RE: Resolving localhost on Windows 7 (for exim)

["Pierre A. Humblet" <Pierre dot Humblet at ieee dot org>]

> -----Original Message-----
> From: Corinna Vinschen
> Sent: Thursday, January 08, 2015 08:24
> 
> Hi Pierre,
> 
> On Jan  5 09:03, Pierre A. Humblet wrote:
> > While porting exim to Windows 64 I have observed strange results when
> > resolving localhost
> >
> > On Windows XP,
> >
> > Resolv: search "localhost" type 28
> > Resolv: query "localhost" type 28
> > Resolv: DnsQuery: 0 (Windows)
> > Resolv: localhost Section 0 Type 28 Windows Record Length 16
> > 08:02:06  3760 DNS lookup of localhost (AAAA) succeeded
> > Resolv: search "localhost" type 1
> > Resolv: query "localhost" type 1
> > Resolv: DnsQuery: 0 (Windows)
> > Resolv: localhost Section 1 Type 1 Windows Record Length 4
> > 08:44:13  5552 DNS lookup of localhost (A) succeeded
> >
> > We see that for IPV4 localhost things are fine.
> > Windows returns an answer section (1) and Cygwin processes it correctly.
> >
> > However for IPV6 it returned a question section (0) but with data in it.
> > Cygwin essentially drops that.
> > That's why above the application tried an A record after getting the
> > AAAA record, which was empty.
> >
> >
> > However of Windows 7
> > CYGWIN_NT-6.1 Dell3020 1.7.33-2(0.280/5/3) 2014-11-13 15:47 x86_64
> > Cygwin
> >
> > Resolv: search "localhost" type 28
> > Resolv: query "localhost" type 28
> > Resolv: DnsQuery: 0 (Windows)
> > Resolv: localhost Section 0 Type 28 Windows Record Length 16
> > 08:22:24 140244 DNS lookup of localhost (AAAA) succeeded
> > Resolv: search "localhost" type 1
> > Resolv: query "localhost" type 1
> > Resolv: DnsQuery: 0 (Windows)
> > Resolv: localhost Section 0 Type 1 Windows Record Length 4
> >
> > We see that Windows returns question sections in both cases and
> > localhost is never resolved.
> > From what I have seen Windows never returns question section in normal
> > cases so I suggest inserting the following on line 251 of
> > minires-os-if.c, to essentially turn question sections in answer
> > sections (after  while (rr) { )
> >
> > if ((rr->Flags.DW & 0x3) == 0) {
> >         DPRINTF(debug, "Got section 0 %s %d with data length %d\n",
> > DomName, Type, rr->wDataLength);
> >         if (rr->wDataLength > 0)
> >                 rr->Flags.DW |= 1; // Make it an answer section as
> > there is data }
> 
> Can you please send at least a real patch?  Without the formatting matching
> the surrounding code I'm totally unsure where to apply this code.  A
> ChangeLog entry would be helpful as well.

Here they are. 

cvs diff -up minires-os-if.c
Index: minires-os-if.c
===================================================================
RCS file: /cvs/src/src/winsup/cygwin/libc/minires-os-if.c,v
retrieving revision 1.15
diff -u -p -r1.15 minires-os-if.c
--- minires-os-if.c     23 Apr 2013 09:44:35 -0000      1.15
+++ minires-os-if.c     12 Jan 2015 03:39:27 -0000
@@ -249,6 +249,13 @@ static int cygwin_query(res_state statp,
    rr = pQueryResultsSet;
    section = 0;
    while (rr) {
+    /* Some Windows versions return questions when providing locally
+       generated answers, for example for "localhost" or for the computer name */
+    if (((rr->Flags.DW & 0x3) == DnsSectionQuestion) &&
+       (rr->wDataLength > 0)) {
+      DPRINTF(debug, "Changing record below from question to answer\n");
+      rr->Flags.DW ^= DnsSectionQuestion ^ DnsSectionAnswer;
+    }
      if (!counts[0] && (rr->Flags.DW & 0x3)) {
        /* No question. Adopt the first name as the name in the question */
        if ((len = dn_comp(rr->pName, ptr, AnsLength - 4,

2015-01-11  Pierre A. Humblet <pierre@phumblet.no-ip.org>

         * minires-os-if.c (cygwin_query): Change questions into answers.


> > It would be nice if this would be tried ASAP.
> > However I am not setup currently to build cygwin.
> 
> It's not exactly tricky to set this up...

Right, I had it all downloaded and working in one evening.
CYGWIN_NT-6.1 Dell3020 1.7.34(0.283/5/3) 2015-01-11 18:48 x86_64 Cygwin

With the above changes localhost is resolved fine.
Now the bad news:  the exim daemon crashes.

The reason is this:
$ getent passwd exim
NT SERVICE+exim:*:376394:376394:U-NT SERVICE\exim,S-1-5-80-3213360373-4072665756-2198108471-1641386292-839958090:/:/sbin/nologin

So even though I am requesting just "exim" I am getting an entry for "NT SERVICE+exim"
Talk about aliasing.
The way the exim code works, when an "exim" user exists (per getpwnam) the daemon setuids to it.
Here it's trying to setuid to a service. 
This would break every exim installation.

Pierre



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