topic Re: SSH is not working 10.10.20.20 DevBox: CentOS 7 Workstation in DevNet Sandbox

SSH is not working 10.10.20.20 DevBox: CentOS 7 Workstation

nileshbhadane123 — Tue, 04 Jun 2019 09:40:04 GMT

When I am trying to do SSH to 10.10.20.20 DevBox: CentOS 7 Workstation, it is not not responding and getting hang. I am following steps from https://learninglabs.cisco.com/lab/nxos_telemetry_part1/step/2. But this devbox is responding to ping command. Please help me to fix this issue.

Re: SSH is not working 10.10.20.20 DevBox: CentOS 7 Workstation

zowu — Fri, 22 Jun 2018 05:19:41 GMT

Hi Nilesh,

The Devbox should be 10.10.20.25 other than 10.10.20.20. I've tried a new reservation and the devbox is working here.

Thanks,

Bob

Re: SSH is not working 10.10.20.20 DevBox: CentOS 7 Workstation

Adrian2 — Mon, 25 Jun 2018 10:59:20 GMT

Hi,

I have got the same problem. I am using MacOS X High Sierra and the problem is that i can't ssh to 10.10.20.20 and RDP to 10.10.20.161. I did some troubleshooting and Ping does work with no problems. Also when I am doing telnet to 10.10.20.20 on port 22 then port is open. The problem for me looks like it's locally or because of VPN connection.

Interesting thing is that when I start windows on the same computer via VMware then I can access everything, but I do prefer to learn from my main system which is MacOS X of course.

In Addition there is no device under 10.10.20.25

Could you please help me investigate the problem?

PS: I suppose that all MAC users will have similar problem

Thanks

Adrian

Re: SSH is not working 10.10.20.20 DevBox: CentOS 7 Workstation

jokearns1 — Mon, 25 Jun 2018 12:22:16 GMT

Hi Adrian,

What Sandbox lab are you using?

Joe

Re: SSH is not working 10.10.20.20 DevBox: CentOS 7 Workstation

admin11111 — Tue, 26 Jun 2018 13:07:54 GMT

Hi Joe,

I am using followind lab: NX-OS with Nexus 9Kv Lab (devnetsandboxlabs.cisco.com/rave36)

In Addition I did debug ssh on my MACBOOK and it stuck in: ssh2_msg_kexinit sent

Adrian

Re: SSH is not working 10.10.20.20 DevBox: CentOS 7 Workstation

admin11111 — Fri, 29 Jun 2018 11:12:31 GMT

Hi Joe,

Have you dad time to have a look in to this what I wrote?

In addition I have found that there is Packet fragmentation and My question is if may be there is a problem.

Max mtu without fragmentation is 12xx

Re: SSH is not working 10.10.20.20 DevBox: CentOS 7 Workstation

admin11111 — Sun, 01 Jul 2018 07:50:11 GMT

Hi,

I have solved the issue and I would like to share with you hot to fix it.

When you connect to VPN it also negotiate the MTU and in my case it negotiated 1406 MTU.

When I did ping with DF bit set and with my negotiated MTU. Ping didn't work:

root@xxx:~# ping -s 1406 10.10.20.20

PING 10.10.20.20 (10.10.20.20) 1406(1434) bytes of data.

After that I have changed MTU on tunnel interface:

root@xxx:~# ifconfig tun0 mtu 1355

I set maximum possible MTU with no fragmentation.

Now I am able to connect to 10.10.20.20 with no problems

I believe such problem will be not only for Mac users but also for linux users. So this solution will be working for linux based users as well.

Re: SSH is not working 10.10.20.20 DevBox: CentOS 7 Workstation

admin11111 — Mon, 09 Jul 2018 03:44:24 GMT

Hi, this solution worked for me but not with that MTU (1355), for me had to be something under the 12xx range.

Also a quick note for anyone testing this, you should test the ping with the -D flag to avoid fragmentation since Jumbo packets are enabled and you can actually ping all over 8184:

```

daniel$ ping -s 8184 10.10.20.20

PING 10.10.20.20 (10.10.20.20): 8184 data bytes

8192 bytes from 10.10.20.20: icmp_seq=0 ttl=63 time=147.845 ms

8192 bytes from 10.10.20.20: icmp_seq=1 ttl=63 time=147.635 ms

8192 bytes from 10.10.20.20: icmp_seq=2 ttl=63 time=147.429 ms

```