Iptables rules to allow/block ssh incoming/outgoing connection in Linux

Let me show you some iptable rules which can be used to allow or block ssh connection from a specific host or network

Block 192.168.1.10 from connecting your localhost 192.168.1.6

[root@test1 ~]# iptables -I INPUT -s 192.168.1.10 -p tcp --dport ssh -j REJECT

Let us try to connect 192.168.1.6

[root@test ~]# ssh 192.168.1.6 -v
OpenSSH_5.3p1, OpenSSL 1.0.0-fips 29 Mar 2010
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: Applying options for *
debug1: Connecting to 192.168.1.6 [192.168.1.6] port 22.
debug1: connect to address 192.168.1.6 port 22: Connection refused
ssh: connect to host 192.168.1.6 port 22: Connection refused

So it straight away throws "Connection refused"

Let us check the packet received attempts on 192.168.1.6 made by 192.168.1.10

[root@test1 ~]# iptables -L INPUT -v
Chain INPUT (policy ACCEPT 19 packets, 1263 bytes)
pkts bytes target prot opt in out source destination
1 120 REJECT tcp -- any any 192.168.1.10 anywhere tcp dpt:ssh reject-with icmp-port-unreachable

So as per the log 1 attempt was made to connect to our machine from 192.168.1.10

Let us try some other ways to block ssh connection from a specific host

[root@test1 ~]# iptables -I INPUT -s 192.168.1.10 -p tcp --dport ssh -j DROP

Let us try to connect our 192.168.1.6 from 192.168.1.10

[root@test ~]# ssh 192.168.1.6 -v
OpenSSH_5.3p1, OpenSSL 1.0.0-fips 29 Mar 2010
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: Applying options for *
debug1: Connecting to 192.168.1.6 [192.168.1.6] port 22.
debug1: connect to address 192.168.1.6 port 22: Connection timed out
ssh: connect to host 192.168.1.6 port 22: Connection timed out

As you see a new message as compare to last rule.

Here we are accepting SYN signal from the remote host but we are not responding to it so there was no successful connection made between both the hosts.

Lets see no. of packets sent in an attempt to connect 192.168.1.6

[root@test1 ~]# iptables -L INPUT -v
Chain INPUT (policy ACCEPT 22 packets, 1552 bytes)
pkts bytes target prot opt in out source destination
3 180 DROP tcp -- any any 192.168.1.10 anywhere tcp dpt:ssh

3 packets or attempts were made by 192.168.1.10 before throwing "Connection timed out" error

Block ssh connection for ESTABLISHED state from 192.168.1.10

[root@test1 ~]# iptables -I INPUT -s 192.168.1.10 -p tcp --dport ssh -m state --state ESTABLISHED -j REJECT

Let us make an attempt for the same from 192.168.1.10

[root@test ~]# ssh 192.168.1.6 -v
OpenSSH_5.3p1, OpenSSL 1.0.0-fips 29 Mar 2010
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: Applying options for *
debug1: Connecting to 192.168.1.6 [192.168.1.6] port 22.
debug1: Connection established.
debug1: permanently_set_uid: 0/0
debug1: identity file /root/.ssh/identity type -1
debug1: identity file /root/.ssh/id_rsa type -1
debug1: identity file /root/.ssh/id_dsa type -1

The attempts gets stucked here after establishing a connection

Allow ssh connection from 192.168.1.10

# iptables -I INPUT -s 192.168.1.10 -p tcp --dport ssh -j ACCEPT

Let us make an attempt for the same from 192.168.1.10

[root@test ~]# ssh 192.168.1.6
root@192.168.1.6's password:
Last login: Thu Mar 6 11:19:32 2014 from 192.168.1.2
[root@test1 ~]#

We have successfully connected

Much more strict rule to block or allow the same (replace ACCEPT with REJECT to block)

[root@test1 ~]# iptables -I INPUT -s 192.168.1.10 -p tcp --dport ssh -m state --state NEW,ESTABLISHED,RELATED -j ACCEPT

State	Description
NEW	meaning that the packet has started a new connection, or otherwise associated with a connection which has not seen packets in both directions
ESTABLISHED	meaning that the packet is associated with a connection which has seen packets in both directions
RELATED	meaning that the packet is starting a new connection, but is associated with an existing connection, such as an FTP data transfer, or an ICMP error

Block ssh connection from all the host EXCEPT 192.168.1.2

# iptables -I INPUT ! -s 192.168.1.2 -p tcp --dport ssh -m state --state NEW,ESTABLISHED,RELATED -j REJECT

Let us also log this message to verify our rule
# iptables -I INPUT ! -s 192.168.1.2 -p tcp --dport ssh -m state --state NEW,ESTABLISHED,RELATED -j LOG --log-prefix "BLOCK SSH "

To know more about how to log iptable messages follow the below link
How to log iptables messages in different log file

Make an attempt to do ssh from any other machine in 192.168.1.0/24 network

[root@test ~]# ssh 192.168.1.6
ssh: connect to host 192.168.1.6 port 22: Connection refused

Verify in the logs

# cat /var/log/iptables | grep "BLOCK SSH"
Mar 6 14:03:55 test1 kernel: BLOCK SSH IN=eth3 OUT= MAC=00:0c:29:51:aa:e1:00:0c:29:a3:f5:fa:08:00 SRC=192.168.1.10 DST=192.168.1.6 LEN=60 TOS=0x00 PREC=0x00 TTL=64 ID=43914 DF PROTO=TCP SPT=35026 DPT=22 WINDOW=14600 RES=0x00 SYN URGP=0

Block outgoing ssh connection for 192.168.1.0/24 subnet

# iptables -I OUTPUT -d 192.168.1.0/24 -p tcp --dport 22 -j REJECT

Verify

# ssh -v 192.168.1.6
OpenSSH_5.3p1, OpenSSL 1.0.1e-fips 11 Feb 2013
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: Applying options for *
debug1: Connecting to 192.168.1.6 [192.168.1.6] port 22.
debug1: connect to address 192.168.1.6 port 22: Connection refused
ssh: connect to host 192.168.1.6 port 22: Connection refused

So as you see localhost is not allowed to make ssh connection to 192.168.1.0/24 subnet