[Nottingham] Schooling Laptops Donations, and mobile SSH timeouts - Re: [Talk] *TODAY* 8:30pm Thursday 28/01/2021: Going Mobile with SSH and Dodging Timeouts...

[Martin]

Folks,

A good do on the night as always.

And a very warm welcome to Andy.


Following on from Andy's push to help with providing schoolchildren with 
devices (and support?) to get them online for their remote schooling, we 
have locally here the article:

*Nottinghamshire schools appeal for donations to provide laptops for 
home learning*

'Sadly some students have no device to work on at home'

https://www.nottinghampost.com/news/local-news/nottinghamshire-schools-appeal-donations-provide-4902524


Notes from that are:

"Archway Learning Trust which operates the five Bluecoat sites and The 
Nottingham Emmanuel School had launched a crowdfunding appeal to be able 
to buy laptops ... A further nine local schools - listed below - are 
also appealing for donations."

"Archway Learning Trust, which educates more than 25 percent of 
secondary school students in Nottingham, has moved its learning 
provision online..."

"... an additional 500 laptops are still needed to help these students, 
at a *cost of £300 each* ."

"The Government has said it will be providing one million laptops and 
tablets for schools to allocate locally to students who need them most."

" *More than 54,000 4G routers* have also been delivered to schools and 
colleges in England, to ensure every child has internet access."

Phew?!


Meanwhile for practising my preaching for my new settings for ssh over a 
mobile connection:

The new settings worked a treat, helped immensely by the use of byobu.

After a 'good few hours of use', there was just the one reassignment of 
IP address for the mobile connection for whatever reason.

At least for my one example, using a mobile 'broadband' connection works 
well.


Minor aside: The connection lost by the mobile connection being 
reassigned a new IP address still has that ssh connection lingering on 
the server. On my netbook upon reconnecting, I got the irksome ssh 
"kexchange" error which as newly configured, cleared after the one hour 
timeout. I reconnected to byobu and all continued unperturbed.

So why has sshd not killed the lingering dead connection?


/etc/ssh/ssh_config:

# Connection timeouts
#
# NB: Typical NAT/Firewall timeouts are:
#       TCP WWW         60s
#       TCP SYN         60s
#       TCP             86400s
#       UDP             180s - 30s
#       ICMP            10s
#
#       Assume the worst case TCP and go for 55s
ServerAliveCountMax 2
ServerAliveInterval 53s
TCPKeepAlive no

# Cipher key timeouts (Also avoid the forever kexchange hangup on 
reconnecting a broken connection?)
# SFR requires rekey after 1GByte /aggregate/ data or after 1hour. 
OpenSSH is coded for 1GByte per data direction!... Hence must set for 
worst case for <512MByte
RekeyLimit 222M 3559s



/etc/ssh/sshd_config:

# Connection timeouts
#
# NB: Typical NAT/Firewall timeouts are:
# 	TCP WWW		60s
# 	TCP SYN		60s
# 	TCP		86400s
# 	UDP		180s - 30s
# 	ICMP		10s
#
# 	Assume the worst case TCP and go for 55s
ClientAliveCountMax 2
ClientAliveInterval 53s
TCPKeepAlive no

# Cipher key timeouts (Also avoid the forever kexchange hangup on 
reconnecting a broken connection?)
# SFR requires rekey after 1GByte /aggregate/ data or after 1hour. 
OpenSSH is coded for 1GByte per data direction!... Hence must set for 
worst case for <512MByte
RekeyLimit 222M 3559s


Aside: 53s is used for the sake of being prime and also for gaining a 
little extra margin.


?

Cheers,
Martin


byobu:

https://en.wikipedia.org/wiki/Byobu_(software)


primegen:

http://cr.yp.to/primegen.html
https://en.wikipedia.org/wiki/Daniel_J._Bernstein#Mathematics
https://en.wikipedia.org/wiki/Sieve_of_Atkin
https://github.com/jbarham/primegen



On 28/01/2021 01:09, Martin via Nottingham wrote:
> Folks,
> 
>> Carrying-on on the dance around the Coronavirus... Continuing onwards 
>> virtual until we evolve soon vaccinated around the SARS-CoV-2 virus 
>> and the resultant COVID-19...
>>
>>
>> We have:
> 
> 
> *NLUG is Socially Online* !
> 
> 
> *TODAY* 8:30pm Thursday 28/01/2021 (That's 8:30pm _GMT_  )
> 
> NLUG 'Talk': *Going Mobile with SSH and Dodging Timeouts*