Using traceroute

[Trenix25]

It seems traceroute uses ICMP echo requests, but the man page says I can tell it to use ICMP echo requests using the -I option which implies that it can use something else instead if I don't. What will it use without using the -I option? Is there any way to get this to work if networks block ping requests? I'm getting a lot of stars when trying to trace the path to my favorite web sites. It can be handy when my network connection to my ISP goes down though. It makes it possible to find a clue about just where the network has failed.

Signed,

Matthew Campbell

 

[dos2unix]

Without the -I option, traceroute typically uses UDP packets by default. It sends UDP packets to high-numbered ports (starting at 33434) and listens for ICMP “port unreachable” messages from the destination. This is different from the ICMP echo requests used when the -I option is specified.

 

[Trenix25]

Without the -I option, traceroute typically uses UDP packets by default. It sends UDP packets to high-numbered ports (starting at 33434) and listens for ICMP “port unreachable” messages from the destination. This is different from the ICMP echo requests used when the -I option is specified.

Hmmm, this is going to be a problem considering my ISP blocks UDP packets, except those used by NTP.

I can send UDP packets, as far as I know, but I can't receive them. If I send UDP probes will I still get ICMP messages?

Signed,

Matthew Campbell

 

[dos2unix]

If I send UDP probes will I still get ICMP messages?

The short answer is no. UDP is a "connectionless" protocol. Meaning it doesn't ever send replies.
It's "fire and forget". or "best effort". You send packets, but there is no way of knowing if they got through, because there is no
reply or no acknowledgment that the packet was received.

User Datagram Protocol - Wikipedia

en.wikipedia.org

It's usually a little faster, and a little more lightweight because it doesn't have the overhead of acknowledgment replies.
But less reliable, because you never know if the data made it through or not.

 

[Trenix25]

So if I cannot receive UDP packets because of the ISP firewall that blocks them when they are sent to me then I will have to use ICMP every time? I know what a UDP packet is. How does it cause a response like I would otherwise get from an ICMP packet?

Signed,

Matthew Campbell

 

[dos2unix]

Some distro's have something called tcptraceroute. It is very similar to traceroute.

 

[Trenix25]

Some distro's have something called tcptraceroute. It is very similar to traceroute.

I have that. I'll give that a try. Thanks.

I recently tried to ping a remote host, but another host answered the ping request. The intended remote host was in a foreign country, the the host that answered was in the US. Any idea how that could happen?

Signed,

Matthew Campbell

 

[KGIII]

Any idea how that could happen?

CDN perhaps?

 

[Trenix25]

CDN perhaps?

It's a university in New Mexico that is responding to a ping request sent to a website in the Middle East.

Signed,

Matthew Campbell