openssh unix dev - Aug 2016 - [PATCH] Make "ssh" try different configuration filenames

To provide a bit more backwards-compatible (which is nice for eg. NFS-
shared /home directories) try a few version-number based names.

Eg., for "OpenSSH_7.3" the strings that are tried after
"~/.ssh/config"
are "_7.3", "_7", and "".
---
 ssh.c | 37 ++++++++++++++++++++++++++++++-------
 1 file changed, 30 insertions(+), 7 deletions(-)

diff --git a/ssh.c b/ssh.c
index 03a23fb..25359fe 100644
--- a/ssh.c
+++ b/ssh.c
@@ -464,7 +464,8 @@ static void
 process_config_files(const char *host_arg, struct passwd *pw, int post_canon)
 {
 	char buf[PATH_MAX];
-	int r;
+	char *version_postfix;
+	int r, len;
 
 	if (config != NULL) {
 		if (strcasecmp(config, "none") != 0 &&
@@ -473,12 +474,34 @@ process_config_files(const char *host_arg, struct passwd
*pw, int post_canon)
 			fatal("Can't open user config file %.100s: "
 			    "%.100s", config, strerror(errno));
 	} else {
-		r = snprintf(buf, sizeof buf, "%s/%s", pw->pw_dir,
-		    _PATH_SSH_USER_CONFFILE);
-		if (r > 0 && (size_t)r < sizeof(buf))
-			(void)read_config_file(buf, pw, host, host_arg,
-			    &options, SSHCONF_CHECKPERM | SSHCONF_USERCONF |
-			    (post_canon ? SSHCONF_POSTCANON : 0));
+		version_postfix = strchr(SSH_VERSION, '_');
+		if (!version_postfix)
+			version_postfix = "";
+
+		/* Find the best fitting config file,
+		 * Ie. try "_7.3", "_7", and "". */
+		len = strlen(version_postfix);
+		while (1) {
+			r = snprintf(buf, sizeof buf, "%s/%s%.*s", pw->pw_dir,
+					_PATH_SSH_USER_CONFFILE,
+					len, version_postfix);
+			if (r > 0 && (size_t)r < sizeof(buf))
+				if (read_config_file(buf, pw, host, host_arg,
+							&options, SSHCONF_CHECKPERM | SSHCONF_USERCONF |
+							(post_canon ? SSHCONF_POSTCANON : 0)))
+					break;
+
+			/* Nothing to look at */
+			if (!len)
+				break;
+
+			/* Try a smaller fit; skip last digits, then non-digits. */
+			len--;
+			while (len &&  isdigit(version_postfix[len-1]))
+				len--;
+			while (len && !isdigit(version_postfix[len-1]))
+				len--;
+		}
 
 		/* Read systemwide configuration file after user config. */
 		(void)read_config_file(_PATH_HOST_CONFIG_FILE, pw,
-- 
2.9.3

On Mon, 29 Aug 2016, Philipp Marek wrote:
> To provide a bit more backwards-compatible (which is nice for eg. NFS-
> shared /home directories) try a few version-number based names.
I'm not sure about this. We already have an IgnoreUnknown directive to
skip keywords that aren't supported. Perhaps we could consider adding a
percent_expand() to the include directive or a "localversion" clause
to
the "Match" keyword to get this capability in a more general form.
E.g.

Match localversion 7.*
  Include ~/.ssh/config/config-7x

-d

Hi Damien,


thanks for the quick feedback!

> > To provide a bit more backwards-compatible (which is nice for eg. NFS-
> > shared /home directories) try a few version-number based names.
> 
> I'm not sure about this. We already have an IgnoreUnknown directive to
> skip keywords that aren't supported.
    "Bad configuration option: IgnoreUnknown"

> Perhaps we could consider adding a
> percent_expand() to the include directive or a "localversion"
clause to
> the "Match" keyword to get this capability in a more general
form. E.g.
> 
> Match localversion 7.*
>   Include ~/.ssh/config/config-7x
That wouldn't help with old versions...
My patch would allow a newer SSH to read "its" config file, which
(among
other things) "Include"s the files for older versions as well...



Thanks for thinking about the merits!