Category: bpf

Zero Trust Networks

Instead of  the “inside” and “outside” notion of traditional firewalls and perimeter defense technologies, the Zero Trust Network notion has its origin in the Cloud+Mobile first world where a person carrying a mobile device can be anywhere in the world (inside/outside the enterprise) and needs to be seamlessly and securely connected to online services.

The essential idea appears to be device authentication coupled with a second factor in the shape of an easy to remember password, with backend security smarts to identify the accessing device. More importantly, every service that is access externally needs to be authenticated, instead of some services being treated as internal services and being less protected.

Some properties of zero trust networks:

  • Network locality based access control is insufficient
  • Every device, user and service is authenticated
  • Policies are dynamic – they gather and utilize data inputs for making access control decisions
  • Attacks from trusted insiders are mitigated against

This is a big change from many networks which have network based defense at the core (for good reason, as it was cost effective). To create a zero-trust network, a startin point is to identify, enumerate and sequence all network flows.

I attended a talk by Centrify on this topic, which resonated with experiences in cloud, mobile and fog systems.

Related effort in Kubernetes – Progress Toward Zero Trust Kubernetes Networks, Istio Service Mesh , API Gateway to Service Mesh.  One can contrast the API gateway as being present only at the ingress point of a cloud, whereas with a Zero-trust/Service-mesh/Sidecar approach every microservice building-block has its own external proxy and ‘API’ for management added to it. The latter would add to latency concerns for real-time applications, as the new sidecar proxies are in the data path. One benefit of the service mesh is a mechanism to put in service to service security in a uniform manner.

The key original motivation behind Istio, in the second presentation by Lyft above, was greater observability and reliability across a complex cluster of microservices. This strikes me as a greater motivating use-case of this technology, than added security.  From the security point of view, there is a parallel of the Istio approach with the SDN problem statement of a horizontal and ubiquitous security layer.   Greater visibility is also a motivation behind the P4 programming language presented in disaggregated storage talk on protocol independant switch architecture or PISA here – one of the things it enables is inband telemetry.

Linux Bridges and Firewalls for Containers

Hubs and Repeaters work on bits on Layer1. Bridges and Switches work on frames, the protocol data unit for Layer2. A Hub supports a star configuration – like a splitter it simply forwards traffic received on one segment to all the others. A Repeater amplifies signal for longer distances. A Bridge is a smarter type of Hub, usually implemented in software, which looks at the destination MAC address of the packet before forwarding to the segment with MAC matching that destination, thereby avoiding unnecessary network traffic on all the other segments. It does not look at IP addresses. A Switch is a reincarnation of a Bridge in hardware with several ports.  The basic operation of a Bridge/Switch is L2 forwarding, which includes ARL (Address Resolution Logic) table learning and ageing.  An L3 Router switches packets based on the IP address instead of the frame address.

A linux bridge from a VM to the Host, does not need an IP address, but it is a good idea if you want to talk to the host for management or set firewall rules on it.

To create a bridge br0 between two interfaces eth0 and eth1, one runs commands: brctl addbr br0, brctl stp br0 on, brctl addif br0 eth0, brctl addif br0 eth1.

Since a bridge is an L2 construct, how can one filter IP addresses with it. A quote: An Ethernet interface cannot usefully have an IP address if it is also attached to a bridge. However it is possible to achieve the same effect by binding an address to the bridge itself:

ifconfig br0 192.168.0.1

An example of firewall configuration on a bridge.

Troubleshooting bridges:

brctl showmacs br0

Docker creates a few bridges by default. A primer on Docker Container networking and the bridges it creates are here and  here. Here are three common ones.

bridge: docker0 bridge . This is the default, allowing all containers to talk to each other

host: container listens to host, without any isolation.

Some problems with host iptables rules reveal themselves in working with docker.

A look at firewalld, ebtables, ipset, nftables at https://www.slideshare.net/albertspijkers/red-hat-enterprise-linux-next-generation-firewall . The command line tools talk to the kernel over a netlink socket.

A more extensible approach is with Berkeley Packet Filter (BPF), the classical one now called cBPF, the extended successor called eBPF.   A very basic problem is filtering HTTP or DNS packets based on the destination hostname, rather than the IP address.  This is discussed in an example by Cloudflare here.