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Pol Hallen
Asked: 2015-04-22 07:37:27 +0800 CST

Default arp cache timeout

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I didn't found how check and change the default arp cache timeout on Linux. I using debian with kernel 3.x (but I supposed it same with older kernels)

linux

2 Answers

  1. I acknowledge that as I write this, this is a three year old question. But I came across this question while researching the same subject, and in the process of corroborating watchmansky's answer (https://serverfault.com/a/684381/188907), I learned a bit more about the situation, at least as it exists today.

    According to https://linux.die.net/man/7/arp, the setting

    gc_stale_time

    effects how often the ARP cache is checked for stale entries. (Or garbage collected, hence the "gc_" at the start of the setting name.)

    Meanwhile, the value

    base_reachable_time_ms

    actually controls how long an ARP cache entry is valid, and it defaults to 30000 milliseconds. But each new ARP cache entry will actually receive a time to live value randomly set somewhere between base_reachable_time_ms / 2 and 3*base_reachable_time_ms / 2*.

    This means each new cached ARP entry will have a starting timeout between 15 and 45 seconds, unless the value of base_reachable_time_ms is changed.

    This assumes that the current timeout value for a cached ARP entry is validated before use, and that the garbage collection rate does not influence the effective validity of the cache entries.

    (*Confirmed by reading the code at https://elixir.bootlin.com/linux/v4.17.11/source/net/core/neighbour.c#L115)

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  2. Best Answer

    60 seconds

    you can check it with:

    cat /proc/sys/net/ipv4/neigh/ethX/gc_stale_time

    and change it with

    echo timeout > /proc/sys/net/ipv4/neigh/ethX/gc_stale_time

    timeout is new value

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