[avr-gcc-list] avr1.org server misconfiguration

[Erik Walthinsen]

I have been unable to send email to the avr-gcc-list for a very long 
time, and I finally figured out why.  Specifically, if I telnet to port 
25 (SMTP) of avr1.org (202.37.230.18) from my server, I got Connection 
Refused.  Yet doing the same thing from my laptop worked just fine...

What I realized is that my laptop has TCP ECN (Early Congestion 
Notification, http://www.icir.org/floyd/ecn.html) turned off, whereas my 
home server has it turned on.  Turning it off allowed sendmail to 
connect and finally deliver all the pending mail.  This is why you're 
all now getting a flood of email from me that's rather back-dated and 
out of order (yay for threaded email clients, go Thunderbird!).

However, ECN is considered to be a standard feature of the modern 
Internet, having been rolled out in force 2, 3, or even maybe 4 years 
ago (not sure exactly).  Some checking around with tcptraceroute 
indicates that the problem most likely exists at the avr1.org server 
itself, and not somewhere up the router chain.

This is usually due to a really old Linux kernel that predates proper 
ECN functionality, which puts it back in the early 2.4.x series (not 
even 2.4.xx as far as I remember).  All recent (<3 years old) versions 
of Linux, Windows, IOS, etc. have it turned on by default, as it's 
expected that 99++% of all routers and servers on the net deal with it 
properly.

If anyone else is having problems sending mail to this list, check your 
mail server and turn ECN off until this is fixed.  In Linux you want to 
set /proc/sys/net/ipv4/tcp_ecn to 0.  And notify any remaining sites out 
on the net that don't work properly with ECN, as it means they need to 
upgrade either their router or server to a more recent version, ECN or 
not (lots and lots of vulnerabilities in older Linux, IOS, etc. versions 
besides not playing with ECN right).

TTYL,
    Omega
    aka Erik Walthinsen
    address@hidden