Chapter 21. Troubleshooting

This chapter covers common problems and solutions for Red Hat Enterprise Linux 6 virtualization issues.
Read this chapter to develop an understanding of some of the common problems associated with virtualization technologies. Troubleshooting takes practice and experience which are difficult to learn from a book. It is recommended that you experiment and test virtualization on Red Hat Enterprise Linux 6 to develop your troubleshooting skills.
If you cannot find the answer in this document there may be an answer online from the virtualization community. Refer to Section B.1, “Online Resources” for a list of Linux virtualization websites.

21.1. Debugging and Troubleshooting Tools

This section summarizes the System Administrator applications, the networking utilities, and debugging tools. You can employ these standard System administration tools and logs to assist with troubleshooting:
These networking tools can assist with troubleshooting virtualization networking problems:
  • ifconfig
  • tcpdump
    The tcpdump command 'sniffs' network packets. tcpdump is useful for finding network abnormalities and problems with network authentication. There is a graphical version of tcpdump named wireshark.
  • brctl
    brctl is a networking tool that inspects and configures the Ethernet bridge configuration in the Linux kernel. You must have root access before performing these example commands: 
    # 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
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: connect to a VNC server running on your server or a virtual machine. Install vncviwer using the yum install tigervnc command.
  • vncserver: start a remote desktop on your server. Gives you the ability to run graphical user interfaces such as virt-manager via a remote session. Install vncserver using the yum install tigervnc-server command.