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Monday, January 27, 2025

get router to answer ARP requests from swap so i can handle it


I’ve an internet managed swap on my community. It’s related to a router, together with the host PC i’m on. I can’t ping it by means of the router.

Router is Fedora server.

the router’s relevant interfaces are:

enp3s0 wan interface
enp4s0 with static IP 192.168.2.2/24
enp6s0 with static IP 10.2.4.1/24

routing desk on the router has relevant entries:

Vacation spot     Gateway         Genmask         Flags Metric Ref    Use Iface
0.0.0.0         192.168.2.2     0.0.0.0         UG    100    0        0 enp4s0
0.0.0.0         10.2.4.1        0.0.0.0         UG    103    0        0 enp6s0
10.2.4.0        0.0.0.0         255.255.255.0   U     103    0        0 enp6s0
192.168.2.0     0.0.0.0         255.255.255.0   U     100    0        0 enp4s0

the hosts are straight hooked up to those interfaces with these ips:

          IP                    MAC                  hooked up if
swap    192.168.2.1 (static)  60:be:b4:13:28:e1    enp4s0
laptop        10.2.4.5    (dhcp)    1c:2a:a3:1e:74:df    enp6s0

when i ping from the host laptop, i get timeouts. so i ran tcpdump from the router on enp4s0

sudo tcpdump -i enp4s0 -n

16:18:06.345052 IP 10.2.4.5 > 192.168.2.1: ICMP echo request, id 48, seq 430, size 64
16:18:07.334961 ARP, Request who-has 192.168.2.2 inform 192.168.2.2, size 28
16:18:07.369062 IP 10.2.4.5 > 192.168.2.1: ICMP echo request, id 48, seq 431, size 64
16:18:08.361151 ARP, Request who-has 192.168.2.2 inform 192.168.2.2, size 28
16:18:08.393080 IP 10.2.4.5 > 192.168.2.1: ICMP echo request, id 48, seq 432, size 64
16:18:09.385150 ARP, Request who-has 192.168.2.2 inform 192.168.2.2, size 28
16:18:09.417072 IP 10.2.4.5 > 192.168.2.1: ICMP echo request, id 48, seq 433, size 64
[ ... ]
16:18:17.609124 IP 10.2.4.5 > 192.168.2.1: ICMP echo request, id 48, seq 441, size 64
16:18:18.601152 ARP, Request who-has 192.168.2.1 inform 192.168.2.2, size 28
16:18:18.601366 ARP, Reply 192.168.2.1 is-at 1c:2a:a3:1e:74:df, size 46
16:18:18.633088 IP 10.2.4.5 > 192.168.2.1: ICMP echo request, id 48, seq 442, size 64

discernible details:

  • the packets journey the router’s nftables ahead chain from enp6s0 to enp4s0
  • the swap then asks who has 192.168.2.2, the gateway. it will get no response.
  • the router later asks who has 192.168.2.1. the swap responds with its mac.
  • the router’s arp desk information it.
  • the swap nonetheless doesn’t know who has 192.168.2.2

routers relevant arp entries

Tackle                  HWtype  HWaddress           Flags Masks            Iface
192.168.2.1              ether   1c:2a:a3:1e:74:df   C                     enp4s0
10.2.4.5                 ether   04:7c:16:4d:0a:84   C                     enp6s0

the swap isn’t responding to pings, and moreover, to my net requests to handle it. when i ship an internet request from the browser, i instantly get a bunch extra “Request who-has 192.168.2.2”, which reveals that the http request reached the swap and is aware of to answer to that IP, however it nonetheless does not know who has that IP. it retains asking.

right here is the host laptop relevant arp desk entries, displaying the router responded to the PC’s arp request, so why not then to the swap?

Tackle                  HWtype  HWaddress           Flags Masks            Iface
10.2.4.1                 ether   60:be:b4:13:28:e3   C                     enp12s0

what would trigger the router to not reply to the swap’s arp request so i can ultimately ping and handle it.

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