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A routing puzzle for you
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| Question: is there ever a reason to ARP for an IP outside of any local subnets? | |
| Update: I like @bmastenbrook's answer: https://twitter.com/bmastenbrook/status/1069415501296586752 | |
| # my subnet is a /24, and 10.0.10.10 is totally not on the same /24 as 10.0.0.42. | |
| xomg% netstat -rn -f inet | |
| Routing tables | |
| Internet: | |
| Destination Gateway Flags Refs Use Netif Expire | |
| default 10.0.10.10 UGScI 3 0 en0 | |
| 10/24 link#8 UCS 0 0 en0 ! | |
| 10.0.0.42/32 link#8 UCS 0 0 en0 ! | |
| 10.0.10.10/32 link#8 UCS 1 0 en0 ! | |
| 10.0.10.10 link#8 UHLWIir 1 0 en0 ! | |
| 127 127.0.0.1 UCS 0 0 lo0 | |
| 127.0.0.1 127.0.0.1 UH 1 10797 lo0 | |
| 169.254 link#8 UCS 0 0 en0 ! | |
| 224.0.0/4 link#8 UmCS 1 0 en0 ! | |
| 224.0.0.251 1:0:5e:0:0:fb UHmLWI 0 0 en0 | |
| 255.255.255.255/32 link#8 UCS 0 0 en0 ! | |
| # A view from SystemConfiguration: | |
| xomg% networksetup -getinfo "Wi-Fi" | |
| Manual Configuration | |
| IP address: 10.0.0.42 | |
| Subnet mask: 255.255.255.0 | |
| Router: 10.0.10.10 | |
| IPv6: Off | |
| Wi-Fi ID: 8c:85:90:8d:48:11 | |
| # no funny business | |
| xomg% networksetup -getadditionalroutes "Wi-Fi" | |
| There are no additional IPv4 routes on Wi-Fi. | |
| # and yet, here is macOS ARPing for 10.0.10.10 (which happens when I attempt to ping a non-local IP): | |
| xomg% sudo tcpdump -i all -e -q -n arp | |
| tcpdump: data link type PKTAP | |
| tcpdump: verbose output suppressed, use -v or -vv for full protocol decode | |
| listening on all, link-type PKTAP (Apple DLT_PKTAP), capture size 262144 bytes | |
| 16:20:23.073750 8c:85:90:8d:48:11 > ff:ff:ff:ff:ff:ff, ARP, length 42: Request who-has 10.0.10.10 tell 10.0.0.42, length 28 | |
| 16:20:24.074042 8c:85:90:8d:48:11 > ff:ff:ff:ff:ff:ff, ARP, length 42: Request who-has 10.0.10.10 tell 10.0.0.42, length 28 | |
| ^C | |
| 2 packets captured | |
| 12 packets received by filter | |
| 0 packets dropped by kernel |
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