On 6/30/05, Nate Carlson <tclug at natecarlson.com> wrote: > On Thu, 30 Jun 2005, Scot Jenkins wrote: > > I have several systems that I recently upgraded from Debian woody to > > sarge. On systems where I've upgraded the kernel, I have a strange > > networking issue. I do secondary MX for a friend (we'll call his host > > friendmail) and after the OS upgrades on my end, I started spooling mail > > for him. I contacted him thinking his mail server was down but it > > turned out that from my Debian mail server, I couldn't telnet to > > friendmail:25. I could not telnet to any other open ports on his IP, > > 80, 22, etc. I was able to telnet to other random hosts on various > > ports just fine. > > Do you have ECN enabled? > cat /proc/sys/net/ipv4/tcp_ecn That was it. It looks like the 2.4.18 kernel had an option to enable ECN and another to disable it by default: CONFIG_INET_ECN=y CONFIG_INET_ECN_DISABLED=y The "disabled" option appears to be gone in newer kernels. One can toggle it off a couple of ways: echo "0" > /proc/sys/net/ipv4/tcp_ecn --OR-- add this line to /etc/sysctl.conf if your distro supports sysctl: net.ipv4.tcp_ecn = 0 Thanks a million Nate! Scot