diff options
Diffstat (limited to 'README')
| -rw-r--r-- | README | 214 |
1 files changed, 214 insertions, 0 deletions
@@ -0,0 +1,214 @@ +Background +========== + +'usbmuxd' stands for "USB multiplexing daemon". This daemon is in charge of +multiplexing connections over USB to an iPhone or iPod touch. To users, it means +you can sync your music, contacts, photos, etc. over USB. To developers, it +means you can connect to any listening localhost socket on the device. usbmuxd +is not used for tethering data transfer, which uses a dedicated USB interface as +a virtual network device. + +Multiple connections to different TCP ports can happen in parallel. An example +(and useful) tool called 'iproxy' is included that allows you to forward +localhost ports to the device---allows SSH over USB on jailbroken devices, or +allowing access the lockdown daemon (and then to all of the file access, sync, +notification and backup services running on the device). + +The higher-level layers are handled by libimobiledevice. 'ifuse' is then able +to sit on top of this and mount your device's AFC filesystem share. + +This package contains the usbmuxd communication interface library 'libusbmuxd'. + +There is also a Python implementation of the client library in the python-client +directory, and an example tcprelay.py which performs a similar function to iproxy. +This implementation supports OSX and Windows and the new iTunes plist-based +usbmuxd protocol, so it is portable and will run on those operating systems with +no modification, using Apple's native usbmuxd. This is useful if you need to +tunnel to your device from another OS in a pinch. Run python tcpclient.py --help +for usage information. + +License +======= + +The contents of this package are licensed under the GNU Lesser General Public +License, version 2.1 or, at your option, any later version (see COPYING.LGPLv2.1). +If a more permissive license is specified at the top of a source file, it takes +precedence over this. + +Legal +===== + +Apple, iPhone, and iPod touch are trademarks of Apple Inc., registered in the +U.S. and other countries. + +Building +======== + + ./autogen.sh + make + sudo make install + +Running (with magic) +==================== + + (Unplug + replug your jailbroken device) + ./iproxy 2222 22 & + ssh -p 2222 root@localhost + +Hopefully you get the normal SSH login prompt. You may still lots of debugging +output for the moment. If this is getting in the way of your ssh login, then +run the 'ssh' command from a different xterminal or virtual console. Of course, +you need to have OpenSSH installed on your jailbroken device for this to work. + +If you have iFuse, you can run "ifuse <mountpoint">. This doesn't require +iproxy and works on all devices, jailbroken or not. + +Running (without magic) +======================= + +If 'udev' is _not_ automatically running on your machine and picking up the new +.rules file, you will need to start usbmuxd by hand first. Check it's running +and that there is only one copy with 'ps aux | grep +usbmuxd'. + + sudo usbmuxd -U -v -v & + ./iproxy 2222 22 & + ssh -p 2222 root@localhost + +Tip: Starting SSH if disabled +============================= + +If your device is rooted, but SSH isn't started and you _cannot_ (for instance, +cracked/broken screen) get to the Services control panel on the device, then you +can start the SSH service over the USB by mounting the (jailbroken) filesystem. + +You will need to mount it using 'ifuse --afc2' (to access the root directory of +the device), and then edit: + + /Library/LaunchDaemons/com.openssh.sshd.plist + +to _remove_ the lines: + + <key>Disabled</key> + <true/> + +Reboot the device and then sshd should be running. + +TODO +==== + +The server currently assumes that the device is well-behaved and does not do a +bunch of checks like looking for the expected SEQ and ACK numbers from it. This +is normally not an issue, but it's annoying for debugging because lost packets +(which shouldn't happen, but can happen if the code is buggy) mean that stuff +gets out of sync and then might crash and burn dozens of packets later. + +The server needs more testing, and some optimizing. + +Someone should probably do some edge-case testing on the TCP stuff. + +The outgoing ACK handling on the server probably needs some thought. Currently, +when there's an outstanding ACK, we send it after a timeout (to avoid sending +a no-payload ACK packet for everything the phone sends us). However, there's +probably a better way of doing this. + +Architecture information +======================== + +The iPhone / iPod Touch basically implements a rather strange USB networking +system that operates at a higher level. It is of course completely proprietary. +Generally speaking, this is what goes on in a typical usage scenario: + +0. iTunes opens a connection to usbmuxd and asks it for device notifications +1. User inserts phone into computer +2. usbmuxd notices the phone and pings it with a version packet +3. phone replies +4. usbmuxd now considers the phone to be connected and tells iTunes +5. iTunes opens another separate connection to usbmuxd and asks it to connect + to, say, the afc port on the device +6. usbmuxd sends a pseudo-TCP SYN packet to the phone +7. the phone's kernel