Sharing a connection with a server using a unique ip

OK, the issue here is that you have set up your laptop as the gateway. Presumably, you chose the "Automatic" rather than the "Manual" approach on the guide you followed. You need to go back to network settings on the sever and set it to manual:

In the above picture, I have set the following:

• Address: 192.168.1.50 : this is the IP address of the server. The important part is the 192.168.1, this will make sure that your server is on the same network as the rest of your local network. The last number, 50, I chose randomly, all that is needed is that it be a number beyond the range of addresses that your DHCP server allocates. This is usually your router and you should check the settings just to be sure, but 50 should be fine.

• Netmask: 255.255.255.0 : This is to make sure that the server is on the same subnet, for more details see here.

• Gateway: 192.168.1.1 : This is the address of your router, based on the ifconfig output you posted, this should be correct.

• DNS: 212.27.40.240 : This is the address of the DNS server, the server you contact in order to translate numeric IPs into host names like google.com. I have chosen the OpenDNS server but you can use another, just copy the settings from your laptop's connection.

Once you have done that, just reconnect and your server should be visible at 192.168.1.50.

If you don't have a GUI (you can install one very easily on the Server edition by the way), you'll need to define all this in the file /etc/network/interfaces. Open it with a text editor (using nano as an example but you can use emacs or whatever you are most comfortable with):

sudo nano /etc/network/interfaces

It probably looks like this:

# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).

# The loopback network interface
auto lo
iface lo inet loopback

# The primary network interface
auto eth0
iface eth0 inet dhcp

Make it look like this:

# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).

# The loopback network interface
auto lo
iface lo inet loopback

auto eth0
iface eth0 inet static
    address 192.168.1.50
    netmask 255.255.255.0
    gateway 192.168.1.1

Save the file (with nano: Ctrl+O, then Enter, then Ctrl+X)

That sets up the network settings, the DNS server might not be necessary. If after making the changes above you can't connect to http://google.com but you can connect to, for example 74.125.195.139 (that's google.fr in case you're wondering) you will need to configure the DNS servers. To do this, open the file /etc/resolv.conf with nano:

sudo nano /etc/resolv.conf

And add this line:

nameserver 212.27.40.240

Now, disconnect, reconnect and you should be fine.

This answer presumes several things (like IP's and that you have bash) and may not work out as you it's intended, please proceed with care.

If I read your question correctly, you want that the port 80 on your server is available to the LAN/wlan0 device, and that the server can just connect to internet fine. Well, first of all, reconfigure everything as it's shown in the question otherwise I can not guarantee this will work out. Second, do backups of the configuration just in case you break something along the way. This includes a dump of your iptables. Just using iptables-save. This script needs to be saved on your laptop.

#!/bin/bash
INET_IFACE="wlan0"
INET_ADDRESS="192.168.1.4"

# Local Interface Information
LOCAL_IFACE="eth0"
LOCAL_IP="10.42.0.1"
LOCAL_NET="10.42.0.0/24"
LOCAL_BCAST="10.42.0.255"

# Localhost Interface

LO_IFACE="lo"
LO_IP="127.0.0.1"

# Save and Restore arguments handled here
if [ "$1" = "save" ]
then
    echo -n "Saving firewall to /etc/sysconfig/iptables ... "
    /usr/sbin/iptables-save > /etc/sysconfig/iptables
    echo "done"
    exit 0
elif [ "$1" = "restore" ]
then
    echo -n "Restoring firewall from /etc/sysconfig/iptables ... "
    /usr/sbin/iptables-restore < /etc/sysconfig/iptables
    echo "done"
    exit 0
fi

###############################################################################
#
# Load Modules
#

echo "Loading kernel modules ..."

# You should uncomment the line below and run it the first time just to
# ensure all kernel module dependencies are OK.  There is no need to run
# every time, however.

