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Re: iptables and Remote Desktop Connection problems



Have you tried disabling the firewall to see if it works???

Also certain types of Remote Desktop software send GRE type packets that
can't be forwarded via NAT.

I hope some of this might help.

On Thu, 2004-10-28 at 21:05, Edward wrote:
> I'm sorry to ask a Windows based question, but I believe it is the Linux 
> iptables firewall that is causing the problem.
> 
> My brother and wife will be in Europe as of tomorrow, wanting to be able 
> to go to an internet cafe, and with no other requisites, take over a 
> Windows box on their local LAN.
> 
> Ofcourse they leave today, and asked me to set this up yesterday, so any 
> urgency is appreciated.
> 
> I'm running Fedora core 1 on the box with the firewall blocking anything 
> suspect I know of.
> 
> I've set up a no-ip address for their dynamic address. This part is 
> working fine.
> 
> I've set up Remote Desktop Connection via the web on the target (host) 
> PC, and this is also working flawlessly...INTERNALLY...
> 
> I hit the box with the URL (names changed to protect the immense 
> security risk I'm creating but my brother refuses to care about):
> 
> http://<dynamicaddress>.hopto.org:<port>/tsweb/
> 
> This works flawlessly internally. <dynamicaddress>.hopto.org resolves 
> correctly, and the box pops up in my browser.
> 
> However, externally it's another story. I get the RDC screen where you 
> put in the computer name and screen size, after it downloads the 
> required ActiveX control from the host box.
> 
> If I put in the computer name - it tells me it can't find that host on 
> the network.
> 
> If I try to put in the internal ip address, lets call it <internalip>, 
> it tells me the host is busy or a network problem.
> 
> According to ALL the docs I've read on how to set this up, all I need to 
> forward are ports 3389 and <port> which defaults to 80, but can be any 
> arbitrary number.
> 
> (I've tried with both 80 AND <port>, just to eliminate stupidity).
> 
> Here's my iptables script:
> 
> ---
> 
> #!/bin/sh
> #
> # rc.firewall-2.4
> FWVER=0.73
> #
> #               Initial SIMPLE IP Masquerade test for 2.4.x 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:
> #       0.73 - 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 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
> INSMOD=/sbin/insmod
> FPMAIN=<internalip>
> RDPPORT=<port>
> 
> 
> #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="ppp0"
> INTIF="ath0"
> echo "   External Interface:  $EXTIF"
> echo "   Internal Interface:  $INTIF"
> 
> 
> #======================================================================
> #== 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, "
> $INSMOD 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, "
> $INSMOD 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, "
> $INSMOD 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, "
> $INSMOD 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, "
> $INSMOD 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, "
> $INSMOD ip_nat_ftp
> 
> 
> #Loads the IRC NAT functionality into the core IPTABLES code
> # Require to support NAT of IRC DCC requests
> #
> # Disabled by default -- remove the "#" on the next line to activate
> #
> #echo -e "ip_nat_irc"
> #$INSMOD ip_nat_irc
> 
> echo 
> "----------------------------------------------------------------------"
> 
> # Just to be complete, here is a list of the remaining kernel modules
> # and their function.  Please note that several modules should be only
> # loaded by the correct master kernel module for proper operation.
> # --------------------------------------------------------------------
> #
> #    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.
> #
> #    iptable_mangle - this target allows for packets to be manipulated
> #                     for things like the TCPMSS option, etc.
> 
> 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)
> #
> 
> LSMOD=/sbin/lsmod
> DEPMOD=/sbin/depmod
> INSMOD=/sbin/insmod
> GREP=/bin/grep
> AWK=/bin/awk
> SED=/bin/sed
> IFCONFIG=/sbin/ifconfig
> EXTIP="`$IFCONFIG $EXTIF | $AWK \
>   /$EXTIF/'{next}//{split($0,a,":");split(a[2],a," ");print a[1];exit}'`"
> echo " I think the external IP is: " $EXTIP
> 
> 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
> 
> #Added for RDP
> echo "Setting up FPMain for Remote Desktop Connection"
> $IPTABLES -A FORWARD -p tcp -i $EXTIF -o $INTIF -d $FPMAIN --dport 3389 
> -j ACCEPT
> $IPTABLES -A FORWARD -p tcp -i $EXTIF -o $INTIF -d $FPMAIN --dport 
> $RDPPORT -j ACCEPT
> $IPTABLES -t nat -A PREROUTING -i $EXTIF -p tcp -d $EXTIP --sport 
> 1024:65535 --dport 3889 -j DNAT --to $FPMAIN:3389
> $IPTABLES -t nat -A PREROUTING -i $EXTIF -p tcp -d $EXTIP --sport 
> 1024:65535 --dport $RDPPORT -j DNAT --to $FPMAIN:$RDPPORT
> 
> echo "	Dropping NetBIOS Requests from outside"
> $IPTABLES -A INPUT -i $EXTIF -p tcp --dport 137:139 -j DROP
> $IPTABLES -A INPUT -i $EXTIF -p udp --dport 137:139 -j DROP
> 
> echo "	Dropping ICMP from outside"
> $IPTABLES -A INPUT -i $EXTIF -p icmp -j DROP
> $IPTABLES -A FORWARD -j LOG
> 
> echo "	Dropping SSH from outside"
> $IPTABLES -A INPUT -i $EXTIF -p tcp --dport 22 -j DROP
> 
> echo "	Dropping outside SMTP"
> $IPTABLES -A INPUT -i $EXTIF -p tcp --dport 25 -j DROP
> 
> echo "	Dropping outside POP"
> $IPTABLES -A INPUT -i $EXTIF -p tcp --dport 110 -j DROP
> 
> echo "	Dropping outside Ident"
> $IPTABLES -A INPUT -i $EXTIF -p tcp --dport 113 -j DROP
> 
> echo "	Dropping outside Remote Grab"
> $IPTABLES -A INPUT -i $EXTIF -p tcp --dport 7000 -j DROP
> 
> echo "   Enabling SNAT (MASQUERADE) functionality on $EXTIF"
> $IPTABLES -t nat -A POSTROUTING -o $EXTIF -j MASQUERADE
> 
> 
> 
> echo -e "\nDone.\n"
> 
> ---
> 
> I've used the same sort of "forwarding" idea on another server to send 
> emule packets to my Windows box and that works.
> 
> I'm using the same syntax here to forward 3398 and <port> to 
> <internalip> internally. (I think).
> 
> Any idea what could be wrong?
> 
> Oh, and obviously <dynamicaddress>, <internalip>, and <port> are real 
> values, I've just hidden them here because I'm anal about security.
> 
> Regards,
> Ed.


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