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‎Dockerfile

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FROM golang:alpine
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COPY . /app
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WORKDIR /app
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RUN apk add --no-cache make build-base libpcap-dev openssh-client tcpdump
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RUN go mod download
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RUN go build ./cmd/pcap-broker
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ENTRYPOINT ["./pcap-broker"]

‎LICENSE

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‎README.md

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# pcap-broker
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`pcap-broker` is a tool to capture network traffic and make this available to one or more clients via PCAP-over-IP.
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PCAP-over-IP can be useful in situations where low latency is a priority, for example during Attack and Defend CTFs.
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More information on PCAP-over-IP can be found here:
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* https://www.netresec.com/?page=Blog&month=2022-08&post=What-is-PCAP-over-IP
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`pcap-broker` supports the following features:
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* Distributing packet data to one or more PCAP-over-IP listeners
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* Execute a command to capture traffic, usually `tcpdump` (expects stdout to be pcap data)
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* `pcap-broker` will exit if the capture command exits
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## Building
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To build `pcap-broker`:
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```shell
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$ go build ./cmd/pcap-broker
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$ ./pcap-broker --help
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```
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Or you can build the Docker container:
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```shell
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$ docker build -t pcap-broker .
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$ docker run -it pcap-broker --help
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```
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## Running
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```shell
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$ ./pcap-broker --help
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Usage of ./pcap-broker:
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-cmd string
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command to execute for pcap data (eg: tcpdump -i eth0 -n --immediate-mode -s 65535 -U -w -)
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-listen string
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listen address for pcap-over-ip (eg: localhost:4242)
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-n disable reverse lookup of connecting PCAP-over-IP client IP address
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```
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Arguments can be passed via commandline:
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```shell
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$ ./pcap-broker -cmd "sudo tcpdump -i eth0 -n --immediate-mode -s 65535 -U -w -"
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```
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Or alternatively via environment variables:
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```shell
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LISTEN_ADDRESS=:4242 PCAP_COMMAND='sudo tcpdump -i eth0 -n --immediate-mode -s 65535 -U -w -' ./pcap-broker
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```
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Using environment variables is useful when you are using `pcap-broker` in a Docker setup.
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Now you can connect to it via TCP and stream PCAP data using `nc` and `tcpdump`:
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```shell
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$ nc -v localhost 4242 | tcpdump -nr -
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```
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Or use a tool that natively supports PCAP-over-IP, for example `tshark`:
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```shell
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$ tshark -i TCP@localhost:4242
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```
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# Acquiring PCAP data over SSH
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One use case is to acquire PCAP from a remote machine over SSH and make this available via PCAP-over-IP.
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Such a use case, including an example SSH command to bootstrap this, has been documented in the `docker-compose.yml.example` file:
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```yaml
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version: "3.2"
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services:
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pcap-broker-remote-host:
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image: pcap-broker:latest
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restart: always
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volumes:
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# mount local user's SSH key into container
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- ~/.ssh/id_ed25519:/root/.ssh/id_ed25519:ro
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ports:
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# make the PCAP-over-IP port also available on the host on port 4200
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- 4200:4242
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environment:
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# Command to SSH into remote-host and execute tcpdump and filter out it's own SSH client traffic
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PCAP_COMMAND: ssh root@remote-host -o StrictHostKeyChecking=no 'IFACE=$$(ip route show to default | grep -Po1 "dev \K\w+") && BPF=$$(echo $$SSH_CLIENT | awk "{printf \"not (host %s and port %s and %s)\", \$$1, \$$2, \$$3;}") && tcpdump -U --immediate-mode -ni $$IFACE $$BPF -s 65535 -w -'
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LISTEN_ADDRESS: "0.0.0.0:4242"
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```
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## Background
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This tool was initially written for Attack & Defend CTF purposes but can be useful in other situations where low latency is preferred, or whenever a no-nonsense PCAP-over-IP server is needed. During the CTF that Fox-IT participated in, `pcap-broker` allowed the Blue Team to capture network data once and disseminate this to other tools that natively support PCAP-over-IP, such as:
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* [Arkime](https://arkime.com/)
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* [Tulip](https://github.com/OpenAttackDefenseTools/tulip) (after we did some custom patches)
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* WireShark's dumpcap and tshark

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