You want to set up an old laptop as a portable network diagnostic station. What should you have on it?
This is a fine and endlessly useful thing to have. It doesn't have to be a super-duper brand-new laptop; any one of reasonably recent vintage that supports USB 2.0 and Linux will do. It should have:
Two wired Ethernet interfaces and one wireless
Modem
USB 2.0 ports
Serial port
Serial terminal
Most laptops don't have a serial port, so you can use a USB-to-serial adapter instead.
Another great thing to have is a PATA/SATA-to-USB 2.0 adapter for rescuing failing hard drives. This lets you plug in either 2.5" or 3.5" PATA or SATA hard drives, and then do a direct copy to save your data. Use the excellent GNU ddrescue utility for this. If your primary hard drive isn't big enough to hold the data, hook up a second one with another PATA/SATA-to-USB 2.0 adapter, or copy it over your network. Why not just copy it over the network in the first place? Because a failing drive is going to take the networking stack down along with everything else.
Install whatever Linux distribution you want, and these applications:
Secure remote administration.
Securely mount remote filesystems.
Insecurely login to servers; useful for several kinds of tests.
Port scanner and network exploration.
Show routes taken to other hosts.
Packet sniffers.
Show listening and connected ports.
Display NAT connections.
Send ICMP ECHO_REQUEST to network hosts.
Test that a server is listening.
Test multicast connectivity.
Packet sniffer that does plaintext and regular expression filtering, rather than filtering on hosts, protocols, and TCP flags.
Send Wake-on-LAN packets to WOL-compliant computers.
Console-based network statistics utility.
Ping-like program for http-requests.
Display bandwidth usage on an interface.
Measure TCP and UDP bandwidth performance.
Find hostnames or IP addresses.
Query name servers.
Send ARP REQUEST to check for duplicate IP addresses, and to see if a host is up.
Excellent dd-type block copier for rescuing failing hard drives. GNU ddrescue is written by Antonio Diaz; don't confuse it with the older dd-rescue, authored by Kurt Garloff. That is also a good rescue utility, but the newer GNU ddrescue is faster and does a better job.
See the Introduction to Chapter 6 for additional information on the net-tools and iproute2 packages.
Get these utilities for wireless troubleshooting:
802.11b wireless network sniffer.
Userspace tools for Linux wireless extensions.
Userspace tools for the Atheros Wireless driver.
Wireless authenticator.
Crack and recover WEP/WPA passwords.
WLAN sniffer.
Key negotiation with your WEP/WPA Authenticator.
Doubtless you will find others that you must have; just fling 'em in there and go to work.
Don't forget to pay extra attention to security. Be sure to keep all of your packages updated, especially security updates, and be finicky with access controls. You can always run a firewall, but this often gets in the way, so your best strategy is to configure it as though you were always going to run it without a firewall. You shouldn't need to run any services anyway, except sshd, so a firewall isn't strictly necessary.