Date: Wed, 20 Jul 2011 12:23:42 -0700
From: Ray Van Dolson <rvandolson@esri.com>
To: linux-nfs@vger.kernel.org
Subject: NFS server responds to SYN with ACK only
Message-ID: <20110720192341.GA18167@esri.com>
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Sender: linux-nfs-owner@vger.kernel.org
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We have a couple legacy CentOS (RHEL)-based appliances with slightly
dated NFS implementations.

Server (CentOS 4 based):

    nfs-utils-1.0.6-70.EL4
    Kernel 2.6.9-42.0.10.plus.c4smp

Client (CentOS 5 based):

    nfs-utils-1.0.9-42.el5.x86_64
    Kernel 2.6.18-164.15.1.el5

The client has a long-lived NFSv3 mount to the server that sometimes
stops responding (blocks).  We can lazy unmount it, but subsequent
mount requests hang and the following is observed via tcpdump:

    1. Client GETPORT for NFS service succeeds.
    2. Client GETPORT for MOUNT succeeds
    3. Client MNT call succeeds (server gives valid response including
       file handle)
    4. Client sends a SYN packet to NFS port on server
    5. Server responds with ACK *only*

When we bounce the NFS daemon on the server, everything starts working
and in step 5 above, we get a SYN,ACK as expected in response to #4,
and everything proceeds along nicely.

Does this jog anybody on a long-ago fixed bug?  I'm thinking updating
the kernel and nfs-utils on the server will likely help, but would love
to find where behavior like the above is referenced as a "bug".

Thanks,
Ray