[PATCH v3 1/1] fs/smb/client/fs_context: Add hostname option for CIFS module to work with domain-based dfs resources with Kerberos authentication

[Steve French]

tentatively merged into cifs-2.6.git for-next pending more review and testing.

Maria,
Have you verified that behavior hasn't changed when hostname not
specified on mount - the fallback to default hostname code was a
little confusing to read?

On Tue, Dec 30, 2025 at 10:48 AM Maria Alexeeva <alxvmr at altlinux.org> wrote:
>
> Paths to domain-based dfs resources are defined using the domain name
> of the server in the format:
> \\<DOMAIN.NAME>\<dfsroot>\<path>
>
> The CIFS module, when requesting a TGS, uses the server name
> (<DOMAIN.NAME>) it obtained from the UNC for the initial connection.
> It then composes an SPN that does not match any entities
> in the domain because it is the domain name itself.
>
> To eliminate this behavior, a hostname option is added, which is
> the name of the server to connect to and is used in composing the SPN.
> In the future this option will be used in the cifs-utils development.
>
> Suggested-by: Ivan Volchenko <ivolchenko86 at gmail.com>
> Signed-off-by: Maria Alexeeva <alxvmr at altlinux.org>
> ---
>  fs/smb/client/fs_context.c | 35 +++++++++++++++++++++++++++++------
>  fs/smb/client/fs_context.h |  3 +++
>  2 files changed, 32 insertions(+), 6 deletions(-)
>
> diff --git a/fs/smb/client/fs_context.c b/fs/smb/client/fs_context.c
> index d4291d3a9a48..f0d1895fe4bb 100644
> --- a/fs/smb/client/fs_context.c
> +++ b/fs/smb/client/fs_context.c
> @@ -177,6 +177,7 @@ const struct fs_parameter_spec smb3_fs_parameters[] = {
>         fsparam_string("password2", Opt_pass2),
>         fsparam_string("ip", Opt_ip),
>         fsparam_string("addr", Opt_ip),
> +       fsparam_string("hostname", Opt_hostname),
>         fsparam_string("domain", Opt_domain),
>         fsparam_string("dom", Opt_domain),
>         fsparam_string("srcaddr", Opt_srcaddr),
> @@ -866,16 +867,23 @@ static int smb3_fs_context_validate(struct fs_context *fc)
>                 return -ENOENT;
>         }
>
> +       if (ctx->got_opt_hostname) {
> +               kfree(ctx->server_hostname);
> +               ctx->server_hostname = ctx->opt_hostname;
> +               pr_notice("changing server hostname to name provided in hostname= option\n");
> +       }
> +
>         if (!ctx->got_ip) {
>                 int len;
> -               const char *slash;
>
> -               /* No ip= option specified? Try to get it from UNC */
> -               /* Use the address part of the UNC. */
> -               slash = strchr(&ctx->UNC[2], '\\');
> -               len = slash - &ctx->UNC[2];
> +               /*
> +                * No ip= option specified? Try to get it from server_hostname
> +                * Use the address part of the UNC parsed into server_hostname
> +                * or hostname= option if specified.
> +                */
> +               len = strlen(ctx->server_hostname);
>                 if (!cifs_convert_address((struct sockaddr *)&ctx->dstaddr,
> -                                         &ctx->UNC[2], len)) {
> +                                         ctx->server_hostname, len)) {
>                         pr_err("Unable to determine destination address\n");
>                         return -EHOSTUNREACH;
>                 }
> @@ -1591,6 +1599,21 @@ static int smb3_fs_context_parse_param(struct fs_context *fc,
>                 }
>                 ctx->got_ip = true;
>                 break;
> +       case Opt_hostname:
> +               if (strnlen(param->string, CIFS_NI_MAXHOST) == CIFS_NI_MAXHOST) {
> +                       pr_warn("host name too long\n");
> +                       goto cifs_parse_mount_err;
> +               }
> +
> +               kfree(ctx->opt_hostname);
> +               ctx->opt_hostname = kstrdup(param->string, GFP_KERNEL);
> +               if (ctx->opt_hostname == NULL) {
> +                       cifs_errorf(fc, "OOM when copying hostname string\n");
> +                       goto cifs_parse_mount_err;
> +               }
> +               cifs_dbg(FYI, "Host name set\n");
> +               ctx->got_opt_hostname = true;
> +               break;
>         case Opt_domain:
>                 if (strnlen(param->string, CIFS_MAX_DOMAINNAME_LEN)
>                                 == CIFS_MAX_DOMAINNAME_LEN) {
> diff --git a/fs/smb/client/fs_context.h b/fs/smb/client/fs_context.h
> index 7af7cbbe4208..ff1a04661ef5 100644
> --- a/fs/smb/client/fs_context.h
> +++ b/fs/smb/client/fs_context.h
> @@ -184,6 +184,7 @@ enum cifs_param {
>         Opt_pass,
>         Opt_pass2,
>         Opt_ip,
> +       Opt_hostname,
>         Opt_domain,
>         Opt_srcaddr,
>         Opt_iocharset,
> @@ -214,6 +215,7 @@ struct smb3_fs_context {
>         bool gid_specified;
>         bool sloppy;
>         bool got_ip;
> +       bool got_opt_hostname;
>         bool got_version;
>         bool got_rsize;
>         bool got_wsize;
> @@ -226,6 +228,7 @@ struct smb3_fs_context {
>         char *domainname;
>         char *source;
>         char *server_hostname;
> +       char *opt_hostname;
>         char *UNC;
>         char *nodename;
>         char workstation_name[CIFS_MAX_WORKSTATION_LEN];
> --
> 2.50.1
>


-- 
Thanks,

Steve