The Linux iptables/ip6tables firewalls are enabled by default.

• ICMP incoming traffic is accepted, and all outgoing traffic for any protocol is also accepted by default.
• Rules also apply for IPv6, which benefits from the same filtering rules as IPv4.
• Firewall has rejection rules at the end of the INPUT and OUTPUT chains, to properly disallow non-matching traffic. This implies that new rules appended to the chain will never be matched.

If you want to add new rules, either make sure to add them to a file in /etc/firewall.d/, or if you add them in your application, prepend them to the chain by using “-I” (insert) instead of “-A” (append).

By default, the flows allowed, for IPv4 and IPv6, are: DHCP, DNS, ICMP, SSH, NTP, HTTP (TCP/80), HTTPS.

Custom rules can be added in /etc/firewall.d/, in the iptables-save/dump format.
Some tables are already created with default rules. These files can be edited by the user to add or remove rules. Files named *.rules are used for IPv4, and *.v6rules for IPv6.

A table is dedicated to the web interface access: /etc/firewall.d/webaw.rules and /etc/firewall.d/webaw.v6rules. Don't modify these files.

To apply new rules, reboot the gateway or restart the firewall with the command /etc/init.d/firewall restart.

To check the rules, launch the command: iptables -L for IPv4 rules and ip6tables -L for IPv6 rules.

These rules can allow UDP traffic to go through and from remote port 1600 on UDP:

-A INPUT -p udp --sport 1600 -j ACCEPT
-A OUTPUT -p udp --dport 1600 -j ACCEPT

Add these rules in the /etc/firewall.d/00_default.rules file.

# Default IPv4 firewall rules for Keros
* filter
# Open output port for DNS request
-A OUTPUT -p udp --dport domain -j ACCEPT
-A INPUT  -p udp --sport domain -j ACCEPT
-A OUTPUT -p tcp --dport domain -j ACCEPT
-A INPUT  -p tcp --sport domain ! --syn -j ACCEPT
 
# Allows loopback
-A INPUT  -i lo -j ACCEPT
-A OUTPUT -o lo -j ACCEPT
 
# SSH
-A OUTPUT -p tcp --dport ssh -j ACCEPT
-A INPUT  -p tcp --sport ssh ! --syn -j ACCEPT
-A OUTPUT -p tcp --sport ssh -j ACCEPT
-A INPUT  -p tcp --dport ssh -j ACCEPT
 
# NTP Out
-A OUTPUT -p udp --dport ntp -j ACCEPT
-A INPUT  -p udp --sport ntp -j ACCEPT
 
# outbound HTTP + HTTPS
-A OUTPUT -p tcp --dport http -j ACCEPT
-A INPUT  -p tcp --sport http ! --syn -j ACCEPT
-A OUTPUT -p tcp --dport https -j ACCEPT
-A INPUT  -p tcp --sport https ! --syn -j ACCEPT
 
# ICMP
-A OUTPUT -p icmp -j ACCEPT
-A INPUT  -p icmp -j ACCEPT
 
# TCP resets that we use to REJECT connection attempts
-A OUTPUT -p tcp --tcp-flags RST RST -j ACCEPT
# or that we receive (useful to terminate connection if no symmetric rule exist on INPUT)
-A INPUT  -p tcp --tcp-flags RST RST -j ACCEPT
 
# DHCP client and server
-A INPUT -p udp --dport bootps:bootpc --sport bootps:bootpc -j ACCEPT
 
# My new rules 
-A OUTPUT -p udp --dport 1600 -j ACCEPT
-A INPUT -p udp --sport 1600 -j ACCEPT
 
COMMIT

Restart the firewall to apply the rules:

# /etc/init.d/firewall restart
Flushing iptable rules.
Set default policy to 'ACCEPT'
Flushing ip6table rules.
Setting default IPv6 policy to 'ACCEPT'
Flushing iptable rules.
Applying IPv4 firewall config file /etc/firewall.d//00_default.rules
Applying IPv4 firewall config file /etc/firewall.d//zz_reject.rules
Flushing ip6table rules.
Applying IPv6 firewall config file /etc/firewall.d//00_default.v6rules
Applying IPv6 firewall config file /etc/firewall.d//zz_reject.v6rules
root@klk-lpbs-04018B:/user/rootfs_rw/etc/firewall.d # 

Check the result with:

# iptables -L
Chain INPUT (policy ACCEPT)
target     prot opt source               destination         
ACCEPT     udp  --  anywhere             anywhere             udp spt:domain
ACCEPT     tcp  --  anywhere             anywhere             tcp spt:domain flags:!FIN,SYN,RST,ACK/SYN
ACCEPT     all  --  anywhere             anywhere            
ACCEPT     tcp  --  anywhere             anywhere             tcp spt:ssh flags:!FIN,SYN,RST,ACK/SYN
ACCEPT     tcp  --  anywhere             anywhere             tcp dpt:ssh
ACCEPT     udp  --  anywhere             anywhere             udp spt:ntp
ACCEPT     tcp  --  anywhere             anywhere             tcp spt:http flags:!FIN,SYN,RST,ACK/SYN
ACCEPT     tcp  --  anywhere             anywhere             tcp spt:https flags:!FIN,SYN,RST,ACK/SYN
ACCEPT     icmp --  anywhere             anywhere            
ACCEPT     tcp  --  anywhere             anywhere             tcp flags:RST/RST
ACCEPT     udp  --  anywhere             anywhere             udp spts:bootps:bootpc dpts:bootps:bootpc
ACCEPT     udp  --  anywhere             anywhere             udp spt:1600
REJECT     tcp  --  anywhere             anywhere             reject-with tcp-reset
REJECT     all  --  anywhere             anywhere             reject-with icmp-port-unreachable
Chain FORWARD (policy ACCEPT)
target     prot opt source               destination         
Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination         
ACCEPT     udp  --  anywhere             anywhere             udp dpt:domain
ACCEPT     tcp  --  anywhere             anywhere             tcp dpt:domain
ACCEPT     all  --  anywhere             anywhere            
ACCEPT     tcp  --  anywhere             anywhere             tcp dpt:ssh
ACCEPT     tcp  --  anywhere             anywhere             tcp spt:ssh
ACCEPT     udp  --  anywhere             anywhere             udp dpt:ntp
ACCEPT     tcp  --  anywhere             anywhere             tcp dpt:http
ACCEPT     tcp  --  anywhere             anywhere             tcp dpt:https
ACCEPT     icmp --  anywhere             anywhere            
ACCEPT     tcp  --  anywhere             anywhere             tcp flags:RST/RST
ACCEPT     udp  --  anywhere             anywhere             udp dpt:1600
REJECT     tcp  --  anywhere             anywhere             reject-with tcp-reset
REJECT     all  --  anywhere             anywhere             reject-with icmp-port-unreachable