rfc: add initial draft of pcap-cf

toc/rfc/rfc-draft-pcap-cf.md

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+# Meta
+[meta]: #meta
+- Name: Integrate pcap feature for Cloud Foundry applications
+- Start Date: 2025-09-10
+- Author(s): @maxmoehl @peanball
+- Status: Draft
+- RFC Pull Request: [community#1309](https://github.com/cloudfoundry/community/issues/1309)
+
+## Summary
+
+Add a feature to Cloud Foundry that provides application developers with the
+ability to perform packet capture (tcpdump) operations within their
+application containers for debugging purposes. This RFC complements
+[RFC-0019 (pcap-bosh)](rfc-0019-pcap-bosh.md) by extending packet capture
+capabilities from BOSH-level infrastructure debugging to application-level
+debugging.
+
+The main goal is to allow app developers to capture network traffic from
+their applications when troubleshooting issues that require elevated
+privileges, while maintaining security through operator controls and a
+reduced-functionality custom tool.
+
+## Problem
+
+Application developers frequently need to perform in-depth troubleshooting
+of their applications when deployed via Cloud Foundry buildpacks. Network analysis
+tasks, such as packet captures (tcpdump), connectivity checks and performance
+checks, require elevated privileges as the captured data may be sensitive.
+Currently, there is no possibility for app developers to perform any privileged
+actions within their application containers, which also excludes such network
+analysis tasks.
+
+[RFC-0019 (pcap-bosh)](rfc-0019-pcap-bosh.md) addresses packet capture
+at the BOSH infrastructure level for platform operators. Application developers
+and operators, an equally important group of users, need similar capabilities,
+scoped to their individual applications.
+
+While platform traffic can be secured from eavesdropping, e.g. via TLS, access
+to the network traffic in a container will contain sensitive and possibly
+private information. 
+
+Elevated privileges of any kind can easily be misused, and it is paramount that
+the security model of Cloud Foundry remains intact, where applications usually
+run in isolated, unprivileged containers.
+
+Any solution must consider that network capture is a privilege that has to be
+enabled explicitly, not given by default and can be forbidden altogether.
+
+## Proposal: Custom Packet Capturing Tool
+
+This RFC proposes implementing a reduced-functionality packet capturing tool
+written in Go using the [gopacket][gopacket] library. This custom tool would be
+injected into application containers via Diego, similar to how [diego-ssh][diego-ssh]
+currently provides SSH access.
+
+### Key Features
+
+The tool reduces the attack surface to a minimum by only exposing the options
+which are strictly necessary to capture packets, namely:
+* Specifying a network interface
+* Applying packet filters (pcap-filter format)
+* Setting snapshot length (snaplen)
+
+The tool would be injected via Diego using a mechanism similar to
+[diego-ssh][diego-ssh]. This ensures that the tool is available in all apps
+regardless of the deployment method, and it follows established patterns that
+are already known by maintainers.
+
+The tool has the necessary file capability `CAP_NET_RAW` assigned
+to be able to capture packets. It remains in the namespaces of
+the application to ensure a vulnerability in the tool does not enable a
+container escape.
+
+The tool is only accessible if SSH is enabled for the application.
+
+## Other Options Considered
+
+#### Option 1: `sudo` Access
+
+Adding a platform switch to make the `vcap` user a sudoer was considered but
+discarded due to:
+* Significant security risks of providing full root access in the app container.
+* Potential for privilege escalation beyond intended packet capture use
+* Inconsistency with Cloud Foundry's [security-first design principles][cf-sec]
+
+Even considering that we could configure sudo in a way that the vcap user is
+only allowed to perform verify specific tasks (like running tcpdump) this still
+suffers from potential privilege escalation as explained in the next section.
+
+#### Option 2: `setcap` on `tcpdump` Binary
+
+Setting capabilities on the existing `tcpdump` binary was considered but
+discarded because:
+* `tcpdump` offers functionality to run arbitrary code in its security
+  context
+* Broader attack surface compared to a purpose-built tool
+
+## Implementation Considerations
+
+Based on the proposed solution the following sections detail out the changes
+which have to be made to the individual components of Cloud Foundry to implement
+it.
+
+### diego-release: Custom Packet Capturing Tool
+
+A new package will be added to diego-release which implements packet capturing
+in go through the [gopacket][gopacket] library. The resulting binary will be included in
+the various lifecycle archives that are added to the final app container and
+the necessary capability `CAP_NET_RAW` will be assigned
+to the executable via file capabilities. This allows regular users to gain those
+capabilities when executing the binary.
+
+The functionality is limited to network capturing with the aforementioned scope
+of selected network interface, filtering expression in pcap-filter format and
+length of captured packets. This reduces the attack surface, compared to
+invoking a full `tcpdump`.
+
+### CF CLI: New `pcap` command
+
+Similar to the `bosh pcap` command, a `cf pcap` command will be added. Like its
+counterpart, it will connect to the desired instances via SSH and execute the new
+packet capturing tool and stream back the captured packets via stdout and thus via SSH. If there
+are multiple streams, the CLI will merge them and write them out to a single
+file in the pcap format.
+
+Examples usages and output:
+
+```bash
+$ cf pcap myapp --output capture.pcap --interface eth0 --filter "tcp port 80" --snaplen 1500
+Starting capture on all instances.
+Capturing, press ^C to stop...
+^C
+Saved capture to 'capture.pcap'.
+$ cf pcap myapp -o capture.pcap -i 1 -f "host database.example.com"
+Starting capture on instances: 1.
+Capturing, press ^C to stop...
+^C
+Saved capture to 'capture.pcap'.
+```
+
+## References
+
+* Prior discussions:
+  * https://github.com/cloudfoundry/diego-release/issues/1023
+  * https://cloudfoundry.slack.com/archives/C0DEQSW9W/p1744034414856129?thread_ts=1744034414.856129&cid=C0DEQSW9W
+  * https://cloudfoundry.slack.com/archives/C033RE5D6/p1744299390390049
+* https://github.com/cloudfoundry/pcap-release
+* https://github.com/cloudfoundry/community/blob/main/toc/rfc/rfc-0019-pcap-bosh.md
+* https://github.com/gopacket/gopacket
+
+[gopacket]: https://github.com/gopacket/gopacket
+[diego-ssh]: https://github.com/cloudfoundry/diego-ssh
+[cf-sec]: https://docs.cloudfoundry.org/concepts/security.html