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Talk a bit about sockets that are bound before ppp changes

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its IP number.
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Brian Somers 1998-01-29 00:40:02 +00:00
parent 4206e13f22
commit 15d57161a2
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svn path=/head/; revision=2401
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FAQ/network.sgml
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@ -1,4 +1,4 @@
<!-- $Id: network.sgml,v 1.6 1998-01-20 22:48:19 brian Exp $ -->
<!-- $Id: network.sgml,v 1.7 1998-01-29 00:40:02 brian Exp $ -->
<!-- The FreeBSD Documentation Project -->
<sect>
@ -630,6 +630,54 @@ default 10.0.0.2 UGSc 0 0 tun0
other bits and pieces such as what actually caused the dump and
the addresses & values of the relevant variables.
<sect2>
<heading>
The process that forces a dial in auto mode never connects
</heading>
<p>This is a known problem when <bf/ppp/ is set up to negotiate
a dynamic local IP number with the peer. When that initial program
calls <htmlurl url="http://www.freebsd.org/cgi/man.cgi?connect"
name="connect(2)">, the IP number of the tun interface is
assigned to the socket endpoint. The kernel creates the first
outgoing packet and writes it to the tun device. <bf/Ppp/ then
reads the packet and establishes a connection. If, as a result
of <bf/ppp/s dynamic IP assignment, the interface address is changed,
the original socket endpoint will be invalid. Any subsequent
packets sent to the peer will usually be dropped. Even if
they aren't, any responses will not route back to the originating
machine as the IP number is no longer owned by that machine.
<p>There are several theoretical ways to approach this problem.
It would be nicest if the peer would re-assign the same IP number
if possible <tt/:-)/
<p>The easiest method from our side would be to never change the
tun interface IP number, but instead to change all outgoing packets
so that the source IP number is changed from the interface IP to
the negotiated IP on the fly. This is essentially what
<htmlurl url="http://www.freebsd.org/cgi/man.cgi?libalias"
name="libalias(3)"> (and ppp's <bf/-alias/ switch) already does.
<p>Another alternative (and probably the most reliable) is to
implement a system call that changes all bound sockets from one
IP to another. <bf/Ppp/ would use this call to modify the
sockets of all existing programs when a new IP number is
negotiated.
<p>A third possibility is to allow an interface to be brought
up without an IP number. Outgoing packets would be given
an IP number of 255.255.255.255 up until the first SIOCAIFADDR
ioctl is done. This would result in fully binding the socket. It
would be up to <bf/ppp/ to change the source IP number, but only if
it's set to 255.255.255.255, and only the IP number and IP checksum
would need to change. This, however is a bit of a hack as
the kernel would be sending bad packets to an improperly
configured interface, on the assumption that some other mechanism
is capable of fixing things retrospectively.
<p>None of these solutions has been implemented (yet).
<sect2>
<heading>None of this helps - I'm desperate !</heading>