VNC Viewer 请求超时
问题
连接时提示：Timed out waiting for a response from the computer
在通过VNC Viewer连接虚拟桌面时，出现了连接超时的现象，一般出现这种问题，首先考虑一下是不是服务器IP 或 VNC端口号 输入错误导致。
经过排查后，确认IP和端口号都是正确的，尝试各种方法后发现问题所在：
服务器可能打开了防火墙，通过服务器外部IP无法访问某些服务，导致连接超时。
解决方案
只需要在服务器中手动开启相应的端口号，VNC Viewer就能够访问了。
首先查看当前防火墙的规则：
#查看当前的规则和对应的编号，需要使用root权限
sudo iptables -L -n --line-number
下面还有很多规则，图中的两个端口是与VNC相关的端口，一个是52号桌面的端口，一个是9号桌面的端口，之前的规则中只有对09的规则，图中的52是后来添加的。
由于之前的规则中并没有开放52号桌面的端口，所以手动进行开启。
首先要获取52号桌面的端口号，VNC的默认端口是从5901开始的，即1号桌面的端口为5901，相应的52号桌面的端口号为5952，如果不确定或为了稳妥，可以使用下面的命令查看个人用户下所有VNC的端口号。
netstat -lp|grep -i vnc
可以得到，52号桌面的端口号为：5952
然后，手动将开启52端口的规则，加入防火墙规则的命令如下：
sudo iptables -I INPUT -p tcp --dport 5952 -j ACCEPT
开启端口后VNC Viewer就可以访问端口52对应的VNC了。
gnome桌面灰屏
注：在gnome环境下的安装不一定能够成功，如果你的系统是16.04或18.04，直接向后翻，看xfce4的方案。
打开桌面后为灰屏状态，鼠标为 ‘X’ 状，无法进行任何操作。
如果你之前已经配置了xstartup文件，且运行正常，可以尝试一下命令，安装gnome需要的包，或对这些包进行更新。
sudo apt-get install gnome-panel gnome-settings-daemon metacity nautilus gnome-terminal
在安装或更新后，如果还是灰屏，说明是xstartup的问题。
我们打开 amax-sys:52.log 文件，查看桌面的日志：
Xvnc Free Edition 4.1.1 - built Feb 25 2015 23:02:21
Copyright (C) 2002-2005 RealVNC Ltd.
See http://www.realvnc.com for information on VNC.
Underlying X server release 40300000, The XFree86 Project, Inc
Mon Nov 8 14:25:29 2021
vncext: VNC extension running!
vncext: Listening for VNC connections on port 5952
vncext: created VNC server for screen 0
error opening security policy file /etc/X11/xserver/SecurityPolicy
Could not init font path element /usr/X11R6/lib/X11/fonts/Type1/, removing from list!
Could not init font path element /usr/X11R6/lib/X11/fonts/Speedo/, removing from list!
Could not init font path element /usr/X11R6/lib/X11/fonts/misc/, removing from list!
Could not init font path element /usr/X11R6/lib/X11/fonts/75dpi/, removing from list!
Could not init font path element /usr/X11R6/lib/X11/fonts/100dpi/, removing from list!
Could not init font path element /usr/share/fonts/X11/75dpi/, removing from list!
Could not init font path element /usr/share/fonts/X11/100dpi/, removing from list!
sh: /home/liushuo/.vnc/xstartup: Permission denied
从最后一句中，得到xstartup文件的权限不足，查看xstartup的权限：
可以看到xstartup只有读写权限，因此我们为xstartup添加执行权限。
chmod +x .vnc/xstartup
然后重新生成52号桌面
vncserver -kill :52
vncserver -geometry 1920x1080 :52
打开桌面后，菜单栏可以正常显示，程序和终端都可以正常运行，但是桌面和图标无法显示，且程序运行很慢。
查看日志文件：
Xvnc Free Edition 4.1.1 - built Feb 25 2015 23:02:21
Copyright (C) 2002-2005 RealVNC Ltd.
See http://www.realvnc.com for information on VNC.
