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ipp-usb

GitHub Go Report Card

Introduction

IPP-over-USB allows using the IPP protocol, normally designed for network printers, to be used with USB printers as well.

The idea behind this standard is simple: It allows to send HTTP requests to the device via a USB connection, so enabling IPP, eSCL (AirScan) and web console on devices without Ethernet or WiFi connections.

Unfortunately, the naive implementation, which simply relays a TCP connection to USB, does not work. It happens because closing the TCP connection on the client side has a useful side effect of discarding all data sent to this connection from the server side, but it does not happen with USB connections. In the case of USB, all data not received by the client will remain in the USB buffers, and the next time the client connects to the device, it will receive unexpected data, left from the previous abnormally completed request.

Actually, it is an obvious flaw in the IPP-over-USB standard, but we have to live with it.

So the implementation, once the HTTP request is sent, must read the entire HTTP response, which means that the implementation must understand the HTTP protocol, and effectively implement a HTTP reverse proxy, backed by the IPP-over-USB connection to the device.

And this is what the ipp-usb program actually does.

Features in detail

  • Implements HTTP proxy, backed by USB connection to IPP-over-USB device
  • Full support of IPP printing, eSCL scanning, and web admin interface
  • DNS-SD advertising for all supported services
  • DNS-SD parameters for IPP based on IPP get-printer-attributes query
  • DNS-SD parameters for eSCL based on parsing GET /eSCL/ScannerCapabilities response
  • TCP port allocation for device is bound to particular device (combination of VendorID, ProductID and device serial number), so if the user has multiple devices, they will receive the same TCP port when connected. This allocation is persisted on a disk
  • Automatic DNS-SD name conflict resolution. The finally chosen device's network name is persisted on a disk
  • Can be started by UDEV or run in standalone mode
  • Can share printer to other computers on a network, or use the loopback interface only
  • Can generate very detailed logs for possible troubleshooting

Under the hood

Though looks simple, ipp-usb does many non obvious things under the hood

  • Client-side HTTP connections are completely decoupled from printer-side HTTP-over-USB connections
  • HTTP requests are sanitized, missed headers are added
  • HTTP protocol upgraded from 1.0 to 1.1, if needed
  • Attempts to upgrade HTTP connection to winsock, if unwisely made by web console, are prohibited, because it can steal USB connection for a long time
  • Client HTTP requests are fairly balanced between all available 2-3 USB connections, regardless of number and persistence of client connections
  • Dropping connection by client properly handled in all cases, even in a middle of sending. In a worst case, printer may receive truncated document, but HTTP transaction will always be performed correctly

Memory footprint

Being written on Go, ipp-usb has a large executable size. However, its memory consumption is not very high. When single device is connected, ipp-usb RSS is similar or even slightly less in comparison to ippusbxd. And because ipp-usb handles all devices in a single process, it uses noticeably less memory that ippusbxd, when serving 2 or more devices.

External dependencies

This program has very few external dependencies, namely:

  • libusb for USB access
  • libavahi-common and libavahi-client for DNS-SD
  • Running Avahi daemon

Binary packages

Binary packages available for the following Linux distros:

  • Debian (10)
  • Fedora (29, 30, 31 and 32)
  • openSUSE (Tumbleweed)
  • Ubuntu (18.04, 19.04, 19.10 and 20.04)

Linux Mint users may use Ubuntu packages:

  • Linux Mint 18.x - use packages for Ubuntu 16.04
  • Linux Mint 19.x - use packages for Ubuntu 18.04

Follow this link for downloads: https://download.opensuse.org/repositories/home:/pzz/

Not only Linux

We are glad to announce that ipp-usb was recently included into the FreeBSD ports: https://www.freshports.org/print/ipp-usb/

Hope, NetBSD/OpenBSD support will be added as well, so technology becomes not Linux-only, but UNIX-wide.

The ipp-usb Snap

ipp-usb is also available as a Snap in the Snap Store: https://snapcraft.io/ipp-usb

Before you install the Snap, uninstall any already existing installation of ipp-usb.

Simply install it via any GUI client for the Snap Store (Like "Ubuntu Software") or via command line:

sudo snap install --edge ipp-usb

Now you can connect and disconnect IPP-over-USB devices and ipp-usb gets started by the Snap whenever needed. Also devices which are already connected during boot, start, or update of the Snap are considered.

You can also use

ipp-usb status

to check the status of the running ipp-usb daemon (supported device must be connected for the ipp-usb daemon to be running, accesses only the ipp-usb daemon of the Snap) and

ipp-usb check

to scan the USB for the presence of potentially supported USB devices (7/1/4 interface protocol). This command requires access to the raw USB and therefore on many systems root privileges are required.

The Snap is automatically updated when further development on ipp-usb happens.

The configuration file is here:

/var/snap/ipp-usb/common/etc/ipp-usb.conf

You can edit it and afterwards restart the Snap to use the changed configuration.

