firewall NAT tables with dynamic assign IP
Allan Spagnol Comar
allan.comar at gmail.com
Sat Oct 21 13:51:07 UTC 2006
Hi noah, I do not understand your doubt, if you want the DHCP ip to be
changed from the internal network to the external network you don´t
need the $IPTABLES -t nat -A POSTROUTING -o $EXTIF -j SNAT --to $EXTIP
line, you just need
$IPTABLES -t nat -A POSTROUTING -o $EXTIF -j MASQUERADE
holpe it helps, Allan
On 10/20/06, Noah <admin2 at enabled.com> wrote:
>
>
> Hi there,
>
> So I am wanting to learn how to modify my firewall-tables script to figure
> out the current dynamically assigned IP address on fxp1 and also update the
> firewall-tables if the dhcpd running on fxp1 reassigns a new IP.
>
>
> Here is the script:
>
> #!/bin/sh
> #
> #firewall-iptables
> FWVER= 0.76
> #
> # Initial SIMPLE IP Masquerade test for 2.6 / 2.4 kernels
> # using IPTABLES.
> #
> # Once IP Masquerading has been tested, with this simple
> # ruleset, it is highly recommended to use a stronger
> # IPTABLES ruleset either given later in this HOWTO or
> # from another reputable resource.
> #
> #
> #
> # Log:
> # 0.76 - Added comments on why the default policy is ACCEPT
> # 0.75 - Added more kernel modules to the comments section
> # 0.74 - the ruleset now uses modprobe vs. insmod
> # 0.73 - REJECT is not a legal policy yet; back to DROP
> # 0.72 - Changed the default block behavior to REJECT not DROP
> # 0.71 - Added clarification that PPPoE users need to use
> # "ppp0" instead of "eth0" for their external interface
> # 0.70 - Added commented option for IRC nat module
> # - Added additional use of environment variables
> # - Added additional formatting
> # 0.63 - Added support for the IRC IPTABLES module
> # 0.62 - Fixed a typo on the MASQ enable line that used eth0
> # instead of $EXTIF
> # 0.61 - Changed the firewall to use variables for the internal
> # and external interfaces.
> # 0.60 - 0.50 had a mistake where the ruleset had a rule to DROP
> # all forwarded packets but it didn't have a rule to ACCEPT
> # any packets to be forwarded either
> # - Load the ip_nat_ftp and ip_conntrack_ftp modules by default
> # 0.50 - Initial draft
> #
>
> echo -e "\n\nLoading simple rc.firewall-iptables version $FWVER..\n"
>
>
> # The location of the iptables and kernel module programs
> #
> # If your Linux distribution came with a copy of iptables,
> # most likely all the programs will be located in /sbin. If
> # you manually compiled iptables, the default location will
> # be in /usr/local/sbin
> #
> # ** Please use the "whereis iptables" command to figure out
> # ** where your copy is and change the path below to reflect
> # ** your setup
> #
> IPTABLES=/sbin/iptables
> #IPTABLES=/usr/local/sbin/iptables
> DEPMOD=/sbin/depmod
> MODPROBE=/sbin/modprobe
>
>
> #Setting the EXTERNAL and INTERNAL interfaces for the network
> #
> # Each IP Masquerade network needs to have at least one
> # external and one internal network. The external network
> # is where the natting will occur and the internal network
> # should preferably be addressed with a RFC1918 private address
> # scheme.
> #
> # For this example, "eth0" is external and "eth1" is internal"
> #
> #
> # NOTE: If this doesnt EXACTLY fit your configuration, you must
> # change the EXTIF or INTIF variables above. For example:
> #
> # If you are a PPPoE or analog modem user:
> #
> # EXTIF="ppp0"
> #
> #
> EXTIF="eth1"
> INTIF="eth0"
> #INTIF2="eth0"
> echo " External Interface: $EXTIF"
> echo " Internal Interface: $INTIF"
> #echo " Internal Interface: $INTIF2"
>
> #EXTIP="your external IP address"
> EXTIP="<dhcpd_assigned_ip>"
> echo " External IP: $EXTIP"
>
> #======================================================================
> #== No editing beyond this line is required for initial MASQ testing ==
>
>
> echo -en " loading modules: "
>
> # Need to verify that all modules have all required dependencies
> #
> echo " - Verifying that all kernel modules are ok"
> $DEPMOD -a
>
> # With the new IPTABLES code, the core MASQ functionality is now either
> # modular or compiled into the kernel. This HOWTO shows ALL IPTABLES
> # options as MODULES. If your kernel is compiled correctly, there is
> # NO need to load the kernel modules manually.
