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Re: postgres 9 bind address for replication

From: Adam Crews <adam(dot)crews(at)gmail(dot)com>
To: Andrew Dunstan <andrew(at)dunslane(dot)net>
Cc: "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: postgres 9 bind address for replication
Date: 2012-07-24 02:02:45
Message-ID: -799162477685392392@unknownmsgid (view raw or flat)
Thread:
Lists: pgsql-cluster-hackerspgsql-generalpgsql-hackers
On Jul 23, 2012, at 11:45 AM, Andrew Dunstan <andrew(at)dunslane(dot)net> wrote:

>
> On 07/23/2012 02:23 PM, Adam Crews wrote:
>> On Mon, Jul 23, 2012 at 8:34 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>>> On Sat, Jul 21, 2012 at 1:24 AM, Adam Crews <adam(dot)crews(at)gmail(dot)com> wrote:
>>>> Hello,
>>>>
>>>> I'm sorry for cross-posting, however I originally posted this to
>>>> pgsql-general list, but didnt get any replies.
>> Then I posted to pgsql-cluster-hackers..., and now here.
>>
>>>>
>>>> I’m using pg 9.1.3 on CentOS 5 and have a few slave databases setup
>>>> using the built in streaming replication.
>>>>
>>>> On the slaves I set the “listen_addresses” config option to an ip
>>>> address for a virtual alias on my network interfaces.  The host has an
>>>> address of 10.1.1.10, and there is a virtual alias of 10.1.1.40 that
>>>> the slave postmaster binds to.
>>>>
>>>> When the slave makes it connection to the master to start replication
>>>> the source address for the connection is the host address, not the
>>>> virtual alias address.  Connections appear to come from 10.1.1.10,
>>>> instead of the slave postmaster address of 10.1.1.40.
>>>>
>>>> This seems like a bug to me.  I could understand that if the
>>>> postmaster is listening on all interfaces, then it should use whatever
>>>> the IP is for the for the host, but in an instance where the
>>>> postmaster has been configured to listen to a specific address it
>>>> seems like the call to start the replication should be passed that
>>>> address so connections come from the slave postmaster’s IP, instead of
>>>> the host.
>>>>
>>>> Is there a config option that can be used to adjust this?  I've looked
>>>> in the docs, but haven't found one yet.
>>>>
>>>> Is this perhaps a bug, or lack of feature?
>>> I don't think it's a bug, because the behavior you're hoping for might
>>> not be what everyone would want in a similar situation.  It might
>>> qualify as an unimplemented feature.
>>>
>>> This mailing list isn't heavily used and this seems a bit off-topic
>>> for it anyway; you might want to try a different one for further
>>> discussion of this issue.
>>>
>> So, I think this, as Robert states, an unimplemented feature.
>>
>> For my situation it would be very useful to have an option to be able
>> to specify the source address for replication.
>>
>> I discovered this because I bind the listen address for postgres to a
>> single address even though the host system may have multiple
>> addresses.  I then use that single address in iptables rules on other
>> systems.  Since I expect the slave to be at a .40 address, but the
>> replication comes from the primary address of the interface (in this
>> case .10), my iptables rules were missing the access for the slave to
>> connect to the master.
>>
>> This site http://linux-ip.net/html/routing-saddr-selection.html
>> describes the behavior I'm seeing.
>>
>> How do I go about requesting a config option that would allow me to
>> specify the source address for the replication connections?
>>
>
> You just have :-)
>
> You could just add an iptables rule redirecting .10 packets on port 5432 (or whatever you're using) appropriately.
>
> We don't have any provision for binding the local end of any connection AFAIK. So the first question is "Do we want to?" and the second is "If yes, when and how?" I don't see that replication should be a special case - if this is worth providing for it should be applicable to all clients, ISTM.
>
> cheers
>
> andrew

I've worked around the issue for now by just allowing the host address
instead of the postmaster ip address in my iptables.

I agree that if implemented it should be an option available to all
clients, not just the replicator.

To give a little background, here's what I'm doing. In my environment
we have dedicated ip addresses to specific postmasters, then sometimes
run multiple postmasters on the same server.  Our data files are
stored on san storage and can be moved between different servers.
With this setup I've abstracted my database from the underlying
hardware and can move the postmasters between hosts with minimal
downtime. (3 minutes to stop the db, unmount the storage, mount on the
new host, start the postmaster).  The ip travels to the new host as a
virtual interface along with the database files.  To make sure clients
can't access a db they are not supposed to, I control access via
iptables and pg_hba.  I can now do system patching with little impact
to the application.

Thanks,
-Adam

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