See inline -
Janne Huttunen wrote:
Hi!
The selection of anonymous port in FreeBSD seems to act
a bit weird (bug?). This was first observed on actual
use on FreeBSD 6.2, but I have verified that the it
behaves the same on a December snapshot of CURRENT too.
1. A process creates an UDP socket and sends a packet
from it (at which point a local port is assigned
for it).
2. Another process creates an UDP socket, sets
SO_REUSEADDR (or SO_REUSEPORT) and sends a packet
from it (at which point a local port is assigned
for it).
Every now and then it happens that the second process
gets the same local port as the first one. If the
second process doesn't set the socket option this
won't happen. Note however, that the first process
does not have to cooperate in any way i.e. it does
not set any options.
Now, I'm fairly newbie when it comes to the FreeBSD
IP stack, but it seems to me that this phenomenon is
caused by the code in in_pcbconnect_setup(). If the
local port is zero the in_pcbbind_setup() is called
to select a port. That routine is called with the
local address set to the source address selected for
the outgoing packet, but when the port has been
selected, it is committed with INADDR_ANY as the
local address. Then when the second process in
in_pcbbind_setup() tries to check if the port is
already in use, it won't match the INADDR_ANY and
assigns the same port again.
Well it has been almost 20 years since I first ran across
this issue and was told back then that it was "as designed".
I believe you will see that this only happens when INADDR_ANY
is in effect. If instead you use a specific IP address as
your source it should not happen. I have not had a chance
to really go over the FreeBSD TCP/IP stack since the beginnings
of FreeBSD back in the early 90's (we were using basically the
same code for our product on a different architecture).
As an example of what the person was explaining he pointed to
the BIND code which expressly binds to each interface IP address
instead of too INADDR_ANY to prevent snooping.
I apologize if I am somewhat off base, having only re-entered
playing with FreeBSD in the last few months.
Patrick
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