Subject: Re: Socket hangs in read() when putting interface down

From: Rick Jones <[email-masked]>
Date: Tue, 05 Jun 2007 10:41:51 -0700

chris schlund wrote:
> Using read() on a socket(AF_INET, SOCK_STREAM) connected via any network
> interface (e.g eth0) and putting this interface down (ifconfig) while
> the socket is in read() produces a hanger (the read() will not return
> anymore).
> Even if I install sigaction signal handler I got no signal when the
> interface is removed.

I'd not think there was a direct tie-in between an application signal 
handler and an interface being downed.

> This may be ok on a major interface like eth0 but it is a problem when
> using a ppp interface via GSM data or GPRS connection.
> Killing the pppd while the socket waits in read() produces the same
> behaviour. Also a broken connection looks the same.
> It is reproducable on several 2.6 kernel versions including the lastest
> stable 2.6.21.
> 
> On my understanding "ifconfig down" should lead into a read() error.

Why?  Can't traffic for the endpoint arrive via some other interface on 
the system?  I would think that getting out of a blocking read() 
requires the application to either have a timer running (and so get out 
of read() with EINTR) or to have keepalives enabled on the TCP 
connection (I'm assuming TCP for an AF_INET, SOCK_STREAM although I 
suppose it could be SCTP)

rick jones