Security risk associated with a NIC's promiscuous mode?

Chuck Swiger cswiger at
Fri Oct 7 08:03:12 PDT 2005

John Conover wrote:
[ ... ]
>> A mild one.  For example, I believe there was recently a security bug in 
>> tcpdump's string handling which could be exploited by tcpdump seeing a 
>> maliciously-crafted packet.  Running the NIC in promisc mode means that packet 
>> just has to go by, rather than being sent specificly to the machine running the 
>> sniffer...
>> In other words, it's not a great idea to run a sniffer on your most important 
>> fileserver or whatever, rather than an isolated laptop or other test system.
> Thanks, Chuck; That's the kind of stuff I was concerned about.

Sure, you're welcome.  The issue is much like building out (or writing code 
for) a firewall or router or IDS.

Anything which gets exposed to untrusted traffic ought to be considered "at 
risk", and the software ought to be written with extreme care to not trust the 
data-- don't trust an IP packet to really tell you what size it is (you may not 
have sniffed all of that data, depending on how you configured PCAP or BPF), 
don't trust Content-length headers in email or HTTP traffic to be valid without 
double-checking (trying to memcpy -1 bytes makes programs unhappy [1]), don't 
trust headers to be of reasonable size, etc.

If at all possible, anything running a sniffer ought to be dedicated for the 
purpose, or only used for brief periods by a human sysadmin (ie, not running 
forever under the incurious gaze of a daemon).  If possible and convenient, the 
rest of your network should not trust or depend on your sniffer box at all.


[1]: On the other hand, being able to drop -1 gold pieces in Hack could be 
considered a feature, at least to most people.  :-)

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