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Common Patterns For Generators


The purpose of this document is to describe common patterns for new Aerleon Generators.

Security based requirements:

Inet_version ‘Mixed’ Platform Support

When the platform does not support “mixed” in a single access-list


When the inet_version is set to ‘mixed’ it implies that the resultant policy should contain addresses from both families. Some platforms do not support both address families to exist in the same filter therefore leading to Aerleon generators needing to handle the output differently for those platforms. Cisco is an example platform that does not support a mixed filter being generated, and therefore it requires two separate access-list filters.

Platforms that support mixed family filters will simply generate filters that contain both address families. Some platforms (such as GCE) support "mixed" in a single access-list, but do not support "mixed" addresses in the same rule.

Desired Approach

The desired approach will be to have Aerleon output two filters, one for each address family, for platforms that do not support ‘mixed’. This currently occurs already with which outputs two access-lists one that contains IPv4 and another that contains IPv6 addresses. This solves a problem of having to potentially maintain two different .pol so that in cases where vendor syntax of filter name is derived from .pol a syncing between v4 and v6 .pol do not need to be maintained.

This may be misleading at first because when using Aerleon the user expects that the output will be a single policy, but it is actually their lack of understanding about the vendor syntax that causes this belief.

If the user does not want this output, then the user can simply issue two headers to Aerleon one for IPv4 and one for IPv6.

When the platform supports “mixed” in a single access-list

This will require the policy to be generated correctly for the following permutations of address family, and when “mixed” is supported, and with valid tests:

  1. MIXED_TO_V4
  2. V4_TO_MIXED
  3. MIXED_TO_V6
  4. V6_TO_MIXED
  8. V4_TO_V4
  9. V6_TO_V6
  10. V4_TO_V6
  11. V6_TO_V4

The junipersrx, junipermsmpc, paloalto and cloudarmor generators support this.

When the platform supports “mixed” in a single access-list, but not "mixed" addresses in a single rule

This will require a "mixed" policy to be generated by rendering a single rule for both "inet" and "inet6", to end up with 2 resultant rules, one with IPv4 addresses and one with IPv6 respectively. This processing should consider the following:

  1. Logically similar IPv4 and IPv6 rules should be close to each other to be easy to read and reason about in the generated policy. So if the IPv4 and IPv6 rules originate from a single "mixed" rule, they should be close to each other. It is thus preferable to process each rule, once for "inet" and then "inet6", over processing all rules in the policy for "inet", and then all the rules for "inet6". This also helps adhere to requirements regarding rule priority.
  2. Special handling of "mixed" such as not re-using the code for "inet" and "inet6" processing may result in inconsistent handling of rules when dealing with "mixed" versus "inet6". As an example, the expected outcome is that a rule containing IPv6 addresses only, when processed under "mixed" or "inet6", should result in the same generated rule.
  3. Rule names for such processing should preserve semantic meaning, but also differentiate between the IPv4 and IPv6 variants of the rule. Suffixes for the IPv6 rules (such as "-ipv6") are preferred to preserve sorting order of rules.

Processing rules with no explicit IP addresses

There are several reasons why a source_address or destination_address might be empty while processing it in a generator. It is important to understand which case is being encountered, in order to correctly translate policy intent. This list may not be exhaustive.

  1. When a source_address or destination_address is not explicitly set in a .pol file.
    1. This implies allowing all addresses of the address family (inet_version) the policy is being rendered for.
  2. source_address/destination_address is empty because it got filtered out due to the inet_version, since it has no addresses left of that specific address family.
    1. This should not render the term for that specific inet_version. Logging is preferred.
  3. source_address/destination_address is empty because it got removed due to source_address_exclude/destination_address_exclude, and has no addresses left.
    1. Log, and do not render the term.
    2. If it is a deny term, the log level should be a warning since the deny term is not being rendered.

noverbose option is supported correctly

noverberse must be supported if it makes sense for the platform. Noverbose removes all comments from the ACE terms, policies, etc.

Sample.pol file is present

A sample pol file should exist for that generator. Some examples are here. The sample.pol file for the new generator should have examples for all the filter option types used, with all supported IP types, along with any custom fields for that generator.