# /sbin/depmod -a

# Unless you have kernel module auto-loading disabled, you should not
# need to manually load each of these modules.  Other than ip_tables,
# ip_conntrack, and some of the optional modules, I've left these
# commented by default.  Uncomment if you have any problems or if
# you have disabled module autoload.  Note that some modules must
# be loaded by another kernel module.

# core netfilter module
/sbin/modprobe ip_tables

# the stateful connection tracking module
/sbin/modprobe ip_conntrack

# filter table module
# /sbin/modprobe iptable_filter

# mangle table module
# /sbin/modprobe iptable_mangle

# nat table module
# /sbin/modprobe iptable_nat

# LOG target module
# /sbin/modprobe ipt_LOG

# This is used to limit the number of packets per sec/min/hr
# /sbin/modprobe ipt_limit

# masquerade target module
/sbin/modprobe ipt_MASQUERADE

# filter using owner as part of the match
# /sbin/modprobe ipt_owner

# REJECT target drops the packet and returns an ICMP response.
# The response is configurable.  By default, connection refused.
# /sbin/modprobe ipt_REJECT

# This target allows packets to be marked in the mangle table
# /sbin/modprobe ipt_mark

# This target affects the TCP MSS
# /sbin/modprobe ipt_tcpmss

# This match allows multiple ports instead of a single port or range
# /sbin/modprobe multiport

# This match checks against the TCP flags
# /sbin/modprobe ipt_state

# This match catches packets with invalid flags
# /sbin/modprobe ipt_unclean

# The ftp nat module is required for non-PASV ftp support
/sbin/modprobe ip_nat_ftp

# the module for full ftp connection tracking
/sbin/modprobe ip_conntrack_ftp

# the module for full irc connection tracking
/sbin/modprobe ip_conntrack_irc


###############################################################################
#
# Kernel Parameter Configuration
#
# See http://ipsysctl-tutorial.frozentux.net/chunkyhtml/index.html
# for a detailed tutorial on sysctl and the various settings
# available.

# Required to enable IPv4 forwarding.
# Redhat users can try setting FORWARD_IPV4 in /etc/sysconfig/network to true
# Alternatively, it can be set in /etc/sysctl.conf
/sbin/sysctl -w net.ipv4.ip_forward="1"

# This enables dynamic address hacking.
# This may help if you have a dynamic IP address \(e.g. slip, ppp, dhcp\).
#if [ "/sbin/sysctl -w" = "" ]
#then
#    echo "1" > /proc/sys/net/ipv4/ip_dynaddr
#else
#    /sbin/sysctl -w net.ipv4.ip_dynaddr="1"
#fi

# This enables SYN flood protection.
# The SYN cookies activation allows your system to accept an unlimited
# number of TCP connections while still trying to give reasonable
# service during a denial of service attack.

/sbin/sysctl -w net.ipv4.tcp_syncookies="1"


# This enables source validation by reversed path according to RFC1812.
# In other words, did the response packet originate from the same interface
# through which the source packet was sent?  It's recommended for single-homed
# systems and routers on stub networks.  Since those are the configurations
# this firewall is designed to support, I turn it on by default.
# Turn it off if you use multiple NICs connected to the same network.

/sbin/sysctl -w net.ipv4.conf.all.rp_filter="1"


# This option allows a subnet to be firewalled with a single IP address.
# It's used to build a DMZ.  Since that's not a focus of this firewall
# script, it's not enabled by default, but is included for reference.
# See: http://www.sjdjweis.com/linux/proxyarp/ 
#if [ "/sbin/sysctl -w" = "" ]
#then
#    echo "1" > /proc/sys/net/ipv4/conf/all/proxy_arp
#else
#    /sbin/sysctl -w net.ipv4.conf.all.proxy_arp="1"
#fi

# The following kernel settings were suggested by Alex Weeks. Thanks!

# This kernel parameter instructs the kernel to ignore all ICMP
# echo requests sent to the broadcast address.  This prevents
# a number of smurfs and similar DoS nasty attacks.

