passt/README.md

<span style="font-weight: bold; color: red;">While functional and tested to some extent, this project is still in early development phase: don't use in production or critical environments yet.</span>

# passt: Plug A Simple Socket Transport

_passt_ implements a translation layer between a Layer-2 network interface and
native Layer-4 sockets (TCP, UDP, ICMP/ICMPv6 echo) on a host. It doesn't
require any capabilities or privileges, and it can be used as a simple
replacement for Slirp.

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# pasta: Pack A Subtle Tap Abstraction

_pasta_ (same binary as _passt_, different command) offers equivalent
functionality, for network namespaces: traffic is forwarded using a tap
interface inside the namespace, without the need to create further interfaces on
the host, hence not requiring any capabilities or privileges.

It also implements a tap bypass path for local connections: packets with a local
destination address are moved directly between Layer-4 sockets, avoiding Layer-2
translations, using the _splice_(2) and _recvmmsg_(2)/_sendmmsg_(2) system calls
for TCP and UDP, respectively.

<img src="/builds/latest/web/pasta_overview.png" class="bright" style="z-index: 20; position: relative;">

- [Motivation](#motivation)
- [Non-functional Targets](#non-functional-targets)
- [Interfaces and Environment](#interfaces-and-environment)
- [Services](#services)
- [Addresses](#addresses)
- [Protocols](#protocols)
- [Ports](#ports)
- [Demo](#demo)
- [Continuous Integration](#continuous-integration)
- [Performance](#performance)
- [Try it](#try-it)
- [Contribute](#contribute)

## Motivation

### passt

When container workloads are moved to virtual machines, the network traffic is
typically forwarded by interfaces operating at data link level. Some components
in the containers ecosystem (such as _service meshes_), however, expect
applications to run locally, with visible sockets and processes, for the
purposes of socket redirection, monitoring, port mapping.

To solve this issue, user mode networking, as provided e.g. by _libslirp_,
can be used. Existing solutions implement a full TCP/IP stack, replaying traffic
on sockets that are local to the pod of the service mesh. This creates the
illusion of application processes running on the same host, eventually separated
by user namespaces.

While being almost transparent to the service mesh infrastructure, that kind of
solution comes with a number of downsides:

* three different TCP/IP stacks (guest, adaptation and host) need to be
  traversed for every service request
* addressing needs to be coordinated to create the pretense of consistent
  addresses and routes between guest and host environments. This typically needs
  a NAT with masquerading, or some form of packet bridging
* the traffic seen by the service mesh and observable externally is a distant
  replica of the packets forwarded to and from the guest environment:
  * TCP congestion windows and network buffering mechanisms in general operate
    differently from what would be naturally expected by the application
  * protocols carrying addressing information might pose additional challenges,
    as the applications don't see the same set of addresses and routes as they
    would if deployed with regular containers

_passt_ implements a thinner layer between guest and host, that only implements
what's strictly needed to pretend processes are running locally. The TCP
adaptation doesn't keep per-connection packet buffers, and reflects observed
sending windows and acknowledgements between the two sides. This TCP adaptation
is needed as _passt_ runs without the `CAP_NET_RAW` capability: it can't create
raw IP sockets on the pod, and therefore needs to map packets at Layer-2 to
Layer-4 sockets offered by the host kernel.

The problem and this approach are illustrated in more detail, with diagrams,
[here](https://gitlab.com/abologna/kubevirt-and-kvm/-/blob/master/Networking.md).

### pasta

On Linux, regular users can create network namespaces and run application
services inside them. However, connecting namespaces to other namespaces and to
external hosts requires the creation of network interfaces, such as `veth`
pairs, which needs in turn elevated privileges or the `CAP_NET_ADMIN`
capability. _pasta_, similarly to _slirp4netns_, solves this problem by creating
a tap interface available to processes in the namespace, and mapping network
traffic outside the namespace using native Layer-4 sockets.

