Distributed tcpdump

ddump is a small utility to run multiple network captures on multiple targets and live-merging the captured streams. This is useful when investigating network issues in a distributed system.

The main process runs as a command line program and outputs the merged stream on stdout for easy integration.

The worker processes run as a long-running HTTP server, suitable for integration behind a streaming (i.e. non buffering) HTTP reverse proxy. Under the hood, it automatically filters out its own traffic from the captures to avoid capture amplification.

While ddump can authenticate using Basic Auth and Mutual TLS, the worker itself does not support authentication nor authorization. This task is left to a reverse proxy.



sudo apt install libpcap-dev


Build all:

# Build

Run & test the worker:

# Allow running without root
sudo setcap cap_net_raw=eip ddump-worker

# Run

# Query
curl http://localhost:8475/capture -vo ./full-capture.pcap
curl http://localhost:8475/capture?interface=eth0&filter=tcp%20port%2080 -vo ./ethernet-http-capture.pcap
curl http://localhost:8475/capture\?filter\=icmp -sN | tcpdump -vnl -r-

Run & test the CLI:

./ddump \
    "" \
    "" \
    | tcpdump -vnlr-

The command above will start 2 capture streams on a locally running worker:

  • Ping to/from
  • Ping to/from

The command then outputs to stdout the merged capture stream in a format suitable for ingestion by tcpdump (as demonstrated in the example).



The main ddump process supports basic Auth as well as mutual TLS authentication.

  • Basic Auth: To enable Basic Auth, set the “user:[email protected]” on the target URL. Please note that this will be visible in to any process with /proc access. In scenarios where security is critical, please use mutual TLS authentication instead.
  • Server TLS authentication: By default, when using HTTPS, ddump validates the server certificate using the system’s CA bundle or the bundle targeted by the SSL_CERT_FILE or SSL_CERT_DIR standard environment variables. The CA to use can be customized using the --ca flag.
  • Client TLS authentication: To enable mutual TLS authentication, set the --cert and --key parameters to respectively the target PEM encoded client certificate full chain and the key associated to the leaf certificate.

The worker ddump-worker process does not support TLS nor Basic Auth by itself. Instead, it is strongly advised to run the worker behind a reverse proxy. Additionally, the proxy could parse and filter the client’s provided filter for additional security. This is out of the scope of this project.


The main ddump process does not require any specific privileges. It only connects to the workers over HTTP(S), merges the streams and outputs the merged stream to stdout.

The worker ddump-worker process requires CAP_NET_RAW. This basically gives full access to all network packets reaching any of the network interfaces supported by Linux’s packet capture. Please note that this includes packets that would eventually be dropped by the firewall as the capture hook is before the filtering. It is strongly advised to use the capabilities and NEVER run as root.

The capabilities can be configured on the binary using:

sudo setcap cap_net_raw=eip ddump-worker

CLI reference

Command line

The command line is of the form:


Where target URLs are of the form PROTOCOL://[USER[:PASSWORD]@]HOST[:PORT(8475)][?filter=FILTER]:

  • PROTOCOL: One of http, https
  • USER: Optional username, if going through and authenticating reverse proxy
  • PASSWORD: Optional password, if going through and authenticating reverse proxy
  • HOST: IP address or domain name of the target capture agent
  • PORT: Optional TCP port, if the capture agent listens on a non-standard port
  • FILTER: Optional url-encoded capture filter

And the following flags are supported:

  • -ca: Path to the trusted CA certificate bundle. (default: Use system CA)
  • -cert: Path to the client certificate to use for mutual TLS authentication. (default: disable mTLS)
  • -key: Path to the client key associated to the cert. (default: disable mTLS)


The main output stream is the merged pcap stream. Messages and errors are sent to stderr.

Worker Reference


  • -listen: Address to listen on. It may be an “<address>:<port>” or “unix:<path>“. (default: :8475)


GET /capture

Start a network capture.

Request parameters:

  • filter: Tcpdump BPF filter. (default: no filter)

Note: There is no “interface” parameter. This is intentional. Indeed, to merge multiple pcap streams, all streams must have the same type. This implies the use of a generic link layer. This layer could be one of Linux’s SLL or SLL2. While SLL2 would have allowed per-interface filters, it is not (yet) supported by the underlying gopacket library.


  • Headers:
    • Content-Type: application/vnd.tcpdump.pcap
  • Body: Raw pcap stream


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