3270 devices

The 3270 is the classic ‘green-screen’ console of the mainframes (see the IBM 3270 Wikipedia article).

The 3270 data stream is not implemented within QEMU; the device only provides TN3270 (a telnet extension; see RFC 854 and RFC 1576) and leaves the heavy lifting to an external 3270 terminal emulator (such as x3270) to make a single 3270 device available to a guest. Note that this supports basic features only.

To provide a 3270 device to a guest, create a x-terminal3270 linked to a tn3270 chardev. The guest will see a 3270 channel device. In order to actually be able to use it, attach the x3270 emulator to the chardev.

Example configuration

  • Make sure that 3270 support is enabled in the guest’s Linux kernel. You need CONFIG_TN3270 and at least one of CONFIG_TN3270_TTY (for additional ttys) or CONFIG_TN3270_CONSOLE (for a 3270 console).

  • Add a tn3270 chardev and a x-terminal3270 to the QEMU command line:

    -chardev socket,id=ch0,host=0.0.0.0,port=2300,nowait,server,tn3270
    -device x-terminal3270,chardev=ch0,devno=fe.0.000a,id=terminal0
    
  • Start the guest. In the guest, use chccwdev -e 0.0.000a to enable the device.

  • On the host, start the x3270 emulator:

    x3270 <host>:2300
    
  • In the guest, locate the 3270 device node under /dev/3270/ (say, tty1) and start a getty on it:

    systemctl start serial-getty@3270-tty1.service
    

    This should get you an additional tty for logging into the guest.

  • If you want to use the 3270 device as the Linux kernel console instead of an additional tty, you can also append conmode=3270 condev=000a to the guest’s kernel command line. The kernel then should use the 3270 as console after the next boot.

Restrictions

3270 support is very basic. In particular:

  • Only one 3270 device is supported.
  • It has only been tested with Linux guests and the x3270 emulator.
  • TLS/SSL is not supported.
  • Resizing on reattach is not supported.
  • Multiple commands in one inbound buffer (for example, when the reset key is pressed while the network is slow) are not supported.