ICMP, Ping, and Traceroute – What I wish I was taught (2020) • Hacker News Board

Re: ICMP, Ping, and Traceroute – What I wish I was taught (2020)

#11

Post by atoav » Thu, Jul 28, 2022, 7:52 AM UTC

Another thing that can be useful is to use tcpdump on the other side to see if the stuff sent to e.g. a TCP port is actually received:

  tcpdump -i any src port 5432 or dst port 5432

If it is now received you know you need to investigate on your receiving side, if it is not, the problem might be firewall or network.

Re: ICMP, Ping, and Traceroute – What I wish I was taught (2020)

#12

Post by Aachen » Thu, Jul 28, 2022, 7:54 AM UTC

tedd4u wrote: post #8

Tip for tracerouters ... use mtr instead brew install mtr Try tapping 'd' twice to enable a stripchart ? for help My traceroute [v0.95] xxxxxxxx.local (172.16.0.2) -> news.ycombinator.com (50.1122022-07-27T23:37 Keys: Help Display mode Restart statistics Order of fields quit Packets Pings Host Loss% Snt Last Avg Best Wrst StDev 1. 172.16.0.1 0.0% 14 15.4 17.6 14.1 20.5 2.2 2. (waiting for reply) 3. 162.158.164.5 14.3…

Yeah I tried mtr again recently. After maybe 30 seconds of working fine, it'll exit with an error like "no route to host" (even though it was getting responses before). Yeah, packet loss is exactly why I'm opening mtr: to see where on the path it occurs... how in the world does that make it crash

Re: ICMP, Ping, and Traceroute – What I wish I was taught (2020)

#13

Post by Aachen » Thu, Jul 28, 2022, 7:58 AM UTC

I'm interested in the topic so skimmed the whole thing but it's all just basics. Most people won't know them all, but so FYI: skip the lengthy article if you know how traceroute and ARP and ICMP work in general, what the difference is between connection times out and connection refused, and why sometimes traceroute returns more than one host for a given hop.

Re: ICMP, Ping, and Traceroute – What I wish I was taught (2020)

#14

Post by owlbynight » Thu, Jul 28, 2022, 8:05 AM UTC

ianai wrote: post #5

Telnet to the port sometime establishes connectivity too.

In lieu of telnet, you can use curl as well:

curl -v telnet://example.com:8000

Re: ICMP, Ping, and Traceroute – What I wish I was taught (2020)

#15

Post by layer8 » Thu, Jul 28, 2022, 8:46 AM UTC

tedd4u wrote: post #8

Tip for tracerouters ... use mtr instead brew install mtr Try tapping 'd' twice to enable a stripchart ? for help My traceroute [v0.95] xxxxxxxx.local (172.16.0.2) -> news.ycombinator.com (50.1122022-07-27T23:37 Keys: Help Display mode Restart statistics Order of fields quit Packets Pings Host Loss% Snt Last Avg Best Wrst StDev 1. 172.16.0.1 0.0% 14 15.4 17.6 14.1 20.5 2.2 2. (waiting for reply) 3. 162.158.164.5 14.3…

There’s also WinMTR for Windows: https://github.com/White-Tiger/WinMTR

Re: ICMP, Ping, and Traceroute – What I wish I was taught (2020)

#16

Post by oakwhiz » Thu, Jul 28, 2022, 10:02 AM UTC

Aachen wrote: post #12

tedd4u wrote: post #8

Tip for tracerouters ... use mtr instead brew install mtr Try tapping 'd' twice to enable a stripchart ? for help My traceroute [v0.95] xxxxxxxx.local (172.16.0.2) -> news.ycombinator.com (50.1122022-07-27T23:37 Keys: Help Display mode Restart statistics Order of fields quit Packets Pings Host Loss% Snt Last Avg Best Wrst StDev 1. 172.16.0.1 0.0% 14 15.4 17.6 14.1 20.5 2.2 2. (waiting for reply) 3. 162.158.164.5 14.3…

Yeah I tried mtr again recently. After maybe 30 seconds of working fine, it'll exit with an error like "no route to host" (even though it was getting responses before). Yeah, packet loss is exactly why I'm opening mtr: to see where on the path it occurs... how in the world does that make it crash

I was under the impression that this issue had been fixed years prior.

Re: ICMP, Ping, and Traceroute – What I wish I was taught (2020)

#17

Post by MauranKilom » Thu, Jul 28, 2022, 10:18 AM UTC

So maybe I misunderstood the article, but I would expect that (assuming my router is on 192.168.0.123) that

  ping 192.168.0.123

followed by

  arp -a 192.168.0.123

should tell me the MAC address of the router. But instead I get "No ARP entries found." - what's going on here?

Re: ICMP, Ping, and Traceroute – What I wish I was taught (2020)

#18

Post by Hikikomori » Thu, Jul 28, 2022, 10:19 AM UTC

Aachen wrote: post #13

I'm interested in the topic so skimmed the whole thing but it's all just basics. Most people won't know them all, but so FYI: skip the lengthy article if you know how traceroute and ARP and ICMP work in general, what the difference is between connection times out and connection refused, and why sometimes traceroute returns more than one host for a given hop.

I wouldn't call the details he goes into with traceroute "just basics", its common even for network engineers to come to the wrong conclusion based on traceroute output, or not realising that they need more information to truly know what is going on (traceroute from both sides).

Re: ICMP, Ping, and Traceroute – What I wish I was taught (2020)

#19

Post by manaskarekar » Thu, Jul 28, 2022, 10:29 AM UTC

Another useful tool, fping - https://linux.die.net/man/8/fping

Re: ICMP, Ping, and Traceroute – What I wish I was taught (2020)

#20

Post by voltagex_ » Thu, Jul 28, 2022, 11:22 AM UTC

layer8 wrote: post #15

tedd4u wrote: post #8

Tip for tracerouters ... use mtr instead brew install mtr Try tapping 'd' twice to enable a stripchart ? for help My traceroute [v0.95] xxxxxxxx.local (172.16.0.2) -> news.ycombinator.com (50.1122022-07-27T23:37 Keys: Help Display mode Restart statistics Order of fields quit Packets Pings Host Loss% Snt Last Avg Best Wrst StDev 1. 172.16.0.1 0.0% 14 15.4 17.6 14.1 20.5 2.2 2. (waiting for reply) 3. 162.158.164.5 14.3…

There’s also WinMTR for Windows: https://github.com/White-Tiger/WinMTR

Last updated 9 years ago.