Underlying X server release 40300000, The XFree86 Project, Inc
Mon Nov 8 17:17:22 2021
vncext: VNC extension running!
vncext: Listening for VNC connections on port 5951
vncext: created VNC server for screen 0
error opening security policy file /etc/X11/xserver/SecurityPolicy
Could not init font path element /usr/X11R6/lib/X11/fonts/Type1/, removing from list!
Could not init font path element /usr/X11R6/lib/X11/fonts/Speedo/, removing from list!
Could not init font path element /usr/X11R6/lib/X11/fonts/misc/, removing from list!
Could not init font path element /usr/X11R6/lib/X11/fonts/75dpi/, removing from list!
Could not init font path element /usr/X11R6/lib/X11/fonts/100dpi/, removing from list!
Could not init font path element /usr/share/fonts/X11/75dpi/, removing from list!
Could not init font path element /usr/share/fonts/X11/100dpi/, removing from list!
/home/liushuo/.vnc/xstartup: line 16: gnome-settings-daemon: command not found
metacity-Message: 17:17:25.351: could not find XKB extension.
原因是没有 gnome-settings-daemon 命令，也就是gnome-settings-daemon没有安装成功。
我通过 apt-get 安装没有出现问题，但是通过 ps -A | grep gnome 查找不到gnome-settings-daemon的信息。
搞了半天不知道问题出在哪里，知道解决方案的大佬可以交流一下。
我的xstartup内容如下：
#!/bin/sh
# Uncomment the following two lines for normal desktop:
# unset SESSION_MANAGER
# exec /etc/X11/xinit/xinitrc
# dbus-launch startxfce4
[ -x /etc/vnc/xstartup ] && exec /etc/vnc/xstartup
[ -r $HOME/.Xresources ] && xrdb $HOME/.Xresources
xsetroot -solid grey
vncconfig -iconic &
gnome-panel &
gnome-settings-daemon &
metacity &
nautilus &
gnome-terminal &
如果你的gnome-settings-daemon正常，修改xstartup的权限后，桌面应该就出来了。
如果你和我出现一样的问题，且Ubuntu系统为16.04或18.04，可以选择安装另一种桌面环境：Xfce4。
xfce4方案
安装方式也非常简单，此处默认系统已经安装了vncserver，如果没有安装，使用如下命令进行安装：
sudo apt-get install vnc4server
然后安装xfce
sudo apt-get install xfce4
可以使用如下命令，查看是否安装成功：
ps -A | grep xfce4
修改配置文件xstartup为：
#!/bin/sh
# Uncomment the following two lines for normal desktop:
# unset SESSION_MANAGER
# exec /etc/X11/xinit/xinitrc
dbus-launch startxfce4
[ -x /etc/vnc/xstartup ] && exec /etc/vnc/xstartup
[ -r $HOME/.Xresources ] && xrdb $HOME/.Xresources
xsetroot -solid grey
vncconfig -iconic &
x-terminal-emulator -geometry 80x24+10+10 -ls -title "$VNCDESKTOP Desktop" &
x-window-manager &
xrdb $HOME/.Xresources
然后重新生成52号桌面即可
vncserver -kill :52
vncserver -geometry 1920x1080 :52
附当前正常使用的VNC的log文件：
Xvnc Free Edition 4.1.1 - built Feb 25 2015 23:02:21
Copyright (C) 2002-2005 RealVNC Ltd.
See http://www.realvnc.com for information on VNC.
Underlying X server release 40300000, The XFree86 Project, Inc
Mon Nov 8 19:08:30 2021
vncext: VNC extension running!
vncext: Listening for VNC connections on port 5951
vncext: created VNC server for screen 0
error opening security policy file /etc/X11/xserver/SecurityPolicy
Could not init font path element /usr/X11R6/lib/X11/fonts/Type1/, removing from list!
Could not init font path element /usr/X11R6/lib/X11/fonts/Speedo/, removing from list!
Could not init font path element /usr/X11R6/lib/X11/fonts/misc/, removing from list!
Could not init font path element /usr/X11R6/lib/X11/fonts/75dpi/, removing from list!
Could not init font path element /usr/X11R6/lib/X11/fonts/100dpi/, removing from list!
