Date: Tue, 26 Jul 2016 08:59:15 -0700 (PDT) From: Don Lewis <truckman@FreeBSD.org> To: freebsd-net@FreeBSD.org Subject: IPv6 -> IPv4 fallback broken in serf, kernel bug? Message-ID: <201607261559.u6QFxF8a081339@gw.catspoiler.org>
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Serf has some code to fall back from IPv4 if an IPv6 and more generally
try different addresses on multi-homed servers if connection attempts
fail, but it does not work properly on recent versions of FreeBSD. I've
tested both recent FreeBSD 10.3-STABLE and HEAD.
The way that it is supposed to work is that serf creates a socket, sets
it non-blocking, calls connect(), and then passes the fd to poll(). When
the connection attempt fails, it expects to see a POLLERR event. The
POLLERR event handler will then call getsockopt(fd, SOL_SOCKET,
SO_ERROR, &error, ...). If the returned error is ECONNREFUSED or one of
a couple of other errors, then serf will move on to the next address.
Instead what happens is that serf also(?) sees POLLIN set, which it
processes first by calling read(), which returns an ECONNREFUSED error.
That not a documented error return from read().
An easy way to test this is to truss svn and attempt to do an http
checkout from a host that has both IPv6 and IPv4 addresses, but is not
listening on port 80. The only connection attempt will be to the IPv6
address.
socket(PF_INET6,SOCK_STREAM|SOCK_CLOEXEC,6) = 4 (0x4)
fcntl(4,F_GETFL,) = 2 (0x2)
fcntl(4,F_SETFL,O_NONBLOCK|0x2) = 0 (0x0)
setsockopt(0x4,0x6,0x1,0x7fffffffdda4,0x4) = 0 (0x0)
gettimeofday({ 1469515046.979461 },0x0) = 0 (0x0)
connect(4,{ AF_INET6 [xxxx:xxxx:xxxx:xxxx::xxxx]:80 },28) ERR#36 'Operation now in progress'
gettimeofday({ 1469515046.979614 },0x0) = 0 (0x0)
kevent(3,{ 4,EVFILT_READ,EV_ADD,0x0,0x0,0x805491300 },1,0x0,0,0x0) = 0 (0x0)
kevent(3,{ 4,EVFILT_WRITE,EV_ADD,0x0,0x0,0x805491300 },1,0x0,0,0x0) = 0 (0x0)
kevent(3,0x0,0,{ 4,EVFILT_READ,EV_EOF,NOTE_LOWAT|0x3c,0x0,0x805491300 4,EVFILT_WRITE,EV_EOF,NOTE_LOWAT|0x3c,0x8000,0x805491300 },32,{ 0.500000000 }) = 2 (0x2)
read(4,0x80549c064,8000) ERR#61 'Connection refused'
kevent(3,{ 4,EVFILT_READ,EV_DELETE,0x0,0x0,0x0 },1,0x0,0,0x0) = 0 (0x0)
kevent(3,{ 4,EVFILT_WRITE,EV_DELETE,0x0,0x0,0x0 },1,0x0,0,0x0) = 0 (0x0)
kevent(3,{ 4,EVFILT_READ,EV_DELETE,0x0,0x0,0x0 },1,0x0,0,0x0) ERR#2 'No such file or directory'
kevent(3,{ 4,EVFILT_WRITE,EV_DELETE,0x0,0x0,0x0 },1,0x0,0,0x0) ERR#2 'No such file or directory'
close(4) = 0 (0x0)
close(3) = 0 (0x0)
svn: E170013: Unable to connect to a repository at URL ...
It looks like it should be possible to patch serf to handle this, but:
* Should POLLIN be set for this event?
* What errno value should read() return in this case, if it is
ECONNREFUSED, then that should be documented.
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