[PATCH] SSH: do not send spoofable TCP keep alive messages

["Peter Müller" <peter.mueller@ipfire.org>]

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By default, both SSH server and client rely on TCP-based keep alive
messages to detect broken sessions, which can be spoofed rather easily
in order to keep a broken session opened (and vice versa).

Since we rely on SSH-based keep alive messages, which are not vulnerable
to this kind of tampering, there is no need to double-check connections
via TCP keep alive as well.

This patch thereof disables using TCP keep alive for both SSH client and
server scenario. Further, {Client,Server}AliveCountMax default to 3,
which is sufficient (3 * 10 sec. = broken SSH connections die after 30
seconds), so we can omit that option. 60 seconds won't have any
advantage here.

Signed-off-by: Peter Müller <peter.mueller(a)ipfire.org>
---
 config/ssh/ssh_config  | 11 +++++++----
 config/ssh/sshd_config |  7 ++++---
 2 files changed, 11 insertions(+), 7 deletions(-)

diff --git a/config/ssh/ssh_config b/config/ssh/ssh_config
index 2e2ee60c3..ab0967086 100644
--- a/config/ssh/ssh_config
+++ b/config/ssh/ssh_config
@@ -5,7 +5,7 @@
 
 # Set some basic hardening options for all connections
 Host *
-        # Disable Roaming as it is known to be vulnerable
+        # Disable undocumented roaming feature as it is known to be vulnerable
         UseRoaming no
 
         # Only use secure crypto algorithms
@@ -13,15 +13,18 @@ Host *
         Ciphers chacha20-poly1305(a)openssh.com,aes256-gcm(a)openssh.com,aes128-gcm(a)openssh.com,aes256-ctr,aes192-ctr,aes128-ctr
         MACs hmac-sha2-512-etm(a)openssh.com,hmac-sha2-256-etm(a)openssh.com,umac-128-etm(a)openssh.com,hmac-sha2-512,hmac-sha2-256,umac-128(a)openssh.com
 
-        # Always visualise server host keys (but helps to identify key based MITM attacks)
+        # Always visualise server host keys (helps to identify key based MITM attacks)
         VisualHostKey yes
 
         # Use SSHFP (might work on some up-to-date networks) to look up host keys
         VerifyHostKeyDNS yes
 
-        # send keep-alive messages to connected server to avoid broken connections
+        # Send SSH-based keep alive messages to connected server to avoid broken connections
         ServerAliveInterval 10
-        ServerAliveCountMax 6
+
+	# Disable TCP keep alive messages since they can be spoofed and we have SSH-based
+	# keep alive messages enabled; there is no need to do things twice here
+	TCPKeepAlive no
 
         # Ensure only allowed authentication methods are used
         PreferredAuthentications publickey,keyboard-interactive,password
diff --git a/config/ssh/sshd_config b/config/ssh/sshd_config
index bea5cee53..a9eb5ff14 100644
--- a/config/ssh/sshd_config
+++ b/config/ssh/sshd_config
@@ -47,11 +47,12 @@ AllowTcpForwarding no
 AllowAgentForwarding no
 PermitOpen none
 
-# Detect broken sessions by sending keep-alive messages to clients via SSH connection
+# Send SSH-based keep alive messages every 10 seconds
 ClientAliveInterval 10
 
-# Close unresponsive SSH sessions which fail to answer keep-alive
-ClientAliveCountMax 6
+# Since TCP keep alive messages can be spoofed and we have the SSH-based already,
+# there is no need for this to be enabled as well
+TCPKeepAlive no
 
 # Add support for SFTP
 Subsystem	sftp	/usr/lib/openssh/sftp-server
-- 
2.26.2