How to instruct SSHD to use its absolute path when forking?

[nolo...@gmail.com]

I'm having troubles with an updated OpenSSH server installed in `--prefix=/usr/local`. SSHD's path is `/usr/local/sbin/sshd`. When the server receives a connection, the connection fails.

The client displays the following when run with `-d`: "ssh_exchange_identification: Connection closed by remote host".

The server displays the following when run with `-d`: "sshd re-exec requires execution with an absolute path".

There were no configuration operations related to forking behavior. `./configure --help | egrep "(fork|re-exec)` returns nothing. Grepping the source code for interesting defines related to forking returned extraneous results. For example, `egrep -IR -i "(exec|fork)" * | grep define` returns about 10 hits, but none of them look like they control the behavior.

How do I tell SSHD to use its absolute path when forking?

[nolo...@gmail.com]

On Sunday, August 23, 2015 at 1:56:07 PM UTC-4, nolo...@gmail.com wrote:
> I'm having troubles with an updated OpenSSH server installed in `--prefix=/usr/local`. SSHD's path is `/usr/local/sbin/sshd`. When the server receives a connection, the connection fails.
>
> The client displays the following when run with `-d`: "ssh_exchange_identification: Connection closed by remote host".
>
> The server displays the following when run with `-d`: "sshd re-exec requires execution with an absolute path".
>

It appears the error message "sshd re-exec requires execution with an absolute path" is caused by the following in `sshd.c`. It is present near line 1625:

if (!test_flag && (rexec_flag && (av[0] == NULL || *av[0] != '/')))
fatal("sshd re-exec requires execution with an absolute path");

Changing the code to print av[0] shows there is no executable or path:

if (!test_flag && (rexec_flag && (av[0] == NULL || *av[0] != '/')))
fatal("sshd re-exec requires execution with an absolute path (re-exec with \"%s\")", (av[0] ? av[0] : "<NULL>"));

The new message (because of the changes above) includes "(re-exec with "-i -d")"

This is OS X 10-5 on a old PowerMac, and the service is being started by Launchd and a plist file. Unfortunately, changing the plist to explicitly provide the absolute path in argv[0] (the full cat is shown below):

<key>ProgramArguments</key>
<array>
<string>/usr/local/sbin/sshd -i -d</string>
</array>

Results in the following. Notice sshd.c still stripped the absolute path I entered at argv[0]:

$ sudo grep 'sshd' /var/log/* 2>/dev/null
/var/log/system.log:Aug 23 15:51:22 riemann.local sshd -i -d[1243]: error: Bind to port 22 on 0.0.0.0 failed: Address already in use.
/var/log/system.log:Aug 23 15:51:22 riemann.local sshd -i -d[1243]: error: Bind to port 22 on :: failed: Address already in use.
/var/log/system.log:Aug 23 15:51:22 riemann.local sshd -i -d[1243]: fatal: Cannot bind any address.

> How do I tell SSHD to use its absolute path when forking?

The problem seems a little deeper than simply adding an absolute path at argv[0].

Help would be appreciated.

-----

$ cat ssh-7.1.plist
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
<key>Disabled</key>
<true/>
<key>Label</key>
<string>com.openssh.sshd-v7.1</string>
<key>Program</key>
<string>/usr/local/sbin/sshd</string>
<key>ProgramArguments</key>
<array>
<string>/usr/local/sbin/sshd -i -d</string>
</array>
<key>Sockets</key>
<dict>
<key>Listeners</key>
<dict>
<key>SockServiceName</key>
<string>1522</string>
</dict>
</dict>
<key>inetdCompatibility</key>
<dict>
<key>Wait</key>
<false/>
</dict>
<key>StandardErrorPath</key>
<string>/var/log/sshd.log</string>
<key>SHAuthorizationRight</key>
<string>system.preferences</string>
</dict>
</plist>

[Simon Tatham]

<nolo...@gmail.com> wrote:
> This is OS X 10-5 on a old PowerMac, and the service is being started by
> Launchd and a plist file. Unfortunately, changing the plist to
> explicitly provide the absolute path in argv[0] (the full cat is shown
> below):
>
> <key>ProgramArguments</key>
> <array>
> <string>/usr/local/sbin/sshd -i -d</string>
> </array>

I don't know the plist semantics for sure, but if I've made sense of
the format, surely that would set the following argument list:

argc = 1
argv[0] = "/usr/local/sbin/sshd -i -d"

whereas what you wanted was

argc = 3
argv[0] = "/usr/local/sbin/sshd"
argv[1] = "-i"
argv[2] = "-d"

so that argv[0] is _just_ the absolute path.

What would happen if you did this instead?

<array>
<string>/usr/local/sbin/sshd</string>
<string>-i</string>
<string>-d</string>
</array>

--
for k in [pow(x,37,0x1a1298d262b49c895d47f) for x in [0x50deb914257022de7fff,
0x213558f2215127d5a2d1, 0x90c99e86d08b91218630, 0x109f3d0cfbf640c0beee7,
0xc83e01379a5fbec5fdd1, 0x19d3d70a8d567e388600e, 0x534e2f6e8a4a33155123]]:
print "".join([chr(32+3*((k>>x)&1))for x in range(79)]) # <ana...@pobox.com>

[nolo...@gmail.com]

On Sunday, August 23, 2015 at 5:34:06 PM UTC-4, Simon Tatham wrote:
> <nolo...@gmail.com> wrote:
> > This is OS X 10-5 on a old PowerMac, and the service is being started by
> > Launchd and a plist file. Unfortunately, changing the plist to
> > explicitly provide the absolute path in argv[0] (the full cat is shown
> > below):
> >
> > <key>ProgramArguments</key>
> > <array>
> > <string>/usr/local/sbin/sshd -i -d</string>
> > </array>
>
> I don't know the plist semantics for sure, but if I've made sense of
> the format, surely that would set the following argument list:
>
> argc = 1
> argv[0] = "/usr/local/sbin/sshd -i -d"
>
> whereas what you wanted was
>
> argc = 3
> argv[0] = "/usr/local/sbin/sshd"
> argv[1] = "-i"
> argv[2] = "-d"
>

Yep, that was it. Thanks.