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7. Example masquerading firewall scripts

7.1 Kernel 2.0, ipfwadm

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#!/bin/sh 
#04/04/1999 
#example rc.firewall script for the 2.0 kernels using ipfwadm 
#I cant take full credit for this script.  I had found it a few 
#years ago and made slight modifications. 
#Send questions or comments to [email protected]. 

#--------------------------------------------------------------------- 
#Variables 
#--------------------------------------------------------------------- 

#local ethernet interface 
localip= 
localif=eth0 

#static ethernet interface 
staticip= 
staticif=eth1 

PATH="/bin:/sbin:/usr/bin:/usr/sbin:/usr/local/bin:/usr/local/sbin" 

#--------------------------------------------------------------------- 
#Incoming Firewall Policies 
#--------------------------------------------------------------------- 
#flush incoming firewall policies 
/sbin/ipfwadm -I -f 

#set incoming firewall policy default to deny 
/sbin/ipfwadm -I -p deny 

#--------------------------------------------------------------------- 

#local interface, local machines, going anywhere is valid 
/sbin/ipfwadm -I -a accept -V $localip -S $localip/24 -D 0.0.0.0/0 
#remote interface, claiming to be local machines (IP spoofing) deny and log 
/sbin/ipfwadm -I -a deny -V $staticip -S $localip/24 -D 0.0.0.0/0 -o 
#remote interface, any source, going to staticip address is valid 
/sbin/ipfwadm -I -a accept -V $staticip -S 0.0.0.0/0 -D $staticip/32 
#loopback interface is valid 
/sbin/ipfwadm -I -a accept -V 127.0.0.1 -S 0.0.0.0/0 -D 0.0.0.0/0 
#all other incoming is denied and logged 
/sbin/ipfwadm -I -a deny -S 0.0.0.0/0 -D 0.0.0.0/0 -o 

#--------------------------------------------------------------------- 
#Outgoing Firewall Policies 
#--------------------------------------------------------------------- 

#flush outgoing firewall policies 
/sbin/ipfwadm -O -f 

#set outgoing firewall policy default to deny 
/sbin/ipfwadm -O -p deny 

#--------------------------------------------------------------------- 

#local interface, any source going to local net is valid 
/sbin/ipfwadm -O -a accept -V $localip -S 0.0.0.0/0 -D $localip/24 
#outgoing to localnet on static interface, stuffed routing, deny 
/sbin/ipfwadm -O -a deny -V $staticip -S 0.0.0.0/0 -D $localip/24 -o 
#outgoing from localnet on static interface, stuffed masquerading, deny 
/sbin/ipfwadm -O -a deny -V $staticip -S $localip/24 -D 0.0.0.0/0 -o 
#outgoing to localnet on static interface, stuffed masquerading, deny 
/sbin/ipfwadm -O -a deny -V $staticip -S 0.0.0.0/0 -D $localip/24 -o 
#anything else outgoing on remote interface is valid 
/sbin/ipfwadm -O -a accept -V $staticip -S $staticip/32 -D 0.0.0.0/0 
#loopback interface is valid 
/sbin/ipfwadm -O -a accept -V 127.0.0.1 -S 0.0.0.0/0 -D 0.0.0.0/0 
#all other outgoing is denied and logged 
/sbin/ipfwadm -O -a deny -S 0.0.0.0/0 -D 0.0.0.0/0 -o 

#-------------------------------------------------------------------------- 
#Forwarding firewall policies 
#-------------------------------------------------------------------------- 

#flush forwarding policies 
/sbin/ipfwadm -F -f 

#set forwarding policy default to deny 
/sbin/ipfwadm -F -p deny 

#masquerade from localnet on local interface to anywhere 
/sbin/ipfwadm -F -a masquerade -W $staticif -S $localip/24 -D 0.0.0.0/0 
#all other forwarding is denied 
/sbin/ipfwadm -F -a deny -S 0.0.0.0/0 -D 0.0.0.0/0 

exit 0 

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7.2 Kernel 2.1/2.2, ipchains

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#!/bin/sh 
#04/04/1999 
#example rc.firewall script for the newer 2.1/2.2 kernels using ipchains
#that creates user defined chains for each interface.  There are firewall
#rules for spoofing protection which may be unnecessary since the newer
#kernels can have kernel spoofing protection enabled.  You might say it's
#super paranoid checking. 
#Send questions or comments to [email protected]. 

