameserver=1
Note: substitute the appropriate information about the DNS that your
Linux host uses
9. Save your config.tel file
10.
Telnet to the linux box to test the network connection: telnet
192.168.1.1
3.3.6. Configuring MacOS Based System Running MacTCP
1. If you haven't installed the appropriate driver software for your
Ethernet adapter, now would be a very good time to do so.
2. Open the MacTCP control panel. Select the appropriate network
driver (Ethernet, NOT EtherTalk) and click on the 'More...' button.
3. Under 'Obtain Address:', click 'Manually'.
4. Under 'IP Address:', select class C from the popup menu. Ignore the
rest of this section of the dialog box.
5. Fill in the appropriate information under 'Domain Name Server
Information:'.
6. Under 'Gateway Address:', enter 192.168.1.1
7. Click 'OK' to save the settings. In the main window of the MacTCP
control panel, enter the IP address of your Mac (192.168.1.x, 1 < x
< 255) in the 'IP Address:' box.
8. Close the MacTCP control panel. If a dialog box pops up notifying
you to do so, restart the system.
9. You may optionally ping the Linux box to test the network
connection. If you have the freeware program MacTCP Watcher, click
on the 'Ping' button, and enter the address of your Linux box
(192.168.1.1) in the dialog box that pops up. (This is only a LAN
connection testing, you can't ping the outside world yet.)
10.
You can optionally create a Hosts file in your System Folder so
that you can use the hostnames of the machines on your LAN. The
file should already exist in your System Folder, and should contain
some (commented-out) sample entries which you can modify according
to your needs.
3.3.7. Configuring MacOS Based System Running Open Transport
1. If you haven't installed the appropriate driver software for your
Ethernet adapter, now would be a very good time to do so.
2. Open the TCP/IP Control Panel and choose 'User Mode ...' from the
Edit menu. Make sure the user mode is set to at least 'Advanced'
and click the 'OK' button.
3. Choose 'Configurations...' from the File menu. Select your
'Default' configuration and click the 'Duplicate...' button. Enter
'IP Masq' (or something to let you know that this is a special
configuration) in the 'Duplicate Configuration' dialog, it will
probably say something like 'Deafault copy'. Then click the 'OK'
button, and the 'Make Active' button
4. Select 'Ethernet' from the 'Connect via:' pop-up.
5. Select the appropriate item from the 'Configure:' pop-up. If you
don't know which option to choose, you probably should re-select
your 'Default' configuration and quit. I use 'Manually'.
6. Enter the IP address of your Mac (192.168.1.x, 1 < x < 255) in the
'IP Address:' box.
7. Enter 255.255.255.0 in the 'Subnet mask:' box.
8. Enter 192.168.1.1 in the 'Router address:' box.
9. Enter the IP addresses of your domain name servers in the 'Name
server addr.:' box.
10.
Enter the name of your Internet domain (e.g. 'microsoft.com') in
the 'Starting domain name' box under 'Implicit Search Path:'.
11.
The following procedures are optional. Incorrect values may cause
erratic behavior. If your not sure, it's probably better to leave
them blank, unchecked and/or un- selected. Remove any information
from those fields, if necessary. As far as I know there is no way
through the TCP/IP dialogs, to tell the system not to use a
previously select alternate "Hosts" file. If you know, I would be
interested.
Check the '802.3' if your network requires 802.3 frame types.
12.
Click the 'Options...' button to make sure that the TCP/IP is
active. I use the 'Load only when needed' option. If you run and
quit TCP/IP applications many times without rebooting your machine,
you may find that unchecking the 'Load only when needed' option
will prevent/reduce the effects on your machines memory management.
With the item unchecked the TCP/IP protocol stacks are always
loaded and available for use. If checked, the TCP/IP stacks are
automatically loaded when needed and un- loaded when not. It's the
loading and unloading process that can cause your machines memory
to become fragmented.
13.
