Ssh – On AWS do I have to open ports in an EC2 instance’s firewall as well as security group

amazon ec2amazon-web-servicesiptablesredhatssh

If I change my SSH port from 22 to 23453, I can no longer ssh in.

In more detail, I'm using a Red Hat EC2 instance on Amazon Web Services. This is the second change I've on a fresh install (first change was to add a non-root user).

I can ssh in fine using Git Bash and a local .ssh/config file, I edit the line in /etc/ssh/sshd_config that currently says

#Port 23453

to say

Port 23453

then restart sshd with

sudo service sshd restart

I then add a line "Port 23453" my .ssh/config file

Host foo 
Hostname my-ec2-public-DNS
Port 23453
IdentityFile my ssl key

If I open another Git Bash shell (without closing my existing connection) and attempt to ssh into my instance (with ssh foo) I see the following error:

ssh: connect to host my-ec2-public-DNS port 23453: Bad file number

The security group attached to this instance has two entries, both TCP

22 (SSH) 0.0.0.0/0

23453 0.0.0.0/0

My best guess is that the port is still blocked by my firewall.

The output of sudo iptables -L is as follows

Chain INPUT (policy ACCEPT)
target     prot opt source               destination
ACCEPT     all  --  anywhere             anywhere            state RELATED,ESTABLISHED
ACCEPT     icmp --  anywhere             anywhere
ACCEPT     all  --  anywhere             anywhere
ACCEPT     tcp  --  anywhere             anywhere            state NEW tcp dpt:ssh
REJECT     all  --  anywhere             anywhere            reject-with icmp-host-prohibited

Chain FORWARD (policy ACCEPT)
target     prot opt source               destination
REJECT     all  --  anywhere             anywhere            reject-with icmp-host-prohibited

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination

Which looks pretty open to me.

UPDATE

After adding an iptables rule

iptables -A INPUT -p tcp --dport 23453 -j ACCEPT

and trying again, still no luck.

Output of iptables -L

Chain INPUT (policy ACCEPT)
target     prot opt source               destination
ACCEPT     all  --  anywhere             anywhere            state RELATED,ESTABLISHED
ACCEPT     icmp --  anywhere             anywhere
ACCEPT     all  --  anywhere             anywhere
ACCEPT     tcp  --  anywhere             anywhere            state NEW tcp dpt:ssh
REJECT     all  --  anywhere             anywhere            reject-with icmp-host-prohibited
ACCEPT     tcp  --  anywhere             anywhere            tcp dpt:23453

Chain FORWARD (policy ACCEPT)
target     prot opt source               destination
REJECT     all  --  anywhere             anywhere            reject-with icmp-host-prohibited

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination

Which looks sufficiently open. I'm not entirely sure how to look for packets coming in or activity on the port. But the output of netstat -ntlp (as root)

Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address               Foreign Address             State PID/Program name
tcp        0      0 0.0.0.0:56137               0.0.0.0:*                   LISTEN      948/rpc.statd
tcp        0      0 0.0.0.0:111                 0.0.0.0:*                   LISTEN      930/rpcbind
tcp        0      0 127.0.0.1:631               0.0.0.0:*                   LISTEN      1012/cupsd
tcp        0      0 127.0.0.1:25                0.0.0.0:*                   LISTEN      1224/master
tcp        0      0 0.0.0.0:23453               0.0.0.0:*                   LISTEN      32638/sshd
tcp        0      0 :::36139                    :::*                        LISTEN      948/rpc.statd
tcp        0      0 :::111                      :::*                        LISTEN      930/rpcbind
tcp        0      0 ::1:631                     :::*                        LISTEN      1012/cupsd
tcp        0      0 :::23453                    :::*                        LISTEN      32638/sshd

Which seems to me to show sshd on 23453.

I've checked again that the instance has the port open in the security group (Port: 23453, Protocol: tcp, Source: 0.0.0.0/0)

What else can be causing the failure to connect via SSH?

Cheers

POSTMORTEM

I can now connect. It was a missing rule in iptables. The output of iptables -L now looks like this:

Chain INPUT (policy ACCEPT)
target     prot opt source               destination
ACCEPT     all  --  anywhere             anywhere            state RELATED,ESTABLISHED
ACCEPT     icmp --  anywhere             anywhere
ACCEPT     tcp  --  anywhere             anywhere            tcp dpt:23453 state NEW
ACCEPT     all  --  anywhere             anywhere
ACCEPT     tcp  --  anywhere             anywhere            state NEW tcp dpt:ssh
REJECT     all  --  anywhere             anywhere            reject-with icmp-host-prohibited

Chain FORWARD (policy ACCEPT)
target     prot opt source               destination
REJECT     all  --  anywhere             anywhere            reject-with icmp-host-prohibited

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination

Best Answer

Your instance firewall doesn't have this port open. Try the following command:

iptables -I INPUT 3 -s 0.0.0.0/0 -d 0.0.0.0/0 -p tcp --dport 23453 -m state --state New -j ACCEPT

Note that iptables rules need to be saved to persist after a reboot. On RHEL that's:

/sbin/service iptables save

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Linux – iptables port forward forwarding

You're sending the traffic to 10.52.208.221. Given the config you posted, your problem is the webserver, not the firewall. Your rules look to be correct. FORWARD and INPUT are redirected to RH-Firewall-1-INPUT where your first rule is to allow all traffic. As somebody else pointed out, you could be allowing all traffic on eth1, while the world is actually coming in eth0. Secondary, you have to NAT the traffic as it goes back out to the world

iptables --table nat --append POSTROUTING --proto tcp --dport 80 --jump MASQUERADE -o OUT_INTERFACE

Your traffic originating from the router will never hit the input or forward chains, but instead traverse the output chain on to the webserver. Other systems in that subnet will similarly go directly to the webserver. Systems out on the world at large however will traverse the INPUT / FORWARD chains and need SNAT as well as DNAT so that it appears to the world to be one machine. You still cannot test from within your network. you must test from the opposite interface from the webserver. Get me your IP addresses and I'll point you to the proper configs.

Linux – Unable to make outbound SNMP connections when IPTables is enabled

You must put

ACCEPT     udp  --  anywhere             anywhere            udp spts:snmp:snmptrap dpts:1024:65535 state ESTABLISHED

before RH-Firewall-1-INPUT because of on RH-Firewall-1-INPUT rules there is REJECT on the end of line, the iptables read from top to down.

 Chain INPUT (policy ACCEPT)
 target     prot opt source               destination
 1 RH-Firewall-1-INPUT  all  --  anywhere             anywhere
 2 ACCEPT     udp  --  anywhere             anywhere            udp spts:snmp:snmptrap dpts:1024:65535 state ESTABLISHED

If you want to add using command line, you can use:

 iptables -I OUTPUT 1 -p udp -s 0/0 --sport 1024:65535 -d 0/0 --dport 161:162 -m state --state NEW,ESTABLISHED -j ACCEPT
 iptables -I INPUT 1 -p udp -s 0/0 --sport 161:162 -d 0/0 --dport 1024:65535 -m state --state ESTABLISHED -j ACCEPT

It should be like:

 Chain INPUT (policy ACCEPT)
 target     prot opt source               destination
 1 ACCEPT     udp  --  anywhere             anywhere            udp spts:snmp:snmptrap dpts:1024:65535 state ESTABLISHED
 2 RH-Firewall-1-INPUT  all  --  anywhere             anywhere

Best Answer

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