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Appendix A. Troubleshooting

This chapter covers common problems and solutions for Red Hat Enterprise Linux 7 virtualization issues.
Read this chapter to develop an understanding of some of the common problems associated with virtualization technologies. It is recommended that you experiment and test virtualization on Red Hat Enterprise Linux 7 to develop your troubleshooting skills.
If you cannot find the answer in this document, there may be an answer online from the virtualization community. See Section D.1, “Online Resources” for a list of Linux virtualization websites.
In addition, you will find further information on troubleshooting virtualization in RHEL 7 in the Red Hat Knowledgebase.

A.1. Debugging and Troubleshooting Tools

This section summarizes the system administrator applications, the networking utilities, and debugging tools. You can use these standard system administration tools and logs to assist with troubleshooting:
These networking utilities can assist with troubleshooting virtualization networking problems:
  • ip addr, ip route, and ip monitor
  • tcpdump - diagnoses packet traffic on a network. This command is useful for finding network abnormalities and problems with network authentication. There is a graphical version of tcpdump, named wireshark.
  • brctl - A networking utility that inspects and configures the Ethernet bridge configuration in the Linux kernel. For example:
    $ brctl show
    bridge-name    bridge-id          STP  enabled  interfaces
    virtbr0             8000.feffffff       yes        eth0
    $ brctl showmacs virtbr0
    port-no           mac-addr                  local?       aging timer
    1                 fe:ff:ff:ff:ff:           yes            0.00
    2                 fe:ff:ff:fe:ff:           yes            0.00
    $ brctl showstp virtbr0
    bridge-id              8000.fefffffffff
    designated-root        8000.fefffffffff
    root-port              0                   path-cost             0
    max-age                20.00               bridge-max-age        20.00
    hello-time             2.00                bridge-hello-time     2.00
    forward-delay          0.00                bridge-forward-delay  0.00
    aging-time            300.01
    hello-timer            1.43                tcn-timer             0.00
    topology-change-timer  0.00                gc-timer              0.02
Listed below are some other useful commands for troubleshooting virtualization:
  • strace is a command which traces system calls and events received and used by another process.
  • vncviewer connects to a VNC server running on your server or a virtual machine. Install vncviewer using the yum install tigervnc command.
  • vncserver starts a remote desktop on your server. Gives you the ability to run graphical user interfaces, such as virt-manager, using a remote session. Install vncserver using the yum install tigervnc-server command.
In addition to all the commands listed above, examining virtualization logs can be helpful. For more information, see Section A.6, “Virtualization Logs”.