[ale] Linux box as a router w/DHCP

[Marvin, International Martian of Mystery]

Bob Kruger wrote:
> All;
>
> I would like to use a Linux box as both a router and as a firewall for a small network going out to the internet.  The issue is that the IP address to the ISP varies as it is assigned via a DHCP server.  This makes static routing commands problematic.
>
> Does anyone know of a good "howto" or tutorial that addresses this and would allow for some sort of dynamic routing configuration when the IP address assigned by the ISP changes?
>
> Thanks in advance for any assistance.
>
> V/r
>
> Bob
>
>
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>
>   
This is what I use:



#  ###  NAT STUFF
#  # EXTERNAL -> external network interface
#  # INTERNAL -> internal network interface
  EXTERNAL=eth1
  INTERNAL=eth0
  echo 1 > /proc/sys/net/ipv4/ip_forward
  echo "Setting up NAT (Network Address Translation)..."
# by default, nothing is forwarded.
#  iptables -P FORWARD DROP

#  # Allow all connections OUT and only related ones IN
  iptables -A FORWARD -i $EXTERNAL -o $INTERNAL -m state --state 
ESTABLISHED,R
  iptables -A FORWARD -i $INTERNAL -o $EXTERNAL -j ACCEPT
  # enable MASQUERADING
  iptables -t nat -A POSTROUTING -o $EXTERNAL -j MASQUERADE

 iptables -A INPUT -p tcp --dport 1723 -j ACCEPT
 iptables -A INPUT -p 47 -j ACCEPT


##  ### forward some ports ###################################
iptables -t nat -A PREROUTING -p tcp -d 71.56.13.176 \
    --dport  5900  -j DNAT --to 192.168.2.1:5900