p2p: Service for sharing files between Chrome OS devices

About

p2p is a software package for advertising and discovering content on the local network (LAN). It is built on top of HTTP and mDNS/DNS-SD. p2p is written specifically for Chrome OS and uses services and conventions specific to that OS. It is not intended to be portable to other OSes.

Files

Content to be shared with other devices on the LAN is to stored in an encrypted part of the stateful partition. The following directory is used:

/var/cache/p2p

In the following this directory shall be referred to as $(P2P_DIR).

Theory of Operation

Files to be shared are advertised via a DNS-SD service with the service type _cros_p2p._tcp.

The address and port number advertised in the DNS-SD service refer to a HTTP server running on the same machine. To simplify firewall management, port 16725 (“AU”) is always used but in the future this may by dynamic.

Any file with a .p2p file extension in $(P2P_DIR) is advertised as a TXT record in a DNS-SD response where its value is the length of the file (decimal encoding). For example, if the file $(P2P_DIR)/some_payload.bin.p2p exists and is 123456789 bytes long, the DNS-SD response will advertise a TXT record id_some_payload.bin=123456789 (the id_ prefix is there just for namespacing purposes). By convention, this allows any client to download the file using the URL

http://<IP>:<PORT>/some_payload.bin

where IP and PORT is taken from the DNS-SD response. Additionally, to allow sharing files that are not completely downloaded yet, one can set the user.cros-p2p-filesize extended attribute with the final size. This will make the HTTP server block until content is available. Note that the mDNS/DNS-SD response always returns the size of the file on disk - this is to allow a peer to pick the peer with e.g. the most downloaded bytes.

Note that the size set in the user.cros-p2p-filesize extended attribute - if present - is always constant (because it contains the final size of the file). In contrast, the size conveyed via the mDNS/DNS-SD response reflects the actual size of the file which grows as more and more data is being downloaded. As such, this value is not necessarily constant and in fact, by virtue of how mDNS/DNS-SD works, changes to the value are propagated on the local network. To conserve bandwidth with frequently changing files, file size changes are propagated at most every ten seconds.

The $(P2P_DIR) directory is the sole interface for other software on the local system to share files with other peers. For example, if the update_engine program (used for updating Chrome OS) is downloading an payload it can create a file, say $(P2P_DIR)/some_update_xyz.bin.p2p.tmp and start writing to it as it downloads the rest of the payload. When the update_engine program has verified that the file is authentic (by e.g. checking a cryptographic signature in the beginning of the file) and it knows the final size, it can

• sets the user.cros-p2p-filesize xattr to the size
• rename the .tmp extension away
• proceed to write payload to the file

and, hey presto, the file some_update_xyz.bin is now shared with the rest of the local network.

In addition to advertising files, the _cros_p2p._tcp DNS-SD service will also advertise the current number of HTTP connections in the num-connections TXT attribute. This can used (in a cooperative manner) to limit the number of simultaneous downloads in the LAN.

Programs

The p2p package is comprised of three programs.

p2p-server

The primary purpose of this program is to monitor the $(P2P_DIR) directory and advertise .p2p files via mDNS/DNS-SD (it uses the Avahi package to do this). When there is a non-zero number of .p2p files, it will start the p2p-http-server program and when the number of .p2p files drop to zero it will terminate kill the instance.

This program is run as a daemon (long-running process) and is usually started via an Upstart job, p2p, to ensure the firewall is properly configured (specifically, opening the TCP port that the HTTP server will listen on), its dependencies (e.g. Avahi) has started and that the program is launched with appropriate privileges (using minijail0 to drop privileges).

p2p-http-server

The purpose of this program is to serve .p2p files via HTTP. It is started and stopped by the p2p-server server, when needed.

p2p-client

This is a simple program to discover content available on the LAN. Given a file identifier, it looks on the LAN for _cros_p2p._tcp DNS-SD services. If one or more peers have the file, p2p-client picks one of them and prints the URL on stdout.

Users and Permissions

p2p User and Group

The p2p user (and p2p group) is used to run p2p-server and p2p-http-server without root privileges.

Permissions on $(P2P_DIR)

The $(P2P_DIR) is owned by root and its permissions are rwxr-xr-x, that is

• any (non-jailed) user on the system can read files in $(P2P_DIR)
• only root can write files

In the future a more sophisticated ACL scheme may be introduced to allow known unprivileged programs (say, update_engine) to share content.

Frequently Asked Questions

(TODO: write me)