https://bugzilla.mindrot.org/show_bug.cgi?id=2855
Bug ID: 2855
Summary: Opening tun devices fails in OpenSSH v7.7
Product: Portable OpenSSH
Version: -current
Hardware: All
OS: Linux
Status: NEW
Severity: normal
Priority: P5
Component: ssh
Assignee: unassigned-b...@mindrot.org
Reporter: nschi...@adversec.com
I've experienced some strange behaviour in OpenSSH portable v7.7 (on
ArchLinux) when I tried to create a layer-3 VPN connection. Running ssh
with "-w 444:444" (or any other argument, even "any:any") failed with
"Tunnel device open failed.". I traced this error back to the following
check [1]:
"""
if (ifname != NULL && (*ifname = strdup(ifr.ifr_name)))
goto failed;
"""
In my case this check is always true, resulting in the error message
mentioned above. The following strace output shows how ssh successfully
opens and configures the tun device, but ultimately goes to "failed":
"""
[pid 6229] write(2, "debug1: Requesting tun unit 444 "..., 43debug1:
Requesting tun unit 444 in mode 1
) = 43
[pid 6229] openat(AT_FDCWD, "/dev/net/tun", O_RDWR) = 3
[pid 6229] ioctl(3, TUNSETIFF, 0x7ffca40818b0) = 0
[pid 6229] write(2, "debug1: sys_tun_open: tun444 mod"..., 42debug1:
sys_tun_open: tun444 mode 1 fd 3
) = 42
[pid 6229] close(3) = 0
[pid 6229] write(2, "Tunnel device open failed.\r\n", 28Tunnel device
open failed.
"""
The write call after ioctl corresponds to the debug output here [2].
Where everything seems to be fine, but then it just closes the fd and
returns "an error". I don't see why the check in [1] should fail, I
haven't run it in gdb yet. But maybe the issue is more obvious to you
guys?
Rolling back to 7.6 resolves the issue, seems like this has been
introduced in 7.7 (the files have also been renamed).
[1]
https://github.com/openssh/openssh-portable/blob/V_7_7/openbsd-compat/port-net.c#L188-L189
[2]
https://github.com/openssh/openssh-portable/blob/V_7_7/openbsd-compat/port-net.c#L186
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