i just made some quick changes to the ipcheck script (see below) in the repo (mentioned above) as it did not work for me and also used the ip as seen by the n900 and not as seen from the outside world.
i also dislike having passwords readable to anyone who gets their hands on the phone since "/home/user/.ipcheckrc" is unprotected. yes you can chmod it but why leave it in the user folder then? removing the line ". /home/user/.ipcheckrc" and running "chmod go-r /etc/ipcheckrc" should do the trick after adding a little script (dyndns_autoupdate.sh) -- which updates dyndns every 10 mins -- to /etc/init.d (see below).
This overcomes the issue of /etc/network/if-up.d/ipcheck not always being run when the connection is changed and additionally ensures that your dyndns entry is regularly updated.

on a sidenote.. the sudoers configuration of the n900 is VERY lacking. almost anything that is installed and requires elevated user status will be run as root with no password request. run "sudo -l" to see all the progs that can do this. but i digress.

ofc. a more elegant ipcheck solution would check something other than $IP_ADDRESS before runing ipcheck.py and thus be more efficient but this also does the trick and the script is only called every now and then (its not like you are switching your connection every few ms or anything).

/etc/network/if-up.d/ipcheck
/etc/init.d/dyndns_autoupdate.sh
Edit: i just noticed. the n900 does not use init it uses upstart as mentioned in http://talk.maemo.org/showthread.php?t=35567
but the idea is the same.

this is easily fixed by adding a script to /etc/event.d that runs the init script in /etc/init.d. naturally you could just write the script into an event.d script but again both options work and the reasoning for the hack is as above (and i am too lazy to backtrack ).

/etc/event.d/dyndns

Thanks for resurrecting this thread.

I hope someone brings a GUI-based application for DynDNS to Maemo N900.

well, i would be interested in doing some programming. but i don't have the time at the moment

Wouldn't www.no-ip.com solve this. It's setup is in CLI.

Thank you Crocodile
nice infos, used for my asterisk server =)

i just used this command instead
python ipcheck.py -i -l -r checkip.dyndns.org:8245 user pass dynhost.dyndns.org


works great

A method to overcome both the IP and the NAT problems could be to set up a reverse tunneling towards an always-on machine, i.e. to exec this line on the n900 every time it connects to internet:

ssh -R 9000:localhost:22 dest_user@dest_machine

where dest_user@dest_machine is an sshd enabled account WITH the n900/.ssh/id_rsa.pub file included in authorized_keys (meaning that no password is needed for ssh-ing from n900).

Then it's easy to connect to the n900 from that machine WITHOUT KNOWING THE N900 IP:

ssh -p 9000 user@localhost

simple like that

Has the OP considered using the OpenVPN client on the N900 so that it gets into a vlan?
Then SSH is possible no matter which machine in the vlan you're working from.

This method is not useful if what you want is to be able to ssh into the n900 from any random *nix machine. But since you mentioned key pairs, I thought OpenVPN should satisfy and exceed your requirements.

Why not? From any PC you can ssh to the dest_user@dest_machine (on port 22) and then ssh to n900. Very simple IMO.

Yes, it is possible to log in to an SSH server from any machine.
From a security point of view key pairs are typically to be used to machines that you trust. If you're doing keypairs for any random machine then you are compromising the security of your SSH server from the moment you install a key pair on an untrusted machine.
ssh password logins are only slightly better - there are keyloggers to subvert that.

Since openvpn does key pairs, I wouldn't recommend using it on untrusted machine, just in the same way that I don't recommend using ssh on an untrusted machine.
In fact the above statement is needlessly repetitive - because OpenVPN uses ssl as its underlying transport.

It is understood that the reverse tunneling that I proposed requires a machine that you trust, typically your home pc, because from there you can connect to n900 without the need of a password!

I use dyndns on my pc so the trick works fine: every time it connects to the net it sets up the tunnel to my pc (on a dedicated account). In the case of my n900 being stolen I could connect to it and find out its latest gps position