dns query id not changing

Adam Denenberg straightflush at gmail.com
Fri Dec 17 19:55:43 UTC 2004

one other thing that i came across is that when i change the
nsswitch.conf file from

passwd: files ldap
group:  files ldap


passwd: files ldap
group: files

 the problem goes away, and all the transaction IDs get incremented.=20
For some reason when enabling libnss_ldap.so thru nsswitch.conf for
groups, i get this redundant transaction ID that is causing the

 can anyone explain this?


On Fri, 17 Dec 2004 10:24:03 -0500, Adam Denenberg
<straightflush at gmail.com> wrote:
> yeah i understand it will be reused.  My issue is that in the
> application, every time i run tcpdump and check, the 3rd request for a
> certain A record is always the same as the 2nd (same Transaction ID).
> So my conern is why the resolver is doing this.  This is reproducable
> behavior and is inconsistent with the way other queries are run.
> I wrote a small test script to do a gethostbyname_r() in a for loop of
> 100 queries and this never happens.  However in the application
> (pam_ldap/nss_ldap) the 3rd Transaction ID is always the same as the
> 2nd.
> i am trying to find the cause of this behavior, b/c it is breaking the
> applicatoin when dns requests traverse the FW.
> thanks
> adam
> On Fri, 17 Dec 2004 07:13:38 +0000 (UTC), phn at icke-reklam.ipsec.nu
> <phn at icke-reklam.ipsec.nu> wrote:
> > Adam Denenberg <straightflush at gmail.com> wrote:
> > > Well the issue is that that same "tuple" is coming in as the firewall
> > > is tearing down the connection.  The firewall needs about 60ms for th=
> > > connection to expire according to Cisco.
> >
> > > I agree that there is definitely an issue with the firewall, but it
> > > was also my understadning  the resolver used random query IDs for eac=
> > > request.  Out of nowhere, the resolver appears to re-use a query ID
> > > that it just used in a very short span (20ms) and it confuses the
> > > firewall b/c the firewall has not torn down the previous connection.
> > > Shouldnt this behavior be somewhat consistent?
> >
> > > We have an application (pam_ldap) that needs to make 3 or 4 DNS
> > > requests. The 3rd dns request is _always_ using the same ID as the 2n=
> > > DNS request and occurs in about 10ms before the firewall has torn dow=
> > > the original connection and thus causes a timeout (the FW drops it).
> >
> > >  I guess what i am trying to figure out is why the behavior of the
> > > resolver for creating a query ID is not consistent.  It should either
> > > reuse the same one, or create totally random ones.  It is doing a
> > > little bit of both and thus breaking the app.
> >
> > As this id os a 16-bit value it will be reused fast.
> >
> > Do as others suggested, fix the pix! Change vendor if needed.
> >
> > --
> > Peter H=E5kanson
> >         IPSec  Sverige      ( At Gothenburg Riverside )
> >            Sorry about my e-mail address, but i'm trying to keep spam o=
> >            remove "icke-reklam" if you feel for mailing me. Thanx.
> >
> >

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