Correct auth, but based on stale nonce received from

Looks like this:

[Oct  7 23:27:30] NOTICE[2056]: chan_sip.c:8866 check_auth: Correct auth, but based on stale nonce received from '<sip:ac009602@21.11.2.1>'

It means that a SIP device is re-using an old authentication challenge. If it still registers and can place calls, there's no problem to worry about. It's just a warning. source

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It can be that someone wants to register with a sniffed SIP packet.

It's basically the nonce="" value is not the same Asterisk sent for that REGISTER session. source

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This NOTICE is only seen when sip debug is enabled. From chan_sip.c:

if (sipdebug)
    ast_log(LOG_NOTICE, "Correct auth, but based on stale nonce received from '%s'\n", get_header(req, "To"));