Existing approaches typically implement a full, generic TCP/IP stack for this
translation between data and transport layers, without the possibility of
speeding up local connections, and usually requiring NAT. _pasta_:
* avoids the need for a generic, full-fledged TCP/IP stack by coordinating TCP
connection dynamics between sender and receiver
* offers a fast bypass path for local connections: if a process connects to
another process on the same host across namespaces, data is directly forwarded
using pairs of Layer-4 sockets
* with default options, maps routing and addressing information to the
namespace, avoiding any need for NAT

## Non-functional Targets

Security and maintainability goals:

* no dynamic memory allocation
* ~5 000 LoC target
* no external dependencies

## Interfaces and Environment

_passt_ exchanges packets with _qemu_ via UNIX domain socket, using the `socket`
back-end in qemu. Currently, qemu can only connect to a listening process via
TCP. Two temporary solutions are available:

* a [patch](https://passt.top/passt/tree/qemu) for qemu
* a wrapper, [qrap](https://passt.top/passt/tree/qrap.c), that connects to a
  UNIX domain socket and starts qemu, which can now use the file descriptor
  that's already opened

This approach, compared to using a _tap_ device, doesn't require any security
capabilities, as we don't need to create any interface.

_pasta_ runs out of the box with any recent (post-3.8) Linux kernel.

## Services

_passt_ and _pasta_ provide some minimalistic implementations of networking
services:

* [ARP proxy](https://passt.top/passt/tree/arp.c), that resolves the address of
  the host (which is used as gateway) to the original MAC address of the host
* [DHCP server](https://passt.top/passt/tree/dhcp.c), a simple implementation
  handing out one single IPv4 address to the guest or namespace, namely, the
  same address as the first one configured for the upstream host interface, and
  passing the nameservers configured on the host
* [NDP proxy](https://passt.top/passt/tree/ndp.c), which can also assign prefix
  and nameserver using SLAAC
* [DHCPv6 server](https://passt.top/passt/tree/dhcpv6.c): a simple
  implementation handing out one single IPv6 address to the guest or namespace,
  namely, the the same address as the first one configured for the upstream host
  interface, and passing the nameservers configured on the host

## Addresses

For IPv4, the guest or namespace is assigned, via DHCP, the same address as the
upstream interface of the host, and the same default gateway as the default
gateway of the host. Addresses are translated in case the guest is seen using a
different address from the assigned one.

For IPv6, the guest or namespace is assigned, via SLAAC, the same prefix as the
upstream interface of the host, the same default route as the default route of
the host, and, if a DHCPv6 client is running in the guest or namespace, also the
same address as the upstream address of the host. This means that, with a DHCPv6
client in the guest or namespace, addresses don't need to be translated. Should
the client use a different address, the destination address is translated for
packets going to the guest or to the namespace.

### Local connections with _passt_

For UDP and TCP, for both IPv4 and IPv6, packets from the host addressed to a
loopback address are forwarded to the guest with their source address changed to
the address of the gateway or first hop of the default route. This mapping is
reversed on the other way.

### Local connections with _pasta_

Packets addressed to a loopback address in either namespace are directly
forwarded to the corresponding (or configured) port in the other namespace.
Similarly as _passt_, packets from the non-init namespace addressed to the
default gateway, which are therefore sent via the tap device, will have their
destination address translated to the loopback address.

## Protocols

_passt_ and _pasta_ support TCP, UDP and ICMP/ICMPv6 echo (requests and
replies). More details about the TCP implementation are available
[here](https://passt.top/passt/tree/tcp.c), and for the UDP
implementation [here](https://passt.top/passt/tree/udp.c).

An IGMP/MLD proxy is currently work in progress.

## Ports

### passt

To avoid the need for explicit port mapping configuration, _passt_ can bind to
all unbound non-ephemeral (0-49152) TCP and UDP ports. Binding to low ports
(0-1023) will fail without additional capabilities, and ports already bound
(service proxies, etc.) will also not be used. Smaller subsets of ports, with
port translations, are also configurable.

UDP ephemeral ports are bound dynamically, as the guest uses them.

If all ports are forwarded, service proxies and other services running in the
container need to be started before _passt_ starts.

### pasta

With default options, _pasta_ scans for bound ports on init and non-init
namespaces, and automatically forwards them from the other side. Port forwarding
is fully configurable with command line options.

