Security 201

Centralised accounts

LDAP and Kerberos

Active Directory

NIS, NIS+, YP, Hesiod

Firewalls and packet filters

host vs network

“Crunchy outer shell, soft center is bad”

In general terms, implementing defense-in-depth strategies is always a sensible practice. The concept in which multiple layers of security controls (defense) are placed throughout an information technology (IT) system. Its intent is to provide redundancy in the event a security control fails or a vulnerability is exploited.

Implementing a firewall on the network and host-based packet filters provides defense-in-depth layers to your infrastructure. In today’s landscape, the firewall aspect could be a service like ec2 security groups with host-based packet filters such as iptables, Windows Firewall (or WFAS). Although this can add additional complexity to deployment, that is not a reason to not implement it where appropriate.

The defense-in-depth concept is mostly regarded in terms of attack and compromise, however in ops it also safeguards us as everyone makes mistakes. Sometimes, we ourselves or our colleagues are the point of failure.

Strange as it may seem, people often make the mistake of disabling “rules” when something is not working and they cannot figure out why. The just checking test. This is always the first mistake. In real world operations these things do happen, whether it is a just checking mistake, an incorrect configuration or action, sometimes we make serious mistakes, to err is human.

In all these situations using a firewall/other and host-based packet filters comes to the fore:

  • It protects us, the people working on the systems, from ourselves.
  • It protects the organisation’s assets in the event of a failure or compromise at either layer, whether it be user error or a systematic failure.
  • It keeps us proficient and in-hand on both the network specific security implementation, the host-based security practices and the applications related to their management.
  • It is good practice.

An old skool, real world example:

  • You have a MSSQL server running on Windows Server 2000 (no “firewall” back then & ip filters are not enabled) - it has both private and public network interfaces.
  • The MSSQL server has a public NIC because it has run replication with your customer’s MSSQL server sometimes for dev purposes and catalog updates.
  • You have rules on the diversely routed, mirrored NetScreen NS1000 firewalls that allows port 1433 between the MSSQL servers only on the public interface.
  • Your colleague has an issue that cannot be resolved and quickly just sets the firewall/s to “Allows from all”, the just checking test.
  • Immediate result - network unreachable.
  • SQL Slammer had just arrived and proceeded to gobble up 2Gbps of public T1 bandwidth.
  • All points into the network and the network itself are saturated until you debug the issue via the serial port on the firewall and figure out what happened.

The synopsis is that a practice of disabling rules was implemented, as it had always been the last line in debugging. It was a practice that had been done many times by the engineers in the organisation at the time with no “apparent” consequences in the past. This time differed in that the MSSQL server had a public interface added to allow for replication with the customer and SQL Slammer was in the wild.

If the MSSQL server had ip filtering enabled as well the situation would have been mitigated. Needless to say, it was the last time “Allow from all” debug strategy was ever implemented. However it is useful to note, that because this was done all the time, the engineer in question did not even tie the action of “Allow from all” and the network becoming unreachable together at the time, because the action previously had never resulted in the outcome that was experienced in this instance.

Stateful vs stateless

IPTables: Adding and deleting rules

pf: Adding and deleting rules

Public Key Cryptography


What is PKI? What uses it? Why is it important?

Using public and private keys for SSH authentication

Two factor authentication

Building systems to be auditable

Data retention

Log aggregation

Log and event reviews

Role accounts vs individual accounts

Network Intrusion Detection

Host Intrusion Detection

Defense practices

Risk and risk management

Compliance: The bare minimum

What is compliance and why do you need it?

What kinds of data can’t you store without it?

Legal obligations

Dealing with security incidents

ACLs and extended attributes (xattrs)



Data placement

Eg, local vs cloud, the implications, etc

Additional reading

Ken Thompson, Reflections on Trusting Trust: