15 Sep 09 Linux DHCP Server

DHCP is an acronym for Dynamic Host Configuration Protocol. It allows a host to broadcast a request for it’s IP settings. Hopefully, a DHCP server like the one we’ll be configuring will respond. Running tcpdump shows a dhcp request looks like:

17:26:02.003956 00:00:00:00:00:00 > ff:ff:ff:ff:ff:ff, ethertype IPv4 (0×0800), length 342: 0.0.0.0.68 > 255.255.255.255.67: BOOTP/DHCP, Request, length 300

Configuration is easy, to start with, just run ‘apt-get install dhcpd’
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Tags: apt-get, dhcp, dhcpd, Linux, ps, syslog, tcpdump

15 Dec 08 Some simple filtering and sniffing with tcpdump

tcpdump is one of the best network debugging tools available. In it’s most basic form, it will print network traffic in terms of a source and destination address to the console, more advanced uses include printing out captured ASCII and simple but powerful filtering.

What we see is:

And this is repeated over and over. Now this is a feedback loop. As we are connected via port 22 (SSH), this loop will continue, and we must therefore filter it out:

Now we can cleanly monitor traffic. What happens though if we want to view SSH traffic, but not our own?

We can build this filter up as much as we wish. Let’s start watching HTTP (tcp port 80) traffic only:

Finally, let’s set the ’snaplen’ to 1500 bytes, and print out the captured data in ASCII:

And from this we can see all HTTP traffic. As you can see, it’s that easy to capture and decode plaintext traffic. We can do the same on port 110 (POP3):

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Tags: ascii, http, https, IMAP, imaps, POP3, pop3s, sniffing http, tcpdump

15 Sep 08 Quick Linux and Windows OpenVPN HOWTO and tutorial, including VPN routing

OpenVPN is a popular Windows/Linux VPN Server/Client pair. I think there’s a separate GUI available for it if you’re so minded. This howto will cover command line usage only.

I’ll provide example configuration based on a Linux server and a Windows client, however the same applies pretty easily if you wanted to mix and match.

On debian, apt-get install openvpn. On any other linux distro, use your own package manager or alternatively download from source and compile.
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Tags: 10.8.0.1, 10.8.0.2, apt-get, client, compile, compression, daemon, daemonize, debian, default gateway, distro, established, forward, generate, interface, IP address, iptables, ip_forward, keepalive, Linux, linux server, masquerade, nat, openvpn, package manager, ping, related, remote, route, route add, route delete, route print, routing, secret key, server, source, tap, tcp, tcpdump, traffic, tun, udp, VPN, vpn client, vpn server, windows, windows client