Linux 4.20.0 kernel bug affecting Nmap

[Daniel Miller <bonsaiviking () gmail com>]

Hi all,

Recently, users of Arch Linux and other distributions which use very recent
Linux kernel versions reported on Github [1] that Nmap ARP scan was not
working, with the error message:

WARNING: eth_send of ARP packet returned -1 rather than expected 42
(errno=22: Invalid argument)

It took a bit of searching, but we've determined that this issue was due to
a bug in the Linux kernel, which was fixed [2] in Linux 4.20.1.
Essentially, 4.20 added some validation for addresses passed to sendto(),
but since we don't pass such an address, the validation was failing. The
fix changes the validation to only happen when an address is passed.

Users report that upgrading to Linux 4.20.1 or downgrading to any version
prior to 4.20 will restore functionality.

Dan

[1] Original bug report for Nmap: https://github.com/nmap/nmap/issues/1423
[2] Linux kernel commit that fixes the bug:
https://github.com/torvalds/linux/commit/6b8d95f1795c42161dc0984b6863e95d6acf24ed

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