firewalk - Active Reconnaissance Network Security Tool with
Extreme Prejudice
firewalk [-dhinprSsTtvx] target_gateway metric
Firewalk is an active reconnaissance network security tool that
attempts to determine what layer 4 protocols a given IP forwarding device
will pass. Firewalk works by sending out TCP or UDP packets with a TTL one
greater than the targeted gateway. If the gateway allows the traffic, it
will forward the packets to the next hop where they will expire and elicit
an ICMP_TIME_EXCEEDED message. If the gateway hostdoes not allow the
traffic, it will likely drop the packets on the floor and we will see no
response.
To get the correct IP TTL that will result in expired packets one
beyond the gateway we need to ramp up hop-counts. We do this in the same
manner that traceroute works. Once we have the gateway hopcount (at that
point the scan is said to be `bound`) we can begin our scan.
It is significant to note the fact that the ultimate destination
host does not have to be reached. It just needs to be somewhere downstream,
on the other side of the gateway, from the scanning host. Please see
http://www.wiley.com/cda/product/0,,0471205443,00.html for more information
on Firewalking and networking security tools in general.
If an option takes an argument, it proceedes the option letter,
with the default in parenthesis.
- -d 1-65535 (34434)
- Specify the initial destination port to use during the network discovery
(aka TTL ramping) phase.
- -h
- Program help.
- -i interface_name
- Specify interface to use. Only necessary on multi-homed machines.
- -n
- Do not resolve IP addresses into hostnames. This saves a DNS lookup and
speeds the scans (mainly during network discovery).
- -P 1-2000 (0)
- Set a network writing pause which may be necessary to keep the program
from flooding the network.
- -p TCP, UDP (UDP)
- Type of scan to perform.
- -r
- Strict RFC 793 compliance. This only comes into play when doing a TCP scan
when your packets have an expire vector of one and your metric host is one
hop from your gateway. Since the packets will reach their destination,
they will not expire, so we look for terminal responses. For a TCP port in
the listen state, we will get back a SYN|ACK with the ACK as our SEQ + 1.
However, for a closed port, the response is stack dependent. If the host
is RFC compliant we will receive an RST|ACK with the ACK as our SEQ + 1.
However, if the host is not compliant (ie: microsoft) then the best we can
do is inverse tuple matching (which is the default).
- -S 1-65535,...
(1-130,139,1025)
- Specify the ports for the scan. Ports may be specified in ranges,
delimited by dashes, and multiple ranges may be specified, delimited by
commas. Omitting the terminating port number is shorthand for 65535.
- -s 1-65535 (53)
- Specify the source port for the scan (both phases).
- -T 1-2000 (2)
- Network packet reading timeout. This is the time firewalk will spend
waiting for a response before timing out.
- -t 1-25 (1)
- Set the initial IP time to live (TTL) value. If a target gateway is known
to be (at least) n hops from the source host, the TTL can be preloaded to
facilitate a faster scan.
- -v
- Dump program version and exit.
- -x expire vector (1)
- The expire vector is the number of hops that the scanning probes will
expire, past the gateway host. The binding hopcount is the hopcount of the
gateway + the expire vector.
Mike D. Schiffman <mike@infonexus.com>
Please send bug reports to mike@infonexus.com