Re: Assorted LVS troubles

To: " users mailing list." <lvs-users@xxxxxxxxxxxxxxxxxxxxxx>
Subject: Re: Assorted LVS troubles
From: Joseph Mack NA3T <jmack@xxxxxxxx>
Date: Thu, 23 Feb 2006 06:46:02 -0800 (PST)
On Thu, 23 Feb 2006, Tobias Klausmann wrote:

1) It seems like Linux does not ARP as often as I expect it to.

by watching tcpdump, my machines send arp requests about every 90sec. TCP/IP illustrated (Stevens), v1, section 4.5 says that Berkeley derived implementations (which Linux used to be) have timeouts of 20mins. I guess it's implementation dependant.

I thought no matter how much communication happens between two hosts, there usually is an ARP request (who-has) every five minutes. But in the case of keepalived, which checks the Realservers, I see no ARP requests at all, just normal communication. If I flush the arp cache for an IP, there's one ARP request (and reply), then no more.

presumably it gets replies? (and then is happy)

Does normal TCP communication "refresh" the ARP table entries?

no it's a separate layer.

There are no hardcoded ARP entries in my setup.

The logical network setup is like this:

[[RSs]] <-------- [LB] <-------- [client]
  |      VLAN A          VLAN B    ^
  |                                |

I see no ARP requests on VLAN A. I.e. the OS on LB learns the MAC
address of the RSs once, then keeps that knowledge forever.


I realize that this *probably* isn't an IPVS problem in and of
itself, but haven't seen it anywhere else.

I assume you're asking if what you see is OK. It looks OK to me.

2) When I last tested IPVS on 2.6, I sometimes saw "stuck"
connections. The LB had connections in its counters (and probably
in the IPVS conntrack table, too) that expired ages ago. Even
days or weeks after the last packet for such a farm was sent to
the LB, they were still there. Sometimes, even deleting the
entire farm and adding it again didn't help. While the farm is in
use, the number of those connections steadily increases. Is this
problem known (and/or fixed)?

are you using persistence? If so, this can be part of the behaviour - look at the lengthy section in the HOWTO on persistence and how to kill connections.


Joseph Mack NA3T EME(B,D), FM05lw North Carolina
jmack (at) wm7d (dot) net - azimuthal equidistant map
generator at Homepage It's GNU/Linux!

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