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

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 responsable 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:

active_target
Optional. iptables jump target for valid connections. Defaults to ACCEPT.
invalid_target
Optional. iptables jump target for invalid connections. Defaults to DROP.
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).