/sbin/sysctl -w net.ipv4.icmp_echo_ignore_broadcasts="1"

# This option can be used to accept or refuse source routed
# packets.  It is usually on by default, but is generally
# considered a security risk.  This option turns it off.

/sbin/sysctl -w net.ipv4.conf.all.accept_source_route="0"

# This option can disable ICMP redirects.  ICMP redirects
# are generally considered a security risk and shouldn't be
# needed by most systems using this generator.
#if [ "/sbin/sysctl -w" = "" ]
#then
#    echo "0" > /proc/sys/net/ipv4/conf/all/accept_redirects
#else
#    /sbin/sysctl -w net.ipv4.conf.all.accept_redirects="0"
#fi

# However, we'll ensure the secure_redirects option is on instead.
# This option accepts only from gateways in the default gateways list.

/sbin/sysctl -w net.ipv4.conf.all.secure_redirects="1"

# This option logs packets from impossible addresses.

/sbin/sysctl -w net.ipv4.conf.all.log_martians="1"

###############################################################################
#
# Flush Any Existing Rules or Chains
#

echo "Flushing Tables ..."

# Reset Default Policies
/usr/sbin/iptables -P INPUT ACCEPT
/usr/sbin/iptables -P FORWARD ACCEPT
/usr/sbin/iptables -P OUTPUT ACCEPT
/usr/sbin/iptables -t nat -P PREROUTING ACCEPT
/usr/sbin/iptables -t nat -P POSTROUTING ACCEPT
/usr/sbin/iptables -t nat -P OUTPUT ACCEPT
/usr/sbin/iptables -t mangle -P PREROUTING ACCEPT
/usr/sbin/iptables -t mangle -P OUTPUT ACCEPT

# Flush all rules
/usr/sbin/iptables -F
/usr/sbin/iptables -t nat -F
/usr/sbin/iptables -t mangle -F

# Erase all non-default chains
/usr/sbin/iptables -X
/usr/sbin/iptables -t nat -X
/usr/sbin/iptables -t mangle -X

if [ "$1" = "stop" ]
then
    echo "Firewall completely flushed!  Now running with no firewall."
    exit 0
fi

###############################################################################
#
# Rules Configuration
#

###############################################################################
#
# Filter Table
#
###############################################################################

# Set Policies

/usr/sbin/iptables -P INPUT DROP
/usr/sbin/iptables -P OUTPUT DROP
/usr/sbin/iptables -P FORWARD DROP

###############################################################################
#
# User-Specified Chains
#
# Create user chains to reduce the number of rules each packet
# must traverse.

echo "Create and populate custom rule chains ..."

# Create a chain to filter INVALID packets

/usr/sbin/iptables -N bad_packets

# Create another chain to filter bad tcp packets

/usr/sbin/iptables -N bad_tcp_packets

# Create separate chains for icmp, tcp (incoming and outgoing),
# and incoming udp packets.

/usr/sbin/iptables -N icmp_packets

# Used for UDP packets inbound from the Internet
/usr/sbin/iptables -N udp_inbound

# Used to block outbound UDP services from internal network
# Default to allow all
/usr/sbin/iptables -N udp_outbound

# Used to allow inbound services if desired
# Default fail except for established sessions
/usr/sbin/iptables -N tcp_inbound

# Used to block outbound services from internal network
# Default to allow all
/usr/sbin/iptables -N tcp_outbound

###############################################################################
#
# Populate User Chains
#

# bad_packets chain
#

# Drop packets received on the external interface
# claiming a source of the local network
/usr/sbin/iptables -A bad_packets -p ALL -i $INET_IFACE -s $LOCAL_NET -j LOG \
    --log-prefix "Illegal source: "

/usr/sbin/iptables -A bad_packets -p ALL -i $INET_IFACE -s $LOCAL_NET -j DROP

# Drop INVALID packets immediately
/usr/sbin/iptables -A bad_packets -p ALL -m conntrack --ctstate INVALID -j LOG \
    --log-prefix "Invalid packet: "

