IIS 7 Web logs : GET request with no browser type

We have a IIS7 server and regularly create reports of visitor stats using 'Weblog expert lite' anayliser and the IIS log files.

Gradually over the last month of IIS logs were getting increasing percentage of users with Browser : 'other' and OS 'other'
Looking at the IIS logs there a number of GET requests without a browser defined - instead there is a dash '-' for example:
ET /pages/VSearch.aspx - 443 - 222.333.444.444 HTTP/1.1 - ASP.NET_SessionId=f443434bs155erk0op55 - toast.pancake.org 200 0 0 21282 183 998

IIS 7 server 2008R2....

When I navigate to the site in IE11/Chrome/Firefox I can see it recognises my browser verison - any ideas what could be causing this?
Spikeuk30Asked:
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Dave BaldwinFixer of ProblemsCommented:
There is a lot of software that can be used to access your site without listing an OS or a browser.  cURL and PHP curl will only send what you tell it to send as a "User-agent' string which is where these things are listed.  Other than people trying to scan your sites, you might see those kinds of listings if you are providing 'web services' that are accessed by programs other than web browsers.

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Spikeuk30Author Commented:
ok - its very strange how its suddenly grown to thousands of visitors - with currently around 70% of traffic not having a user agent string?

Are there any steps I can take to try and track down what exactly these are?
Dave BaldwinFixer of ProblemsCommented:
That sounds like someone is either trying to take down your site by overloading it or trying to break in by overloading it.  Looks like the first step is to make sure 'Request Filtering' is enabled.  http://www.iis.net/configreference/system.webserver/security/requestfiltering/filteringrules  It's not obvious how to block blank user agents.
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