You may ping the Linux box to test the network connection. If you
have the freeware program MacTCP Watcher, click on the 'Ping'
button, and enter the address of your Linux box (192.168.1.1) in
the dialog box that pops up. (This is only a LAN connection
testing, you can't ping the outside world yet.)
14.
You can create a Hosts file in your System Folder so that you can
use the hostnames of the machines on your LAN. The file may or may
not already exist in your System Folder. If so, it should contain
some (commented-out) sample entries which you can modify according
to your needs. If not, you can get a copy of the file from a
system running MacTCP, or just create your own (it follows a subset
of the Unix /etc/hosts file format, described on page 33 of RFC
1035). Once you've created the file, open the TCP/IP control
panel, click on the 'Select Hosts File...' button, and open the
Hosts file.
15.
Click the close box or choose 'Close' or 'Quit' from the File menu,
and then click the 'Save' button to save the changes you have made.
16.
The changes take effect immediately, but rebooting the system won't
hurt.
3.3.8. Configuring Other Systems such as OS/2
They should be following the same theory for setup. Check the
sections above. If you're interested in writing about any of these
systems such as OS/2, or any variations of UNIX based system, please
send a detail setup instruction to achau@wwonline.com.
3.4. Configuring IP Forwarding Policies
At this point, you should have your kernel and other required packages
installed, as well as your modules loaded. Also, the IP addresses,
gateway, and DNS should be all set on the OTHER machines.
Now, the only thing left to do is to use ipfwadm to forward
appropriate packets to the appropriate machine:
ipfwadm -F -p deny
ipfwadm -F -a m -S yyy.yyy.yyy.yyy/x -D 0.0.0.0/0
where x is one of the following numbers according to the class of your
subnet, and yyy.yyy.yyy.yyy is your network address.
netmask | x | Subnet
~~~~~~~~~~~~~~~~|~~~~|~~~~~~~~~~~~~~~
255.0.0.0 | 8 | Class A
255.255.0.0 | 16 | Class B
255.255.255.0 | 24 | Class C
255.255.255.255 | 32 | Point-to-point
For example, if I'm on a class C subnet, I would have entered:
ipfwadm -F -p deny
ipfwadm -F -a m -S 192.168.1.0/24 -D 0.0.0.0/0
The second command could have either -V 192.168.1.1 or -W eth0 added
to it to ensure that the masqueraded packets came in through the
appropriate interface of the system - if you are at all security
concious (otherwise known as justifiably paranoid) then you will want
to do this.
You can also do it on a per machine basis. For example, if I want
192.168.1.2 and 192.168.1.8 to have access to the Internet, but not
the other machines, I would have entered:
ipfwadm -F -p deny
ipfwadm -F -a m -S 192.168.1.2/32 -D 0.0.0.0/0
ipfwadm -F -a m -S 192.168.1.8/32 -D 0.0.0.0/0
Alternately, you can type the netmask instead of the value, e.g.
192.168.1.0/255.255.255.0
What appears to be a common mistake is to make the first command be
this
ipfwadm -F -p masquerade
Do not make your default policy be masquerading - otherwise someone
who can manipulate their routing will be able to tunnel straight back
through your gateway, using it to masquerade their identity!
Again, you can add these lines to the /etc/rc.local files, one of the
rc files you prefer, or do it manually every time you need ip_masq.
Please read section 4.4 for a detail guide on Ipfwadm
3.5. Testing IP Masquerade
It's time to give it a try, after all these hard work. Make sure the
connection of your Linux hosts to the Internet is okay.
You can try browsing some 'INTERNET!!!' web sites on your OTHER
machines, and see if you get it. I recommend using an IP address
rather than a hostname on your first try, because your DNS setup may
not be correct.
For example, you can access Netscape's site http://home.netscape.com
with an entry of http://198.95.249.78
If you see that nice sailboat, then congratulations! It's working! You
may then try one with hostname entry, and then telnet, ftp, Real
Audio, True Speech, whatever..... but *not* PING yet, remember!!!
So far, I have no trouble with the above settings, and it's full
credit to the people who spend their time making this wonderful
feature working.
