Re: Question regarding expected behavior of two udp sockets with SO_REUSEADDR set

[Eric Dumazet <eric.dumazet@gmail.com>]

Le vendredi 19 novembre 2010 à 19:48 -0500, Neil Horman a écrit :
> Hey all-
> 
> 	Got a question regarding expected/desired behavior of $SUBJECT
> 
> 
> I have a report of a problem with a program that opens two sockets:
> 
> The first socket is UDP and binds to 127.0.0.1 on a randomly selected port
> 
> The second socket is UDP and calls connect, sending to the first socket
> 
> Both sockets are part of the same process and have SO_REUSEADDR set
> 
> After the connect the second socket sends a message to the first socket.  The
> first socket waits for the message by calling select().
> 
> Its observed that occasionally the first socket fails to receive the message,
> which is odd, given that the system is unloaded, and this is the only message
> being sent.  A little investigation shows that when this happens, the client and
> the server wind up bound to the same port.
> 
> This happens because the second socket calls inet_autobind during the connect
> call, and since both it and the server have SO_REUSEADDR set, it is possible
> that the autobind will select the same port that the first socket is bound to.
> When this happens the sendmsg path can get confused.  Specifically, when the skb
> is delivered to the destination socket, the hash lookup might find the wrong
> entry and enqueue the skb to the second socket instead of the first.
> 
> Questions:
> 
> 1) Is that expected?
> 

Is SO_REUSADDR used on both sockets ?

May I ask why SO_REUSEADDR is set in the first place on UDP sockets ?

I use it before a bind() on a given port (non null), but apparently your
program doesnt bind() the 2nd socket before its connect() ?


> 2) If not, what do you think the best way to fix it is?
> 
> 	a) Deny autobinds to the same port when SO_REUSEADDR is set, but allow
> explicity binds to the same port?
> 
> 	b) Deny both autobinds and explicit binds to the same port/addr,
> effectively disablind SO_REUSEADDR with UDP, kind of like with listening TCP
> sockets
> 
> 	c) Add magic to udp_rcv to detect skbs originating from local sockets,
> and _dont_ deliver to the socket it originated from

Why ? Its a valid use case IMHO, even with a single socket.

> 
> I'm inclined to say, no this is not expected behavior, and that we should fix it
> with option A, but I'm interested in getting other opinions before I go down any
> particular path.
> 

autobind certainly is a problem, we tried to 'fix' it in recent past and
had to revert some patches. We tried to allow more sockets to be used
but we failed.