URL Scan

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URL Scan

jmsc
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Re: URL Scan

barry rowlingson
On Sun, Apr 17, 2011 at 9:40 PM, jmsc <[hidden email]> wrote:

> I am wondering why when I try to input data from the first site listed below
> into R using the scan() function, a different page is read in instead (the
> second site listed):
>
> http://data.visionappraisal.com/CanterburyCT/parcel.asp?pid=1242
>
> http://www.visionappraisal.com/databases/
>
> I am wondering if this is an issue with R or something in the source code of
> the web page that I am not familiar with. Since I can access the first site
> directly, I assume it is not within the source code. Any help would be
> appreciated.

 I can't access the first URL directly - even from my web browser
without R being involved at all. Is that "pid" a parcel ID that you
need to be logged in to see? Or not a valid parcel id anymore?

 If you want to access a web site from R that needs a login/password
then you need to send the appropriate login form info from R and keep
the cookie session info that gets returned. Web sessions from R and
from a web browser are independent.

Barry

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Re: URL Scan

jmsc
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Re: URL Scan

barry rowlingson
On Sun, Apr 17, 2011 at 11:56 PM, jmsc <[hidden email]> wrote:
> The site does not require a login/password. Another way to access the first
> site would be to go to the second site, click Connecticut, click Canterbury,
> CT, enter the online database, click search under Query by Location with
> nothing in the search fields, and click the first property. Viewing the
> frame source on this page redirects to the second site.

 it doesn't require a login/pass, but it uses session cookies to
simulate a logged-in user (there's even a log out button that clears
the session).

> Also, could you direct me to or give me some instructions on scanning from
> sites that do require a login/password? Thanks.

 I had a quick look for R-help posts on this ( RSiteSearch("cookies"),
RSiteSearch("session") etc) but didn't find much. You probably want to
install  RCurl and look at the examples.

 Generally what happens is that a successful login, or in this case
just visiting the database front page, causes the web server to send
back a 'cookie' with a long ID number in it. For every further access
to that web site your browser includes the cookie. The server then
looks up the ID, goes 'yup, this is a valid session', and sends you
the page you want. If the cookie isn't there, or the ID isn't valid
(and the ID numbers are big enough to make guessing impractical), then
you get the default page.

Barry

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Re: URL Scan

jmsc
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