On 25.06.21 14:46, Eric Robinson wrote:
> Olaf and Scott --
> Thanks to both of you for your comments. I may have asked my question poorly, 
> since what you both described is the way I understand TCP to work. There is 
> no correlation between an incoming connection to tomcat and its outgoing 
> connection to a database backend, nor would I expect there to be.
> Perhaps a simpler way to ask my question is: when a server has multiple IPs, 
> which one does tomcat use as its source IP when it initiates a three-way 
> handshake with a remote machine?
> For example, suppose my server has IP addresses and, and my 
> tomcat connector looks like this...
> <Connector
>     port="8080"
>     protocol="HTTP/1.1"
>     address=""
>     connectionTimeout="20000"
>     redirectPort="8443"
>   />
> Tomcat is now listening on IP
> But here's the question. If tomcat needs to initiate a TCP session to a 
> remote machine (acting as a TCP client), will it use or as 
> the source IP of the outbound connection? I'm assuming it will use the same 
> IP that the connector is configured to listen on.
Hi Eric,

again: There's no correlation. Your question boils down to a
context-free "which source IP does tomcat use for outgoing
connections?". In fact, Tomcat doesn't use any. It just asks the runtime
environment (ultimately I'd expect the OS) for a connection to a
particular destination, then it uses that.

How the connection is then established will depend on

* available network adapters
* best route to the target address
* OS or network configuration

It will /not/ depend on any of Tomcat's Connector-configurations whatsoever


>> -----Original Message-----
>> From: Olaf Kock <tom...@olafkock.de>
>> Sent: Friday, June 25, 2021 3:01 AM
>> To: users@tomcat.apache.org
>> Subject: Re: Re-Use TCP Source Ports if the Socket is Unique?
>> On 25.06.21 05:19, Eric Robinson wrote:
>>> Thanks for the feedback, Daniel.
>>> I guess the answer depends on whether the socket libraries use the tomcat
>> listening port as the source IP. If you have three tomcat instances 
>> listening on
>> three different IPs, each instance should be able to open a client connection
>> using the same source port, as long as each tomcat uses its listening IP as 
>> the
>> source IP of the socket.
>>> That's the part I'm still not sure about.
>> My expectation is that database connections do not have any correlation
>> with the listening port: Technically, DB connection pools can be shared 
>> across
>> all contained Hosts and Connectors /within a single tomcat/, and when
>> multiple processes are added to the game, it doesn't really change anything.
>> In fact, it's not uncommon that there's a public facing network adapter,
>> where a http-connector listens, but a completely different network adapter
>> for any backend communication - e.g. to the database. All that I expect a
>> database driver to do is to specify where it wants to connect to, and the OS
>> figures out how that connection needs to be routed.
>> That's utterly independent of any http connection that comes in to the same
>> process.
>> So: Don't expect any correlation, and you're safe.
>> (Note: There /may/ be ways to configure a db-driver to specify a source
>> address, but I'd expect that rather to add a potential failure rather than
>> anything that I'd want to control. If you interpret such a situation 
>> differently:
>> Please elaborate)
>> Best,
>> Olaf
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