/usr/sbin/iptables -A bad_packets -p ALL -m conntrack --ctstate INVALID -j DROP

# Then check the tcp packets for additional problems
/usr/sbin/iptables -A bad_packets -p tcp -j bad_tcp_packets

# All good, so return
/usr/sbin/iptables -A bad_packets -p ALL -j RETURN

# bad_tcp_packets chain
#
# All tcp packets will traverse this chain.
# Every new connection attempt should begin with
# a syn packet.  If it doesn't, it is likely a
# port scan.  This drops packets in state
# NEW that are not flagged as syn packets.

# Return to the calling chain if the bad packets originate
# from the local interface. This maintains the approach
# throughout this firewall of a largely trusted internal
# network.
/usr/sbin/iptables -A bad_tcp_packets -p tcp -i $LOCAL_IFACE -j RETURN

# However, I originally did apply this filter to the forward chain
# for packets originating from the internal network.  While I have
# not conclusively determined its effect, it appears to have the
# interesting side effect of blocking some of the ad systems.
# Apparently some ad systems have the browser initiate a NEW
# connection that is not flagged as a syn packet to retrieve
# the ad image.  If you wish to experiment further comment the
# rule above. If you try it, you may also wish to uncomment the
# rule below.  It will keep those packets from being logged.
# There are a lot of them.
# /usr/sbin/iptables -A bad_tcp_packets -p tcp -i $LOCAL_IFACE ! --syn \
#     -m conntrack --ctstate NEW -j DROP

/usr/sbin/iptables -A bad_tcp_packets -p tcp ! --syn -m conntrack --ctstate NEW -j LOG \
    --log-prefix "New not syn: "
/usr/sbin/iptables -A bad_tcp_packets -p tcp ! --syn -m conntrack --ctstate NEW -j DROP

/usr/sbin/iptables -A bad_tcp_packets -p tcp --tcp-flags ALL NONE -j LOG \
    --log-prefix "Stealth scan: "
/usr/sbin/iptables -A bad_tcp_packets -p tcp --tcp-flags ALL NONE -j DROP

/usr/sbin/iptables -A bad_tcp_packets -p tcp --tcp-flags ALL ALL -j LOG \
    --log-prefix "Stealth scan: "
/usr/sbin/iptables -A bad_tcp_packets -p tcp --tcp-flags ALL ALL -j DROP

/usr/sbin/iptables -A bad_tcp_packets -p tcp --tcp-flags ALL FIN,URG,PSH -j LOG \
    --log-prefix "Stealth scan: "
/usr/sbin/iptables -A bad_tcp_packets -p tcp --tcp-flags ALL FIN,URG,PSH -j DROP

/usr/sbin/iptables -A bad_tcp_packets -p tcp --tcp-flags ALL SYN,RST,ACK,FIN,URG -j LOG \
    --log-prefix "Stealth scan: "
/usr/sbin/iptables -A bad_tcp_packets -p tcp --tcp-flags ALL SYN,RST,ACK,FIN,URG -j DROP

/usr/sbin/iptables -A bad_tcp_packets -p tcp --tcp-flags SYN,RST SYN,RST -j LOG \
    --log-prefix "Stealth scan: "
/usr/sbin/iptables -A bad_tcp_packets -p tcp --tcp-flags SYN,RST SYN,RST -j DROP

/usr/sbin/iptables -A bad_tcp_packets -p tcp --tcp-flags SYN,FIN SYN,FIN -j LOG \
    --log-prefix "Stealth scan: "
/usr/sbin/iptables -A bad_tcp_packets -p tcp --tcp-flags SYN,FIN SYN,FIN -j DROP

# All good, so return
/usr/sbin/iptables -A bad_tcp_packets -p tcp -j RETURN

# icmp_packets chain
#
# This chain is for inbound (from the Internet) icmp packets only.
# Type 8 (Echo Request) is not accepted by default
# Enable it if you want remote hosts to be able to reach you.
# 11 (Time Exceeded) is the only one accepted
# that would not already be covered by the established
# connection rule.  Applied to INPUT on the external interface.
# 
# See: http://www.ee.siue.edu/~rwalden/networking/icmp.html
# for more info on ICMP types.
#
# Note that the stateful settings allow replies to ICMP packets.
# These rules allow new packets of the specified types.

