6 ms·

> Aside: asking how traceroute works is one of my interview questions, most people don't know... Curious what type of roles you interview for, are they network

> Aside: asking how traceroute works is one of my interview questions, most people don't know... Curious what type of roles you interview for, are they networking-centric? Iirc this is CCNA-level material, I'd expect anyone working in networking to be able to describe how traceroute works. I've used it more as a smoke test question than a question that most people don't know.

js2 3y ago
Unix sys admin, build & release, developer tooling, operations. We use the Internet every day. I like folks to have an idea how it works, to be intellectually curious, and to be generally informed about the technology they use. The traceroute man page explains how it works. FWIW, my CS degree included a networking class.
- sterlind 3y ago
  fwiw, networking wasn't a required class at my school. neither was databases, weirdly, nor anything web-adjacent (no JS, no HTML or CSS.) ...huh. I'm realizing my CS department was pretty weak. but tbf I knew how traceroute worked by high school, so it didn't matter.
Hikikomori 3y ago
Describing how it works and using it to troubleshoot real problems and drawing the correct conclusions are very different things. If you believe it is simple then you have a shallow understanding of traceroute, which is why it's a good interview question as you can go really deep on your answer. Even seasoned network engineers get it wrong. https://youtu.be/WL0ZTcfSvB4?si=dtihJG1pGJwiyj1u https://youtu.be/WL0ZTcfSvB4?si=dtihJG1pGJwiyj1u
- bwann 3y ago
  +1 for this, I always recommend ras' tutorials at NANOG. He goes into a lot of detail people just hand wave over or don't even consider
  - qohen 3y ago
    Hadn't come across NANOG before -- thanks for mentioning them. Here's a link to their videos: https://www.youtube.com/playlist?list=PLO8DR5ZGla8iVN2v3UKkRH6MkOzXobdvX https://www.youtube.com/playlist?list=PLO8DR5ZGla8iVN2v3UKkR... This is the ras' "Troubleshooting with Traceroute" tutorial: https://www.youtube.com/watch?v=WL0ZTcfSvB4&list=PLO8DR5ZGla8iVN2v3UKkRH6MkOzXobdvX&index=12 https://www.youtube.com/watch?v=WL0ZTcfSvB4&list=PLO8DR5ZGla... Slides: https://archive.nanog.org/meetings/nanog47/presentations/Sunday/RAS_Traceroute_N47_Sun.pdf https://archive.nanog.org/meetings/nanog47/presentations/Sun... (There are one or two other traceroute tutorials, not sure how different they are from the above, e.g. https://www.youtube.com/watch?v=4dUqVlZ6trU&list=PLO8DR5ZGla8iVN2v3UKkRH6MkOzXobdvX&index=81 https://www.youtube.com/watch?v=4dUqVlZ6trU&list=PLO8DR5ZGla... ).
- 0x0000000 3y ago
  Your linked 1+ hour video includes things like IATA airport codes for geolocation and such. None of that is relevant to the fact that all traceroute implementations make use of very basic networking fundamentals: in particular, by sending out a series of packets with artificially limited TTL, to [hopefully] receive ICMP type 11 code 0 "TTL expired" packets from the hops along the path to your target. Traceroute is simple. Sure, there's lots you can do to enrich the data you receive (e.g, reverse DNS and geolocation), or sending multiple sequences to identify equal cost multipath. But these are not inherent or necessary to perform a traceroute. And understanding why different protocols exhibit different behavior / observe different metrics, or why some nodes don't send ICMP TTL expired, is important. But that's more in line with what you call "using it to troubleshoot", which is not "how it works." But "how traceroute works" is simple: First you send a packet with TTL=1, then you send a packet with TTL=2, and so on. That's it, that's how it works.
  - js2 3y ago
    The typical candidate I might ask doesn't know that IP packets include a TTL field. So we start simpler: "What is a packet?" "What is a router?" "How does a packet get to the destination?" "What if there's a routing loop?" Some candidates throw up their hands immediately, which makes it a short interview. Some candidates already know, which makes it a useless question and we move on to other things. For everyone else, I think it's a good interview question.
    - 0x0000000 3y ago
      Yup I saw your other reply and totally makes sense -- networking related or adjacent, but not quite networking-proper type roles. Thanks for your replies.
    - hruzgar 3y ago
      I asked chatgpt to give me answers to these questions as I'm also in the process of learning these things. What do you think of these answers? Would chatgpt pass the interview? 1. *"What is a packet?"* - A packet is a data unit sent over a network, encapsulated within protocols like IP and TCP/UDP. It contains both the payload (actual data) and control information such as source and destination IP addresses. Packets enable efficient routing and reassembly at the destination in network communication. 2. *"What is a router?"* - A router is a device that forwards data packets between networks, operating at the network layer. It uses IP addresses, routing tables, and algorithms to determine the best path for packet forwarding, connecting different network types and managing traffic between internal and external networks. 3. *"How does a packet get to the destination?"* - A packet reaches its destination through routing. It passes through routers that consult their routing tables to forward the packet. The packet traverses multiple networks, with IP protocols guiding it towards the destination, ensuring error checking and congestion handling. 4. *"What if there's a routing loop?"* - In a routing loop, a packet is passed continuously between routers. The Time-To-Live (TTL) field in IP packets prevents endless loops by decrementing each time a packet passes a router. If TTL hits zero before the destination is reached, the packet is discarded to prevent network clogging. Routing protocols also have mechanisms to detect and prevent loops.

js2 3y ago

Unix sys admin, build & release, developer tooling, operations. We use the Internet every day. I like folks to have an idea how it works, to be intellectually curious, and to be generally informed about the technology they use. The traceroute man page explains how it works. FWIW, my CS degree included a networking class.

sterlind 3y ago
fwiw, networking wasn't a required class at my school. neither was databases, weirdly, nor anything web-adjacent (no JS, no HTML or CSS.) ...huh. I'm realizing my CS department was pretty weak. but tbf I knew how traceroute worked by high school, so it didn't matter.