Perform truncating of names for term name or comment based on max length

This is to ensure that truncation of terms and comments that exceed a max_width, is supported by the generator. Not all platforms have length limitations, if the generator’s platform has none, then this requirement can be skipped. This applies only when the term name and comments are being incorporated into the policy. If they are actual # comments (which are not applied to the policy), then this requirement does not apply. This could be done using the existing WrapWords() or it may be done by a truncate function within the generator using a custom function such as in This wrapping should also be present for the term name, and should be using Aerleon's FixTermLength().

Logging is supported correctly for different types of logging

There are different values of logging already created in

Not all are supported by every platform.

  • LOG_BOTH is for logging session-init as well as session-close, which is currently supported by JuniperSRX.
  • DISABLE is a negative logging action.
  • Every other option are considered positive actions.

General rules:

  • The generator should support all the types of logging it can.
  • For all unsupported positive logging actions, the generator should enable logging.
  • For DISABLE, if the platform can turn off logging, it must; if the platform does not support it, then it does nothing. Key part is DISABLE must not enable logging.

DSMO Support

DSMO is Discontinuous Subnet Masks and is used to save on TCAM space. This is supported by certain platforms such as Cisco, but is not supported by most platforms. If DSMO is not supported by a platform, this requirement can be safely ignored. If DSMO is supported by a platform it must be fully implemented and carefully unit tested.

The usage of good and meta Unified Direction names

Good unified direction names for the ACE terms, that are meta and not specific to that platform, are preferred. This is only for platforms that require direction. The meta directions supported by Aerleon are “in”, “out” and “”, but these should not be used as is. Different platforms use “ingress”, “egress” or “both” such as GCE. Do not rely on platform specific names for directions. “ingress” and “egress” are the preferred directions to be used, which can then be converted to the platform’s required specific names for directions.

Term Expirations are handled correctly

Term expirations need to be handled correctly in code. An example from Juniper is that when the term is close to expiration, an INFO message is logged; and when it is expired, a WARNING message is logged and the term is not generated. This is also done similarly across other platforms such as Cisco/GCE.

ICMP and ICMPv6 handling

The generator needs to handle ICMP and ICMPv6 correctly. This is a broad requirement, but ICMP and ICMPv6 requires careful handling to avoid rendering icmp terms under inet6, and icmpv6 under inet. One commit that implements this for gcp_hf is here. Some generators do not support icmp and icmp6 when the address family is mixed, such as The expected correct behavior is that when “mixed” is specified in the inet_version, the rule for ICMP should only contain IPv4 addresses, and the rule for ICMPv6 should contain only IPv6 addresses. Tests must ensure that types and codes used are valid for the given address family.

In the future, we hope to refactor the code to allow for general ICMP support, but for now this functionality is implemented per-platform in each generator.

Note: In a related requirement, IGMP does not apply to IPv6, and thus rules containing IGMP should not be generated with IPv6 addresses.

Makes an explicit determination about statefulness

The generator author should check for “Am I stateful?”. It should clearly state in generator the result of this as a comment somewhere If it is, it should make sure that it is doing the right thing for terms. For example, for Juniper SRX, it is possible to skip TCP-established because it is stateful. You would also want to do the stateful check probably early in the code rather than later, since this may impact efficiency by being able to skip further code/ checks. A pro of checking early would be being able to skip any processing of terms not necessary for a stateful firewall such as skipping TCP-established. In contrast, in, the check is made later, while formatting the terms to modify the term to allow established and related terms.

Syntax of the config from the generator and on-device should match when cryptographically verified

The ACL that is generated from the generator, and the ACL that is obtained from the device when a show configuration command is used, should match bit-by-bit, such that it should be possible to run a hashing function (such as SHA-1) and obtain the same hash for both the ACL configurations. Another variant of this requirement is that the diff between these two policies should be empty.

There can be certain exceptions, such as if there is a policy header comment that cannot be handled by Cisco devices and is thus not present in the Cisco ACL. This can be handled by checking the diff between them and skipping over the known mismatches that are acceptable because of the device’s incapability. Another example of an exception is the Juniper control sequence such as ‘replace’, which indicates the device to replace, rather than merge the contents of the ACL, which does not show up on the device ACL.