# ICMP packets should fit in a Layer 2 frame, thus they should
# never be fragmented.  Fragmented ICMP packets are a typical sign
# of a denial of service attack.
/usr/sbin/iptables -A icmp_packets --fragment -p ICMP -j LOG \
    --log-prefix "ICMP Fragment: "
/usr/sbin/iptables -A icmp_packets --fragment -p ICMP -j DROP

# Echo - uncomment to allow your system to be pinged.
# Uncomment the LOG command if you also want to log PING attempts
# 
# /usr/sbin/iptables -A icmp_packets -p ICMP -s 0/0 --icmp-type 8 -j LOG \
#    --log-prefix "Ping detected: "
# /usr/sbin/iptables -A icmp_packets -p ICMP -s 0/0 --icmp-type 8 -j ACCEPT

# By default, however, drop pings without logging. Blaster
# and other worms have infected systems blasting pings.
# Comment the line below if you want pings logged, but it
# will likely fill your logs.
/usr/sbin/iptables -A icmp_packets -p ICMP -s 0/0 --icmp-type 8 -j DROP

# Time Exceeded
/usr/sbin/iptables -A icmp_packets -p ICMP -s 0/0 --icmp-type 11 -j ACCEPT

# Not matched, so return so it will be logged
/usr/sbin/iptables -A icmp_packets -p ICMP -j RETURN

# TCP & UDP
# Identify ports at:
#    http://www.chebucto.ns.ca/~rakerman/port-table.html
#    http://www.iana.org/assignments/port-numbers

# udp_inbound chain
#
# This chain describes the inbound UDP packets it will accept.
# It's applied to INPUT on the external or Internet interface.
# Note that the stateful settings allow replies.
# These rules are for new requests.
# It drops netbios packets (windows) immediately without logging.

# Drop netbios calls
# Please note that these rules do not really change the way the firewall
# treats netbios connections.  Connections from the localhost and
# internal interface (if one exists) are accepted by default.
# Responses from the Internet to requests initiated by or through
# the firewall are also accepted by default.  To get here, the
# packets would have to be part of a new request received by the
# Internet interface.  You would have to manually add rules to
# accept these.  I added these rules because some network connections,
# such as those via cable modems, tend to be filled with noise from
# unprotected Windows machines.  These rules drop those packets
# quickly and without logging them.  This prevents them from traversing
# the whole chain and keeps the log from getting cluttered with
# chatter from Windows systems.
/usr/sbin/iptables -A udp_inbound -p UDP -s 0/0 --destination-port 137 -j DROP
/usr/sbin/iptables -A udp_inbound -p UDP -s 0/0 --destination-port 138 -j DROP


# Not matched, so return for logging
/usr/sbin/iptables -A udp_inbound -p UDP -j RETURN

# udp_outbound chain
#
# This chain is used with a private network to prevent forwarding for
# UDP requests on specific protocols.  Applied to the FORWARD rule from
# the internal network.  Ends with an ACCEPT


# No match, so ACCEPT
/usr/sbin/iptables -A udp_outbound -p UDP -s 0/0 -j ACCEPT

# tcp_inbound chain
#
# This chain is used to allow inbound connections to the
# system/gateway.  Use with care.  It defaults to none.
# It's applied on INPUT from the external or Internet interface.


# Not matched, so return so it will be logged
/usr/sbin/iptables -A tcp_inbound -p TCP -j RETURN

# tcp_outbound chain
#
# This chain is used with a private network to prevent forwarding for
# requests on specific protocols.  Applied to the FORWARD rule from
# the internal network.  Ends with an ACCEPT


# No match, so ACCEPT
/usr/sbin/iptables -A tcp_outbound -p TCP -s 0/0 -j ACCEPT

###############################################################################
#
# INPUT Chain
#

echo "Process INPUT chain ..."

