Network tracing

Network problems could be detected by enabling packet logging at the guest side or the host side, but using the built-in capability of VirtualBox to create pcap files might provide even more useful information because it contains a log of really all packets received and sent by guest.

To enable network tracing do the following:

# VBoxManage modifyvm [your-vm] -nictrace[adapter-number] on -nictracefile[adapter-number] file.pcap
# VirtualBox -startvm [your-vm]

For example

# VBoxManage modifyvm "ubuntu" -nictrace1 on -nictracefile1 file.pcap
# VirtualBox -startvm "ubuntu"

Note: Don't forget to disable tracing because your hard disk will quickly be filled by pcap files in case of intensive network usage. The port-forwarding configuration might be affected by this setting as well.
Note: In case situation is 100% reproducible and it takes several steps to reproduce. It's recommended following steps in pcap file creation:

1. prepare environment to test, "lead" your application very close to the state when the 
     problem appears.
2. press host-S (take snapshot)
3. press host-Q (select shutdown with discard to snapshot, it's important )
4. run vm again and finish the test