How to set up QoS bandwidth rate limit on Vyatta router

If you are running a network shared by multiple devices, you probably want to set up QoS policies (e.g., average bandwidth rate, burst size), so that network bandwidth is properly shared by them. Vyatta software router supports powerful QoS settings. You can easily define any QoS policy, and bind the policy to a specific network interface/port, or to specific types of traffic, etc.Read more »

How to benchmark virtual machines

When you are comparing various hypervisors, you may want to compare the performance of virtual machines (VMs) running on different hypervisors. Also, if you are comparing various public cloud providers, you will want to benchmark VMs offered by available providers. Whatever the reason may be, it is useful to know how to benchmark VMs properly.Read more »

How to check openvswitch version

Open vSwitch (OVS) is an open-source virtual switch, featuring programmable switch forwarding capability. The forwarding plane of OVS can be dynamically controlled by an external network controller, such as OpenFlow controller, which enables key innovation called software-defined networking in large-scale virtual environments.Read more »

How to find a rogue DHCP server

If your company has a large corporate network shared by many employees, you may have encountered the case where your host machine is getting an IP address assigned by some unknown DHCP server not under the administrative control of the corporate network, which in turn causes various connectivity issues for your host. When a host machine is trying obtain an IP address via DHCP, it accepts whichever DHCP offer message arrives first. Hence, if a rogue DHCP server is interfering with a legitimate DHCP server by sending out DHCP offers of its own, it can cause various networking related problems for connected hosts by supplying them with unreachable default gateway, private IP addresses, bogus DNS resolver, etc.Read more »

How to find the IP address of a DHCP server

When a network interface set up for DHCP is being enabled, a host machine broadcasts DHCP discovery message to discover available DHCP servers on the network. Once the host has received a DHCP offer message from any DHCP server, it then sends a DHCP request message to the DHCP server to finalize the lease of an IP address.Read more »

/etc/hosts vs. /etc/resolv.conf

When an application performs DNS lookup as part of its operations on Linux, it can leverage both /etc/hosts and /etc/resolv.conf configuration files to resolve DNS names. How these configuration files are used for DNS lookup can vary among different applications, complicating system administration. But Linux libc library and the GNU C Library (glibc) provides a clean solution to this problem, by defining DNS lookup orders with Name Service Switch (NSS). NSS defines a set of databases and their lookup order in NSS configuration file (/etc/nsswitch.conf in Linux). Applications built with those libraries will then perform DNS lookups in a consistent manner.Read more »

How to set up QoS bandwidth rate limit on XenServer VM's network interfaces

In a multi-tenant VM environment, you would need to allocate network bandwidth appropriately to all existing tenants/VMs, such that no one can consume available network resources by himeself/herself. Even in a single-user environment, you may want to define QoS policies for your VM (such as per-interface bandwidth caps) for various reasons. If you are using XenServer as your hypervisor, XenServer actually allows you to rate limit outgoing traffic of VM's virtual interfaces. If you would like to set up QoS bandwidth rate limit on XenServer VM's network interfaces, follow the instructions here.Read more »