cannot ssh into a box with DHCP assigned IP address

Drew Tomlinson drew at
Tue Mar 19 21:34:13 UTC 2013

On 2/20/2013 5:55 AM, Anton Shterenlikht wrote:
> 	From feenberg at Wed Feb 20 13:39:28 2013
> 	> 	From: Fleuriot Damien <ml at>
> 	> 	To: mexas at
> 	> 	Subject: Re: cannot ssh into a box with DHCP assigned IP address
> 	> 	Date: Wed, 20 Feb 2013 10:31:22 +0100
> 	> 	Cc: freebsd-questions at
> 	>
> 	> 	On Feb 20, 2013, at 10:28 AM, Anton Shterenlikht <mexas at> wrote:
> 	>
> 	> 	> I have a laptop with FreeBSD -current,
> 	> 	> with ip address assigned via DHCP.
> 	> 	> The laptop has neither a static ip address,
> 	> 	> nor a domain.
> 	> 	>
> 	> 	> I can ping the laptop fine, but cannot

I doubt that you can ping from as is private IP address space and is not routed across the 

> 	> 	> ssh into it. The sshd is running, /etc/ssh/ssd_config
> 	> 	> seems fine, /etc/hosts.allow is fine.
> 	> 	> However, /etc/hosts is just the default:
> 	While on the problem machine, can you ssh to localhost? ssh to the IP
> 	address?
> yes to both
> 	I would suspect the problem is in /etc/hosts.allow
> 	 or /etc/hosts.deny,
> The first non-comment line in /etc/hosts.allow is
> ALL : ALL : allow
> and I don't have /etc/hosts.deny:
> root at zzz:~ # ls /etc/hosts*
> /etc/hosts              /etc/hosts.equiv
> /etc/hosts.allow        /etc/hosts.lpd
> root at zzz:~ #
> 	or perhaps the subnet mask is incorrect.
> Well.. what should it be?
> I have on the problem box (ssh server):
> wlan0: flags=8943<UP,BROADCAST,RUNNING,PROMISC,SIMPLEX,MULTICAST> metric 0 mtu 1
> 500
>          ether 00:21:5c:50:68:c3
>          inet netmask 0xfffffc00 broadcast
>          media: IEEE 802.11 Wireless Ethernet OFDM/54Mbps mode 11g
>          status: associated
>          ssid eduroam channel 1 (2412 MHz 11g) bssid 00:3a:98:62:cd:a0
>          country US authmode WPA2/802.11i privacy ON deftxkey UNDEF
>          AES-CCM 2:128-bit AES-CCM 3:128-bit txpower 14 bmiss 10 scanvalid 450
>          bgscan bgscanintvl 300 bgscanidle 250 roam:rssi 7 roam:rate 5
>          protmode CTS wme roaming MANUAL
> I'm trying to ssh from
> I wonder, perhaps it somehow built into the
> Eduroam wireless, provided by the University,
> that the devices connected to it cannot be
> accessible. They can only initiate outgoing
> connections, but all incoming connections are
> somehow blocked? Given that the majority of
> the devices will be unsecured MS boxes, maybe
> the university thought that this is wise idea
> for safety. Perhaps I can investigate this
> with my IT guys.
> Or I might be talking complete nonsense here, not my area at all.

It is kind of "built in" as you say.  The Eduroam wireless network 
appears to be a private network sitting behind a NAT gateway.  Thus what 
happens when you access the Internet is that your laptop sends that 
request to the NAT gateway on the Eduroam network.  The NAT gateway 
strips off your private IP address and replaces it with a public IP 
address , marks the connection in its table, and sends it on it's way.  
Then when the answer comes back from the Internet, the NAT gateway 
strips off the public IP address and replaces with your private IP 
address, and sends it to your laptop.

While on the Eduroam network, go to  You will see 
that your IP is not  It will be a public IP address that 
Eduroam uses.

Bottom line is that the only way you could ssh to your laptop from the 
Internet is if you got the university to give you a public IP address 
and port and then they set up NAT and port forwarding on their network 
to point that public IP address to your laptop private IP address ssh port.

Since I doubt you will have much luck with that, I suspect the short 
answer is "you can't ssh to your laptop from the Internet when it's 
connected to your university network".

I'm sure this isn't the answer you wanted but hopefully this will save 
you some frustration.



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