Distributed traces with OBI

Introduction

OBI supports distributed traces for applications with some limitations and kernel version restrictions.

The distributed tracing is implemented through the propagation of the W3C traceparent header value. traceparent context propagation is automatic and it doesn’t require any action or configuration.

OBI reads any incoming trace context header values, tracks the program execution flow and propagates the trace context by automatically adding the traceparent field in outgoing HTTP/gRPC requests. If an application has already added the traceparent field in outgoing requests, OBI uses that value for tracing instead its own generated trace context. If OBI cannot find an incoming traceparent context value, it generates one according to the W3C specification.

Implementation

The trace context propagation is implemented in two different ways:

1. By writing the outgoing header information at network level
2. By writing the header information at library level for Go

Depending on the programming language your service is written in, OBI uses one or both approaches of context propagation. We use these multiple approaches to implement context propagation, because writing memory with eBPF depends on the kernel configuration and the Linux system capabilities granted to OBI. For more details on this topic, see our KubeCon NA 2024 talk So You Want to Write Memory with eBPF?.

The context propagation at network level is disabled by default and can be enabled by setting the environment variable OTEL_EBPF_BPF_CONTEXT_PROPAGATION=all or by modifying the OBI configuration file:

ebpf:
  context_propagation: 'all'

Context propagation at network level

The context propagation at network level is implemented by writing the trace context information in the outgoing HTTP headers as well at the TCP/IP packet level. HTTP context propagation is fully compatible with any other OpenTelemetry based tracing library. This means that OBI instrumented services correctly propagate the trace information, when sending to and receiving from services instrumented with the OpenTelemetry SDKs. We use Linux Traffic Control (TC) to perform the adjustment of the network packets, which requires that other eBPF programs that use Linux Traffic Control chain properly with OBI. For special considerations regarding Cilium CNI, consult our Cilium Compatibility guide.

For TLS encrypted traffic (HTTPS), OBI is unable to inject the trace information in the outgoing HTTP headers and instead it injects the information at TCP/IP packet level. Because of this limitation, OBI is only able to send the trace information to other OBI instrumented services. L7 proxies and load balancers disrupt the TCP/IP context propagation, because the original packets are discarded and replayed downstream. Parsing incoming trace context information from OpenTelemetry SDK instrumented services still works.

gRPC and HTTP/2 are not supported at the moment.

This type of context propagation works for any programming language and doesn’t require that OBI runs in privileged mode or has CAP_SYS_ADMIN granted. For more details, see the Distributed traces and context propagation configuration section.

Kubernetes Configuration

The recommended way to deploy OBI on Kubernetes with distributed tracing support at network level is as DaemonSet.

The following Kubernetes configuration must be used:

• OBI must be deployed as a DaemonSet with host network access (hostNetwork: true).
• The /sys/fs/cgroup path from the host must be volume mounted as local /sys/fs/cgroup path.
• The CAP_NET_ADMIN capability must be granted to the OBI container.

The following YAML snippet shows an example OBI deployment configuration:

spec:
  serviceAccount: beyla
  hostPID: true # <-- Important. Required in DaemonSet mode so OBI can discover all monitored processes
  hostNetwork: true # <-- Important. Required in DaemonSet mode so OBI can see all network packets
  dnsPolicy: ClusterFirstWithHostNet
  containers:
    - name: beyla
      resources:
        limits:
          memory: 120Mi
      terminationMessagePolicy: FallbackToLogsOnError
      image: 'beyla:latest'
      imagePullPolicy: 'Always'
      command: ['/beyla', '--config=/config/beyla-config.yml']
      env:
        - name: OTEL_EXPORTER_OTLP_ENDPOINT
          value: 'http://otelcol:4318'
        - name: OTEL_EBPF_KUBE_METADATA_ENABLE
          value: 'autodetect'
      securityContext:
        runAsUser: 0
        readOnlyRootFilesystem: true
        capabilities:
          add:
            - BPF # <-- Important. Required for most eBPF probes to function correctly.
            - SYS_PTRACE # <-- Important. Allows OBI to access the container namespaces and inspect executables.
            - NET_RAW # <-- Important. Allows OBI to use socket filters for http requests.
            - CHECKPOINT_RESTORE # <-- Important. Allows OBI to open ELF files.
            - DAC_READ_SEARCH # <-- Important. Allows OBI to open ELF files.
            - PERFMON # <-- Important. Allows OBI to load BPF programs.
            - NET_ADMIN # <-- Important. Allows OBI to inject HTTP and TCP context propagation information.
      volumeMounts:
        - name: cgroup
          mountPath: /sys/fs/cgroup # <-- Important. Allows OBI to monitor all newly sockets to track outgoing requests.
        - mountPath: /config
          name: beyla-config
  tolerations:
    - effect: NoSchedule
      operator: Exists
    - effect: NoExecute
      operator: Exists
  volumes:
    - name: beyla-config
      configMap:
        name: beyla-config
    - name: cgroup
      hostPath:
        path: /sys/fs/cgroup

If /sys/fs/cgroup is not mounted as a local volume path for the OBI DaemonSet some requests may not have their context propagated. We use this volume path to listen to newly created sockets.

Kernel version limitations

The network level context propagation incoming headers parsing generally requires kernel 5.17 or newer for the addition and use of BPF loops.

Some patched kernels, such as RHEL 9.2, may have this functionality ported back. Setting OTEL_EBPF_OVERRIDE_BPF_LOOP_ENABLED skips kernel checks in the case your kernel includes the functionality but is lower than 5.17.

Go context propagation by instrumenting at library level

This type of context propagation is only supported for Go applications and uses eBPF user memory write support (bpf_probe_write_user). The advantage of this approach is that it works for HTTP/HTTP2/HTTPS and gRPC with some limitations, however the use of bpf_probe_write_user requires the OBI is granted CAP_SYS_ADMIN or it’s configured to run as privileged container.

Kernel integrity mode limitations

In order to write the traceparent value in outgoing HTTP/gRPC request headers, OBI needs to write to the process memory using the bpf_probe_write_user eBPF helper. Since kernel 5.14 (with fixes backported to the 5.10 series) this helper is protected (and unavailable to BPF programs) if the Linux Kernel is running in integrity lockdown mode. Kernel integrity mode is typically enabled by default if the Kernel has Secure Boot enabled, but it can also be enabled manually.

OBI automatically checks if it can use the bpf_probe_write_user helper, and enables context propagation only if it’s allowed by the kernel configuration. Verify the Linux Kernel lockdown mode by running the following command:

cat /sys/kernel/security/lockdown

If that file exists and the mode is anything other than [none], OBI cannot perform context propagation and distributed tracing is disabled.

Distributed tracing for Go in containerized environments (including Kubernetes)

Because of the Kernel lockdown mode restrictions, Docker and Kubernetes configuration files should mount the /sys/kernel/security/ volume for the OBI docker container from the host system. This way OBI can correctly determine the Linux Kernel lockdown mode. Here’s an example Docker compose configuration, which ensures OBI has sufficient information to determine the lockdown mode:

services:
  ...
  beyla:
    image: grafana/beyla:latest
    environment:
      OTEL_EBPF_CONFIG_PATH: "/configs/beyla-config.yml"
    volumes:
      - /sys/kernel/security:/sys/kernel/security
      - /sys/fs/cgroup:/sys/fs/cgroup

If the /sys/kernel/security/ volume is not mounted, OBI assumes that the Linux Kernel is not running in integrity mode.