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[Bug 1481561] Re: Connect to XDMCP clients on address requests come from if available

To: touch-packages@xxxxxxxxxxxxxxxxxxx
From: Robert Ancell <robert.ancell@xxxxxxxxxxxxx>
Date: Fri, 20 Nov 2015 07:50:43 -0000
Reply-to: Bug 1481561 <1481561@xxxxxxxxxxxxxxxxxx>
Sender: bounces@xxxxxxxxxxxxx

** Also affects: lightdm (Ubuntu Precise)
   Importance: Undecided
       Status: New

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https://bugs.launchpad.net/bugs/1481561

Title:
  Connect to XDMCP clients on address requests come from if available

Status in Light Display Manager:
  Fix Released
Status in Light Display Manager 1.10 series:
  Fix Committed
Status in Light Display Manager 1.14 series:
  Fix Released
Status in Light Display Manager 1.2 series:
  Fix Committed
Status in lightdm package in Ubuntu:
  Fix Released
Status in lightdm source package in Precise:
  New
Status in lightdm source package in Trusty:
  Fix Committed
Status in lightdm source package in Vivid:
  Fix Committed
Status in lightdm source package in Wily:
  Fix Committed

Bug description:
  [Impact]
  Currently LightDM connects to an XDMCP enabled X server using the first suitable address in the XDMCP Request packet. However, some clients may put a number of addresses in this message and potentially they are not all routable. If the address the request message came from is in the list we should use that first as that is more likely to be a routable address. This also matches the behaviour of GDM which ignores the contents of the Request packet anyway.

  [Test Case]
  1. Start LightDM with XDMCP enabled
  2. Connect with a client that puts a number of addresses in the Request packet

  Expected result:
  LightDM prefers the address the Request came from over the other addresses.

  Observed result:
  LightDM always picks the first address.

  [Regression Potential]
  There is a possibility of a behaviour change in existing clients that had two valid addresses, though the particular address shouldn't matter (unless complex firewalling / routing rules are being used). There is a risk that XDMCP behaviour could have been broken by the change (tested with regression tests).

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