opensourcemirror
/
ipfire-2.x
mirror of git://git.ipfire.org/ipfire-2.x.git
1
0
Fork 0
Code Issues Releases Wiki Activity

SSH: do not send spoofable TCP keep alive messages 

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. For usability reasons, a timeout of 5 minutes (10
seconds * 30 keep alive messages = 300 seconds) will be used for both
client and server configuration, as 60 seconds were found to be too
short for unstable connectivity scenarios.

Signed-off-by: Peter Müller <peter.mueller@ipfire.org>
This commit is contained in:
Peter Müller 2022-04-18 20:40:41 +00:00
parent 018f80c6cd
commit 7a981d94cb
2 changed files with 13 additions and 7 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

12
config/ssh/ssh_config
View File

@ -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 +13,19 @@ Host *
Ciphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr
MACs hmac-sha2-512-etm@openssh.com,hmac-sha2-256-etm@openssh.com,umac-128-etm@openssh.com,hmac-sha2-512,hmac-sha2-256,umac-128@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
ServerAliveCountMax 30
# 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

8
config/ssh/sshd_config
View File

@ -46,11 +46,13 @@ 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 to connected clients to avoid broken connections
ClientAliveInterval 10
ClientAliveCountMax 30
# 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