Firewall Rule Definitions

Firewall configuration in debops.ferm is done through a flexible definition of rules. There are a number of variables which are used to reference a set of default rules and can be extended by user defined rules. Here a description of the involved configurations should be given so that everyone can customize the ruleset according to individual requirements.

Default rules
Custom rules
Rule types
Legacy rules

Default rules 

By default debops.ferm configures a number of rules as soon as a host is part of the [debops_all_hosts] Ansible host group. The rules created by default are defined in defaults/main.yml and activated by being listed in ferm__default_rules. They consist of basic rules for setting the iptables default policies, restricting extensive connection attempts, logging and more.

In case a firewall is not required or preferred this behaviour can be disabled by setting ferm__enabled to False in the inventory.

Custom rules 

A custom rule can be enabled by adding a rule definition to one of the predefined rule lists (ferm__rules, ferm__group_rules, ferm__host_rules or ferm__dependent_rules) in the Ansible inventory. Each rule has to be defined as a YAML dict using some of the following keys:

domain: Optional. iptables domain used for the firewall rule. Possible values: ip, ip6. Defaults to ferm__domains.
table: Optional. iptables table to which the rule is added or from which it is removed. Defaults to filter.
chain: Optional. iptables chain to which the rule is added or from which it is removed. Defaults to INPUT.
by_role: Optional. Name of the Ansible role in the format ROLE_OWNER.ROLE_NAME which is responsible for the rule. This will be included as a comment in the generated rule file.

Depending on the chosen type, many additional variables are supported. Please check the individual rule type description below.

Rule types 

There exist a number of predefined rule types for generating firewall rules through ferm. Following a list of the available rule types which can be used to create custom rules.

The 'accept' type 

This rule type can be used to create rules that match interfaces, ports, remote IP addresses/subnets and can accept the packets, reject, or redirect to a different chain. The following type-specific YAML keys are supported:

accept_any: Optional. Match all source addresses by default. Possible values: True or False. Defaults to True. If this option is set to False and item.target is set to REJECT all traffic is blocked by default. As soon as item.saddr is not empty, this configuration doesn't matter anymore.
daddr: Optional. List of destination IP addresses or networks to which the rule is applied.
dport: Optional. List of destination ports to which the rule is applied.
include: Optional. Custom ferm configuration file to include. See ferm include for more details.
interface: Optional. List of network interfaces for incoming packets to which the rule is applied.
interface_present: Optional. Same as item.interface but first check if specified network interfaces exists before adding the firewall rules.
multiport: Optional. Use iptables multiport extension. Possible values: True or False. Defaults to False.
outerface: Optional. List of network interfaces for outgoing packets to which the rule is applied.
outerface_present: Optional. Same as item.outerface but first check if specified network interface exists before adding the firewall rule.
protocol: Optional. Network protocol to which the rule is applied.
protocol_syn: Optional. Match TCP packet with only the SYN flag set. Possible values True or False. If set to False it will match all other packets except the ones with only the SYN flag set. Defaults to unset.
realgoto: Optional. After packet match jump to custom chain. See ferm realgoto for more details.
reject_with: Optional. Define reject message being sent when the rule item.target is set to REJECT. Defaults to icmp-admin-prohibited.
saddr: Optional. List of source IP addresses or networks to which this rule is applied.
sport: Optional. List of source ports to which the rule is applied.
state: Optional. Connection state which should be matched. Possible values: INVALID, ESTABLISHED, NEW, RELATED, UNTRACKED or comma-separated combination thereof.
subchain: Optional. Subchain name. If more than 3 addresses are listed in target.saddr move resulting iptables rules into a separate subchain with the given name. See ferm subchain for more details.
target: Optional. iptables jump target. Possible values: ACCEPT, DROP, REJECT, RETURN, NOP or a custom target. Defaults to ACCEPT.

The 'ansible_controller' type 

Similar to the accept type but defaults to the SSH target port and sets the source address to the host running Ansible if not overwritten through the item.ansible_controllers key. The following type-specific YAML keys are supported:

ansible_controllers: Optional. List of source IP address which are added to item.saddr. Overwrites auto-detection of the Ansible controller address.
daddr: Optional. List of destination IP addresses or networks to which the rule is applied.
dport: Optional. List of destination ports to which the rule is applied. Defaults to ssh.
include: Optional. Custom ferm configuration file to include. See ferm include for more details.
interface: Optional. List of network interfaces for incoming packets to which the rule is applied.
multiport: Optional. Use iptables multiport extension. Possible values: True or False. Defaults to False.
outerface: Optional. List of network interfaces for outgoing packets to which the rule is applied.
protocol: Optional. Network protocol to which the rule is applied. Defaults to tcp.
protocol_syn: Optional. Match TCP packet with only the SYN flag set. Possible values True or False. If set to False it will match all other packets except the ones with only the SYN flag set. Defaults to unset.
realgoto: Optional. After packet match jump to custom chain. See ferm realgoto for more details.
reject_with: Optional. Define reject message being sent when the rule item.target is set to REJECT. Defaults to icmp-admin-prohibited.
saddr: Optional. List of source IP addresses or networks to which this rule is applied.
sport: Optional. List of source ports to which the rule is applied.
state: Optional. Connection state which should be matched. Possible values: INVALID, ESTABLISHED, NEW, RELATED, UNTRACKED or comma-separated combination thereof.
subchain: Optional. Subchain name. If more than 3 addresses are listed in target.saddr move resulting iptables rules into a separate subchain with the given name. See ferm subchain for more details.
target: Optional. iptables jump target. Possible values: ACCEPT, DROP, REJECT, RETURN, NOP or a custom target. Defaults to ACCEPT.

The 'connection_tracking' type 

This type is used to enable connection tracking using the iptables conntrack or iptables state extension. The following type-specific YAML keys are supported:

tracking_active_target: Optional. iptables jump target for valid connections. Defaults to ACCEPT.
tracking_invalid_target: Optional. iptables jump target for invalid connections. Defaults to DROP.
tracking_module: Optional. iptables module used for connection tracking. Possible values: state or conntrack. Defaults to conntrack.
interface: Optional. List of network interfaces for incoming packets to which the rule is applied.
outerface: Optional. List of network interfaces for outgoing packets to which the rule is applied.
interface_not: Optional. List of network interfaces for incoming packets which are excluded from the rule.
outerface_not: Optional. List of network interfaces for outgoing packets which are excluded from the rule.

The 'custom' type 

The type used to define custom ferm rules. The following additional YAML keys are supported:

rules: ferm rule definition, required.
by_role: Optional. Add comment to generated ferm rule definition file that rule is defined in the given Ansible role.

This template is used among others in a debops.libvirtd custom ferm rule.

The 'default_policy' type 

This type is used to define iptables default policies. The following type-specific YAML keys are supported:

policy: iptables chain policy, required.

The 'dmz' type 

This type can be used to enable connection forwarding to another host. If item.port is not specified, all traffic is forwarded. The following type-specific YAML keys are supported:

multiport: Optional. Use iptables multiport extension. Possible values: True or False. Defaults to False.
interface: Optional. List of public network interfaces which accept connections. At least one of interface or public_ip is required.
public_ip: Optional. IPv4 address on the public network which accepts connections. At least one of interface or public_ip is required. Only 1 IP address should be used at a time.
private_ip: IPv4 address of the host on the internal network, required. Only 1 IP address should be used at a time.
protocol(s): Optional. List of protocols to forward. Defaults to tcp.
port(s): Optional. List of ports to forward.
dport: Optional. Destination port to forward to. Only needs to be specified if internal destination port is different from the original destination port.
snat_ip: Optional. IP address for source address translation. If set the source IP is rewritten to this address. This is typically the IP address of the firewall host on the internal network. This is useful if traffic from the private_ip would otherwise not be routed back to the firewall host.

The 'hashlimit' type 

This type is used to define rate limit rules using the iptables hashlimit extension. The following type-specific YAML keys are supported:

daddr: Optional. List of destination IP addresses or networks to which the rule is applied.
dport: Optional. List of destination ports to which the rule is applied.
hashlimit_burst: Optional. Number of packets to match within the expiration time. Defaults to 5.
hashlimit_expire: Optional. Expiration time of hash entries in seconds. Defaults to 1.8.
hashlimit_target: Optional. Jump target used when packet matches the hashlimit rule which means that the rate limit is not reached yet. Defaults to RETURN.
hashlimit_mode: Optional. Options to take into consideration when associating packet streams. Possible values: srcip, srcport, dstip, dstport or a comma-separated list thereof. Defaults to srcip.
interface: Optional. List of network interfaces for incoming packets to which the rule is applied.
interface_present: Optional. Same as item.interface but first check if specified network interfaces exists before adding the firewall rules.
outerface: Optional. List of network interfaces for outgoing packets to which the rule is applied.
outerface_present: Optional. Same as item.outerface but first check if specified network interface exists before adding the firewall rule.
include: Optional. Custom ferm configuration file to include. See ferm include for more details.
log: Optional. Write rate limit hits to syslog. Possible values: True and False. Defaults to True.
protocol: Optional. Network protocol to which the rule is applied.
protocol_syn: Optional. Match TCP packet with only the SYN flag set. Possible values True or False. If set to False it will match all other packets except the ones with only the SYN flag set. Defaults to unset.
reject_with: Optional. Define reject message being sent when the rule item.target is set to REJECT. Defaults to icmp-admin-prohibited.
state: Optional. Connection state which should be matched. Possible values: INVALID, ESTABLISHED, NEW, RELATED, UNTRACKED or comma-separated combination thereof.
subchain: Optional. Subchain name. Move resulting iptables rules into a separate subchain with the given name. See ferm subchain for more details.
target: Optional. iptables jump target in case the rate limit is reached. Defaults to REJECT.

