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I wanted to check other computers on my local network. On running the arp command in the terminal.

What do the "[temp] [reject] [blackhole]" mean?

shiv@Shivs-MacBook-Pro ~ % arp
usage: arp [-n] [-i interface] hostname
       arp [-n] [-i interface] [-l] -a
       arp -d hostname [pub] [ifscope interface]
       arp -d [-i interface] -a
       arp -s hostname ether_addr [temp] [reject] [blackhole] [pub [only]] [ifscope interface]
       arp -S hostname ether_addr [temp] [reject] [blackhole] [pub [only]] [ifscope interface]
       arp -f filename
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  • man arp might help here.
    – nohillside
    Jan 13 at 18:15
  • For checking on other computer, the arp -s commands are irrelevant. I use arp -a.
    – Gilby
    Jan 13 at 21:37
  • @Gilby i always see different number of addresses when i use the arp -a command, is it due to multi casts and broadcasts? also why are arp -s commands irrelevant? what does the temp rejecet blackhold mean here? Jan 14 at 7:35

1 Answer 1

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While not versed in networking, I was able to find this manpage on arp(8) for reference:

-s hostname ether_addr
Create an ARP entry for the host called hostname with the Ethernet address ether_addr. The Ethernet address is given as six hex bytes separated by colons. The entry will be permanent unless the word temp is given in the command. If the word pub is given, the entry will be "published"; i.e., this system will act as an ARP server, responding to requests for hostname even though the host address is not its own. In this case the ether_addr can be given as auto in which case the interfaces on this host will be examined, and if one of them is found to occupy the same subnet, its Ethernet address will be used. If the only keyword is also specified, this will create a "published (proxy only)" entry. This type of entry is created automatically if arp detects that a routing table entry for hostname already exists.

If the reject keyword is specified the entry will be marked so that traffic to the host will be discarded and the sender will be notified the host is unreachable. The blackhole keyword is similar in that traffic is discarded but the sender is not notified. These can be used to block external traffic to a host without using a firewall.

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  • still unsure about what it signifies Jan 13 at 18:51
  • @ShivCheema Do you understand how the ARP protocol and the system ARP table work? If not, you'll need to learn about them before these options make sense. Jan 13 at 21:35

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