> #
> # NOTE: The following items are listed ONLY for informational reasons.
> # There is no reason to manual load these modules unless your
> # kernel is either mis-configured or you intentionally disabled
> # the kernel module autoloader.
> #
>
> # Upon the commands of starting up IP Masq on the server, the
> # following kernel modules will be automatically loaded:
> #
> # NOTE: Only load the IP MASQ modules you need. All current IP MASQ
> # modules are shown below but are commented out from loading.
> #
> ===============================================================
>
> echo
> "----------------------------------------------------------------------"
>
> #Load the main body of the IPTABLES module - "iptable"
> # - Loaded automatically when the "iptables" command is invoked
> #
> # - Loaded manually to clean up kernel auto-loading timing issues
> #
> echo -en "ip_tables, "
> $MODPROBE ip_tables
>
>
> #Load the IPTABLES filtering module - "iptable_filter"
> # - Loaded automatically when filter policies are activated
>
>
> #Load the stateful connection tracking framework - "ip_conntrack"
> #
> # The conntrack module in itself does nothing without other specific
> # conntrack modules being loaded afterwards such as the "ip_conntrack_ftp"
> # module
> #
> # - This module is loaded automatically when MASQ functionality is
> # enabled
> #
> # - Loaded manually to clean up kernel auto-loading timing issues
> #
> echo -en "ip_conntrack, "
> $MODPROBE ip_conntrack
>
>
> #Load the FTP tracking mechanism for full FTP tracking
> #
> # Enabled by default -- insert a "#" on the next line to deactivate
> #
> echo -en "ip_conntrack_ftp, "
> $MODPROBE ip_conntrack_ftp
>
>
> #Load the IRC tracking mechanism for full IRC tracking
> #
> # Enabled by default -- insert a "#" on the next line to deactivate
> #
> echo -en "ip_conntrack_irc, "
> $MODPROBE ip_conntrack_irc
>
>
> #Load the general IPTABLES NAT code - "iptable_nat"
> # - Loaded automatically when MASQ functionality is turned on
> #
> # - Loaded manually to clean up kernel auto-loading timing issues
> #
> echo -en "iptable_nat, "
> $MODPROBE iptable_nat
>
>
> #Loads the FTP NAT functionality into the core IPTABLES code
> # Required to support non-PASV FTP.
> #
> # Enabled by default -- insert a "#" on the next line to deactivate
> #
> echo -en "ip_nat_ftp, "
> $MODPROBE ip_nat_ftp
>
>
> #Loads the IRC NAT functionality into the core IPTABLES code
> # Required to support NAT of IRC DCC requests
> #
> # Disabled by default -- remove the "#" on the next line to activate
> #
> #echo -e "ip_nat_irc"
> #$MODPROBE ip_nat_irc
>
> echo
> "----------------------------------------------------------------------"
>
> # Just to be complete, here is a partial list of some of the other
> # IPTABLES kernel modules and their function. Please note that most
> # of these modules (the ipt ones) are automatically loaded by the
> # master kernel module for proper operation and don't need to be
> # manually loaded.
> #
> --------------------------------------------------------------------
> #
> # ip_nat_snmp_basic - this module allows for proper NATing of some
> # SNMP traffic
> #
> # iptable_mangle - this target allows for packets to be
> # manipulated for things like the TCPMSS
> # option, etc.
> #
> # --
> #
> # ipt_mark - this target marks a given packet for future action.
> # This automatically loads the ipt_MARK module
> #
> # ipt_tcpmss - this target allows to manipulate the TCP MSS
> # option for braindead remote firewalls.