The 'include' type 

This type can be used to include custom ferm configuration files. The following type-specific YAML keys are supported:

include: Optional. Custom ferm configuration file to include. See ferm include for more details.

The 'log' type 

This type can be used to specify logging rules using the iptables log extension. The following type-specific YAML keys are supported:

include: Optional. Custom ferm configuration file to include. See ferm include for more details.
log_burst: Optional. Burst limit of packets being logged. Defaults to ferm__log_burst.
log_ip_options: Optional. Log IP options of packet. Possible values: True or False. Defaults to True.
log_level: Optional. Log level for firewall messages. Possible values are: emerg, alert, crit, error, warning, notice, info or debug. Defaults to warning.
log_limit: Optional. Rate limit of packets being logged. Defaults to ferm__log_limit.
log_prefix: Optional. Prefix (up to 29 characters) for firewall log messages. Defaults to iptables-log:
log_target: Optional. Select how iptables performs logging. Possible values: LOG, ULOG, NFLOG. Defaults to LOG.
log_tcp_options: Optional. Log TCP options of packet. Possible values: True or False. Defaults to False.
log_tcp_sequence: Optional. Log TCP sequence of packet. Possible values: True or False. Defaults to False.
realgoto: Optional. After packet match jump to custom chain. See ferm realgoto for more details.
reject_with: Optional. Define reject message being sent when the rule item.target is set to REJECT. Defaults to icmp-admin-prohibited.
target: Optional. iptables jump target for logged packets.

The 'recent' type 

This type can be used to track connections and respond accordingly by using the iptables recent extension. The following type-specific YAML keys are supported:

dport: Optional. List of destination ports to which the rule is applied.
include: Optional. Custom ferm configuration file to include. See ferm include for more details.
multiport: Optional. Use iptables multiport extension. Possible values: True or False. Defaults to False.
protocol: Optional. Network protocol to which the rule is applied.
protocol_syn: Optional. Match TCP packet with only the SYN flag set. Possible values True or False. If set to False it will match all other packets except the ones with only the SYN flag set. Defaults to unset.
recent_hitcount: Optional. Must be used in combination with item.recent_update. Match if address is in the list and at least the given number of packets were received so far.
recent_log: Optional. Log packets matching the rule. Possible values: True or False. Defaults to ferm__log. If this is set to True ferm__log must be enabled too for the packet to be logged.
recent_name: Optional. Name of the list. Defaults to DEFAULT.
recent_remove: Optional. Remove address from the list. Possible values: True or False. Defaults to False. Mutually exclusive with item.recent_update.
recent_seconds: Optional. Must be used in combination with item.recent_update. Match if address is in the list and was last seen within the given number of seconds.
recent_set_name: Optional. Add the source address of a matching packet to the given list. This must correspond with item.recent_name of a second rule which would potentially act on the packet, e. g. reject it.
recent_target: Optional. iptables jump target when packet has hit the recent list. Possible values: ACCEPT, DROP, REJECT, RETURN, NOP or a custom target. Defaults to NOP.
recent_update: Optional. Update "last-seen" timestamp. Possible values: True or False. Defaults to False. Mutually exclusive with item.recent_remove.
reject_with: Optional. Define reject message being sent when the rule item.target is set to REJECT. Defaults to icmp-admin-prohibited.
state: Optional. Connection state which should be matched. Possible values: INVALID, ESTABLISHED, NEW, RELATED, UNTRACKED or comma-separated combination thereof.
subchain: Optional. Subchain name. Move resulting iptables rules into a separate subchain with the name given. See ferm subchain for more details.

When using the recent type make sure to always define two rules:

One for matching the packet against the address list using the item.recent_update feature. If this filter matches you likely want to set the item.recent_target to DROP or REJECT.
To clear the source address from the list again in case the connection restrictions are not met, add a second role using item.recent_remove.

The 'reject' type 

This type is used to reject all traffic. It can be added for example as a final rule in a custom chain.

Legacy rules 

Legacy rules are the (old) deprecated way to configure firewall rules using a simpler less flexible syntax than described above. As support for these is likely going to be removed in the future, they shouldn't be used anymore.

Support for legacy rules is still enabled by default. However, they are stored in a separate iptables INPUT chain called debops-legacy-input-rules. In case you haven't defined any legacy rules and none of the DebOps roles you are using are still depending on it, disable support completely by setting ferm__include_legacy to False which will avoid the additional chain from being created.

If you're not sure if you still have legacy rules defined, look for variable names with only on '_' after the ferm prefix (e. g. ferm_input_list and ferm_input_dependent_list).