How do I debug a firewall causing connection problems with my Droplet?

FirewallD is an interface for managing a netfilter firewall designed to be user friendly. It is available for most Linux operating systems. If your Droplet runs FirewallD, you can view the current traffic rules with a sequence of checks.

First, check whether FirewallD is active using the --state flag:

sudo firewall-cmd --state

If you receive the message running, check the active zones using the --get-active-zones flag:

sudo firewall-cmd --get-active-zones

The command returns any network interfaces FirewallD actively controlled by FirewallD rules.

public
  interfaces: eth0

If FirewallD has active zones, it means that it is evaluating traffic against a set of rules. In the example output, FirewallD is currently managing traffic for the Droplet’s eth0 network interface.

You can display the ports and services associated with each of the active zones with the --info-zone option:

sudo firewall-cmd --info-zone=public 

    
        
            
public (active)
  target: default
  icmp-block-inversion: no
  interfaces: eth0
  sources: 
  services: dhcpv6-client ssh
  ports: 
  protocols: 
  masquerade: no
  forward-ports: 
  sourceports: 
  icmp-blocks: 
  rich rules:

        
    

Check whether any ports are open by looking at:

• The target value. If this is set to DROP or %%REJECT%%, the Droplet denies all traffic regardless of the other settings.
• The ports value. This lists ports explicitly allowed through the firewall.
• The services value. This lists services that are allowed through the firewall. In the example output above, the dhcpv6-client and ssh services are allowed through the firewall.

You can also check the ports associated with these services by typing:

sudo firewall-cmd --permanent --get-ports --service=dhcpv6-client
sudo firewall-cmd --permanent --get-ports --service=ssh

The example commands return the ports and network protocols associated with the services.

546/udp
22/tcp

If you have a DigitalOcean Cloud Firewall set up with conflicting rules, you can disable your firewall via FirewallD by typing:

sudo systemctl stop firewalld
sudo systemctl disable firewalld

If you want to keep your Droplet’s firewall in place to filter types of traffic not covered by DigitalOcean’s Cloud Firewall service, such as SFTP traffic, you should modify its rules to match the cloud firewall settings to ensure there are no conflicting rules between the two firewalls. You can learn how to modify the FirewallD rules by following the How To Set Up a Firewall Using FirewallD on CentOS 7 guide.

IPTables is a utility program that manages firewalls on Linux systems. It’s native to all Linux operating systems. If you are using IPTables to manage the Droplet’s firewall, you can view the current IPv4 filtering rules by typing:

sudo iptables --line-numbers -vL

The --line-numbers flag prepends a num column to the output to make the returned chart more human-readable. The -vl flag returns verbose output.

    
        
            
Chain INPUT (policy DROP 1 packets, 40 bytes)
num   pkts bytes target     prot opt in     out     source               destination         
1        0     0 ACCEPT     all  --  lo     *       0.0.0.0/0            0.0.0.0/0           
2      764 56512 ACCEPT     all  --  *      *       0.0.0.0/0            0.0.0.0/0            ctstate RELATED,ESTABLISHED
3        9   540 ACCEPT     tcp  --  *      *       0.0.0.0/0            0.0.0.0/0            tcp dpt:22
4        0     0 ACCEPT     tcp  --  *      *       0.0.0.0/0            0.0.0.0/0            tcp dpt:80

Chain FORWARD (policy ACCEPT 0 packets, 0 bytes)
num   pkts bytes target     prot opt in     out     source               destination         


Chain OUTPUT (policy ACCEPT 374 packets, 97673 bytes)
num   pkts bytes target     prot opt in     out     source               destination

        
    

The output indicates:

• The default policy for incoming and outgoing traffic (DROP for incoming, and ACCEPT for outgoing).
• The exceptions to the default policies. In the example output above, the policy contains exemptions for incoming TCP traffic to ports 22 and 80.

The iptables command only displays the rules for filtering IPv4 traffic. To show the IPv6 filtering rules, rerun the command using the ip6tables command instead of iptables.

If your cloud firewall has rules that conflict with your iptables firewall, you can disable your iptables firewall by running:

sudo iptables -P INPUT ACCEPT
sudo iptables -P OUTPUT ACCEPT
sudo iptables -P FORWARD ACCEPT
sudo iptables -F
sudo ip6tables -P INPUT ACCEPT
sudo ip6tables -P OUTPUT ACCEPT
sudo ip6tables -P FORWARD ACCEPT
sudo ip6tables -F

If you are using a service like iptables-persistent or have a script loading iptables rules at boot, you may have to disable them to disable the firewall.

If you want to keep your Droplet’s firewall in place to filter types of traffic not covered by DigitalOcean’s Cloud Firewall service, such as SFTP traffic, you should modify its rules to match the cloud firewall settings to ensure there are no conflicting rules between the two firewalls. You can learn how to modify the iptables rules by following the Iptables Essentials: Common Firewall Rules and Commands guide.