google search threshold

Keenan Tims ktims at
Mon Feb 29 16:49:52 UTC 2016

FWIW I have seen the captchas more often on IPv6 both from home and the office than when both networks were using a single shared IPv4; not sure if this is just related to chronology or a real effect. Once a month or so I seem to get them for a couple of days, then they go away.

No idea what's triggering it. It would be *really* helpful if Google could provide some useful technical details beyond a generic FAQ page. As it is I just get annoyed by it and have no way to troubleshoot or correct the constant false positives. How is Google detecting "robots"? My sense is that I tend to trigger the captcha thing when iterating similar search terms (particularly due to removal of the + operator and extremely poor "change my search terms because you think you know better than I do what I want to search for" behaviour. My search patterns haven't really changed since turning up IPv6 everywhere, so I have to think either the captcha trigger has gotten more aggressive, or somehow prefers to blacklist IPv6 users.

In any case, just going to IPv6 is definitely not a complete fix for this. It seems to be related to search behaviour and $blackbox_magic.

Keenan Tims
Stargate Connections
From: NANOG <nanog-bounces at> on behalf of Philip Lavine via NANOG <nanog at>
Sent: February 29, 2016 7:53 AM
To: Damian Menscher
Cc: nanog at
Subject: Re: google search threshold

I have about 2000 users behind a single NAT. I have been looking at netflow, URL filter logs, IDS logs, etc. The traffic seems to be legit.

I am going to move more users to IPv6 and divide some of the subnets into different NATS and see if that alleviates the traffic load.
Thanks for the advice.

      From: Damian Menscher <damian at>
 To: Philip Lavine <source_route at>
Cc: "nanog at" <nanog at>
 Sent: Friday, February 26, 2016 6:05 PM
 Subject: Re: google search threshold

On Fri, Feb 26, 2016 at 3:01 PM, Philip Lavine via NANOG <nanog at> wrote:

Does anybody know what the threshold for google searches is before you get the captcha?I  am trying to decide if I need to break up the overload NAT to a pool.

There isn't a threshold -- if you send automated searches from an IP, then it gets blocked (for a while).

So... this comes down to how much you trust your machines/users.  If you're a company with managed systems, then you can have thousands of users share the same IP without problems.  But if you're an ISP, you'll likely run into problems much earlier (since users like their malware).
Some tips:   - if you do NAT: try to partition users into pools so one abusive user can't get all your external IPs blocked  - if you have a proxy: make sure it inserts the X-Forwarded-For header, and is restricted to your own users  - if you're an ISP: IPv6 will allow each user to have their own /64, which avoids shared-fate from abusive ones
Damian (responsible for DDoS defense)-- Damian Menscher :: Security Reliability Engineer :: Google :: AS15169

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