Assuming lawful purposes, what is the best way to tap a network
undetectable to the surveillance subject, not missing any
relevant data, and not exposing the installer to undue risk?
I'd have to say this depends on the media involved.
ethernet switches allow the monitoring of specific ports (or entire
vlans) in most cases. This can be done without impact (assuming nobody
goofs on the ethernet switch config) to other people and limit the scope
of packets inspected.
Various vendors have their own monitoring solutions and port
replication features. I seem to recall one customer of my employer
saying how much they enjoyed the ability to tcpdump/inspect traffic
on their Juniper routers. (with regards to a DoS attack we were working
on tracking).
- Jared
We've been using Shomiti taps for several years with good effect. All
they do is copy all the data going through a segment (100bT in our case)
to two ports, one for inbound, another for outbound. Now Finisar, they
sell both copper and fiber taps for a variety of media, including Ethernet
from 10Mbps to 10Gbps. They have been rock-solid, never missing a packet,
and isolate the sniffer from the rest of the network.
Of course, you then need to choose a packet analyzer/IDS to use with the
tap.
Doug
Assuming lawful purposes, what is the best way to tap a network
undetectable
The best way to go undetectable is easy, run the sniffer without an IP
address. The best way to tap a network varies with your setup. If your
repeated, just plug in and go. If your switched (which most of us are),
you need to figure out how to get in the middle of the data stream you
want to monitor.
The best solution I've found is to use an Ethernet tap. It allows you to
piggy back off of an existing connection and monitor all the traffic
going to and from that system. Its pretty undetectable, does not use any
additional switch ports, and allows you to run full duplex. A number of
vendors sell them and a Google will give you sites on how to make them.
You can plug a mini-hub in line and use that as a tap point to monitor
the stream. Up side is its cheap and easy. Down side is you have to drop
to half duplex. Not a problem in most situations but in some the drop in
performance can be an issue.
Many switch vendors include a copy or mirror port that allows you to
replicate all traffic to and from a specific port, to some other port
where you can plug in your sniffer. Up side here is ease of
configuration. If you want to start monitoring a different port its a
simple configuration change within your switch. Down side is you could
end up missing packets (I've run into this myself). Seems when some/many
switches get busy the first thing they stop doing is copying packets to
the mirror port.
There are tools out there like Dsniff and Ettercap that allow you to
sniff in a switched environment. I recommend you avoid them because they
tend to either work or hose your network. You don't want to DoS
yourself. 
to the surveillance subject, not missing any
relevant data, and not exposing the installer to undue risk?
Sniffing is a passive function so its always possible you are going to
miss data. It all depends on the capabilities of the box recording the
packets.
As for "risk", that's always there as well. For example check the
Bugtraq archives and you are going to find exploits that work against
tools like Tcpdump and Snort. The attacks go after the way the software
processes the packet. So even if you are running without an IP address
its possible that someone with malicious intent can DoS the box.
HTH,
C
> Assuming lawful purposes, what is the best way to tap a network
> undetectable...
The best solution I've found is to use an Ethernet tap. It allows you to
piggy back off of an existing connection and monitor all the traffic
going to and from that system. Its pretty undetectable, does not use any
additional switch ports, and allows you to run full duplex. A number of
vendors sell them and a Google will give you sites on how to make them.
...
i hadn't thought of making my own -- that sounds like a fun project.
for f-root, we've (isc) been installing the netoptics version of this:
works great. it's basically a hub, but with the interesting feature of
letting you monitor TX and RX separately, and full duplex is preserved.
(it takes 2x100Mbit to fully monitor a full duplex 100Mbit link.) it
also fails into "connected" mode if power is dropped. so if both power
blobs die, you lose monitoring, but not connectivity.
there are also 1000-TX, 1000-SX, DS3, sonet and other versions, plus combos.
i'm fairly sure that this is what law enforcement uses for wiretap warrants.