Keep your IDS sensors safe from attack, while still giving yourself access to their data.
Your IDS sensors are the early warning system that can both alert you to an attack and provide needed evidence for investigating a break-in after one has occurred. You should take extra care to protect them and the data that they collect. One way to do this is to run your IDS sensors in stealth mode.
To do this, simply don’t configure an IP address for the interface that your IDS software will be collecting data from. Putting the interface up, but without specifying an IP address, can do this.
For example:
# tcpdump -i eth1
tcpdump: bind: Network is down
# ifconfig eth1 up promisc
# ifconfig eth1
eth1 Link encap:Ethernet HWaddr 00:DE:AD:BE:EF:00
UP BROADCAST PROMISC MULTICAST MTU:1500 Metric:1
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:100
RX bytes:0 (0.0 b) TX bytes:0 (0.0 b)
Interrupt:11 Base address:0x1c80
# /usr/sbin/tcpdump -i eth1
tcpdump: WARNING: eth1: no IPv4 address assigned
tcpdump: listening on eth1
After you’ve put the interface up, just start your IDS [Hack #82] . Your IDS will run as normal, but since there is no way to directly access the machine, it is very difficult to attack it.
However, just like potential attackers, you will be unable to access the machine remotely. Therefore, if you want to manage the sensor remotely, you’ll need to put in a second network interface. Of course, if you did this and hooked it up to the same network that the IDS sensor is monitoring, it would totally defeat the purpose of running the other interface without an IP address. To keep the traffic isolated, you should create a separate network for managing the IDS sensors. You can of course attach this network to one that is remotely accessible and then firewall it heavily.
Another approach is to access the box using an alternate channel, such as a serial port connected to another machine that does have a network connection. Just run a console on the serial port, and take care to heavily secure the second machine. You could also connect a modem (remember those?) to an unlisted phone number or, better yet, an unlisted extension on your office’s PBX. Depending on your situation, simply using the console for access may be the simplest and most secure method.
Whichever method you decide to use for remote access is a choice you’ll have to make by weighing the value of increased security against the inconvenience of jumping through hoops to access the machine. Security nearly always involves a trade-off between convenience and confidence.