From ab3a7fc6d2ea01cd4a17607398d151bb5fe2e63b Mon Sep 17 00:00:00 2001
From: sebres <serg.brester@sebres.de>
Date: Mon, 17 Feb 2020 16:24:42 +0100
Subject: [PATCH] filter.d/sshd.conf: mode `ddos` (and aggressive) extended to
 detect port scanner sending unexpected ident string after connect

---
 config/filter.d/sshd.conf      | 3 ++-
 fail2ban/tests/files/logs/sshd | 5 +++++
 2 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/config/filter.d/sshd.conf b/config/filter.d/sshd.conf
index c61cf960..12631cb3 100644
--- a/config/filter.d/sshd.conf
+++ b/config/filter.d/sshd.conf
@@ -52,7 +52,7 @@ cmnfailre = ^[aA]uthentication (?:failure|error|failed) for <F-USER>.*</F-USER>
             ^User <F-USER>.+</F-USER> from <HOST> not allowed because a group is listed in DenyGroups%(__suff)s$
             ^User <F-USER>.+</F-USER> from <HOST> not allowed because none of user's groups are listed in AllowGroups%(__suff)s$
             ^<F-NOFAIL>%(__pam_auth)s\(sshd:auth\):\s+authentication failure;</F-NOFAIL>(?:\s+(?:(?:logname|e?uid|tty)=\S*)){0,4}\s+ruser=<F-ALT_USER>\S*</F-ALT_USER>\s+rhost=<HOST>(?:\s+user=<F-USER>\S*</F-USER>)?%(__suff)s$
-            ^(error: )?maximum authentication attempts exceeded for <F-USER>.*</F-USER> from <HOST>%(__on_port_opt)s(?: ssh\d*)?%(__suff)s$
+            ^maximum authentication attempts exceeded for <F-USER>.*</F-USER> from <HOST>%(__on_port_opt)s(?: ssh\d*)?%(__suff)s$
             ^User <F-USER>.+</F-USER> not allowed because account is locked%(__suff)s
             ^<F-MLFFORGET>Disconnecting</F-MLFFORGET>(?: from)?(?: (?:invalid|authenticating)) user <F-USER>\S+</F-USER> <HOST>%(__on_port_opt)s:\s*Change of username or service not allowed:\s*.*\[preauth\]\s*$
             ^Disconnecting: Too many authentication failures(?: for <F-USER>.+?</F-USER>)?%(__suff)s$
@@ -71,6 +71,7 @@ mdre-normal =
 mdre-normal-other = ^<F-NOFAIL><F-MLFFORGET>(Connection closed|Disconnected)</F-MLFFORGET></F-NOFAIL> (?:by|from)%(__authng_user)s <HOST>(?:%(__suff)s|\s*)$
 
 mdre-ddos = ^Did not receive identification string from <HOST>
+            ^kex_exchange_identification: client sent invalid protocol identifier
             ^Bad protocol version identification '.*' from <HOST>
             ^Connection <F-MLFFORGET>reset</F-MLFFORGET> by <HOST>
             ^<F-NOFAIL>SSH: Server;Ltype:</F-NOFAIL> (?:Authname|Version|Kex);Remote: <HOST>-\d+;[A-Z]\w+:
diff --git a/fail2ban/tests/files/logs/sshd b/fail2ban/tests/files/logs/sshd
index 3b4f0a0a..0385f38c 100644
--- a/fail2ban/tests/files/logs/sshd
+++ b/fail2ban/tests/files/logs/sshd
@@ -305,6 +305,11 @@ Jul 17 23:04:01 srv sshd[1300]: Connection closed by authenticating user test 12
 
 # filterOptions: [{"test.condition":"name=='sshd'", "mode": "ddos"}, {"test.condition":"name=='sshd'", "mode": "aggressive"}]
 
+# failJSON: { "match": false }
+Feb 17 17:40:17 sshd[19725]: Connection from 192.0.2.10 port 62004 on 192.0.2.10 port 22
+# failJSON: { "time": "2005-02-17T17:40:17", "match": true , "host": "192.0.2.10", "desc": "ddos: port scanner (invalid protocol identifier)" }
+Feb 17 17:40:17 sshd[19725]: error: kex_exchange_identification: client sent invalid protocol identifier ""
+
 # failJSON: { "time": "2005-03-15T09:21:01", "match": true , "host": "192.0.2.212", "desc": "DDOS mode causes failure on close within preauth stage" }
 Mar 15 09:21:01 host sshd[2717]: Connection closed by 192.0.2.212 [preauth]
 # failJSON: { "time": "2005-03-15T09:21:02", "match": true , "host": "192.0.2.212", "desc": "DDOS mode causes failure on close within preauth stage" }