Hikikomori 3y ago

Describing how it works and using it to troubleshoot real problems and drawing the correct conclusions are very different things. If you believe it is simple then you have a shallow understanding of traceroute, which is why it's a good interview question as you can go really deep on your answer. Even seasoned network engineers get it wrong. https://youtu.be/WL0ZTcfSvB4?si=dtihJG1pGJwiyj1u https://youtu.be/WL0ZTcfSvB4?si=dtihJG1pGJwiyj1u

bwann 3y ago
+1 for this, I always recommend ras' tutorials at NANOG. He goes into a lot of detail people just hand wave over or don't even consider
- qohen 3y ago
  Hadn't come across NANOG before -- thanks for mentioning them. Here's a link to their videos: https://www.youtube.com/playlist?list=PLO8DR5ZGla8iVN2v3UKkRH6MkOzXobdvX https://www.youtube.com/playlist?list=PLO8DR5ZGla8iVN2v3UKkR... This is the ras' "Troubleshooting with Traceroute" tutorial: https://www.youtube.com/watch?v=WL0ZTcfSvB4&list=PLO8DR5ZGla8iVN2v3UKkRH6MkOzXobdvX&index=12 https://www.youtube.com/watch?v=WL0ZTcfSvB4&list=PLO8DR5ZGla... Slides: https://archive.nanog.org/meetings/nanog47/presentations/Sunday/RAS_Traceroute_N47_Sun.pdf https://archive.nanog.org/meetings/nanog47/presentations/Sun... (There are one or two other traceroute tutorials, not sure how different they are from the above, e.g. https://www.youtube.com/watch?v=4dUqVlZ6trU&list=PLO8DR5ZGla8iVN2v3UKkRH6MkOzXobdvX&index=81 https://www.youtube.com/watch?v=4dUqVlZ6trU&list=PLO8DR5ZGla... ).
0x0000000 3y ago
Your linked 1+ hour video includes things like IATA airport codes for geolocation and such. None of that is relevant to the fact that all traceroute implementations make use of very basic networking fundamentals: in particular, by sending out a series of packets with artificially limited TTL, to [hopefully] receive ICMP type 11 code 0 "TTL expired" packets from the hops along the path to your target. Traceroute is simple. Sure, there's lots you can do to enrich the data you receive (e.g, reverse DNS and geolocation), or sending multiple sequences to identify equal cost multipath. But these are not inherent or necessary to perform a traceroute. And understanding why different protocols exhibit different behavior / observe different metrics, or why some nodes don't send ICMP TTL expired, is important. But that's more in line with what you call "using it to troubleshoot", which is not "how it works." But "how traceroute works" is simple: First you send a packet with TTL=1, then you send a packet with TTL=2, and so on. That's it, that's how it works.
- js2 3y ago
  The typical candidate I might ask doesn't know that IP packets include a TTL field. So we start simpler: "What is a packet?" "What is a router?" "How does a packet get to the destination?" "What if there's a routing loop?" Some candidates throw up their hands immediately, which makes it a short interview. Some candidates already know, which makes it a useless question and we move on to other things. For everyone else, I think it's a good interview question.
  - 0x0000000 3y ago
    Yup I saw your other reply and totally makes sense -- networking related or adjacent, but not quite networking-proper type roles. Thanks for your replies.
  - hruzgar 3y ago
    I asked chatgpt to give me answers to these questions as I'm also in the process of learning these things. What do you think of these answers? Would chatgpt pass the interview? 1. *"What is a packet?"* - A packet is a data unit sent over a network, encapsulated within protocols like IP and TCP/UDP. It contains both the payload (actual data) and control information such as source and destination IP addresses. Packets enable efficient routing and reassembly at the destination in network communication. 2. *"What is a router?"* - A router is a device that forwards data packets between networks, operating at the network layer. It uses IP addresses, routing tables, and algorithms to determine the best path for packet forwarding, connecting different network types and managing traffic between internal and external networks. 3. *"How does a packet get to the destination?"* - A packet reaches its destination through routing. It passes through routers that consult their routing tables to forward the packet. The packet traverses multiple networks, with IP protocols guiding it towards the destination, ensuring error checking and congestion handling. 4. *"What if there's a routing loop?"* - In a routing loop, a packet is passed continuously between routers. The Time-To-Live (TTL) field in IP packets prevents endless loops by decrementing each time a packet passes a router. If TTL hits zero before the destination is reached, the packet is discarded to prevent network clogging. Routing protocols also have mechanisms to detect and prevent loops.