4. Other IP Masquerade Issues and Software Support
4.1. Problems with IP Masquerade
Firstly masquerading can only work with ported protocols - such as TCP
or UDP. It specifically will not work with ICMP, so ping and
traceroute will not work (unless your ping and/or traceroute have been
modified to use a different method of working).
Some protocols will not currently work with masquerading because they
either assume things about port numbers, or encode data in their data
stream about addresses and ports - these latter protocols need
specific proxies built into the masquerading code to make them work.
4.2. Incoming services
Masquerading cannot handle incoming services at all. There are a few
ways of allowing them, but they are completely separate from
masquerading, and are really part of standard firewall practice.
If you do not require high levels of security then you can simply
redirect ports. There are various ways of doing this - I use a
modified redir program (which I hope will be available from sunsite
and mirrors soon). If you wish to have some level of authorisation on
incoming connections then you can either use TCP wrappers or Xinetd on
top of redir (0.7 or above) to allow only specific IP addresses
through, or use some other tools. The TIS Firewall Toolkit is a good
place to look for tools and information.
4.3. Supported Client Software and Other Setup Note
Generally, application that uses TCP and UDP should work. However,
ICMP based applications are not supported. If you have any suggestion
about applications that are not compatible with IP Masquerade, please
email to me with the name of client and a brief description.
4.3.1. Clients that Work
General Clients
HTTP
all supported platforms, surfing the web
POP & SMTP
all supported platforms, email client
Telnet
all supported platforms, remote session
FTP
all supported platforms, with ip_masq_ftp.o module (not all
sites work with certain clients; e.g. some sites cannot be
reached using ws_ftp32 but works with netscape)
Archie
all supported platforms, file searching client (not all archie
clients are supported)
NNTP (USENET)
all supported platforms, USENET news client
VRML
Windows(possibly all supported platforms), virtual reality
surfing
traceroute
mainly UNIX based platforms, some variations may not work
anything based on IRC
all supported platforms, with ip_masq_irc.o modules
Gopher client
all supported platforms
WAIS client
all supported platforms
Multimedia Clients
Real Audio Player 2.0
Windows, network streaming audio, with ip_masq_raudio module
loaded
True Speech Player 1.1b
Windows, network streaming audio
Internet Wave Player
Windows, network streaming audio
Worlds Chat 0.9a
Windows, Client-Server 3D chat program
Alpha Worlds
Windows, Client-Server 3D chat program
Internet Phone 3.2
Windows, Peer-to-peer audio communications, people can reach you
only if you initiate the call, but people cannot call you
Powwow
Windows, Peer-to-peer Text audio whiteboard communications,
people can reach you only if you initiate the call, but people
cannot call you
CU-SeeMe
all supported platforms, with cuseeme modules loaded, please see
IP Masquerade Resource <http://www.hwy401.com/achau/ipmasq/> for
detail
Note: Some clients such as IPhone and Powwow may work even if you're
not the one who initiate the call by using ipautofw package (there
will be a section on this soon)
Other Clients
NCSA Telnet 2.3.08
DOS, a suite containing telnet, ftp, ping, etc.
PC-anywhere for windows 2.0
MS-Windows, Remotely controls a PC over TCP/IP, only work if it
is a client but not a host
Socket Watch
uses ntp - network time protocol
Linux net-acct package
Linux, network administration-account package
4.3.2. Clients that do not Work
Ping
with exceptions to ICMP packets that contain information about
an original TCP or UDP packet, such as traceroute
talk,ntalk
will not work - requires a kernel proxy to be written.
WebPhone
Cannot work at present (it makes invalid assumptions about
addresses).
X Untested, but I think it cannot work unless someone builds an X
proxy, which is probably an external program to the masquerading
code. One way of making this work is to use ssh as the link and
use the internal X proxy of that to make things work!