driver receives the SYN packet and itself opens a + TCP connection to localhost on the afc port +8. the phone replies with a pseudo-TCP SYN/ACK indicating that the port is open + and the connection can proceed +7. usbmuxd sends a final ACK to the phone +8. usbmuxd replies to iTunes with a "connection successful" message +9. any data that iTunes writes to the usbmuxd socket from now on is forwarded, + through pseudo-TCP, through USB, back into a more regular TCP connection to + localhost, to the afc daemon on the phone, and vice versa + +The usbmuxd protocol is a relatively simple binary message protocol documented +here: + +http://wikee.iphwn.org/usb:usbmux + +Note that once a connection is established the UNIX socket essentially becomes +a dedicated pipe to the TCP connction and no more high-level control is +possible (closing the socket closes the TCP connection). Ditto for the "listen +for devices" mode - usbmuxd will reject any commands in such mode, and the +socket essentially becomes a dedicated device notification pipe. This means +that you need, at minimum, TWO connections to usbmuxd to do anything useful. + +On Windows, usbmuxd works the same way but a TCP connection to localhost port +27015 replaces the UNIX socket. On OSX, the UNIX socket is /var/run/usbmuxd. The +server and client implemented here default the same /var/run/usbmuxd socket. + +The phone protocol operates over a pair of USB bulk endpoints. There is an outer +layer used for packet size info and a "protocol" (version and TCP are the only +two options), and that header is followed by TCP headers for actual data comms. +However, the protocol isn't actual TCP, just a custom protocol which for some +reason uses a standard TCP header and leaves most fields unused. + +There is no reordering or retransmission. There are no checksums, no URG, no +PSH, no non-ACK, no FIN. What there *is* is the SEQ/ACK/window mechanism used +for flow control, and RST is used as the only connection teardown mechanism (and +also for "connection refused"), and the connection startup is SYN/SYNACK/ACK. + +Windows are constant-scaled by 8 bits. This is legal TCP as long as the +corresponding option is negotiated. Of course, no such negotiation happens on +this protocol. + +Note that, since there are no retransmissions, there is some overlap between ACK +and window for flow control. For example, the server doesn't ever touch its +window size, and just refuses to ACK stuff if its buffers are full and the +client isn't reading data. The phone happily seems to stop sending stuff. + +Also, if the phone RSTs you out of nowhere, look at the packet payload for a +textual error message. Note: if it claims to suffer from amnesia, that probably +means you overflowed its input buffer by ignoring its flow control / window +size. Go figure. Probably a logic bug in the kernel code. + +Note that all normal packets need to have flags set to ACK (and only ACK). There +is no support for, erm, not-acking. Keep the ack number field valid at all +times. + +The usbmuxd CONNECT request port field is byte-swapped (network-endian). This is +even more annoying for the plist based protocol, since it's even true there +(where the field is plain text). So even for the plain text int, you need to +swap the bytes (port 22 becomes <integer>5632</integer>). I have no clue if this +is the case on the new plist protocol on PPC macs (is the newer iTunes available +for those?) + +There are a bunch of gotchas due to the USB framing, and this is even worse +because implementations tend to get it wrong (i.e. libusb, and this is the +reason for the patch). Basically, USB Bulk offers, at the low level, the ability +to transfer packets from 0 to wMaxPacketSize (512 here) bytes, period. There is +no other support for higher level framing of transfers. The way you do those is +by breaking them up into packets, and the final shorter packet marks the end of +the transfer. The critical bit is that, if the transfer happens to be divisible +by 512, you send a zero-length packet (ZLP) to indicate the end of the transfer. +Libusb doesn't set this option by default and the iPhone gets packets stuck to +each other, which it doesn't like. Actually, this framing is sort of redundant +because the usbmux packet header includes a length field, but the phone still +wants the ZLPs or else it breaks. To make matters worse, usbdevfs imposes a max +transfer size of 16k, so libusb breaks transfers into that size. This is okay +for sending as long as the ZLP is only added to the last transfer (the patch +does that), but it can easily cause nasty race conditions on RX due to libusb +doing multiple outstanding reads at the same time and then cancelling the rest +when shorter data arrives (but what if some data got into the other requests +already?), so we only do 16k reads and stick them together ourselves by looking +at the packet size header. We still depend on ZLPs being sent to end transfers +at non-16k boundaries that are multiples of 512, but that seems to work fine. I +guess the ZLPs might cause spurious 0-byte transfers to show up on RX if things +line up right, but we ignore those. By the way, the maximum packet/transfer size +is 65535 bytes due to the 16-bit length header of the usbmux protocol. |