How to find other end of unix socket connection?

Question

I have a process (dbus-daemon) which has many open connection over UNIX sockets. One of these connections is fd #36:

=$ ps uw -p 23284
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
depesz   23284  0.0  0.0  24680  1772 ?        Ss   15:25   0:00 /bin/dbus-daemon --fork --print-pid 5 --print-address 7 --session

=$ ls -l /proc/23284/fd/36 
lrwx------ 1 depesz depesz 64 2011-03-28 15:32 /proc/23284/fd/36 -> socket:[1013410]

=$ netstat -nxp | grep 1013410
(Not all processes could be identified, non-owned process info
 will not be shown, you would have to be root to see it all.)
unix  3      [ ]         STREAM     CONNECTED     1013410  23284/dbus-daemon   @/tmp/dbus-3XDU4PYEzD

=$ netstat -nxp | grep dbus-3XDU4PYEzD
unix  3      [ ]         STREAM     CONNECTED     1013953  23284/dbus-daemon   @/tmp/dbus-3XDU4PYEzD
unix  3      [ ]         STREAM     CONNECTED     1013825  23284/dbus-daemon   @/tmp/dbus-3XDU4PYEzD
unix  3      [ ]         STREAM     CONNECTED     1013726  23284/dbus-daemon   @/tmp/dbus-3XDU4PYEzD
unix  3      [ ]         STREAM     CONNECTED     1013471  23284/dbus-daemon   @/tmp/dbus-3XDU4PYEzD
unix  3      [ ]         STREAM     CONNECTED     1013410  23284/dbus-daemon   @/tmp/dbus-3XDU4PYEzD
unix  3      [ ]         STREAM     CONNECTED     1012325  23284/dbus-daemon   @/tmp/dbus-3XDU4PYEzD
unix  3      [ ]         STREAM     CONNECTED     1012302  23284/dbus-daemon   @/tmp/dbus-3XDU4PYEzD
unix  3      [ ]         STREAM     CONNECTED     1012289  23284/dbus-daemon   @/tmp/dbus-3XDU4PYEzD
unix  3      [ ]         STREAM     CONNECTED     1012151  23284/dbus-daemon   @/tmp/dbus-3XDU4PYEzD
unix  3      [ ]         STREAM     CONNECTED     1011957  23284/dbus-daemon   @/tmp/dbus-3XDU4PYEzD
unix  3      [ ]         STREAM     CONNECTED     1011937  23284/dbus-daemon   @/tmp/dbus-3XDU4PYEzD
unix  3      [ ]         STREAM     CONNECTED     1011900  23284/dbus-daemon   @/tmp/dbus-3XDU4PYEzD
unix  3      [ ]         STREAM     CONNECTED     1011775  23284/dbus-daemon   @/tmp/dbus-3XDU4PYEzD
unix  3      [ ]         STREAM     CONNECTED     1011771  23284/dbus-daemon   @/tmp/dbus-3XDU4PYEzD
unix  3      [ ]         STREAM     CONNECTED     1011769  23284/dbus-daemon   @/tmp/dbus-3XDU4PYEzD
unix  3      [ ]         STREAM     CONNECTED     1011766  23284/dbus-daemon   @/tmp/dbus-3XDU4PYEzD
unix  3      [ ]         STREAM     CONNECTED     1011663  23284/dbus-daemon   @/tmp/dbus-3XDU4PYEzD
unix  3      [ ]         STREAM     CONNECTED     1011635  23284/dbus-daemon   @/tmp/dbus-3XDU4PYEzD
unix  3      [ ]         STREAM     CONNECTED     1011627  23284/dbus-daemon   @/tmp/dbus-3XDU4PYEzD
unix  3      [ ]         STREAM     CONNECTED     1011540  23284/dbus-daemon   @/tmp/dbus-3XDU4PYEzD
unix  3      [ ]         STREAM     CONNECTED     1011480  23284/dbus-daemon   @/tmp/dbus-3XDU4PYEzD
unix  3      [ ]         STREAM     CONNECTED     1011349  23284/dbus-daemon   @/tmp/dbus-3XDU4PYEzD
unix  3      [ ]         STREAM     CONNECTED     1011312  23284/dbus-daemon   @/tmp/dbus-3XDU4PYEzD
unix  3      [ ]         STREAM     CONNECTED     1011284  23284/dbus-daemon   @/tmp/dbus-3XDU4PYEzD
unix  3      [ ]         STREAM     CONNECTED     1011250  23284/dbus-daemon   @/tmp/dbus-3XDU4PYEzD
unix  3      [ ]         STREAM     CONNECTED     1011231  23284/dbus-daemon   @/tmp/dbus-3XDU4PYEzD
unix  3      [ ]         STREAM     CONNECTED     1011155  23284/dbus-daemon   @/tmp/dbus-3XDU4PYEzD
unix  3      [ ]         STREAM     CONNECTED     1011061  23284/dbus-daemon   @/tmp/dbus-3XDU4PYEzD
unix  3      [ ]         STREAM     CONNECTED     1011049  23284/dbus-daemon   @/tmp/dbus-3XDU4PYEzD
unix  3      [ ]         STREAM     CONNECTED     1011035  23284/dbus-daemon   @/tmp/dbus-3XDU4PYEzD
unix  3      [ ]         STREAM     CONNECTED     1011013  23284/dbus-daemon   @/tmp/dbus-3XDU4PYEzD
unix  3      [ ]         STREAM     CONNECTED     1010961  23284/dbus-daemon   @/tmp/dbus-3XDU4PYEzD
unix  3      [ ]         STREAM     CONNECTED     1010945  23284/dbus-daemon   @/tmp/dbus-3XDU4PYEzD

Based on number connections, I assume that dbus-daemon is actually server. Which is OK. But how can I find which process is connected to it - using the connection that is 36th file handle in dbus-launcher? Tried lsof and even greps on /proc/net/unix but I can't figure out a way to find the client process.