# Allow all on localhost interface
/usr/sbin/iptables -A INPUT -p ALL -i $LO_IFACE -j ACCEPT

# Drop bad packets
/usr/sbin/iptables -A INPUT -p ALL -j bad_packets

# DOCSIS compliant cable modems
# Some DOCSIS compliant cable modems send IGMP multicasts to find
# connected PCs.  The multicast packets have the destination address
# 224.0.0.1.  You can accept them.  If you choose to do so,
# Uncomment the rule to ACCEPT them and comment the rule to DROP
# them  The firewall will drop them here by default to avoid
# cluttering the log.  The firewall will drop all multicasts
# to the entire subnet (224.0.0.1) by default.  To only affect
# IGMP multicasts, change '-p ALL' to '-p 2'.  Of course,
# if they aren't accepted elsewhere, it will only ensure that
# multicasts on other protocols are logged.
# Drop them without logging.
/usr/sbin/iptables -A INPUT -p ALL -d 224.0.0.1 -j DROP
# The rule to accept the packets.
# /usr/sbin/iptables -A INPUT -p ALL -d 224.0.0.1 -j ACCEPT

# Rules for the private network (accessing gateway system itself)
/usr/sbin/iptables -A INPUT -p ALL -i $LOCAL_IFACE -s $LOCAL_NET -j ACCEPT
/usr/sbin/iptables -A INPUT -p ALL -i $LOCAL_IFACE -d $LOCAL_BCAST -j ACCEPT


# Inbound Internet Packet Rules

# Accept Established Connections
/usr/sbin/iptables -A INPUT -p ALL -i $INET_IFACE -m conntrack --ctstate ESTABLISHED,RELATED \
     -j ACCEPT

# Route the rest to the appropriate user chain
/usr/sbin/iptables -A INPUT -p TCP -i $INET_IFACE -j tcp_inbound
/usr/sbin/iptables -A INPUT -p UDP -i $INET_IFACE -j udp_inbound
/usr/sbin/iptables -A INPUT -p ICMP -i $INET_IFACE -j icmp_packets

# Drop without logging broadcasts that get this far.
# Cuts down on log clutter.
# Comment this line if testing new rules that impact
# broadcast protocols.
/usr/sbin/iptables -A INPUT -m pkttype --pkt-type broadcast -j DROP

# Log packets that still don't match
/usr/sbin/iptables -A INPUT -m limit --limit 3/minute --limit-burst 3 -j LOG \
    --log-prefix "INPUT packet died: "

###############################################################################
#
# FORWARD Chain
#

echo "Process FORWARD chain ..."

# Used if forwarding for a private network

# Drop bad packets
/usr/sbin/iptables -A FORWARD -p ALL -j bad_packets

# Accept TCP packets we want to forward from internal sources
/usr/sbin/iptables -A FORWARD -p tcp -i $LOCAL_IFACE -j tcp_outbound

# Accept UDP packets we want to forward from internal sources
/usr/sbin/iptables -A FORWARD -p udp -i $LOCAL_IFACE -j udp_outbound

# If not blocked, accept any other packets from the internal interface
/usr/sbin/iptables -A FORWARD -p ALL -i $LOCAL_IFACE -j ACCEPT

# Deal with responses from the internet
/usr/sbin/iptables -A FORWARD -i $INET_IFACE -m conntrack --ctstate ESTABLISHED,RELATED \
     -j ACCEPT

# Port Forwarding is enabled, so accept forwarded traffic
/usr/sbin/iptables -A FORWARD -p tcp -i $INET_IFACE --destination-port 80 \
     --destination 10.42.0.50 -j ACCEPT 

# Log packets that still don't match
/usr/sbin/iptables -A FORWARD -m limit --limit 3/minute --limit-burst 3 -j LOG \
    --log-prefix "FORWARD packet died: "

###############################################################################
#
# OUTPUT Chain
#

echo "Process OUTPUT chain ..."

