Date: Fri, 12 Nov 2010 13:29:13 +1100 From: Lawrence Stewart <lstewart@freebsd.org> To: Julian Elischer <julian@freebsd.org> Cc: freebsd-net@freebsd.org, Christopher Penney <penney@msu.edu>, Andre Oppermann <andre@FreeBSD.org> Subject: Re: NFS + FreeBSD TCP Behavior with Linux NAT Message-ID: <4CDCA679.7020401@freebsd.org> In-Reply-To: <4CDC5490.7030109@freebsd.org> References: <AANLkTikmpXDsi9N36D%2BM1ZFfyNGAZ3A-asaTNm5U7PwK@mail.gmail.com> <4CDC5490.7030109@freebsd.org>
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On 11/12/10 07:39, Julian Elischer wrote: > On 11/11/10 6:36 AM, Christopher Penney wrote: >> Hi, >> >> I have a curious problem I'm hoping someone can help with or at least >> educate me on. >> >> I have several large Linux clusters and for each one we hide the compute >> nodes behind a head node using NAT. Historically, this has worked >> very well >> for us and any time a NAT gateway (the head node) reboots everything >> recovers within a minute or two of it coming back up. This includes NFS >> mounts from Linux and Solaris NFS servers, license server connections, >> etc. >> >> Recently, we added a FreeBSD based NFS server to our cluster resources >> and >> have had significant issues with NFS mounts hanging if the head node >> reboots. We don't have this happen much, but it does occasionally >> happen. >> I've explored this and it seems the behavior of FreeBSD differs a >> bit from >> at least Linux and Solaris with respect to TCP recovery. I'm curious if >> someone can explain this or offer any workarounds. >> >> Here are some specifics from a test I ran: >> >> Before the reboot two Linux clients were mounting the FreeBSD server. >> They >> were both using port 903 locally. On the head node clientA:903 was >> remapped >> to headnode:903 and clientB:903 was remapped to headnode:601. There >> is no >> activity when the reboot occurs. The head node takes a few minutes to >> come >> back up (we kept it down for several minutes). >> >> When it comes back up clientA and clientB try to reconnect to the FreeBSD >> NFS server. They both use the same source port, but since the head >> node's >> conntrack table is cleared it's a race to see who gets what port and this >> time clientA:903 appears as headnode:601 and clientB:903 appears as >> headnode:903 (>>> they essentially switch places as far as the FreeBSD >> server would see<<< ). >> >> The FreeBSD NFS server, since there was no outstanding acks it was >> waiting >> on, thinks things are ok so when it gets a SYN from the two clients it >> only >> responds with an ACK. The ACK for each that it replies with is bogus >> (invalid seq number) because it's using the return path the other >> client was >> using before the reboot so the client sends a RST back, but it never >> gets to >> the FreeBSD system since the head node's NAT hasn't yet seen the full >> handshake (that would allow return packets). The end result is a >> "permanent" hang (at least until it would otherwise cleanup idle TCP >> connections). >> >> This is in stark contrast to the behavior of the other systems we have. >> Other systems respond to the SYN used to reconnect with a SYN/ACK. >> They >> appear to implicitly tear down the return path based on getting a SYN >> from a >> seemingly already established connection. >> >> I'm assuming this is one of the grey areas where there is no specific >> behavior outlined in an RFC? Is there any way to make the FreeBSD system >> more reliable in this situation (like making it implicitly tear down the >> return)? Or is there a way to adjust the NAT setup to allow the RST to >> return to the FreeBSD system? Currently, NAT is setup with simply: >> >> iptables -t nat -A POSTROUTING -s 10.1.0.0/16 -o bond0 -j SNAT --to >> 1.2.3.4 >> >> Where 1.2.3.4 is the intranet address and 10.1.0.0 is the cluster >> network. > > I just added NFS to the subject because the NFS people are thise you > need to > connect with. Skimming Chris' problem description, I don't think I agree that this is an NFS issue and agree with Chris that it's netstack related behaviour as opposed to application related. Chris, I have minimal cycles at the moment and your scenario is bending my brain a little bit too much to give a quick response. A tcpdump excerpt showing such an exchange would be very useful. I'll try come back to it when I I have a sec. Andre, do you have a few cycles to digest this in more detail? Cheers, Lawrence
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