1. Python at Elemental Security EuroPython - June 29, 2005 Guido van Rossum Elemental Security, Inc. guido@elementalsecurity.com guido@python.org

2. June 29, 2005 2 Elemental Security, Inc. Enterprise security software –product: Elemental Compliance System (ECS) express, monitor and enforce security policies for any computer connecting to the network (cross-platform) scored 9.3 in recent InfoWorld Test Center Startup (no longer in stealth mode!) C round just closed; 11M led by Lehman Brothers Using lots of Python (and Java!) We're always hiring! See http://www.elementalsecurity.com Now a real website :-)

3. June 29, 2005 3 ECS Application Structure One Central Server –Java, J2EE (Tomcat), some Python, Oracle –front-end: rich web UI (JavaScript + XML-RPC) –back-end: agent connector (HTTP+SSL) Many Agents –Python and C –runs on Windows, Solaris, Linux,... –main components: scheduler server connector policy engine – I'll get back to this later packet filter – nearly the only part written in C

4. June 29, 2005 4 Why Does Elemental Use Python? A. Because I'm There :-) B. Python is the best tool for the job –small footprint –runs everywhere (or almost runs :-) –access to platform-specific APIs (e.g. registry) –much of what we do is "script-like" gather various configuration information about the host check specific policy rules –this is so important we have a custom language for it! –application changes frequently we continually learn to understand the problem better quickly refactor code as needed

5. June 29, 2005 5 ElementClass – a Simpler XML API Use cases: –exchange data with central server policies, reports, etc. –persist structured data within agent policies, schedule, etc. –tool to manage policy definitions (Tkinter UI) XML an obvious choice Want better mapping between Python & XML example: –XML: –Py: sch.start+sch.offset #not int(sch.getattr("start"))

6. June 29, 2005 6 ElementClass – Example Input

7. June 29, 2005 7 ElementClass – Example Code from xmlparse import ElementClass, String, Integer class Employee(ElementClass): __element__ = "employee" __attributes__ = {"name": String, "age": Integer} class Group(ElementClass): __element__ = "group" __attributes__ = {"name": String} __children__ = {Employee: "employees[]"} group = Group.__parseFile__(filename) minors = [e for e in group.employees if e.age < 18] group.employees = minors f = open(filename, "w"); group.__render__(f); f.close()

8. June 29, 2005 8 Element Class – Example Output <employee age="17" name="Ben" /> <employee age="15" name="Dan" />

9. June 29, 2005 9 ElementClass – Limitations, Features No namespace support attribute names must be Python identifiers –(except '-' mapped to '_') Can have CDATA or subelements but not both Subelement choices for #occurrences: –zero or once: Python attribute is None or object –any number: Python attribute is a list, may be empty Ordering of attributes and subelements is lost –except for relative ordering of similar elements All attributes and elements are optional Optionally, can ignore unrecognized attrs/elements

10. June 29, 2005 10 ElementClass – What's Next? Improve the API a bit? –use lists of tuples instead of dicts for metadata this allows specifying attribute/subelement ordering –decide what to do with Unicode values convert to str if ASCII only, or not? –add more attribute data types? currently String, Integer, Boolean, Timestamp add Float; what else? enumerations? –add required attributes, subelements? (which API?) –tidy up output (fewer line breaks) Document it Contribute it to the PSF in time for Python 2.5! –ESI lawyers to look at PSF Contribution Agreement

11. June 29, 2005 11 Really Hammering The Server Server scalability requirement: support 4000 agents –Available: a few dozen test machines –How to do server load testing? Solution 1: run 50 agents on one test machine –test machines overloaded –test machines look too similar –can't quite reach scalability requirement Solution 2: run 500 synthetic agents on one box –skips work that doesn't affect what the server sees –started out as a private hack, adopted very quickly –full potential not yet reached (next: 20K agents!) –can easily inject additional test data into server

12. June 29, 2005 12 The Approach Share as much code as possible with real agent –fortunately, most agent code is in library modules N agent objects, K worker threads (K N) 1 scheduler thread –real-time event queue managed using heapq module –main loop sleeps until next event ready beware: event queue may be updated while sleeping! –distributes events to workers via Queue.Queue –worker main loop: while True: callable, args = workQueue.get() callable(*args) –callable is typically a bound method of an agent object

13. June 29, 2005 13 The Outcome Works really well despite its simplicity –didn't have to use asynchronous I/O Randomized synthetic data sent to server –example: simulate all agents being "nmapped" Probably bounded by number of threads –can't have too many agents per thread Inexplicable slow memory leak (not M2Crypto!)

14. June 29, 2005 14 A Policy Implementation Language ECS is all about policy compliance –each host has a policy compliance score: 0-100% –composed of individual (Boolean) policy rule scores –some (not all) policy rules can also be enforced So what's a policy rule? Examples: –all passwords must be at least 6 characters –ftpd should be disabled –all email must go through server X Elemental has a library of 1000+ policy rules –user selects some and deploys to group of hosts –agent gets rule list, executes rules, uploads results repeat on user-selected schedule (30 min – 7 days)

15. June 29, 2005 15 How To Implement Policy Rules Requirements: –Cost to add another rule must be low –Some rules are relatively complex programming tasks –Rule authors are security experts, not programmers Some possibilities: –shell scripts (Titan) –Perl, Python, etc. –XML –custom language

16. June 29, 2005 16 Why Write Another Language Need a library of policy-checking methods, e.g.: –assert that a file has a specific mode, owner, group –assert that a registry entry has a specific value –parse a configuration file using "name = value" syntax and then check a specific name/value pair Ideal: constraint-based (declarative) language –execution order doesn't matter –compiler can check for conflicts between rules Python would be fine if I were writing all the rules –still fairly low-level; risk of using the wrong approach Compromise: nearly-declarative language –resembles Python except where it doesn't

17. June 29, 2005 17 How Fuel Differs From Python –func has_localhost(host: Host, group: str): bool: for ip in host.gethostgroup(group): if substr(ip, 0, 4) == "127.": return true return false Declarations required; all code is type-checked –interfaces used for library code written in Python Single-assignment language with immutable values –let var [: type] = expr Argument defaults computed dynamically Many Python features left out (e.g. slicing!) Container types: immutable set and struct Fuel is not Turing-complete!

18. June 29, 2005 18 Implementing Fuel Process grammar with pgen –eventually reimplemented pgen in Python Use tokenize.py for tokenization Implemented pgen parsing automaton –as-we-go parse tree reduction Use visitor pattern to translate to Python source Parse tree node classes have grammar in docstrings Run-time library in Python –defines some mutable object types

19. June 29, 2005 19 Challenges in Writing Fuel Not enough users yet to know we're doing it right –yes, we should open-source it! Main challenge is to keep the language expressive without compromising its declarative nature –Fuel 2.0 will tweak the design quite a bit host.runscript("userdel", "-r", acct.name) –admission of defeat – but unavoidable some times Source code organization –linkage between source & hierarchical menu of rules –metadata repeated in source & XML –same rule implemented differently per platform

20. June 29, 2005 20 How We Use Fuel ~1400 policy rules implemented in Fuel Written by about 4 people part-time over 1 year Rules cover Solaris, Linux, Windows (2k+),... Rules cover all areas of security: –accounts, network, filesystem, system, hardware, software, packet filter, trust, authentication, logging

21. June 29, 2005 21 Question Time