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tcptraceroute - A traceroute implementation using TCP packets
tcptraceroute
[-nFE] [ -i interface ] [ -f first ttl ]
[ -l length ] [ -q number of queries ] [ -t tos ]
[ -m max ttl ] [ -p source port ] [ -s source address ]
[ -w wait time ] host [ destination port ] [ length ]
tcptraceroute
is a traceroute implementation using TCP packets.
The more traditional traceroute(8)
sends out either UDP or ICMP ECHO packets with a TTL of one, and increments
the TTL until the destination has been reached. By printing the gateways
that generate ICMP time exceeded messages along the way, it is able to
determine the path packets are taking to reach the destination.
The problem
is that with the widespread use of firewalls on the modern Internet, many
of the packets that traceroute(8) sends out end up being filtered, making
it impossible to completely trace the path to the destination. However,
in many cases, these firewalls will permit inbound TCP packets to specific
ports that hosts sitting behind the firewall are listening for connections
on. By sending out TCP SYN packets instead of UDP or ICMP ECHO packets,
tcptraceroute is able to bypass the most common firewall filters.
It is
worth noting that tcptraceroute never completely establishes a TCP connection
with the destination host. If the host is not listening for incoming connections,
it will respond with an RST indicating that the port is closed. If the
host instead responds with a SYN|ACK, the port is known to be open, and
an RST is sent by the kernel tcptraceroute is running on to tear down the
connection without completing three-way handshake. This is the same half-open
scanning technique that nmap(1) uses when passed the -sS flag.
- -n
- Display
numeric output, rather than doing a reverse DNS lookup for each hop. Reverse
lookups are never attempted on RFC1918 address space, regardless of the
-n flag.
- -f
- Set the initial TTL used in the first outgoing packet. The default
is 1.
- -m
- Set the maximum TTL used in outgoing packets. The default is 30.
- -p
- Use
the specified local TCP port in outgoing packets. The default is to obtain
a free port from the kernel using bind(2). Unlike with traditional traceroute(8),
this number will not increase with each hop.
- -s
- Set the source address for
outgoing packets. See also the -i flag.
- -i
- Use the specified interface for
outgoing packets.
- -q
- Set the number of probes to be sent to each hop. The
default is 3.
- -t
- Set the IP type of service to be used in outgoing packets.
The default is to not set any type of service option.
- -F
- Set the "don't fragment"
bit in outgoing packets.
- -E
- Send ECN SYN packets, as described in RFC2481.
- -w
- Set the timeout, in seconds, to wait for a response for each probe. The
default is 3.
- -l
- Set the total packet length to be used in outgoing packets.
If the length is greater than the minimum size required to assemble the
necessary probe packet headers, this value is automatically increased.
- -d
- Enable
debugging, which may or may not be useful.
Please see the examples.txt
file included in the tcptraceroute distribution for a few real world examples.
To trace the path to a web server listening for connections on port 80:
tcptraceroute webserver
To trace the path to a mail server listening for
connections on port 25:
tcptraceroute mailserver 25
No error checking
is performed on the source address specified by the -s flag, and it is therefore
possible for tcptraceroute to send out TCP SYN packets for which it has
no chance of seeing a response to.
Complete portability to other Unix systems
has not been tested; specifically, tcptraceroute will not function on
systems which modify the IP ID field of packets written to a raw socket.
As of the time of this writing, tcptraceroute is known to compile and
function properly on Linux, OpenBSD, and FreeBSD systems. If you run into
complications on another platform, please let me know.
Michael C. Toren
<mct@toren.net>
For updates, please see:
http://michael.toren.net/code/tcptraceroute/
traceroute(8), ping(8),
nmap(1)
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