Could not init font path element /usr/share/fonts/X11/75dpi/, removing from list!
Could not init font path element /usr/share/fonts/X11/100dpi/, removing from list!
/usr/bin/startxfce4: X server already running on display :51
gpg-agent[42352]: WARNING: "--write-env-file" is an obsolete option - it has no effect
gpg-agent: a gpg-agent is already running - not starting a new one
(xfce4-session:42337): xfce4-session-WARNING **: 19:08:34.025: gpg-agent returned no PID in the variables
(xfce4-session:42337): xfce4-session-WARNING **: 19:08:34.026: xfsm_manager_load_session: Something wrong with /home/liushuo/.cache/sessions/xfce4-session-amax-sys:51, Does it exist? Permissions issue?
vmware-user: could not open /proc/fs/vmblock/dev
(xfwm4:42355): xfwm4-WARNING **: 19:08:34.063: The display does not support the XRender extension.
(xfwm4:42355): xfwm4-WARNING **: 19:08:34.063: The display does not support the XComposite extension.
(xfwm4:42355): xfwm4-WARNING **: 19:08:34.063: The display does not support the XDamage extension.
(xfwm4:42355): xfwm4-WARNING **: 19:08:34.064: The display does not support the XFixes extension.
(xfwm4:42355): xfwm4-WARNING **: 19:08:34.064: Compositing manager disabled.
(xfsettingsd:42371): xfsettingsd-CRITICAL **: 19:08:34.076: RANDR extension is too old, version 1.1. Display settings won't be applied.
Xlib: extension "XInputExtension" missing on display ":51.0".
(xfsettingsd:42371): xfsettingsd-CRITICAL **: 19:08:34.076: XI is not present.
(xfsettingsd:42371): xfsettingsd-CRITICAL **: 19:08:34.076: Failed to initialize the Xkb extension.
(xfsettingsd:42371): xfsettingsd-CRITICAL **: 19:08:34.076: Failed to initialize the Accessibility extension.
** (update-notifier:42372): WARNING **: 19:08:34.100: already running?
xfwm4-Message: 19:08:34.112: Unsupported keyboard modifier '
(polkit-gnome-authentication-agent-1:42374): GLib-CRITICAL **: 19:08:34.115: g_variant_new_string: assertion 'string != NULL' failed
(polkit-gnome-authentication-agent-1:42374): polkit-gnome-1-WARNING **: 19:08:34.116: Failed to register client: GDBus.Error:org.freedesktop.DBus.Error.ServiceUnknown: The name org.gnome.SessionManager was not provided by any .service files
(xfwm4:42355): xfwm4-WARNING **: 19:08:34.117: Cannot find visual format on screen 0
xfwm4-Message: 19:08:34.160: Unsupported keyboard modifier '
(nm-applet:42367): nm-applet-WARNING **: 19:08:34.220: GDBus.Error:org.freedesktop.NetworkManager.AgentManager.PermissionDenied: An agent with this ID is already registered for this user.
(nm-applet:42367): Gtk-WARNING **: 19:08:34.255: Can't set a parent on widget which has a parent
Failure: Module initialization failed
** (xfdesktop:42363): WARNING **: 19:08:34.472: Failed to set the background '/usr/share/backgrounds/xfce/xfce-teal.jpg': GDBus.Error:org.freedesktop.DBus.Error.InvalidArgs: No such interface 'org.freedesktop.DisplayManager.AccountsService'
仍有一些警告，但桌面已经可以正常使用。
参考
端口：
https://www.jianshu.com/p/b3068288d80d
https://www.cnblogs.com/ding2016/p/8031926.html
vnc错误排查：
https://blog.csdn.net/CH_sir/article/details/107930881
https://junchu.blog.csdn.net/article/details/103974974
https://blog.csdn.net/qq695165856/article/details/105677776
灰屏：
https://www.freesion.com/article/85721281890/
xfce4：
https://www.cnblogs.com/kerrycode/p/4790021.html
https://blog.csdn.net/weixin_34174422/article/details/93689750
https://www.linuxidc.com/Linux/2018-08/153435.htm