TCPTRACEROUTE(8) TCPTRACEROUTE(8)
2001 July 31
NAME
tcptraceroute - A traceroute implementation using TCP packets
SYNOPSIS
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 ]
DESCRIPTION
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.
OPTIONS
-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.
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TCPTRACEROUTE(8) TCPTRACEROUTE(8)
2001 July 31
-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.
EXAMPLES
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
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.
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.
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TCPTRACEROUTE(8) TCPTRACEROUTE(8)
2001 July 31
AUTHOR
Michael C. Toren <mct@toren.net>
AVAILABILITY
For updates, please see:
http://michael.toren.net/code/tcptraceroute/
SEE ALSO
traceroute(8), ping(8), nmap(1)
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