If there is a mismatch in the syntax of the 2 ACLs that cannot be fixed, then these mismatches and the technical reasoning behind the lack of a workaround or a fix should be listed in the associated Github issue for this generator.

Apply priority as described in .pol files

If a priority or order exists for the platform, and an input pol file doesn't set priority for ACEs, then autogenerate priorities in top down order based on the ACE order in the .pol file.

Protocol support

Call out support

Make explicit which protocols the platform supports, and support them in the generator. If protocols are not supported by the platform, ensure that the generator explicitly does not support them, and gracefully handles these errors.

Names vs numbers

Names or numbers can be used to represent protocols within a generator. Throughout a given generators use only names or numbers, not a mix of both. The choice should be made based on the default representation for the device platform. (I.e. if the policy once applied to the device will show names in a "show config" command output, then use names within the generator. If the output contains numbers, use numbers within the generator.)

Port support

The following is a list of which IP protocols support ports. When supporting a protocol, make sure that the handling of port or lack thereof is correct.

  • HOPOPT = No
  • ICMP = No
  • IGMP = No
  • GGP = No
  • IPIP = No
  • TCP = Yes
  • EGP = No
  • IGP = No
  • UDP = Yes
  • RDP = Yes (Uses different port ranges though, check RFC)
  • IPV6 = No
  • IPV6_ROUTE = No
  • RSVP = No
  • GRE = No
  • ESP = No
  • AH = No
  • ICMPV6 = No
  • IPV6_NONXT = No
  • IPV6_OPTS = No
  • OSPF = No
  • PIM = No
  • VRRP = No
  • L2TP = No. (only uses UDP 1701)
  • SCTP = Yes
  • UDPLITE = Yes

Note: DCCP also uses ports, but this is not currently supported. Source:

Zone based firewall support

Zone based firewalls should be implemented if supported by the platform. When implementing zone based firewalls, all combinations of zone types must be tested fully. Test for invalid and reserved zone names and illegal combinations of policies (i.e. any-> specific, or any->any).

Address book support

Address books should be implemented if they are present in a platform. The generator should explicitly state whether it is implementing a global or zone based address book. If both are available for the platform, both must be implemented, and an option must be added to choose between the two. When implementing address books, always filter for address family before building the book. Tests should ensure that address books are filtered by address family properly along with their relevant rules.

Coding Style Requirements:

Use builtin libraries

Use only Python standard builtin libraries wherever possible. External dependencies are discouraged and must be justified.

Structure generators for inheritance

See Cisco and Juniper generators for examples. Wherever possible, use base classes, inheritance, etc to allow for common functions between generators in a "family".

Reuse common functions

Use functions from,,, etc wherever possible instead of implementing your own.

If output will be in a common exchange format, use a standard library for rendering

This applies to common standards such as JSON, XML, YAML, protocol buffers, etc. Instead of building up such serialized output using string ops, use a standard library to produce the rendered output instead. (For example, to render JSON, use json.dumps. For XML, use some combination of the standard libraries, such as xml.etree and xml.dom.) This should allow most generator code to interact with objects only, and serialize to a buffer at the end. Unit tests should operate on the object structures. Additional small unit tests should sanity check that the rendering library is producing valid output as expected. If a (JSON, XML, etc.) schema is available this should also be used to validate output in a test.

Check various limits when rendering output

Make sure that line, identifier, full output, etc. limits are applied when rendering final output. Some of these may be specific to a given platform. These should always include but are not limited to:

  • Maximum value of addresses and ports allowed in single rule. Generator must support automatically splitting into new rule when exceeded.
  • Maximum values allowed across entire policy for rule count, address, ports
  • Maximum length for comments, and support splitting across lines the correct way when over. Also check for max per-rule limit if one exists and truncate using the common Aerleon functions if needed.
  • Max term length supported must be 24 or greater, in order to allow for meaningful term names.

Test coverage

General coverage

Aim for as close to 100% test coverage as you can. Tests should cover a wide span of the vendor syntax, not just a single keyword.

Custom exceptions

All custom exceptions types added must be unit tested.