## Demo

### pasta

<p><video id="demo_pasta_video" style="width: 70%; height: auto; max-height: 90%" controls>
 <source src="/builds/latest/web/demo_pasta.webm" type="video/webm">
</video></p>

### passt

<p><video id="demo_passt_video" style="width: 70%; height: auto; max-height: 90%" controls>
 <source src="/builds/latest/web/demo_passt.webm" type="video/webm">
</video></p>

## Continuous Integration

<p><video id="ci_video" style="width: 90%; height: auto; max-height: 90%" controls>
 <source src="/builds/latest/web/ci.webm" type="video/webm">
</video></p>

<script src="/builds/latest/web/ci.js"></script>

Test logs [here](https://passt.top/builds/latest/test/).

## Performance

<script src="/builds/latest/web/perf.js"></script>

## Try it

### passt

* build from source:

        git clone https://passt.top/passt
        cd passt
        make

  * alternatively, static builds for x86_64, with or without AVX2 instructions,
    as of the latest commit are also available for convenience
    [here](https://passt.top/builds/latest/x86_64/avx2/) and
    [here](https://passt.top/builds/latest/x86_64/). Convenience, non-official
    packages for Debian (and derivatives) and RPM-based distributions are also
    available there. These binaries and packages are simply built with:

            CFLAGS="-static" make avx2
            make pkgs
            make static
            make pkgs

* have a look at the _man_ page for synopsis and options:

        man ./passt.1

* run the demo script, that creates a network namespace called `passt`, sets up
  sets up a _veth_ pair and and addresses, together with NAT for IPv4 and NDP
  proxying for IPv6, then starts _passt_ in the network namespace:

        doc/demo.sh

* from the same network namespace, start qemu. At the moment, qemu doesn't
  support UNIX domain sockets for the `socket` back-end. Two alternatives:

  * use the _qrap_ wrapper, which maps a tap socket descriptor to _passt_'s
    UNIX domain socket, for example:

            ip netns exec passt ./qrap 5 qemu-system-x86_64 ... -net socket,fd=5 -net nic,model=virtio ...

  * or patch qemu with [this patch](https://passt.top/passt/tree/qemu/0001-net-Allow-also-UNIX-domain-sockets-to-be-used-as-net.patch)
    and start it like this:

            qemu-system-x86_64 ... -net socket,connect=/tmp/passt.socket -net nic,model=virtio

* alternatively, you can use libvirt, with [this patch](https://passt.top/passt/tree/libvirt/0001-conf-Introduce-support-for-UNIX-domain-socket-as-qem.patch),
  to start qemu (with the patch mentioned above), with this kind of network
  interface configuration:

        <interface type='client'>
          <mac address='52:54:00:02:6b:60'/>
          <source path='/tmp/passt.socket'/>
          <model type='virtio'/>
          <address type='pci' domain='0x0000' bus='0x01' slot='0x00' function='0x0'/>
        </interface>

* and that's it, you should now have TCP connections, UDP, and ICMP/ICMPv6
  echo working from/to the guest for IPv4 and IPv6

* to connect to a service on the VM, just connect to the same port directly
  with the address of the network namespace. For example, to ssh to the guest,
  from the main namespace on the host:

        ssh 192.0.2.2

### passt

* build from source:

        git clone https://passt.top/passt
        cd passt
        make

  * alternatively, static builds for x86_64, with or without AVX2 instructions,
    as of the latest commit are also available for convenience
    [here](https://passt.top/builds/latest/x86_64/avx2/) and
    [here](https://passt.top/builds/latest/x86_64/). Convenience, non-official
    packages for Debian (and derivatives) and RPM-based distributions are also
    available there. These binaries and packages are simply built with:

            CFLAGS="-static" make avx2
            make pkgs
            make static
            make pkgs

* have a look at the _man_ page for synopsis and options:

        man ./pasta.1

* start pasta with:

        ./pasta

* you're now inside a new user and network namespace. For IPv6, SLAAC happens
  right away as _pasta_ sets up the interface, but DHCPv6 support is available
  as well. For IPv4, configure the interface with a DHCP client:

        dhclient

  and, optionally:

        dhclient -6

* and that's it, you should now have TCP connections, UDP, and ICMP/ICMPv6
  echo working from/to the guest for IPv4 and IPv6

* to connect to a service inside the namespace, just connect to the same port
  using the loopback address.

## Contribute

Public bug tracker and mailing lists are coming soon. For the moment being, send
patches and issue reports to [sbrivio@redhat.com](mailto:sbrivio@redhat.com).