4.3.3. Platforms/OS Tested as on OTHER machines
* Linux
* Solaris
* Windows 95
* Windows NT (both workstation and server)
* Windows For Workgroup 3.11 (with TCP/IP package)
* Windows 3.1 (with Chameleon package)
* Novel 4.01 Server
* OS/2 (including Warp v3)
* Macintosh OS (with MacTCP or Open Transport)
* DOS (with NCSA Telnet package, DOS Trumpet works partially)
* Amiga (with AmiTCP or AS225-stack)
* VAX Stations 3520 and 3100 with UCX (TCP/IP stack for VMS)
* Alpha/AXP with Linux/Redhat
* (Anyone tried other platforms?)
4.4. IP Firewall Administration (ipfwadm)
This section provides a more in-depth guide on using ipfwadm.
This is a setup for a firewall/masquerade system behind a PPP link
with a static PPP address follows. Trusted interface is 192.168.255.1,
PPP interface has been changed to protect the guilty :). I listed
each incoming and outgoing interface individually to catch IP spoofing
as well as stuffed routing and/or masquerading. Also anything not
explicitly allowed is forbidden!
#!/bin/sh
#
# /etc/rc.d/rc.firewall, define the firewall configuration, invoked from
# rc.local.
#
PATH=/sbin:/bin:/usr/sbin:/usr/bin
# testing, wait a bit then clear all firewall rules.
# uncomment following lines if you want the firewall to automatically
# disable after 10 minutes.
# (sleep 600; \
# ipfwadm -I -f; \
# ipfwadm -I -p accept; \
# ipfwadm -O -f; \
# ipfwadm -O -p accept; \
# ipfwadm -F -f; \
# ipfwadm -F -p accept; \
# ) &
# Incoming, flush and set default policy of deny. Actually the default policy
# is irrelevant because there is a catch all rule with deny and log.
ipfwadm -I -f
ipfwadm -I -p deny
# local interface, local machines, going anywhere is valid
ipfwadm -I -a accept -V 192.168.255.1 -S 192.168.0.0/16 -D 0.0.0.0/0
# remote interface, claiming to be local machines, IP spoofing, get lost
ipfwadm -I -a deny -V your.static.PPP.address -S 192.168.0.0/16 -D 0.0.0.0/0 -o
# remote interface, any source, going to permanent PPP address is valid
ipfwadm -I -a accept -V your.static.PPP.address -S 0.0.0.0/0 -D
your.static.PPP.address/32
# loopback interface is valid.
ipfwadm -I -a accept -V 127.0.0.1 -S 0.0.0.0/0 -D 0.0.0.0/0
# catch all rule, all other incoming is denied and logged. pity there is no
# log option on the policy but this does the job instead.
ipfwadm -I -a deny -S 0.0.0.0/0 -D 0.0.0.0/0 -o
# Outgoing, flush and set default policy of deny. Actually the default policy
# is irrelevant because there is a catch all rule with deny and log.
ipfwadm -O -f
ipfwadm -O -p deny
# local interface, any source going to local net is valid
ipfwadm -O -a accept -V 192.168.255.1 -S 0.0.0.0/0 -D 192.168.0.0/16
# outgoing to local net on remote interface, stuffed routing, deny
ipfwadm -O -a deny -V your.static.PPP.address -S 0.0.0.0/0 -D 192.168.0.0/16 -o
# outgoing from local net on remote interface, stuffed masquerading, deny
ipfwadm -O -a deny -V your.static.PPP.address -S 192.168.0.0/16 -D 0.0.0.0/0 -o
# outgoing from local net on remote interface, stuffed masquerading, deny
ipfwadm -O -a deny -V your.static.PPP.address -S 0.0.0.0/0 -D 192.168.0.0/16 -o
# anything else outgoing on remote interface is valid
ipfwadm -O -a accept -V your.static.PPP.address -S your.static.PPP.address/32 -D
0.0.0.0/0
# loopback interface is valid.
ipfwadm -O -a accept -V 127.0.0.1 -S 0.0.0.0/0 -D 0.0.0.0/0
# catch all rule, all other outgoing is denied and logged. pity there is no
# log option on the policy but this does the job instead.
ipfwadm -O -a deny -S 0.0.0.0/0 -D 0.0.0.0/0 -o
# Forwarding, flush and set default policy of deny. Actually the default policy
# is irrelevant because there is a catch all rule with deny and log.
ipfwadm -F -f
ipfwadm -F -p deny
# Masquerade from local net on local interface to anywhere.
ipfwadm -F -a masquerade -W ppp0 -S 192.168.0.0/16 -D 0.0.0.0/0