Solution 1

Quite recently I stumbled upon a similar problem. I was shocked to find out that there are cases when this might not be possible. I dug up a comment from the creator of lsof (Vic Abell) where he pointed out that this depends heavily on unix socket implementation. Sometimes so called "endpoint" information for socket is available and sometimes not. Unfortunatelly it is impossible in Linux as he points out.

On Linux, for example, where lsof must use /proc/net/unix, all UNIX domain sockets have a bound path, but no endpoint information. Often there is no bound path. That often makes it impossible to determine the other endpoint, but it is a result of the Linux /proc file system implementation.

If you look at /proc/net/unix you can see for yourself, that (at least on my system) he is absolutelly right. I'm still shocked, because I find such feature essential while tracking server problems.

Solution 2

This answer is for Linux only.

Update for Linux 3.3: As Zulakis wrote in a separate answer (+1 that), you can use ss from iproute2 to get a pair of inode numbers for each socket connection identifying local end and peer. This appears to be based on the same machinery as sock_diag(7) with the UNIX_DIAG_PEER attribute identifying the peer. An answer by Totor over at Unix & Linux Stack Exchange links to the relevant commits in kernel and iproute2 and also mentions the need for the UNIX_DIAG kernel config setting.

Original answer for Linux pre 3.3 follows.

Based on an answer from the Unix & Linux Stack Exchange, I successfully identified the other end of a unix domain socket using in-kernel data structures, accessed using gdb and /proc/kcore. You need to enable the CONFIG_DEBUG_INFO and CONFIG_PROC_KCORE kernel options.

You can use lsof to get the kernel address of the socket, which takes the form of a pointer, e.g. 0xffff8803e256d9c0. That number is actually the address of the relevant in-kernel memory structure or type struct unix_sock. That structure has a field called peer which points at the other end of the socket. So the commands

# gdb /usr/src/linux/vmlinux /proc/kcore
(gdb) p ((struct unix_sock*)0xffff8803e256d9c0)->peer

will print the address of the other end of the connection. You can grep the output of lsof -U for that number to identify the process and file descriptor number of that other end.

Some distributions seem to provide kernel debug symbols as a separate package, which would take the place of the vmlinux file in the above command.

Solution 3

Actually, ss from iproute2 (replacement for netstat, ifconfig, etc.) can show this information.

Here is an example showing an ssh-agent unix domain socket to which a ssh process has connected:

$ sudo ss -a --unix -p
Netid  State      Recv-Q Send-Q Local                             Address:Port          Peer    Address:Port
u_str  ESTAB      0      0      /tmp/ssh-XxnMh2MdLBxo/agent.27402 651026                *       651642                users:(("ssh-agent",pid=27403,fd=4)
u_str  ESTAB      0      0       *                                651642                *       651026                users:(("ssh",pid=2019,fd=4))

Solution 4

Unix sockets usually are assigned numbers in pairs, and are usually consecutive. So the pair for you would likely be 1013410+/-1. See which of those two exists and guess at the culprit.

Solution 5

I wrote a tool which uses MvG's gdb method to reliably get socket peer information, kernel debug symbols not needed.

To get the process connected to a given socket, pass it the inode number:

# socket_peer 1013410
3703 thunderbird 

To find out for all processes at once use netstat_unix, it adds a column to netstat's output:

# netstat_unix
Proto RefCnt Flags       Type       State         I-Node   PID/Program name     Peer PID/Program name  Path
unix  3      [ ]         STREAM     CONNECTED     6825     982/Xorg             1497/compiz            /tmp/.X11-unix/X0
unix  3      [ ]         STREAM     CONNECTED     6824     1497/compiz          982/Xorg                 
unix  3      [ ]         SEQPACKET  CONNECTED     207142   3770/chromium-brows  17783/UMA-Session-R       
unix  3      [ ]         STREAM     CONNECTED     204903   1523/pulseaudio      3703/thunderbird       
unix  3      [ ]         STREAM     CONNECTED     204902   3703/thunderbird     1523/pulseaudio           
unix  3      [ ]         STREAM     CONNECTED     204666   1523/pulseaudio      3703/thunderbird       
...

Try netstat_unix --dump if you need output that's easy to parse.
See https://github.com/lemonsqueeze/unix_sockets_peers for details.

For info, the inode +1/-1 hack isn't reliable. It works most of the time but will fail or (worse) return the wrong socket if you're out of luck.

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Comments

[sandeep seervi]

That is answered on U&L: Who's got the other end of this unix socketpair?

[Admin]

This looks interesting, but requirement to recompile kernel seems to be an overkill. I'm thinking that perhaps it would be possible to do it, without hand-made kernel, and without using gdb, just by peeking at values in kcore and doing some "manual" decoding of values.

[MvG]

@depesz, all you need to know is the offset of the peer member in the unix_sock structure. On my x86_64 system, that offset is 656 bytes, so I could obtain that other end using p ((void**)0xffff8803e256d9c0)[0x52]. You still need CONFIG_PROC_KCORE, obviously.

[estani]

Reference: groups.google.com/forum/#!topic/comp.unix.admin/iZLsq5dHdyI

[SamB]

Hmm. Interesting... I had missed that the "Address:Port" columns can be matched, even though the "Peer" column is totally useless for unix domain sockets.

[i336_]

Note that /proc/net/unix WILL tell you the target file of a random domain socket reference you've dug out of /proc/.../fd/.