This week, I received a request to search for a range of destination addresses that cannot easily done using libpcap conventional macro filters but can be done using an IP protocol filter.  It is quite easy to filter for a CIDR range (i.e. /23, /24) with a libpcap macro filter but when it comes to search for an unusual list of addresses such as 192.168.25.6 to 192.168.25.35, there is no simple macro to easily do it.

The following example illustrates how to find SYN packets directed to natted addresses where an attempt was made to connect or scan a service natted to an internal resource. I used this filter for addresses located in the range 192.168.25.6 to 192.168.25.35.


tcpdump -nr filename '((ip[16:2] = 0xc0a8  and ip[18] = 0x19 and ip[19] > 0x06) and (ip[16:2] = 0xc0a8 and ip[18] = 0x19 and ip[19] < 0x23) and tcp[13] = 0x02)'

The filter breaks down as follow:

Starting IP Range

ip[16:2] = 0xc0a8 -> First two octets of the IP address is 192.168
ip[18] = 0x19       -> Third octet of the IP address is 25
ip[19] > 0x06       -> Last octet of the IP address is greater than 6

Less than IP Range

ip[16:2] = 0xc0a8 -> First two octets of the IP address is 192.168
ip[18] = 0x19       -> Third octet of the IP address is 25
ip[19] < 0x23       -> Last octet of the IP address is less than 35

Last Part of the Filter

tcp[13] = 0x02     -> If there is a successful match, only print those with SYN packets


This filter yielded for example the following results which in this case are inbound scans:

21:38:28.522588 IP xxx.x.221.157.63662 > 192.168.25.7.443: S 1279429613:1279429613(0) win 65535 <mss 1412,nop,nop,sackOK,nop,wscale 1,nop,nop,timestamp 353141033 0>
04:24:34.438872 IP xxx.xxx.253.36.55395 > 192.168.25.7.443: S 3909380827:3909380827(0) win 65535 <mss 1412,nop,nop,sackOK,nop,wscale 1,nop,nop,timestamp 3199762289 0>
04:24:49.700805 IP xx.xxx.160.65.54640 > 192.168.25.7.443: S 520695074:520695074(0) win 65535 <mss 1412,nop,nop,sackOK,nop,wscale 1,nop,nop,timestamp 2595387575 0>
05:20:18.618965 IP xxx.xxx.124.41.49734 > 192.168.25.7.443: S 3595559815:3595559815(0) win 65535 <mss 1412,nop,nop,sackOK,nop,wscale 1,nop,nop,timestamp 1555543940 0>

This same filter could easily be expended to include search for a specific port instead of any ports to further narrow the search.

Update 1:

ISC reader Crist provided  a simpler way to write the above filter by combining all the octets of the source and destination IP address like this: (ip[16:4] > 0xc0a81906) && (ip[16:4] < 0xc0a81923).

This filter is also equivalent where && is equal to and:  (ip[16:4] > 0xc0a81906) and (ip[16:4] < 0xc0a81923)

The complete filter would be like this:

tcpdump -nr filename '((ip[16:4] > 0xc0a81906) and (ip[16:4] < 0xc0a81923) and tcp[13] = 0x02)'

Handler on Duty

Guy

Guy Bruneau IPSS Inc. gbruneau at isc dot sans dot org

Teaching Comprehensive Packet Analysis in Ottawa, ON this coming September