Incompatibilities of particular devices are handled by workarounds defined in the quirk files. You find them here:

/var/snap/ipp-usb/common/quirks

You can add your own quirk files (but if they solve your problem, please report an issue here, with your quirk file attached, so that others with the same problem will get helped, too).

For quick tests you can also edit the existing files, but they will get replaced (and so your changes lost) on the next update of the Snap, as we are changing them on any report of further device incompatibilities.

The log file is here

/var/snap/ipp-usb/common/var/log

and device state files (to assure that each device appears on the same port and with the same DNS-SD service name) are here:

/var/snap/ipp-usb/common/var/dev

You can also build the Snap locally. This is useful when

  • You want to modify ipp-usb
  • You want to learn about snapping Go projects
  • You want to learn about how to use UDEV from within a Snap (note that a Snap cannot install UDEV rules into the system)

To do so, run from the main directory of this source repository

snapcraft snap

and then install the resulting Snap with

sudo snap install --dangerous ipp-usb*.snap

An installed Snap from the Snap Store will get overwritten/replaced by your Snap.

Some technical notes about this Snap:

Snapping the Go project with one Go library taken from upstream (and not from Ubuntu Core) was rather straight-forward. Only observation was that the Go plugin seems not to do "make install". So I had to use an "override-build" to manually install the auxiliary files (ipp-usb.conf, quirk files). I also have adapted the auxiliary file and state directories in paths.go in the "override-build" scriptlet.

The real challenge of this Snap was to trigger ipp-usb on the appearing (and also the presence) of IPP-over-USB devices.

In the classic installation of ipp-usb (via "make install" or RPM/DEB package installation) a UDEV rules file and a systemd service file (in systemd-udev/) are installed, so that the system automatically triggers the launch of ipp-usb when an appropriate device is connected or already present. A Snap is not able to do so. It cannot install any files into the system. It can only bring its own, static file system and create files only in its own state directory. These locations are not scanned for UDEV rules.

So the Snap must discover the devices without its own UDEV rules, but it still can use UDEV. The trick is to do a generic monitoring of UDEV events and filtering out the USB devices with IPP-over-USB interface (7/1/4). If such a device appears, we trigger and ipp-usb launch. We also check on startup of the Snap whether there is such a device already and if so, we also trigger an ipp-usb launch.

ipp-usb is run, as in the classic installation, with "udev" argument. This way it stops by itself when there is no device any more (and we do not need to observe the disappearal events of the devices) and it is assured that only one single instance of ipp-usb is running.

To do this with low coding effort I use the UDEV command line tool udevadm in a shell script (snap/local/run-ipp-usb). Once it runs in "monitor" mode to observe the UDEV events. Then we parse the output lines to only consider the ones for a device appearing and run "udevadm info -q property" on each device path, to get the properties and filter the 7/1/4 interface. In the beginning we use "udevadm trigger" to find the already passed appearal event of a device which is already present. So the shell script is an auxiliary daemon to start ipp-usb when needed.

Installation from source

You will need to install the following packages (exact name depends of your Linux distro):

  • libusb development files
  • libavahi-client and libavahi-common development files
  • gcc
  • Go compiler
  • pkg-config
  • git, make and so on

Building is really simple:

git clone https://github.com/OpenPrinting/ipp-usb.git
cd ipp-usb
make

Then you may make install or just try to run ./ipp-usb directly from the build directory

Avahi Notes (exposing printer to localhost)

IPP-over-USB normally exposes printer to localhost only, hence it requires DNS-SD announces to work for localhost.

This requires Avahi 0.8.0 or newer. Older Avahi versions do not support announcing to localhost.

Some Linux distros (for example recent Ubuntu and Fedora versions) have their Avahi patched to support localhost, others (for example Debian) not.

To determine if your Avahi supports localhost, run the following command in one terminal session:

    avahi-publish -s test _test._tcp 1234

And simultaneously the following command in another terminal session on the same machine:

    avahi-browse _test._tcp -r

If you see localhost in the avahi-browse output, like this:

    =     lo IPv4 test                                          _test._tcp           local
       hostname = [localhost]
       address = [127.0.0.1]
       port = [1234]
       txt = []

your Avahi is OK. Otherwise, update or patching is required.

So users of distros that ship a too old Avahi and without the patch have three possibilities:

  1. Update Avahi to 0.8.0 or newer
  2. Apply the patch by themself, rebuild and reinstall avahi-daemon
  3. Configure ipp-usb to run on all network interfaces, not only on loopback

If you decide to apply the patch, get it as avahi/avahi-localhost.patch in this package or download it here.

The third method is simple to do, just replace interface = loopback with interface = all in the ipp-usb.conf file, but this has the disadvantage of exposing your local USB-connected printer to the entire local network, which can be an unwanted side effect, especially in a big corporative network.