Troubleshooting Commands : tcptraceroute

tcptraceroute
Description
Use the tcptraceroute command to record route information in environments where traditional ICMP traceroute is defeated by firewalls or other filters.
Syntax
tcptraceroute [-nNFSAE] [-i <interface>] [-f <first ttl>] [-l <packet length>][-q <number of queries>] [-t <tos>][-m <max ttl>] [-pP] <source port>] [-s <source address>][-w <wait time>] <host> [destination port] [packet length]
Arguments
 
Specifies the type of tcptraceroute. Select from the following 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. Unlike with traditional traceroute, 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 outgoing 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 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 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.
host <destination port><length>
Defaults
The probe packet length is 40.
Command Mode
User EXEC Mode
Privileged EXEC Mode
Global Configuration Mode
See Also
See the following related commands:
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Usage Guidelines
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tcptraceroute is a traceroute implementation using TCP packets.
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The problem is that with the widespread use of firewalls on the modern Internet, many of the packets that traceroute sends out end up being filtered, making it impossible to completely trace the path to the destination.
However, in many cases, if hosts sitting behind the firewall are listening for connections on specific ports, then these firewalls will permit inbound TCP packets to those ports.
By sending out TCP SYN packets instead of UDP or ICMP ECHO packets, tcptraceroute is able to bypass the most common firewall filters.
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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 uses when passed the -sS flag.
Examples
None.
 

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