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Editorial Team
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Asked: 2026-06-14T09:42:29+00:00

As per arp(7) – Linux man page : base_reachable_time (since Linux 2.2) Once a

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As per arp(7) – Linux man page :

base_reachable_time (since Linux 2.2)
Once a neighbor has been found, the entry is considered to be valid for at least a random value between base_reachable_time/2 and
3*base_reachable_time/2. An entry’s validity will be extended if it
receives positive feedback from higher level protocols. Defaults to 30
seconds. This file is now obsolete in favor of base_reachable_time_ms.
base_reachable_time_ms (since Linux 2.6.12)
As for base_reachable_time, but measures time in milliseconds. Defaults to 30000 milliseconds.

I did not understand this explanation – especially the statement about the positive feed back from higher level protocols. Somebody please clarify ?

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1 Answer

  1. Editorial Team

    From the arp(7) man page,

    When there is no positive feedback for an existing mapping after some
    time (see the /proc interfaces below), a neighbor cache entry is
    considered stale. Positive feedback can be gotten from a higher layer;
    for example from a successful TCP ACK. Other protocols can signal
    forward progress using the MSG_CONFIRM flag to sendmsg(2).     When there
    is no forward progress, ARP tries to reprobe.

    Basically this means that if something like a continues TCP connection is happening with a lot of successful ACKs, then it assumes the IP/MAC pair it has is valid and doesn’t bother doing a new ARP request when the entry in the table would normally expire.

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