Re: bind() udp behavior 2.6.8.1

[Jan Engelhardt]

>Hello,
>
> I am not subscribed to this list so please CC me personally in
>response. 
>
> I am noticing some odd behavior with linux 2.6.8.1 on a redhat 8 box
>when making udp requests.  It seems subsequent udp calls are all
>allocating the same source ephemeral udp port.  I believe the kernel
>should be randomizing these (or incrementing) these ports for subsequent
>requests.

No, you can have a fixed port for any socket. (It's just a question whether 
you actually get the socket, because it might be in use.)

See http://linux01.org:2222/f/UHXT/examples/src/fastsock.c , which contains an 
example on how to choose a fixed port.

>  We ran a test C program that just put a gethostbyname_r call
>in a for loop of 40 calls and all 40 requests used the same UDP source
>port (32789).

Looks normal to me. It might select a random port upon "libc invocation" and 
use it for all further requests. This is in fact very valid, because UDP is 
connectionless; packets can go from anywhere to anywhere without any 
pre-work.

>  This is causing our firewall to drop some packets since
>it thinks it already closed that connection due to too many transactions
>using same udp source/dest port passing thru in too short a time frame.

Then, the firewall UDP implementation is broken. Note, an UDP connection *can 
not be closed*, because it never was "open". If it's trying to do something 
like 
	iptables -p udp -m state --state RELATED
it is doing it wrong, because that is an impossible situation.


Jan Engelhardt
-- 
ENOSPC
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