# catch all rule, all other forwarding is denied and logged. pity there is no
# log option on the policy but this does the job instead.
ipfwadm -F -a deny -S 0.0.0.0/0 -D 0.0.0.0/0 -o
You can block traffic to a particular site using the -I, -O or -F.
Remember that the set of rules are scanned top to bottom and -a means
"append" to the existing set of rules so any restrictions need to come
before global rules. For example (and untested) :-
Using -I rules. Probably the fastest but it only stops the local
machines, the firewall itself can still access the "forbidden" site.
Of course you might want to allow that combination.
# reject and log local interface, local machines going to 204.50.10.13
ipfwadm -I -a reject -V 192.168.255.1 -S 192.168.0.0/16 -D 204.50.10.13/32 -o
# local interface, local machines, going anywhere is valid
ipfwadm -I -a accept -V 192.168.255.1 -S 192.168.0.0/16 -D 0.0.0.0/0
Using -O rules. Slowest because the packets go through masquerading
first but this rule even stops the firewall accessing the forbidden
site.
# reject and log outgoing to 204.50.10.13
ipfwadm -O -a reject -V your.static.PPP.address -S your.static.PPP.address/32 -D
204.50.10.13/32 -o
# anything else outgoing on remote interface is valid
ipfwadm -O -a accept -V your.static.PPP.address -S your.static.PPP.address/32 -D
0.0.0.0/0
Using -F rules. Probably slower than -I and this still only stops
masqueraded machines (i.e. internal), firewall can still get to
forbidden site.
# Reject and log from local net on PPP interface to 204.50.10.13.
ipfwadm -F -a reject -W ppp0 -S 192.168.0.0/16 -D 204.50.10.13/32 -o
# Masquerade from local net on local interface to anywhere.
ipfwadm -F -a masquerade -W ppp0 -S 192.168.0.0/16 -D 0.0.0.0/0
No need for a special rule to allow 192.168.0.0/16 to go to
204.50.11.0, it is covered by the global rules.
There is more than one way of coding the interfaces in the above
rules. For example instead of -V 192.168.255.1 you can code -W eth0,
instead of -V your.static.PPP.address you can use -W ppp0. Personal
choice and documentation more than anything.
4.5. IP Masquerade and Demand-Dial-Up
1. If you would like to setup your network to automatically dial up
the Internet, the diald demand dial-up package will be of great
utility.
2. To setup the diald, please check out the Setting Up Diald for Linux
Page <http://home.pacific.net.sg/~harish/diald.config.html>
3. Once diald and IP masq have been setup, you can go to any of the
client machines and initiate a web, telnet or ftp session.
4. Diald will detect the incoming request, then dial up your ISP and
establish the connection.
5. There is a timeout that will occur with the first connection. This
is inevitable if you are using analog modems. The time taken to
establish the modem link and the PPP connections will cause your
client program to timeout. This can be avoided if you are using an
ISDN connection. All you need to do is to terminate the current
process on the client and restart it.
5. Miscellaneous
5.1. Getting Help
* IP Masquerade Resource page <http://www.hwy401.com/achau/ipmasq/>
should have enough information for setting up IP Masquerade
An alternative <http://www.undergrad.math.uwaterloo.ca/~ach2au>
site for this page
* Joining IP masquerade mailing list (recommended)
To subscribe, send a mail with subject "subscribe" (no quote) to
masq-request@indyramp.com
To unsubscribe, send a mail with subject "unsubscribe" (no quote)
to masq-request@indyramp.com
To get help on using the mailing list, send a mail with subject
"archive help" or "archive dir" (no quote) to masq-
request@indyramp.com
* IP masquerade mailing list archive
<http://www.indyramp.com/masq/list/> contains all the past messages
sent to the mailing list.