> # This automatically loads the ipt_TCPMSS module
> #
> # ipt_limit - this target allows for packets to be limited to
> # to many hits per sec/min/hr
> #
> # ipt_multiport - this match allows for targets within a range
> # of port numbers vs. listing each port individually
> #
> # ipt_state - this match allows to catch packets with various
> # IP and TCP flags set/unset
> #
> # ipt_unclean - this match allows to catch packets that have invalid
> # IP/TCP flags set
> #
> # iptable_filter - this module allows for packets to be DROPped,
> # REJECTed, or LOGged. This module automatically
> # loads the following modules:
> #
> # ipt_LOG - this target allows for packets to be
> # logged
> #
> # ipt_REJECT - this target DROPs the packet and returns
> # a configurable ICMP
> packet back to the
> # sender.
> #
>
> echo -e " Done loading modules.\n"
>
>
>
> #CRITICAL: Enable IP forwarding since it is disabled by default since
> #
> # Redhat Users: you may try changing the options in
> # /etc/sysconfig/network from:
> #
> # FORWARD_IPV4=false
> # to
> # FORWARD_IPV4=true
> #
> echo " Enabling forwarding.."
> echo "1" > /proc/sys/net/ipv4/ip_forward
>
>
> # Dynamic IP users:
> #
> # If you get your IP address dynamically from SLIP, PPP, or DHCP,
> # enable this following option. This enables dynamic-address hacking
> # which makes the life with Diald and similar programs much easier.
> #
> echo " Enabling DynamicAddr.."
> echo "1" > /proc/sys/net/ipv4/ip_dynaddr
>
>
> # Enable simple IP forwarding and Masquerading
> #
> # NOTE: In IPTABLES speak, IP Masquerading is a form of SourceNAT or
> SNAT.
> #
> # NOTE #2: The following is an example for an internal LAN address in the
> # 192.168.0.x network with a 255.255.255.0 or a "24" bit subnet
> mask
> # connecting to the Internet on external interface "eth0". This
> # example will MASQ internal traffic out to the Internet but not
> # allow non-initiated traffic into your internal network.
> #
> #
> # ** Please change the above network numbers, subnet mask, and your
> # *** Internet connection interface name to match your setup
> #
>
>
> #Clearing any previous configuration
> #
> # Unless specified, the defaults for INPUT and OUTPUT is ACCEPT
> # The default for FORWARD is DROP (REJECT is not a valid policy)
> #
> # Isn't ACCEPT insecure? To some degree, YES, but this is our testing
> # phase. Once we know that IPMASQ is working well, I recommend you run
> # the rc.firewall-*-stronger rulesets which set the defaults to DROP but
> # also include the critical additional rulesets to still let you connect
> to
> # the IPMASQ server, etc.
> #
> echo " Clearing any existing rules and setting default policy.."
> $IPTABLES -P INPUT ACCEPT
> $IPTABLES -F INPUT
> $IPTABLES -P OUTPUT ACCEPT
> $IPTABLES -F OUTPUT
> $IPTABLES -P FORWARD DROP
> $IPTABLES -F FORWARD
> $IPTABLES -t nat -F
>
> echo " FWD: Allow all connections OUT and only existing and related ones
> IN"
> $IPTABLES -A FORWARD -i $EXTIF -o $INTIF -m state --state
> ESTABLISHED,RELATED -j ACCEPT
> $IPTABLES -A FORWARD -i $INTIF -o $EXTIF -j ACCEPT
> $IPTABLES -A FORWARD -j LOG
> #$IPTABLES -A FORWARD -i $EXTIF -o $INTIF2 -m state --state
> ESTABLISHED,RELATED \-j ACCEPT
> #$IPTABLES -A FORWARD -i $INTIF -o $INTIF2 -m state --state
> ESTABLISHED,RELATED \-j ACCEPT
> #$IPTABLES -A FORWARD -i $INTIF2 -o $INTIF -m state --state
> ESTABLISHED,RELATED \-j ACCEPT
> #$IPTABLES -A FORWARD -i $INTIF2 -o $EXTIF -j ACCEPT
> $IPTABLES -t nat -A POSTROUTING -o $EXTIF -j SNAT --to $EXTIP
>
>
> echo " Enabling SNAT (MASQUERADE) functionality on $EXTIF"
> $IPTABLES -t nat -A POSTROUTING -o $EXTIF -j MASQUERADE
>
> echo -e "\nrc.firewall-iptables v$FWVER done.\n"
>
>
>
> cheers,
>
> Noah
>
>
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>
>
>
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