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78 Firewall
Table 25 Sample Firewall Rules
#permit: let through and do not log to the host
#alert: let through and log to the host
#deny: DO NOT let through and do not log to the host
#divert: DO NOT let through and log to the host
# S:<precondition>; C:<postcondition> R:<logging>
# A packet is matched if precondition matches the current state of that flow;
# in that case the postcondition is ORed and applied to rewrite the state of that flow;
# A precondition of 1 starts a new flow
# logging should be set to 2 for most cases; see the user manual for R:1
# Topology assumption
#c0 : Unsecured EXTERNAL network
#c1 : Secured INTERNAL network
# specify here your inside networks
#var INTERNAL 192.168.50.0/24
var INTERNAL any
var EXTERNAL any
# specify here your outside DNS servers
#var DNS [10.11.0.1, 10.11.0.2]
var DNS any
var DNSPORT 53
#specify here the services provided from the inside network
#var ALLOWEDPORTS [21,22,25]
var ALLOWEDPORTS 22
#allow INTERNAL network to poke a hole through the firewall for TCP services
permit on c1 tcp $INTERNAL any -> $EXTERNAL any (msg:"Z SYN"; flags:S; S:1; R:2; C:3;)
permit on c0 tcp $EXTERNAL any -> $INTERNAL any (msg:"Z SYNACK"; flags:SA; S:2; R:2; C:4;)
#allow EXTERNAL network to poke a hole through the firewall if accessing any of the ALLOWEDPORTS
permit on c0 tcp $EXTERNAL any -> $INTERNAL $ALLOWEDPORTS (msg:"Z SYN"; flags:S; S:1; R:2; C:3;)
permit on c1 tcp $INTERNAL $ALLOWEDPORTS -> $EXTERNAL any (msg:"Z SYNACK"; flags:SA; S:2; R:2; C:4;)
#allow TCP packets on the established flow/hole (INTERNAL <--> EXTERNAL)
permit on all tcp any any -> any any (msg:"Z TCP flow allowed"; S:4; R:2; C:4;)
#allow INTERNAL network to poke a hole through the firewall for DNS queries
permit on c1 udp $INTERNAL any -> $DNS $DNSPORT (msg:"DNS query"; S:1; R:2; C:9;)
permit on c0 udp $DNS $DNSPORT -> $INTERNAL any (msg:"DNS reply"; S:8; R:2; C:16;)
#allow UDP packets for the established UDP flow/holes (INTERNAL <--> DNS)
permit on all udp any any -> any any (msg:"Z UDP flow allowed"; S:16; R:2; C:16;)
#bad stuff; do not let though and do not log
deny on all tcp any any -> any any (msg:"Z SAPU TCP Flags"; flags:SAPU;)
deny on all tcp any any -> any any (msg:"Z FU TCP Flags"; flags:FU;)
deny on all tcp any any -> any any (msg:"Z PF TCP Flags"; flags:PF;)
deny on all tcp any any -> any any (msg:"Z UP TCP Flags"; flags:UP;)
deny on all tcp any any -> any any (msg:"Z Zero TCP Flags"; flags:0;)