* This Linux IP Masquerade mini HOWTO
<http://www.hwy401.com/achau/ipmasq/ipmasq-HOWTO.html> for kernel
2.x (if you're using a 1.3.x or 2.x kernel)
* IP Masquerade HOWTO for kernel 1.2.x
<http://www.hwy401.com/achau/ipmasq/ipmasq-HOWTO-1.2.x.txt> if
you're using an older kernel
* IP masquerade FAQ <http://www.indyramp.com/masq/ip_masquerade.txt>
has some general information
* X/OS Ipfwadm page <http://www.xos.nl/linux/ipfwadm/> contains
sources, binaries, documentation, and other information about the
ipfwadm package
* LDP Network Administrator's Guide
<http://linuxwww.db.erau.edu/NAG/> is a must for beginners trying
to set up a network
* Linux NET-2 HOWTO
<http://www.caldera.com/LDP/HOWTO/NET-2-HOWTO.html> also has lots
of useful information about Linux networking
* Linux ISP Hookup HOWTO <http://www.caldera.com/LDP/HOWTO/ISP-
Hookup-HOWTO.html> gives you information on how to connect your
Linux host to the Internet
* Linux Ethernet-Howto <http://www.caldera.com/LDP/HOWTO/Ethernet-
HOWTO.html> is a good source of information about setting up a LAN
running ethernet
* You may also be interested in Linux Firewalling and Proxy Server
HOWTO <http://www.caldera.com/LDP/HOWTO/Firewall-HOWTO.html>
* Linux Kernel HOWTO <http://www.caldera.com/LDP/HOWTO/Kernel-
HOWTO.html> will guide you through the kernel compilation process
* Other Linux HOWTOs <http://www.caldera.com/LDP/HOWTO/HOWTO-
INDEX-3.html> such as PPP HOWTO and Kerneld mini HOWTO
* Posting to the USENET newsgroup: comp.os.linux.networking
5.2. Thanks to
* Ed Doolittle, dolittle@math.toronto.edu
on suggestion to -V option in ipfwadm command for improved security
* Matthew Driver, mdriver@cfmeu.asn.au
on helping extensively on this HOWTO, and providing section 3.3.1
(setting up Windows 95)
* Ken Eves, ken@eves.com
on the FAQ that provides invaluable information for this HOWTO
* Ed. Lott, edlott@neosoft.com
for a long list of tested system and software
* Nigel Metheringham, Nigel.Metheringham@theplanet.net
on contributing his version of IP Packet Filtering and IP
Masquerading HOWTO, which make this HOWTO a better and technical
in-depth document
section 4.1, 4.2, and others
* Keith Owens, kaos@audio.apana.org.au
on providing an excellent guide on ipfwadm section 4.2
on correction to ipfwadm -deny option which avoids a security hole,
and clarified the status of ping over ip_masq
* Rob Pelkey, rpelkey@abacus.bates.edu
on providing section 3.3.6 and 3.3.7 (setting up MacTCP and Open
Transport)
* Harish Pillay, h.pillay@ieee.org
on providing section 4.5 (dial-on-demand using diald)
* John B. (Brent) Williams, forerunner@mercury.net
on providing section 3.3.7 (setting up Open Transport)
* developers of IP Masquerade for this great feature
* Nigel Metheringham, Nigel.Metheringham@theplanet.net
* Keith Owens, kaos@audio.apana.org.au
* Jeanette Pauline Middelink, middelin@polyware.iaf.nl
* Miquel van Smoorenburg, miquels@q.cistron.nl
* Jos Vos, jos@xos.nl
* all users sending feedback and suggestion to the mailing list,
especially the ones who reported errors in the document and the
clients that are supported and not supported
5.3. Reference
* IP masquerade FAQ by Ken Eves
* IP masquerade mailing list archive by Indyramp Consulting
* Ipfwadm page by X/OS
* Various networking related Linux HOWTOs
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