RE: [squid-users] WPAD and Internet Explorer 6

On Wed, 20 Apr 2005, Henrik Nordstrom wrote:

> On Tue, 19 Apr 2005, Damien Nguyen wrote:
>
> > I can see name resolution activities from Windows 2000 and XP PCs but not
> > from NT4 workstations.
> >
> > Like I mentioned in my previous post, if I specify the automatic
> > configuration script (the URL to the wpad.dat file) I can see the NT4
> > workstation issue the "GET /WPAD.DAT" command from Ethereal.
> >
> > Again, if I set it back to "automatically detect settings" (even did a
> > reboot after the change), does not work, nothing DNS related in
> > Ethereal...
>
> If you use DHCP then you could try specifying the PAC path in the DHCP
> responses.
>
> SUMMARY OF WPAD DISCOVERY MECHANISMS
>
> +-------------------------+--------+----------+
> | Discovery | | Document |
> | Mechanism | Status | Section |
> +-------------------------+--------+----------+
> | DHCP | MUST | 5.4.1 |
> | SLP | SHOULD | 5.4.2 |
> | "Well Known Alias" | MUST | 5.4.3 |
> | DNS SRV Records | SHOULD | 5.4.4 |
> | DNS TXT "service: URLs" | SHOULD | 5.4.5 |
> +-------------------------+--------+----------+
>
> 5.4.1 DHCP
>
> Client implementations MUST support DHCP. DHCP has widespread
> [...]
>
> The DHCP option code for WPAD is 252 by agreement of the DHC working
> group chair. This option is of type STRING. This string contains a
> URL which points to an appropriate config file. The STRING is of
> arbitrary size.
>
> -> URL as specified in DHCP
>
> 5.4.2 Service Location Protocol /SLP
>
> [RFC2608]
>
> -> URL as specified by SLP

To my knowledge, the WindowsNT DHCP Client never supported this method of
locating the automatic configuration script. It relied solely on the DNS
mechanism used in Internet Explorer 5.5 and early 6.0 versions.

The Windows2000 DHCP Client supports this method but its implementation
appears to be flawed.

The WindowsXP DHCP Client supports this method. This appears to be the
preferred method for WindowsXP based on logged evidence.

>
> 5.4.3 DNS A/CNAME "Well Known Aliases"
>
> Client implementations MUST support this mechanism. This should be
> [...]
>
> The client performs the following DNS lookup:
> QNAME=wpad.TGTDOM., QCLASS=IN, QTYPE=A
>
> -> http://wpad.TGTCOM/wpad.dat

WindowsNT, Windows98, Windows2000, and WindowsXP support this method but
its implementation in WindowsXP appears to be marginal.

One problem with WindowsXP was the autodecrement of the string length that
was returned. It required a symbolic link to be created for the file
name, i.e. wpad.da -> wpad.dat.

> 5.4.5 DNS TXT service: Entries
>
> Client implementations SHOULD support this mechanism. If the
> [...]
>
> The client should attempt to retrieve TXT RRs from the DNS to obtain
> "service: URLs" contained therein. The "service: URL" will be of the
> following format, specifying a complete candidate CURL for each
> record located:
>
> service: wpad:http://<HOST>:<PORT><PATH>
>
> The client should first issue the following DNS query:
> QNAME=wpad.TGTDOM., QCLASS=IN, QTYPE=TXT
>
> -> URL as specified by the DNS TXT service: record

I missed this method. Is the following, the correct TXT entry?

                IN TXT "service: wpad:http://..."

Merton Campbell Crockett

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