#--------------------------------------------------------------------- 
#Variables 
#--------------------------------------------------------------------- 

#local ethernet interface 
localip= 
localif=eth0 

#static ethernet interface 
staticip= 
staticif=eth1 

#loopback interface 
loopback=lo 

PATH="/bin:/sbin:/usr/bin:/usr/sbin:/usr/local/bin:/usr/local/sbin" 

#--------------------------------------------------------------------- 
#Flush built-in input, output, and forward ipchains; set default policy 
#Good policy to deny all packets especially while setting up chains 
#--------------------------------------------------------------------- 

#set incoming firewall policy default to deny 
ipchains -P input DENY 

#flush incoming firewall policies 
ipchains -F input 

#--------------------------------------------------------------------- 

#set outgoing firewall policy default to deny 
ipchains -P output DENY 

#flush outgoing firewall policies 
ipchains -F output 

#--------------------------------------------------------------------- 

#set forwarding firewall policy default to deny 
ipchains -P forward DENY 

#flush forwarding firewall policies 
ipchains -F forward 

#--------------------------------------------------------------------- 
#flush all policies  -redundant for main policies, but also flushes user 
#defined policies 
#ipchains -F 

#remove all user defined policies - you may or may not want to enable this 
#ipchains -X 

#--------------------------------------------------------------------- 
#Incoming Firewall Policies 
#--------------------------------------------------------------------- 

#create new input chain for static ethernet interface 
ipchains -N $staticif"-i" 

#flush all rules in chain (sanity flush) 
ipchains -F $staticif"-i" 

#block incoming tcp SYN packets to all ports on staticif and log 
#this may be a little harsh but its a nice feature 
#ipchains -A $staticif"-i" -j DENY -p tcp -y -i $staticif -s 0/0 \
#-d $staticip : -l 

#remote interface, claiming to be local machines (IP spoofing) deny and log 
ipchains -A $staticif"-i" -j DENY -i $staticif -s $localip/16 -d 0/0 -l 

#remote interface, any source, going to staticip address is valid 
ipchains -A $staticif"-i" -j ACCEPT -i $staticif -s 0/0 -d $staticip/32 

#all other incoming is denied and logged 
ipchains -A $staticif"-i" -j DENY -s 0/0 -d 0/0 -l 

#--------------------------------------------------------------------- 

#create new input chain for local ethernet interface 
ipchains -N $localif"-i" 

#flush all rules in chain (sanity flush) 
ipchains -F $localif"-i" 

#local interface, local machines, going anywhere is valid 
ipchains -A $localif"-i" -j ACCEPT -i $localif -s $localip/24 -d 0/0 

#all other incoming is denied and logged 
ipchains -A $localif"-i" -j DENY -s 0/0 -d 0/0 -l 

#--------------------------------------------------------------------- 

#create new input chain for loopback interface 
ipchains -N $loopback"-i" 

#flush all rules in chain (sanity flush) 
ipchains -F $loopback"-i" 

#loopback interface is valid 
ipchains -A $loopback"-i" -j ACCEPT -i $loopback -s 0/0 -d 0/0 

#all other incoming is denied and logged 
ipchains -A $loopback"-i" -j DENY -s 0/0 -d 0/0 -l 

#-------------------------------------------------------------------------- 
#Forwarding firewall policies 
#-------------------------------------------------------------------------- 

#create new forward chain for static ethernet interface 
ipchains -N $staticif"-f" 

#flush all rules in chain (sanity flush) 
ipchains -F $staticif"-f" 

#masquerade from localnet on static interface to anywhere 
ipchains -A $staticif"-f" -j MASQ -i $staticif -s $localip/24 -d 0/0 

#all other forwarding is denied and logged 
ipchains -A $staticif"-f" -j DENY -s 0/0 -d 0/0 -l 

#--------------------------------------------------------------------- 

#create new forward chain for local ethernet interface 
ipchains -N $localif"-f" 

#flush all rules in chain (sanity flush) 
ipchains -F $localif"-f" 

