How do I send a fd over TCP?
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From: K-mart Cashier on 4 Feb 2010 19:48 Both Machine A and Machine B run FreeBSD. I telnet into machine B from Machine A. Now let's say I have a program running on Machine B. How would I send an open fd from Machine A to Machine B? I'm kind of aware of passing the fd. However, that only seems to work on the same machine.
From: Ben Finney on 4 Feb 2010 20:05 K-mart Cashier <cdalten(a)gmail.com> writes: > Now let's say I have a program running on Machine B. How would I send > an open fd from Machine A to Machine B? A file descriptor is simply an integer. However, it is unique only within a single process running on a single machine. What would you expect to be able to do with the file descriptor on machine B? -- \ “To save the world requires faith and courage: faith in reason, | `\ and courage to proclaim what reason shows to be true.” | _o__) —Bertrand Russell | Ben Finney
From: Ersek, Laszlo on 4 Feb 2010 20:19 In article <b0b6665e-d81c-40ef-889b-03c1caa35adb(a)s33g2000prm.googlegroups.com>, K-mart Cashier <cdalten(a)gmail.com> writes: > Both Machine A and Machine B run FreeBSD. I telnet into machine B from > Machine A. > > Now let's say I have a program running on Machine B. How would I send > an open fd from Machine A to Machine B? I'm kind of aware of passing > the fd. However, that only seems to work on the same machine. Suppose the fd on Machine A refers to a regular file. Suppose you send it over to Machine B. How is Machine B supposed to access the blocks of the file residing on Machine A? Suppose the fd on Machine A refers to a connected TCP socket (the peer being Machine C). Once the fd is sent over to Machine B, what peer IP address will Machine C see, as it was talking to Machine A up to that point? Suppose the fd on Machine A refers to a (connected) pipe... etc. In summary, you can't, because the fd refers to objects living in Machine A's kernel, and in the TCP socket case, the fd is indirectly tied to Machine C's TCP control block and the underlying network infrastructure. It wouldn't be enough to simply copy over the TCP sequence numbers, ports and IP addresses, the routing rules would have to be updated on nodes between Machine B and C. Furthermore, "passing an fd" actually means an inter-process dup(), which allows the original owner to continue using the fd. That would result in a three-pronged TCP "connection" between {A,B} and C. You can't pass the fd because you can't fork() a child in Machine A onto Machine B. Cheers, lacos
From: David Schwartz on 5 Feb 2010 00:51 On Feb 4, 4:48 pm, K-mart Cashier <cdal...(a)gmail.com> wrote: > Now let's say I have a program running on Machine B. How would I send > an open fd from Machine A to Machine B? I'm kind of aware of passing > the fd. However, that only seems to work on the same machine. You will have to decide exactly what you think that means and implement that. It's clear what that means between two processes on the same kernel. They're talking to the same kernel, so when the first process does a 'read' to its kernel, the second process doing a read will be to the same 'kernel', so it's clear how it can access the same file, pipe, connection, or what have you. But it's really not clear what that would mean between two different machines. What would happen if B did a 'read' operation on a fd it got from machine A? How do you expect that 'read' to get to A's kernel? DS
From: Chris Friesen on 5 Feb 2010 11:48
On 02/04/2010 07:05 PM, Ben Finney wrote: > A file descriptor is simply an integer. However, it is unique only > within a single process running on a single machine. There is an exception--you can pass file descriptors between processes via a unix socket. This is still on the same machine though. Chris |