Various keepalive extensions

Add compatibility aliases for ProtocolKeepAlives and SetupTimeOut, supported
in previous versions of Debian's OpenSSH package but since superseded by
ServerAliveInterval.  (We're probably stuck with this bit for
compatibility.)

In batch mode, default ServerAliveInterval to five minutes.

Adjust documentation to match and to give some more advice on use of
keepalives.

Author: Ian Jackson <ian@chiark.greenend.org.uk>
Author: Matthew Vernon <matthew@debian.org>
Author: Colin Watson <cjwatson@debian.org>
Last-Update: 2020-02-21

Patch-Name: keepalive-extensions.patch

3 files changed, 34 insertions, 4 deletions

diff --git a/readconf.c b/readconf.c
index 2ccc48572..431243193 100644
--- a/readconf.c
+++ b/readconf.c
@@ -176,6 +176,7 @@ typedef enum {
         oFingerprintHash, oUpdateHostkeys, oHostbasedKeyTypes,
         oPubkeyAcceptedKeyTypes, oCASignatureAlgorithms, oProxyJump,
         oSecurityKeyProvider,
+        oProtocolKeepAlives, oSetupTimeOut,
         oIgnore, oIgnoredUnknownOption, oDeprecated, oUnsupported
 } OpCodes;
 
@@ -326,6 +327,8 @@ static struct {
         { "ignoreunknown", oIgnoreUnknown },
         { "proxyjump", oProxyJump },
         { "securitykeyprovider", oSecurityKeyProvider },
+        { "protocolkeepalives", oProtocolKeepAlives },
+        { "setuptimeout", oSetupTimeOut },
 
         { NULL, oBadOption }
 };
@@ -1519,6 +1522,8 @@ parse_keytypes:
                 goto parse_flag;
 
         case oServerAliveInterval:
+        case oProtocolKeepAlives: /* Debian-specific compatibility alias */
+        case oSetupTimeOut:       /* Debian-specific compatibility alias */
                 intptr = &options->server_alive_interval;
                 goto parse_time;
 
@@ -2222,8 +2227,13 @@ fill_default_options(Options * options)
                 options->rekey_interval = 0;
         if (options->verify_host_key_dns == -1)
                 options->verify_host_key_dns = 0;
-        if (options->server_alive_interval == -1)
-                options->server_alive_interval = 0;
+        if (options->server_alive_interval == -1) {
+                /* in batch mode, default is 5mins */
+                if (options->batch_mode == 1)
+                        options->server_alive_interval = 300;
+                else
+                        options->server_alive_interval = 0;
+        }
         if (options->server_alive_count_max == -1)
                 options->server_alive_count_max = 3;
         if (options->control_master == -1)
diff --git a/ssh_config.5 b/ssh_config.5
index e2a2359f9..85ab7447f 100644
--- a/ssh_config.5
+++ b/ssh_config.5
@@ -266,9 +266,13 @@ If set to
 .Cm yes ,
 user interaction such as password prompts and host key confirmation requests
 will be disabled.
+In addition, the
+.Cm ServerAliveInterval
+option will be set to 300 seconds by default (Debian-specific).
 This option is useful in scripts and other batch jobs where no user
 is present to interact with
-.Xr ssh 1 .
+.Xr ssh 1 ,
+and where it is desirable to detect a broken network swiftly.
 The argument must be
 .Cm yes
 or
@@ -1604,7 +1608,14 @@ from the server,
 will send a message through the encrypted
 channel to request a response from the server.
 The default
-is 0, indicating that these messages will not be sent to the server.
+is 0, indicating that these messages will not be sent to the server,
+or 300 if the
+.Cm BatchMode
+option is set (Debian-specific).
+.Cm ProtocolKeepAlives
+and
+.Cm SetupTimeOut
+are Debian-specific compatibility aliases for this option.
 .It Cm SetEnv
 Directly specify one or more environment variables and their contents to
 be sent to the server.
@@ -1684,6 +1695,12 @@ Specifies whether the system should send TCP keepalive messages to the
 other side.
 If they are sent, death of the connection or crash of one
 of the machines will be properly noticed.
+This option only uses TCP keepalives (as opposed to using ssh level
+keepalives), so takes a long time to notice when the connection dies.
+As such, you probably want
+the
+.Cm ServerAliveInterval
+option as well.
 However, this means that
 connections will die if the route is down temporarily, and some people
 find it annoying.
diff --git a/sshd_config.5 b/sshd_config.5
index 360e5fb1a..9f093be1f 100644
--- a/sshd_config.5
+++ b/sshd_config.5
@@ -1680,6 +1680,9 @@ This avoids infinitely hanging sessions.
 .Pp
 To disable TCP keepalive messages, the value should be set to
 .Cm no .
+.Pp
+This option was formerly called
+.Cm KeepAlive .
 .It Cm TrustedUserCAKeys
 Specifies a file containing public keys of certificate authorities that are
 trusted to sign user certificates for authentication, or