[HN Gopher] Understanding Traceroute
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       Understanding Traceroute
        
       Author : stonecharioteer
       Score  : 68 points
       Date   : 2026-04-06 03:40 UTC (2 days ago)
        
 (HTM) web link (tech.stonecharioteer.com)
 (TXT) w3m dump (tech.stonecharioteer.com)
        
       | linsomniac wrote:
       | >Many routers deprioritize or drop ICMP to save CPU.
       | 
       | Not exactly.
       | 
       | Most big routers have ASICs (custom silicon) that can handle the
       | bulk of routing decisions, like an interface card will have a
       | chip that can directly determine where a packet needs to go and
       | forwards it there. These are extremely fast, but limited, and are
       | called "fast path".
       | 
       | Aside: Too many ACLs is a common way that packets fall off the
       | fast path, and is why routers on the public Internet will happily
       | forward along bogon traffic that by it's very nature is just
       | wasting bits on the pipes.
       | 
       | There are some things that the fast path cannot handle, and
       | generating ICMP TTL exceeded messages is one of them. Those go
       | over to the router CPU, which historically has been insanely
       | underpowered. Back when I was doing more routing it was common to
       | have host CPUs in the multi-GHz range with multiple cores, but
       | routers of a similar class would have a 100MHz MIPS CPU.
       | 
       | That's why, as the article goes on to explain, "*"s in the
       | traceroute may not indicate a problem. It's not necessarily a
       | literal deprioritization of ICMP.
       | 
       | If you ever see packet loss in a trace at one step but the steps
       | after it aren't showing it, you can ignore that packet loss, it's
       | likely a CPU limitation on a busy router.
        
       | linsomniac wrote:
       | Funny timing of this, just last night I experimented with AI
       | building an "ip intelligence" tool that includes a fast
       | traceroute function and a bunch of IP information (ASN,
       | organization, geolocation, CIDR info, DNSBL, reverse IP... mtr is
       | my go-to for network traces, but (given root permissions) the
       | trace it'll run in less than a second.
       | 
       | https://github.com/linsomniac/ipq
        
       | runjake wrote:
       | I prefer to use mtr these days.
       | 
       | homepage: https://www.bitwizard.nl/mtr/
       | 
       | excellent article on using mtr:
       | https://www.cloudflare.com/learning/network-layer/what-is-mt...
        
         | linsomniac wrote:
         | mtr is spectacular, highly recommended.
        
       | marviio wrote:
       | Once upon a time (early 90s) I read a very funny story on
       | traceroute featuring parrots, one named Polly, and shotguns. But
       | I can't find it. It might have been in a Usenet group. I would
       | love to find it again.
        
       | esbranson wrote:
       | tracepath on Linux does not need the CAP_NET_RAW capability (or
       | full root capabilities) during network data processing because it
       | relies on the Linux socket error queue behavior. I assume
       | traceroute on OpenBSD opens all required sockets and immediately
       | drops privileges.
        
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       (page generated 2026-04-08 23:00 UTC)