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Currently, I must sudo tcpflow -i lo as root user, I want to grant the lo interface and TCP port range 3000-3999 of all interfaces to user1, how to do that?

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1 Answer 1

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You need to be root to do the kind of snooping that tcpflow and tcpdump do. So you either need sudo or a setuid root wrapper.

Well, technically all you need the CAP_NET_RAW capability. If you install the libcap2-bin package¹, you can use the setcap command to create a “setcap-CAP_NET_RAW” wrapper around tcpflow, rather than a setuid wrapper. But that only helps make a security hole in the wrapper be a little less damaging (a user who obtains CAP_NET_RAW access can probably gain root with little effort).

You can use sudo or a setuid root wrapper to let user1 run a command like tcpflow -i eth0 tcp and \( src host and src port 3000 or dst host and src port 3000 \). Note that you can't just let user1 run an arbitrary tcpflow command: that would allow user1 to run things like tcpflow -r … -r /path/to/foo and read (at least partially) /path/to/foo with root permissions.

I don't think you'll be decently able to allow the user to specify an expression to match packets. Rather than give extra permissions to run tcpflow, I suggest giving extra permissions to run a fixed tcpdump command (writing to stdout, let the user redirect into a file if necessary). Then the user can run tcpflow -r to analyze tcpdump output (even in real time, through a pipe).

Now for lo, allow user1 to run this exact command:

tcpdump -i lo tcp

For other interfaces, I think the following command selects tcp ports 3000–3999 (but make extensive tests to be sure):

tcpdump -i eth0 tcp and \( \
  \( src host and 'tcp[0:2] >= 3000' and 'tcp[0:2] <= 3999' \) or
  \( dst host and 'tcp[2:2] >= 3000' and 'tcp[2:2] <= 3999' \) \)

Replace by the IP address associated with the interface. I don't have a good solution to offer if the IP address isn't fixed: if you read it from ifconfig output or otherwise at the beginning of the wrapper script, it's open to a race condition.

My recommendation for a wrapper is not to use a shell script (I don't know if any of the shells in Debian is suitable for writing setuid scripts — shells tend to use a lot of environment variables), but rather Perl (where this kind of thing is explicitly supported), like this:

#!/usr/bin/perl -T
exec '/usr/sbin/tmpdump', '-i', 'lo', 'tcp'

Make the script an ordinary executable (mode 755), and allow user1 to run it through sudo.

¹ For other Linux distributions: you need kernel 2.6.24 or above and the setcap command.

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Very detailed explanation! –  Xiè Jìléi Nov 11 '10 at 1:32

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