C Cheat Sheet

Leave a reply

Array initialization

int a[4] = {[2] = 6, [3] = 7};
int grid[100][100] = [0][0] = 8, [50][25] = 7};

Structure initialization

struct address {
                 int street_no;
                 char *street_name;
                 char *city;
                 char *prov;
                 char *postal_code;
};
struct address temp_address = { .city = "Hamilton", .prov = "Ontario" };

struct a {
         struct b {
              int c;
              int d;
          } e;
         float f;
} g = {.e.c = 3 };

Continue reading →

Equal Cost Multipath

Leave a reply

Equal Cost Multipath (ECMP) is a network load-balancing method that enables the coexistence of multiple network paths form one source node to a destination node. The two or more paths between the nodes have the same routing cost, thus the traffic will be split evenly across, avoiding congestion and increasing the bandwidth.

ECMP is also a network redundancy method. In case one ECMP link fails, the traffic will move on the remaining links with minimal interruption in service.

For ECMP to work, a router will need special support in the forwarding plane and in the routing protocols deployed in the network. For experimentation I will use two Linux virtual machines on my host computer. The Linux kernel has an excellent ECMP implementation, as for the routing protocol I will use OSPF.

The virtual machines are set using virtenv. It is a very light virtualization solution based on Linux containers (LCX) implementation in Linux kernel. Each machine owns a slice of the Linux kernel running on the host computer, with full network and process separation. In each virtual machine I run one instance of RCP100. RCP100 is a router control plane for Linux platforms, supporting among other things OSPF and ECMP. RCP100 also features a CISCO-like command line interface (CLI) which simplifies router operation for people already skilled in configuring commercial routers.

The network diagram is as follows:

ECMP test network

Continue reading →

How to Speed Up Mozilla Firefox

Leave a reply

firefox logo

As the Internet goes slower and slower and your Internet Service Provider refuses to go faster and faster, these are three easy things you can do to speed up Mozilla Firefox:

1. Disable IPv6

For a regular web page such as slashdot.org, Firefox needs to resolve more than 40 domain names. Each domain name is resolved twice, once for an IPv4 address and once for an IPv6 address. This results in lots of DNS requests, slowing down your web access. If you are like 99.999% of the population without IPv6 access, translating domain names in IPv6 addresses is useless.

To disable this functionality, type about:config into the address bar. Type ipv6 into the search bar and toggle network.dns.disableIPv6 to true.

2. Install Adblock Plus extension

The extension will cut down most, if not all, advertisements and annoying banners.

3. Install Ghostery extension

The extension removes “invisible” trackers, web bugs, pixels, and beacons placed on web pages by Facebook, Google Analytics, and over 1,000 other ad networks.

Easy LXC: Running OpenVZ containers in LXC

Leave a reply

ezlxc (Easy LXC) is a small Bash script for running virtual machines (VM) using Linux Containers (LXC). The VM containers are based on the templates provided by OpenVZ project.

In an LXC environment, a single Linux kernel is shared between the host and the virtual machines. Only the essential needed services are run in VMs. The VM is basically a chroot-based environment with the added network/process separation provided by LXC virtualization. Memory requirements for this type of setup is very low – my old dual-core AMD computer with 1GB of RAM memory runs easily 10 VMs.

ezlxc is based on ssh-template script developed and distributed with LXC. I have tested it on a Fedora 17 x86_64 computer and it will probably work without modifications on any recent Linux distribution. Copy the script below in a text editor and save it as ezlxc. Make the file executable (chmod +x ezlxc) and copy it in /usr/local/bin/directory. The copying is performed as root, in fact all the operations below can only be performed as root.

Continue reading →