Local Routing Puzzle

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hi everyone

I’m having a bit of a puzzle on one CentOS system, I have four interfaces:
nm-team1 10.5.6.100
em1 192.168.2.17
em2 192.168.2.18
p3p3 172.25.12.222

I do:
ping 10.5.6.17 -I em1
PING 10.5.6.17 (10.5.6.17) from 192.168.2.17 em1: 56(84)
bytes of data.
64 bytes from 10.5.6.17: icmp_seq=1 ttld time=0.367 ms

the same for -I em2, for -I em1 and naturally for -I
nm-team1 – all good. but:
$ ping 10.5.6.17 -I p3p3
PING 10.5.6.17 (10.5.6.17) from 172.25.12.202 p3p3: 56(84)
bytes of data.

and nothing, ping waits and no reply, Ctrl+C

with such a simple setup rules based routing should not be involved, kernel should figure it out, right?

many thanks. L.

3 thoughts on - Local Routing Puzzle

  • you specifically said to send that packet to an interface on the wrong network, of course, its not going to get through, unless there’s an external route from that network to the destination. I’m presuming there’s a router somewhere else between your 192.168.2.0/24 network and
    10.5.6.17, that would enable those ping -I em1/2 commands to work.
    note that the recipient of the ping needs to have a route to get back to the source, too.

  • I forgot to mentioned, just to be clear, these IFs are all one node, the same one hos, its routing table:

    10.5.6.0 0.0.0.0 255.255.255.0 U 350
    0 0 nm-team1
    172.25.12.0 0.0.0.0 255.255.255.0 U 100
    0 0 p3p3
    192.168.2.0 0.0.0.0 255.255.255.0 U 110
    0 0 em2
    192.168.2.0 0.0.0.0 255.255.255.0 U 111
    0 0 em1

    no default gateways, so you can see these are directly connected networks

    $ traceroute -n 10.5.6.17 -i em1
    traceroute to 10.5.6.17 (10.5.6.17), 30 hops max, 60 byte packets
    1 10.5.6.17 0.426 ms !X 0.393 ms !X 0.311 ms !X
    $ traceroute -n 10.5.6.17 -i em2
    traceroute to 10.5.6.17 (10.5.6.17), 30 hops max, 60 byte packets
    1 10.5.6.17 0.382 ms !X 0.326 ms !X 0.274 ms !X
    $ traceroute -n 10.5.6.17 -i nm-team1
    traceroute to 10.5.6.17 (10.5.6.17), 30 hops max, 60 byte packets
    1 10.5.6.17 0.407 ms !X 0.342 ms !X 0.294 ms !X
    $ traceroute -n 10.5.6.17 -i p3p3
    traceroute to 10.5.6.17 (10.5.6.17), 30 hops max, 60 byte packets
    1 * * *
    2 * * *
    3 * * *
    4 * * *

    I was expecting kernel’s network would know best, what to do, especially that: (enp6s0f0 is 10.5.6.17)

    root@10.5.6.17 ]$ ping 172.25.12.222 -I enp6s0f0
    PING 172.25.12.202 (172.25.12.222) from 10.5.6.17 enp6s0f0:
    56(84) bytes of data.
    64 bytes from 172.25.12.222: icmp_seq=1 ttld time=0.396 ms

    there are two switches and vlans, switches routes auto configured, no default gateways on the switches neither, to try to simplify & troubleshoot.

    regards L.

  • I would not be even bothered that much if at all about that source IF being different net and not pinging, but the real problem is that:
    that host in question is meant to be routing between
    172.25.x.x < => 10.5.x.x and it sort of does but only ICMP
    seems to get through. Anything else, any other port seem to be blocked-filtered and I cannot wrap my head around as to why?
    I’m beginning to think that maybe switch’s trunking/tagging is misconfigured somehow, but it should be simple, gee..

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