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I have a very strange issue SSHing into an Ubuntu server hosted at Amazon Web Services from one IP address.

A few weeks back our ISP made some changes in the building we share with other businesses and as such our IP address changed. No problem, I updated the AWS security groups and thought job done.

Apart from one server, that shares the same security group as server that work fine. The server has a twin, almost identical, in the same location performing the same function (its a web application cluster).

I can SSH into this machine from any location the security group is set to allow EXCEPT the office IP address. The connection times out.

I assumed that the server itself must be blocking the connection but:

  • There is no firewall running
  • There is no anti brute-force running
  • hosts.deny is blank (comments only)
  • I see no mention of the login attempt in auth.log

All non-ssh services seem to connect, for example, if I allow pings it will respond to pings as expected.

Can anybody think of anything I have missed here? What else could be preventing this connection?

Thanks! Jim

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I solved this.

It seems that our ISP made some funky NAT changes and that we in fact have two public IP addresses somehow.

For one reason or another this server saw the other one we were not aware of.

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