ProxyJump may construct erroneous ProxyCommand

[Rob Leslie]

> On Jan 14, 2024, at 2:14 PM, Damien Miller <djm at mindrot.org> wrote:
> 
> On Sat, 13 Jan 2024, Rob Leslie wrote:
> 
>> Hello,
>> 
>> On macOS, Terminal’s “New Remote Connection…” command runs ssh in a new window like this:
>> 
>> login -pfq $USER /usr/bin/ssh $HOST
>> 
>> Here, login executes /usr/bin/ssh with argv[0] set to “-ssh”.
>> 
>> If $HOST has a ProxyJump configuration, the resulting ProxyCommand is:
>> 
>> -ssh -W '[%h]:%p' $JUMP_HOST
>> 
>> Because of the leading hyphen, this fails to execute. If the user’s shell is zsh, the Terminal window shows:
>> 
>> zsh:1: unknown exec flag -s
>> 
>> Would it make sense to ignore any leading hyphen when constructing the ProxyCommand from ProxyJump?
>> 
>> % ssh -V       
>> OpenSSH_9.4p1, LibreSSL 3.3.6
> 
> This sounds more like a problem in OSX Terminal.app than ssh.

I’m not sure why Terminal.app invokes login rather than ssh directly, but I think executing a program with the first character of argv[0] set to a hyphen to indicate a login session is not an uncommon convention.

> We could do something like this:
> 
> 
> diff --git a/ssh.c b/ssh.c
> index 48d93ddf2..7cd498f84 100644
> --- a/ssh.c
> +++ b/ssh.c
> @@ -1313,7 +1313,7 @@ main(int ac, char **av)
>  * Try to use SSH indicated by argv[0], but fall back to
>  * "ssh" if it appears unavailable.
>  */
> - if (strchr(argv0, '/') != NULL && access(argv0, X_OK) != 0)
> + if (access(argv0, X_OK) != 0)
> sshbin = "ssh";
> 
> /* Consistency check */


I was thinking perhaps something like this:


diff --git a/ssh.c b/ssh.c
index 0019281f4..4c80e0df6 100644
--- a/ssh.c
+++ b/ssh.c
@@ -1313,7 +1313,9 @@ main(int ac, char **av)
                 * Try to use SSH indicated by argv[0], but fall back to
                 * "ssh" if it appears unavailable.
                 */
-               if (strchr(argv0, '/') != NULL && access(argv0, X_OK) != 0)
+               if (*sshbin == '-')
+                       ++sshbin;
+               if (strchr(sshbin, '/') != NULL && access(sshbin, X_OK) != 0)
                        sshbin = "ssh";
                 /* Consistency check */