Add DebianBanner server configuration option

Setting this to "no" causes sshd to omit the Debian revision from its
initial protocol handshake, for those scared by package-versioning.patch.

Bug-Debian: http://bugs.debian.org/562048
Forwarded: not-needed
Last-Update: 2020-02-21

Patch-Name: debian-banner.patch

7 files changed, 23 insertions, 5 deletions

diff --git a/kex.c b/kex.c
index f638942d3..2abfbb95a 100644
--- a/kex.c
+++ b/kex.c
@@ -1226,7 +1226,7 @@ send_error(struct ssh *ssh, char *msg)
  */
 int
 kex_exchange_identification(struct ssh *ssh, int timeout_ms,
-    const char *version_addendum)
+    int debian_banner, const char *version_addendum)
 {
         int remote_major, remote_minor, mismatch;
         size_t len, i, n;
@@ -1244,7 +1244,8 @@ kex_exchange_identification(struct ssh *ssh, int timeout_ms,
         if (version_addendum != NULL && *version_addendum == '\0')
                 version_addendum = NULL;
         if ((r = sshbuf_putf(our_version, "SSH-%d.%d-%.100s%s%s\r\n",
-           PROTOCOL_MAJOR_2, PROTOCOL_MINOR_2, SSH_RELEASE,
+           PROTOCOL_MAJOR_2, PROTOCOL_MINOR_2,
+            debian_banner ? SSH_RELEASE : SSH_RELEASE_MINIMUM,
             version_addendum == NULL ? "" : " ",
             version_addendum == NULL ? "" : version_addendum)) != 0) {
                 error("%s: sshbuf_putf: %s", __func__, ssh_err(r));
diff --git a/kex.h b/kex.h
index fe7141414..938dca03b 100644
--- a/kex.h
+++ b/kex.h
@@ -194,7 +194,7 @@ char	*kex_names_cat(const char *, const char *);
 int      kex_assemble_names(char **, const char *, const char *);
 int      kex_gss_names_valid(const char *);
 
-int      kex_exchange_identification(struct ssh *, int, const char *);
+int      kex_exchange_identification(struct ssh *, int, int, const char *);
 
 struct kex *kex_new(void);
 int      kex_ready(struct ssh *, char *[PROPOSAL_MAX]);
diff --git a/servconf.c b/servconf.c
index bf3cd84a4..7bbc25c2e 100644
--- a/servconf.c
+++ b/servconf.c
@@ -194,6 +194,7 @@ initialize_server_options(ServerOptions *options)
         options->fingerprint_hash = -1;
         options->disable_forwarding = -1;
         options->expose_userauth_info = -1;
+        options->debian_banner = -1;
 }
 
 /* Returns 1 if a string option is unset or set to "none" or 0 otherwise. */
@@ -468,6 +469,8 @@ fill_default_server_options(ServerOptions *options)
                 options->expose_userauth_info = 0;
         if (options->sk_provider == NULL)
                 options->sk_provider = xstrdup("internal");
+        if (options->debian_banner == -1)
+                options->debian_banner = 1;
 
         assemble_algorithms(options);
 
@@ -556,6 +559,7 @@ typedef enum {
         sStreamLocalBindMask, sStreamLocalBindUnlink,
         sAllowStreamLocalForwarding, sFingerprintHash, sDisableForwarding,
         sExposeAuthInfo, sRDomain, sPubkeyAuthOptions, sSecurityKeyProvider,
+        sDebianBanner,
         sDeprecated, sIgnore, sUnsupported
 } ServerOpCodes;
 
@@ -719,6 +723,7 @@ static struct {
         { "rdomain", sRDomain, SSHCFG_ALL },
         { "casignaturealgorithms", sCASignatureAlgorithms, SSHCFG_ALL },
         { "securitykeyprovider", sSecurityKeyProvider, SSHCFG_GLOBAL },
+        { "debianbanner", sDebianBanner, SSHCFG_GLOBAL },
         { NULL, sBadOption, 0 }
 };
 
@@ -2382,6 +2387,10 @@ process_server_config_line_depth(ServerOptions *options, char *line,
                         *charptr = xstrdup(arg);
                 break;
 
+        case sDebianBanner:
+                intptr = &options->debian_banner;
+                goto parse_flag;
+
         case sDeprecated:
         case sIgnore:
         case sUnsupported:
diff --git a/servconf.h b/servconf.h
index 3f47ea25e..3fa05fcac 100644
--- a/servconf.h
+++ b/servconf.h
@@ -221,6 +221,8 @@ typedef struct {
         int     expose_userauth_info;
         u_int64_t timing_secret;
         char   *sk_provider;
+
+        int     debian_banner;
 }       ServerOptions;
 
 /* Information about the incoming connection as used by Match */
diff --git a/sshconnect.c b/sshconnect.c
index b796d3c8a..9f2412e0d 100644
--- a/sshconnect.c
+++ b/sshconnect.c
@@ -1292,7 +1292,7 @@ ssh_login(struct ssh *ssh, Sensitive *sensitive, const char *orighost,
         lowercase(host);
 
         /* Exchange protocol version identification strings with the server. */
-        if (kex_exchange_identification(ssh, timeout_ms, NULL) != 0)
+        if (kex_exchange_identification(ssh, timeout_ms, 1, NULL) != 0)
                 cleanup_exit(255); /* error already logged */
 
         /* Put the connection into non-blocking mode. */
diff --git a/sshd.c b/sshd.c
index 65916fc6d..da876a900 100644
--- a/sshd.c
+++ b/sshd.c
@@ -2187,7 +2187,8 @@ main(int ac, char **av)
         if (!debug_flag)
                 alarm(options.login_grace_time);
 
-        if (kex_exchange_identification(ssh, -1, options.version_addendum) != 0)
+        if (kex_exchange_identification(ssh, -1, options.debian_banner,
+            options.version_addendum) != 0)
                 cleanup_exit(255); /* error already logged */
 
         ssh_packet_set_nonblocking(ssh);
diff --git a/sshd_config.5 b/sshd_config.5
index ebd09f891..c926f584c 100644
--- a/sshd_config.5
+++ b/sshd_config.5
@@ -542,6 +542,11 @@ or
 .Cm no .
 The default is
 .Cm yes .
+.It Cm DebianBanner
+Specifies whether the distribution-specified extra version suffix is
+included during initial protocol handshake.
+The default is
+.Cm yes .
 .It Cm DenyGroups
 This keyword can be followed by a list of group name patterns, separated
 by spaces.