My main server (which also does DNS) is "down" (network wise) thus DNS currently does not work on my network. I need to SSH into my switch to make some changes which are required to get the server back up. Problem is I cannot SSH into anything even by IP because from what I'm reading SSH does reverse lookups.

Is there a way around this? Worse case scenario I can maybe console into the switch.

If that is true, then how come I can ssh between all my home computers on my LAN (192.168.0.xxx)? I have no DNS nor /etc/hosts set up for my home computers. I ssh via IP address only. Using OpenSSH.

SSH DNS reverse lookups will timeout after 10 seconds or so, they won't prevent a log in.

Explain more about what is actually happening please.

I want my main Linux server (which does DNS and lot of other functions) to be vlan aware so I can put VMs on different vlans, so I configured the vlan interfaces. I disabled the eth0 "main" interface and lost connectivity. vlan2 is my main vlan but it's set as native on that port. I think all I have to do is go remove it from native and then put it as a normal vlan (have to allow each vlan for the trunk port). however I can't ssh into the switch by IP anymore, nor any other server. I type

ssh 10.1.1.3

It just sits there forever. Googled that, and found lot of stuff related to that being caused by DNS issues. My DNS server is also the server that does reverse lookups. Ex: 10.1.1.3 resolves to the hostname of the switch.

Oddly if I tcpdump I don't see ANY traffic when I type that. So maybe something else is going on. I can ping the host fine or telnet to the SSH port so I know it's not a network issue.

no, it won't be DNS related. Add a few -v's to the ssh command, see if it's connecting on TCP at all. I presume it's not.

Tried that, I get nothing. This worked up until the server went down though.

tried what? got what?

Adding -v. Nothing shows up on the screen. ex:

Oddly there is zero traffic passed when this happens, unless tcpdump filters some stuff like DNS, as it does lookups of it's own... or maybe it too was locking up. Lot of stuff seems to go tits up when reverse DNS lookups don't work. I could not even load wireshark, and at some point my GUI locked up. (KDE).

I'll probably just end up getting my laptop and a console cable to fix the switch that way once I get home from work (internet wont work till I fix it), but was hoping this was something easier to solve this ssh issue.

Guess this teaches me a lesson, should have a secondary DNS.

Sure enough when I fixed the issue on my switch to make the server available to the network again, everything started working. Weird it would do that. I have another issue regarding vlans but I'll make a separate thread.

Out of curiosity though is there anything I could have done at the client end to make it work, such as turn up a temporary DNS server?

As previously stated, it's not a DNS issue, so, no, adding another DNS server would not help.

Your description of your network is unclear, but I suspect the connection issue is due to the subnet that your ssh client is trying to use being currently non-existant.

Nope was able to ping or telnet into the port so I knew routing was ok. I reproduced the issue by turning off DNS on the server just now, as I was curious. The SSH client must be trying to do something related to DNS such as a reverse lookup, and it causes it to hang. Maybe my environment is triggering a bug or something. Most people don't run their own DNS but just use the ISP one, and most of the people that may run their own DNS they probably don't run reverse zones too. So my environment is probably not what is considered typical. Though if you google "SSH hang DNS" or similar terms there are other people who have the same issue.

Video of me reproducing it:

http://www.youtube.com/watch?v=9xEvPZoIa3I

Don't mind the crap quality, youtube asked if it can enhance the colours, I should have said no. :P