[Bug 1949535] [NEW] X-forwarding no longer works

[Robert Dinse]

Public bug reported:

I operate a Linux based Internet hosting service that includes Linux
shell servers of various flavors.  I control these from home using
Ubuntu 21.10 presently.  Until recently, X-forwarding has worked from
all but ancient Redhat 6.2 servers.  I recently upgraded my debian shell
server to Bullet, and after the upgrade ssh -X debian.eskimo.com from my
workstation nanook.eskimo.com ceased to function.  It simply says
"Cannot open DISPLAY".  At the time, all the others continued to work.
Today, when I went to do upgrades, now ssh -X is forwarding across the
board to all flavors of Linux.  I did an ssh -V, nothing obvious wrong
in the connection.

debug1: Authentications that can continue: publickey,password
debug1: Next authentication method: publickey
debug1: Offering public key: /home/nanook/.ssh/id_rsa RSA SHA256:a5bReJXl7L91eGOuCYugHsY2rn2a0WTDXEBTC93YdmA agent
debug1: Server accepts key: /home/nanook/.ssh/id_rsa RSA SHA256:a5bReJXl7L91eGOuCYugHsY2rn2a0WTDXEBTC93YdmA agent
debug1: Authentication succeeded (publickey).
Authenticated to igloo.eskimo.com ([204.122.16.128]:22).
debug1: channel 0: new [client-session]
debug1: Requesting no-more-sessions at openssh.com
debug1: Entering interactive session.
debug1: pledge: network
debug1: client_input_global_request: rtype hostkeys-00 at openssh.com want_reply 0
debug1: Remote: /home/nanook/.ssh/authorized_keys:1: key options: agent-forwarding port-forwarding pty user-rc x11-forwarding
debug1: Remote: /home/nanook/.ssh/authorized_keys:1: key options: agent-forwarding port-forwarding pty user-rc x11-forwarding
debug1: Sending environment.
debug1: Sending env LANG = en_US.UTF-8
debug1: client_input_channel_req: channel 0 rtype exit-status reply 0
debug1: client_input_channel_req: channel 0 rtype eow at openssh.com reply 0
debug1: channel 0: free: client-session, nchannels 1
debug1: fd 2 clearing O_NONBLOCK
Connection to igloo.eskimo.com closed.
Transferred: sent 3748, received 3820 bytes, in 6.4 seconds
Bytes per second: sent 589.4, received 600.8

I don't see anything obviously wrong here but still it does not work, I get:
xclock
Error: Can't open display:

ProblemType: Bug
DistroRelease: Ubuntu 21.10
Package: ssh (not installed)
Uname: Linux 5.13.19 x86_64
ApportVersion: 2.20.11-0ubuntu71
Architecture: amd64
CasperMD5CheckResult: unknown
CurrentDesktop: MATE
Date: Tue Nov  2 17:31:14 2021
SourcePackage: openssh
UpgradeStatus: No upgrade log present (probably fresh install)

** Affects: openssh (Ubuntu)
     Importance: Undecided
         Status: New


** Tags: amd64 apport-bug impish

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https://bugs.launchpad.net/bugs/1949535

Title:
  X-forwarding no longer works

Status in openssh package in Ubuntu:
  New

Bug description:
  I operate a Linux based Internet hosting service that includes Linux
  shell servers of various flavors.  I control these from home using
  Ubuntu 21.10 presently.  Until recently, X-forwarding has worked from
  all but ancient Redhat 6.2 servers.  I recently upgraded my debian
  shell server to Bullet, and after the upgrade ssh -X debian.eskimo.com
  from my workstation nanook.eskimo.com ceased to function.  It simply
  says "Cannot open DISPLAY".  At the time, all the others continued to
  work.  Today, when I went to do upgrades, now ssh -X is forwarding
  across the board to all flavors of Linux.  I did an ssh -V, nothing
  obvious wrong in the connection.

  debug1: Authentications that can continue: publickey,password
  debug1: Next authentication method: publickey
  debug1: Offering public key: /home/nanook/.ssh/id_rsa RSA SHA256:a5bReJXl7L91eGOuCYugHsY2rn2a0WTDXEBTC93YdmA agent
  debug1: Server accepts key: /home/nanook/.ssh/id_rsa RSA SHA256:a5bReJXl7L91eGOuCYugHsY2rn2a0WTDXEBTC93YdmA agent
  debug1: Authentication succeeded (publickey).
  Authenticated to igloo.eskimo.com ([204.122.16.128]:22).
  debug1: channel 0: new [client-session]
  debug1: Requesting no-more-sessions at openssh.com
  debug1: Entering interactive session.
  debug1: pledge: network
  debug1: client_input_global_request: rtype hostkeys-00 at openssh.com want_reply 0
  debug1: Remote: /home/nanook/.ssh/authorized_keys:1: key options: agent-forwarding port-forwarding pty user-rc x11-forwarding
  debug1: Remote: /home/nanook/.ssh/authorized_keys:1: key options: agent-forwarding port-forwarding pty user-rc x11-forwarding
  debug1: Sending environment.
  debug1: Sending env LANG = en_US.UTF-8
  debug1: client_input_channel_req: channel 0 rtype exit-status reply 0
  debug1: client_input_channel_req: channel 0 rtype eow at openssh.com reply 0
  debug1: channel 0: free: client-session, nchannels 1
  debug1: fd 2 clearing O_NONBLOCK
  Connection to igloo.eskimo.com closed.
  Transferred: sent 3748, received 3820 bytes, in 6.4 seconds
  Bytes per second: sent 589.4, received 600.8

  I don't see anything obviously wrong here but still it does not work, I get:
  xclock
  Error: Can't open display:

  ProblemType: Bug
  DistroRelease: Ubuntu 21.10
  Package: ssh (not installed)
  Uname: Linux 5.13.19 x86_64
  ApportVersion: 2.20.11-0ubuntu71
  Architecture: amd64
  CasperMD5CheckResult: unknown
  CurrentDesktop: MATE
  Date: Tue Nov  2 17:31:14 2021
  SourcePackage: openssh
  UpgradeStatus: No upgrade log present (probably fresh install)

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