TRACERT – Windows CMD Command

Trace Route – Find the IP address of any remote host. TRACERT is useful fortroubleshooting large networks where several paths can be taken to arrive at the same point,or where many intermideate systems (routers and bridges)are involved.

The functionality of TRACERT is the same under all versions of windows but the output is cosmetically improved under XP. Tracert uses the IP TTL field and ICMP error messages to determine the route from one host to another through a network. Care must be taken with tracert as it shows the optimal route, not necessarily the actual route.

To be accurate, it is possible to ping from a UNIX machine back to the PC using the -R option to record the route taken – but only if the particular network devices support it. This diagnostic tool determines the path taken to a destination by sending ICMP Echo Request messages with varying Time to Live (TTL) values to the destination.

TTL (Time to Live) calculation

TTL is effectively a count of the (maximum) number of links to the destination host. Each router along the path decrements the TTL in an IP packet by at least 1 before forwarding it.

When the TTL on a packet reaches 0, the router is expected to return an ICMP Time Exceeded message to the source computer.

Tracert determines the path by sending the first Echo Request message with a TTL of 1 and incrementing the TTL by 1 on each subsequent transmission until either the target host responds or the maximum number of hops is reached.

This process relys on intermediate routers to return ICMP Time Exceeded messages. However, some routers do not return Time Exceeded messages for packets with expired TTL values and are invisible to the tracert command.

In this case, a row of asterisks (*) is displayed for that hop. Firewalls Many firewalls will block ICMP traffic by default. If an attacker is able to forge ICMP redirect packets, he or she can alter the routing tables on the host and possibly subvert the security of the host by causing traffic to flow via a path you didn’t intend.


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2 Responses

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