How to set up a transparent proxy on Linux

A transparent proxy sits between clients and the Internet, acting as a gateway for the clients. It's called transparent because clients are not required to configure anything for the proxy. A transparent proxy is useful when it's not possible or desirable to modify client configurations, while it's necessary to intercept client's requests.Read more »

How to use a custom DNS server on Ubuntu Desktop

In Linux, DNS servers are specified in /etc/resolv.conf. However, a proper way to configure custom DNS settings on Ubuntu Desktop is not manually modifying /etc/resolv.conf, but using a separate GUI-based network management program which controls the content of /etc/resolv.conf.Read more »

How to manage a wired network on Ubuntu Desktop

On Ubuntu Desktop, a separate GUI program called NetworkManager is responsible for configuring network settings (e.g., wired network, wireless network, proxy settings). However, on a fresh installation of Ubuntu Desktop, your wired network is NOT managed by NetworkManager, but by /etc/network/interfaces.Read more »

How to configure http proxy exceptions on Linux

When you are behind http proxy, you need to configure proxy settings for your applications in one way or another. Typically you define http_proxy environment variable pointing to your proxy, so that all http requests go through the proxy. But what if you want to bypass http proxy for specific destination web servers? That's when you need to configure http proxy exceptions.Read more »

How to diff remote files over ssh

Diff command line utility compares contents of two files (or directories), and shows how they differ line by line. It's straightforward to use diff if both input files are located on local host. But what if input files located on two different hosts? You can actually diff remote files over ssh. Here is how.Read more »

How to edit a remote file over ssh

When you need to edit a file hosted on a remote host, it will be nice if you can edit the remote file over ssh with vi command, as if it were a local file. Of course you could set up NFS over ssh tunnel to achieve that, but that will be an overkill if this is just one-time usage. So how can I edit a remotely located file over ssh?Read more »

What are popular packet sniffers on Linux?

A packet sniffer captures live packets from networks, decodes them according to protocol specifications, and optionally takes actions according to the parsed packet contents (e.g., in case of security detection systems). It is a very useful tool to diagnose networks or protect against security attacks over networks. The following are several popular open-source packet sniffers available on Linux.Read more »

How to install httptunnel on Linux

Httptunnel is a GNU/GPL-licensed free software that allows one to create a bi-directional tunnel encapsulated by HTTP, between client and server. HTTP-encapsulated tunnels are useful when you want to use games, IM clients, or P2P sharing applications across restrictive firewalls or proxies which tend to block pretty much everything except well known traffic such as HTTP traffic. Httptunnel consists of hts (server) and htc (client) components to establish HTTP tunnels in between.Read more »

How to ping a specific port of a remote host

Ping is a networking utility used to test the reachability and round-trip time (RTT) delay of a remote host over Internet Protocol (IP). Ping utility does so by sending out a series of Internet Control Message Protocol (ICMP) echo request packets to a remote host, and waiting for corresponding ICMP response packets from the host. However, you cannot ping a specific port because ICMP belongs to layer-2 IP layer, not layer-3 transport layer (e.g., TCP/UDP). In order to ping a specific port of a remote host, you need to use layer-3 transport protocols which have a notion of port numbers.Read more »

How to install tcpreplay on Linux

Tcpreplay is a suite of command-line utilities that are used to "replay" previously captured packet dumps in a real network. In order to use tcpreplay, you first need to capture live network traffic using packet capture tools such as tcpdump, rewrite TCP/IP packet headers of the captured packets appropriately (usually MAC and IP addresses), and then re-inject the packets on to any arbitrary network under consideration.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 a DHCP server using dnsmasq

If you are trying to set up a small private network in your home or company, you probably want to set up an internal DHCP/DNS server for your network. Dnsmasq comes in handy in that situation. Dnsmasq is a free lightweight DHCP server for Linux that can serve a small-scale network. It supports static/dynamic DHCP leases and has DNS proxy functionality built in.Read more »

How to set up DHCP and NAT on Vyatta router

Vyatta is a feature-rich enterprise class software router. You can download a free community version of Vyatta as well. Vyatta router can easily be built on a physical machine via a well-packaged LiveCD, or on a VMware/Xen VM via ready-made virtual appliance. As a ready-made software router, Vyatta can be configured extremely easily as a NAT gateway with DHCP, DNS and NAT functionality.Read more »

How to backup files to a remote FTP server using lftp

lftp is a command-line FTP client with several advanced file transfer features. For example, lftp can upload or download a whole directory tree recursively and selectively, or resume interrupted file transfers. A popular use case of lftp is to mirror local files or folders to a remote FTP server. While rsync is a popular mirroring software tool, it uses its own file synchronization protocol, and so does not work over FTP.Read more »

How to enable ssh access on Vyatta router

A fresh installation of Vyatta does not have SSH access enabled by default. Thus, if you would like to SSH to a Vyatta router, you first need to enable SSH service. In order to enable SSH access on Vyatta router, enter the following commands via its console.Read more »

How to set up QoS bandwidth rate limit on OpenWRT

OpenWRT is an operating system used for embedded devices such as commodity home routers to route network traffic. It provides rich feature sets for network management, including firewall, QoS policy, port forwarding, monitoring, etc. One of the useful features of OpenWRT is to configure QoS policy such as bandwidth limiting for connected devices. If you would like to configure upload and download rate limiting on OpenWRT, you can follow the instructions below. This setup has been tested on OpenWRT Kamikaze 8.09 with Linksys WRT54GL router.Read more »

How to synchronize time with NTP

NTP is a networking protocol which enables one to synchronize clock among different computers using IP packets. NTP is commonly used when one maintains a cluster of hosts to operate compute and storage cloud in data centers. NTP is capable of compensating for variable latency over networks, and can achieve clock synchronization with 1 millisecond accuracy in ideal conditions. If you would like to synchronize time with NTP, follow the steps below.Read more »

How to reload config.boot in Vyatta

Vyatta an open-source virtual router operating system based on a customized Debian distribution. Vyatta can easily be installed on virtual machines (VMs) created by standard hypervisors such as Xen, KVM and VMware, to implement virtual networking for VMs. Vyatta offers a specialized management console as well as web-based management interface to configure system settings such as network interfaces, firewall rules, QoS policy, etc. If you are reconfiguring Vyatta via its management console, you may want to know how to reload Vyatta configuration after you are done.Read more »