TCPTRACEROUTE(1)TCPTRACEROUTE(1)NAMEtcptraceroute - A traceroute implementation using TCP packets
SYNOPSIStcptraceroute [-nNFSAE] [ -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 ]
DESCRIPTIONtcptraceroute 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 pack‐
ets 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 des‐
tination. 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 tcptracer‐
oute 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.
OPTIONS-n Display numeric output, rather than doing a reverse DNS lookup
for each hop. By default, reverse lookups are never attempted
on RFC1918 address space, regardless of the -n flag.
-N Perform a reverse DNS lookup for each hop, including RFC1918
addresses.
-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.
-w Set the timeout, in seconds, to wait for a response for each
probe. The default is 3.
-S Set the TCP SYN flag in outgoing packets. This is the default,
if neither -S or -A is specified.
-A Set the TCP ACK flag in outgoing packets. By doing so, it is
possible to trace through stateless firewalls which permit out‐
going TCP connections.
-E Send ECN SYN packets, as described in RFC2481.
-t Set the IP TOS (type of service) to be used in outgoing packets.
The default is not to set any TOS.
-F Set the IP "don't fragment" bit in outgoing packets.
-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.
--dnat
Enable DNAT detection, and display messages when DNAT transi‐
tions are observed. DNAT detection is based on the fact that
some NAT devices, such as some Linux 2.4 kernels, do not cor‐
rectly rewrite the IP address of the IP packets quoted in ICMP
time-exceeded messages tcptraceroute solicits, revealing the
destination IP address an outbound probe packet was NATed to.
NAT devices which correctly rewrite the IP address quoted by
ICMP messages, such as some Linux 2.6 kernels, will not be
detected. For some target hosts, it may be necessary to use
--dnat in conjunction with --track-port. See the examples.txt
file for examples.
--no-dnat
Enable DNAT detection for the purposes of correctly identifying
ICMP time-exceeded messages that match up with outbound probe
packets, but do not display messages when a DNAT transition is
observed. This is the default behavior.
--no-dnat-strict
Do not perform any DNAT detection whatsoever. No attempt will
be made match up ICMP time-exceeded messages with outbound probe
packets, and when tracerouting through a NAT device which does
not rewrite the IP addresses of the IP packets quoted in ICMP
time-exceeded messages, some hops along the path may appear to
be unresponsive. This option should not be needed in the vast
majority of cases, but may be utilized if it is suspected that
the DNAT detection code is misidentifying ICMP time-exceeded
messages.
EXAMPLES
Please see the examples.txt file included in the tcptraceroute distri‐
bution 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
BUGS
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.
AUTHOR
Michael C. Toren <mct@toren.net>
AVAILABILITY
For updates, please see:
http://michael.toren.net/code/tcptraceroute/
SEE ALSOtraceroute(8), ping(8), nmap(1)
2006 March 28 TCPTRACEROUTE(1)
