Section: User Commands (1)
Updated: 2006 March 28
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tcptraceroute - A traceroute implementation using TCP packets
tcptraceroute [-nNFSAE] [ -i
] [ -f
] [ -q
number of queries
] [ -t
] [ -p
] [ -s
is a traceroute implementation using TCP packets.
The more traditional
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
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,
is able to bypass the most common firewall filters.
It is worth noting that
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
is running on to tear down the connection without completing
three-way handshake. This is the same half-open scanning technique
uses when passed the
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.
Perform a reverse DNS lookup for each hop, including RFC1918 addresses.
Set the initial TTL used in the first outgoing packet. The default is 1.
Set the maximum TTL used in outgoing packets. The default is 30.
Use the specified local TCP port in outgoing packets. The default is to
obtain a free port from the kernel using
Unlike with traditional
this number will not increase with each hop.
Set the source address for outgoing packets. See also the -i flag.
Use the specified interface for outgoing packets.
Set the number of probes to be sent to each hop. The default is 3.
Set the timeout, in seconds, to wait for a response for each probe. The
default is 3.
Set the TCP SYN flag in outgoing packets. This is the default, if neither
-S or -A is specified.
Set the TCP ACK flag in outgoing packets. By doing so, it is possible to
trace through stateless firewalls which permit outgoing TCP connections.
Send ECN SYN packets, as described in RFC2481.
Set the IP TOS (type of service) to be used in outgoing packets. The
default is not to set any TOS.
Set the IP "don't fragment" bit in outgoing packets.
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.
Enable debugging, which may or may not be useful.
Enable DNAT detection, and display messages when DNAT transitions are
observed. DNAT detection is based on the fact that some NAT devices,
such as some Linux 2.4 kernels, do not correctly 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
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.
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
Please see the
file included in the
distribution for a few real world examples.
To trace the path to a web server listening for connections on port 80:
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
to send out TCP SYN packets for which it has no chance of seeing a response
Michael C. Toren <email@example.com>
For updates, please see:
- SEE ALSO
linux.jgfs.net manual pages