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List:       linux-ha
Subject:    Re: [Linux-HA] HA-Linux and NTP
From:       Alan Robertson <alanr () unix ! sh>
Date:       2005-02-23 19:16:52
Message-ID: 421CD6A4.3070001 () unix ! sh
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Kashif Shaikh wrote:
> It's not really a bug. If ntp is listening on all interfaces on a multihomed
> box, typically each interface is a separate network. But VIPs are 'grafted'
> onto an existing network. So when a UDP packet is recv'd, a corresponding
> UDP send will *choose* an existing physical interface. In a nutshell
> requests sent to say, eth0:1, will get replies back from eth0. We had this
> problem with NFS over UDP for HA.

A VIP is the same as any other IP.  But applications which never bind to IP 
addresses, always send out via the default address for the interface which 
- in an HA situation - is always wrong.

> Fix #1: Modify ntp sockets to use IP_PKTINFO when recv()'ing and send()'ing
> Fix #2: (Easy fix) Run one ntp for each interface(i.e. bind each ntp
> instance to an IP).

There are plenty of applications which do this right - that is they can 
bind to multiple addresses, and reply using the address they were addressed 
by.  Failure to do what one ought to do constitutes a bug.  Particularly 
for an application as generic and as widely used as xntpd.


> Fix #3: There is a way to add the VIP as a static route to the routing
> table(I forgot how to do this).
 >
> Fix #4: ip firewall rules.

I think you meant to say to change the default source address for the 
interface.  See the IPsrcaddr resource for how to do this with heartbeat.
> 
> #1 is the easiest for deployment, it's not that hard to modify the source
> since its available.

Of course, since we supply IPsrcaddr, #4 is an even easier fix - for some 
applications.

-- 
     Alan Robertson <alanr@unix.sh>

"Openness is the foundation and preservative of friendship...  Let me claim 
from you at all times your undisguised opinions." - William Wilberforce
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