#all other forwarding is denied and logged 
ipchains -A $localif"-f" -j DENY -s 0/0 -d 0/0 -l 

#--------------------------------------------------------------------- 

#create new forward chain for loopback interface 
ipchains -N $loopback"-f" 

#flush all rules in chain (sanity flush) 
ipchains -F $loopback"-f" 

#all other forwarding is denied and logged 
ipchains -A $loopback"-f" -j DENY -s 0/0 -d 0/0 -l 
  

#--------------------------------------------------------------------- 
#Outgoing Firewall Policies 
#--------------------------------------------------------------------- 

#create new output chain for static ethernet interface 
ipchains -N $staticif"-o" 

#flush all rules in chain (sanity flush) 
ipchains -F $staticif"-o" 

#outgoing to localnet on remote interface(stuffed routing) deny & log 
ipchains -A $staticif"-o" -j DENY -i $staticif -s 0/0 -d $localip/24 -l 

#outgoing from local net on remote interface, stuffed masquerading, deny 
ipchains -A $staticif"-o" -j DENY -i $staticif -s $localip/24 -d 0/0 -l 

#anything else outgoing on remote interface is valid 
ipchains -A $staticif"-o" -j ACCEPT -i $staticif -s $staticip/32 -d 0/0 

#all other outgoing is denied and logged 
ipchains -A $staticif"-o" -j DENY -s 0/0 -d 0/0 -l 

#--------------------------------------------------------------------- 

#create new output chain for local ethernet interface 
ipchains -N $localif"-o" 

#flush all rules in chain (sanity flush) 
ipchains -F $localif"-o" 

#local interface, any source going to local net is valid 
ipchains -A $localif"-o" -j ACCEPT -i $localif -s 0/0 -d $localip/24 

#all other outgoing is denied and logged 
ipchains -A $localif"-o" -j DENY -s 0/0 -d 0/0 -l 

#--------------------------------------------------------------------- 

#create new output chain for loopback interface 
ipchains -N $loopback"-o" 

#flush all rules in chain (sanity flush) 
ipchains -F $loopback"-o" 

#loopback interface is valid 
ipchains -A $loopback"-o" -j ACCEPT -i $loopback -s 0/0 -d 0/0 
#all other outgoing is denied and logged 
ipchains -A $loopback"-o" -j DENY -s 0/0 -d 0/0 -l 

#-------------------------------------------------------------------------- 
#make sure forwarding is enabled in the kernel 
#-------------------------------------------------------------------------- 

/bin/echo 1 > /proc/sys/net/ipv4/ip_forward 

#-------------------------------------------------------------------------- 
#Add pointers to built-in chains to enable user defined chains 
#change the order in each chain to optimize filtering for an interface 
#-------------------------------------------------------------------------- 

#add local interface input chain 
ipchains -A input -i $localif -j $localif"-i" 

#add static interface input chain 
ipchains -A input -i $staticif -j $staticif"-i" 

#add loopback interface input chain 
ipchains -A input -i $loopback -j $loopback"-i" 

#------------------------------------------------------------------------- 

#add local interface output chain 
ipchains -A output -i $localif -j $localif"-o" 

#add static interface output chain 
ipchains -A output -i $staticif -j $staticif"-o" 

#add loopback interface output chain 
ipchains -A output -i $loopback -j $loopback"-o" 

#------------------------------------------------------------------------- 

#add local interface forward chain 
ipchains -A forward -i $localif -j $localif"-f" 

#add static interface forward chain 
ipchains -A forward -i $staticif -j $staticif"-f" 

#add loopback interface forward chain 
ipchains -A forward -i $loopback -j $loopback"-f" 

#--------------------------------------------------------------------- 
#Super Paranoid check --- even though default policy is set for deny, 
#block all packets on any interface 
#--------------------------------------------------------------------- 

#all other incoming is denied and logged 
ipchains -A input -j DENY -s 0/0 -d 0/0 -l 

#all other output is denied and logged 
ipchains -A output -j DENY -s 0/0 -d 0/0 -l 

#all other forwarding is denied and logged 
ipchains -A forward -j DENY -s 0/0 -d 0/0 -l 

exit 0

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