# Generally trust the firewall on output

# However, invalid icmp packets need to be dropped
# to prevent a possible exploit.
/usr/sbin/iptables -A OUTPUT -p icmp -m conntrack --ctstate INVALID -j DROP

# Localhost
/usr/sbin/iptables -A OUTPUT -p ALL -s $LO_IP -j ACCEPT
/usr/sbin/iptables -A OUTPUT -p ALL -o $LO_IFACE -j ACCEPT

# To internal network
/usr/sbin/iptables -A OUTPUT -p ALL -s $LOCAL_IP -j ACCEPT
/usr/sbin/iptables -A OUTPUT -p ALL -o $LOCAL_IFACE -j ACCEPT

# To internet
/usr/sbin/iptables -A OUTPUT -p ALL -o $INET_IFACE -j ACCEPT

# Log packets that still don't match
/usr/sbin/iptables -A OUTPUT -m limit --limit 3/minute --limit-burst 3 -j LOG \
    --log-prefix "OUTPUT packet died: "

###############################################################################
#
# nat table
#
###############################################################################

# The nat table is where network address translation occurs if there
# is a private network.  If the gateway is connected to the Internet
# with a static IP, snat is used.  If the gateway has a dynamic address,
# masquerade must be used instead.  There is more overhead associated
# with masquerade, so snat is better when it can be used.
# The nat table has a builtin chain, PREROUTING, for dnat and redirects.
# Another, POSTROUTING, handles snat and masquerade.

echo "Load rules for nat table ..."

###############################################################################
#
# PREROUTING chain
#

# Port Forwarding
# 
# Port forwarding forwards all traffic on a port or ports from
# the firewall to a computer on the internal LAN.  This can
# be required to support special situations.  For instance,
# this is the only way to support file transfers with an ICQ
# client on an internal computer.  It's also required if an internal
# system hosts a service such as a web server.  However, it's also
# a dangerous option.  It allows Internet computers access to
# your internal network.  Use it carefully and only if you're
# certain you know what you're doing.

/usr/sbin/iptables -t nat -A PREROUTING -p tcp -i $INET_IFACE --destination-port 80 \
     -j DNAT --to-destination 10.42.0.50:80


###############################################################################
#
# POSTROUTING chain
#

/usr/sbin/iptables -t nat -A POSTROUTING -o $INET_IFACE \
     -j SNAT --to-source $INET_ADDRESS

###############################################################################
#
# mangle table
#
###############################################################################

# The mangle table is used to alter packets.  It can alter or mangle them in
# several ways.  For the purposes of this generator, we only use its ability
# to alter the TTL in packets.  However, it can be used to set netfilter
# mark values on specific packets.  Those marks could then be used in another
# table like filter, to limit activities associated with a specific host, for
# instance.  The TOS target can be used to set the Type of Service field in
# the IP header.  Note that the TTL target might not be included in the
# distribution on your system.  If it is not and you require it, you will
# have to add it.  That may require that you build from source.

echo "Load rules for mangle table ..."

# Set the TTL in outbound packets to the same consistent value.
# A value around 128 is a good value.  Do not set this too high as
# it will adversely affect your network.  It is also considered bad
# form on the Internet.
/usr/sbin/iptables -t mangle -A OUTPUT -o $INET_IFACE -j TTL --ttl-set 128

Ok, the script just needs to be saved somewhere and just execute it as super user (using sudo). Each time you boot your laptop, you should execute the script again.

Now the script is well commented, but the interesting part is where it says "Forwarding". It figuratively translates package information from one network to other. In this case, it translate everything sent from your server as if comes out from your laptop and everything that reach port 80 to your laptop gets redirected to the server.

The script is based on this helper http://www.slackware.com/~alien/efg/index.php