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I'm trying to connect to my computer's SSH service from outside my network (Internet). The router is a Sagemcom 5655. In the router I configured port-forwarding as I did many times before in my life, but this time is not working.

Protocol: TCP External port: 9092 Forward to internal port: 22 Internal address: 192.168.1.141

The SSH server is working, I use it almost every day on my local network. But when I try to connect from the outside nothing happens.

This is how I connect to the server: my-user@internet-address -p 9092

The only thing I see is a log in /var/log/audit/audit.log that I don't understand:

type=CRYPTO_KEY_USER msg=audit(1574265360.622:648): pid=6212 uid=0 auid=4294967295 ses=4294967295 subj=system_u:system_r:sshd_t:s0-s0:c0.c1023 msg='op=destroy kind=server fp=SHA256:eb:31:df:3b:11:5f:0f:b8:ae:5a:84:65:58:4b:b1:83:48:d8:9a:d3:d6:1c:42:45:d3:9e:2a:3f:75:79:e6:7b direction=? spid=6212 suid=0  exe="/usr/sbin/sshd" hostname=? addr=? terminal=? res=success'UID="root" AUID="unset" SUID="root"
type=CRYPTO_KEY_USER msg=audit(1574265360.622:649): pid=6212 uid=0 auid=4294967295 ses=4294967295 subj=system_u:system_r:sshd_t:s0-s0:c0.c1023 msg='op=destroy kind=server fp=SHA256:25:0f:2c:39:d4:e0:e1:31:fa:cf:4a:2c:4d:1c:91:17:05:b2:94:5c:00:75:6e:28:76:3c:0d:18:48:a3:a3:93 direction=? spid=6212 suid=0  exe="/usr/sbin/sshd" hostname=? addr=? terminal=? res=success'UID="root" AUID="unset" SUID="root"
type=CRYPTO_KEY_USER msg=audit(1574265360.622:650): pid=6212 uid=0 auid=4294967295 ses=4294967295 subj=system_u:system_r:sshd_t:s0-s0:c0.c1023 msg='op=destroy kind=server fp=SHA256:17:2e:8f:02:82:78:ac:ed:eb:06:dd:93:6c:1a:a2:bc:4d:74:ee:c0:18:c1:23:39:5c:e1:a1:0d:04:9a:b6:98 direction=? spid=6212 suid=0  exe="/usr/sbin/sshd" hostname=? addr=? terminal=? res=success'UID="root" AUID="unset" SUID="root"

Is there any chance that something like SE Linux on my Fedora box may be blocking the connection?

I checked /etc/ssh/sshd_config and there's nothing strange. Also, I'm trying to connect with a normal user NOT root.

This is the output of iptables -nvL INPUT

 pkts bytes target     prot opt in     out     source               destination         
21396   14M LIBVIRT_INP  all  --  *      *       0.0.0.0/0            0.0.0.0/0           
16280   14M ACCEPT     all  --  *      *       0.0.0.0/0            0.0.0.0/0            ctstate RELATED,ESTABLISHED
  181 12188 ACCEPT     all  --  lo     *       0.0.0.0/0            0.0.0.0/0           
 4935  271K INPUT_direct  all  --  *      *       0.0.0.0/0            0.0.0.0/0           
 4935  271K INPUT_ZONES  all  --  *      *       0.0.0.0/0            0.0.0.0/0           
   13   520 DROP       all  --  *      *       0.0.0.0/0            0.0.0.0/0            ctstate INVALID
 1582 80252 REJECT     all  --  *      *       0.0.0.0/0            0.0.0.0/0            reject-with icmp-host-prohibited

When I go in the router's management system I can see the "Public IP address" as 200.80.*.*** (the * are just numbers I'm hidding, not wildcards!). I also see that my router is configured to reply to PING. But when I ping or portscan that IP nothing happends.

I have also tested connecting from the outside, connecting my laptop to 4G from my phone's internet connection. Same results.

Thanks!

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8
  • You might also have a firewall enabled: add the output of `iptables -nvL INPUT
    – Piotr P. Karwasz
    Nov 20, 2019 at 17:10
  • Hi, I just added the output. Thanks.
    – Eduardo G.R.
    Nov 20, 2019 at 17:40
  • You have a service which configures your firewall, probably firewalld. To test you can stop the firewall: systemctl stop firewalld.
    – Piotr P. Karwasz
    Nov 20, 2019 at 17:56
  • When you ssh from outside, are you using a different machine? Or is it the same (for example, a laptop that sometimes you connect locally, sometimes remotely)? If you're using a different machine, it's possible that it has a different SSH client that's using a different protocol, encryption or whatnot, that your server refuses for some reason. To test this hypothesis, try to connect with the same machine/client both from inside and outside your local network.
    – msb
    Nov 20, 2019 at 18:42
  • Hi, thanks for your comments. @PiotrP.Karwasz I stopped firewalld as you suggested but nothing changed. In the logs I see the same message.
    – Eduardo G.R.
    Nov 21, 2019 at 12:44

2 Answers 2

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1

Try putting your sshd process into VERBOSE logging mode (eg: LogLevel VERBOSE in /etc/ssh/sshd_config) on your endpoint and then look in /var/log/auth.log (for Ubuntu -- Redhat derivative is probably a different file). This should point to any pubkey errors, protocol, cipher or file/directory permission errors involving the authorized_keys file.

secondly, on the endpoint, install tcpdump and have it listen to the ssh traffic on port 22 (or whatever port you have the daemon on ) with something like this: tcpdump -nn -s0 -A port 22. This will tell you what is connecting to that port regardless of firewall or SELinux issues.

Attempt a connect. Those two pieces should give you some indication of where the problem lies. if tcpdump is quiet, your router is not forwarding the connection properly or there is another hop in between which is blocking the connection.

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4
  • Thanks for the suggestion! Today I did more tests. The logs seem to be OK. If I connect both machines to the LAN, the logs are equal for connections made hiting the LAN IP (192.168****) and PUBLIC (100.80.215-**). Then I connected one machine through a different provider and it seems the public IP (published in the router's software) is not reachable. No ping response, no TCP connections. The message is always "host unreachable". I may need to call my ISP to understand what is going on here. Thanks again for your help.
    – Eduardo G.R.
    Nov 25, 2019 at 16:39
  • Try a whois on your PUBLIC IP address (100.80.x.x). There's an interesting comment I've never seen before: ...can only be used in Service Provider networks or on routing equipment that is able to do address translation across router interfaces when addresses are identical on two different interfaces. Might offer some insight.
    – Server Fault
    Nov 25, 2019 at 16:52
  • Finally I ended up reading about CG-NAT and it resulted that my ISP used it. I called the ISP and ask for a real IP address, 24hs later everything is working as it should. Thanks for all your help!
    – Eduardo G.R.
    Nov 26, 2019 at 16:16
  • You're welcome. An Interesting problem nonetheless!
    – Server Fault
    Nov 26, 2019 at 19:59
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Finally I ended up reading about CG-NAT and it resulted that my ISP used that technology. I called the ISP and ask for a public IP address, 24hs later everything is working as it should.

Thanks for all the help provided!

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