This is from a Fedora 20 client testing to a Centos 6.5 server. I can do a ping6 with no problems, but I cannot 'ssh -6' to the server (ssh -4 works). I have reason to suspect the challenge is in my PPPoE link with a restricted MTU size that v6's PMTU is not working right with. So my question is: Is there a way to specify the MTU size for things like ssh (and http) to use?

On 27.03.2014 20:35, Robert Moskowitz wrote: > On 03/27/2014 12:50 PM, Robert Moskowitz wrote: >> This is from a Fedora 20 client testing to a Centos 6.5 server. >> >> I can do a ping6 with no problems, but I cannot 'ssh -6' to the server >> (ssh -4 works). I have reason to suspect the challenge is in my PPPoE >> link with a restricted MTU size that v6's PMTU is not working right with. >> >> So my question is: Is there a way to specify the MTU size for things >> like ssh (and http) to use? > I was told that: > > To figure out where I could put new MTU values: > “find /sys |grep mtu” > > To set the MTU for my eth0, as root: > “echo 1300 > /sys/devices/pci0000:00/0000:00:03.0/0000:02:00.0/net/eth0/mtu” > > "ifconfig eth0" reports an MTU of 1300 now. > > > And this works. But it is not my problem. Still not getting a TCP connection. Going to have to check with wireshark. Maybe it will help me... > Considering how much data you've provided us. :)

On 03/27/2014 03:40 PM, poma wrote: > On 27.03.2014 20:35, Robert Moskowitz wrote: >> On 03/27/2014 12:50 PM, Robert Moskowitz wrote: >>> This is from a Fedora 20 client testing to a Centos 6.5 server. >>> >>> I can do a ping6 with no problems, but I cannot 'ssh -6' to the server >>> (ssh -4 works). I have reason to suspect the challenge is in my PPPoE >>> link with a restricted MTU size that v6's PMTU is not working right >>> with. >>> >>> So my question is: Is there a way to specify the MTU size for things >>> like ssh (and http) to use? >> I was told that: >> >> To figure out where I could put new MTU values: >> “find /sys |grep mtu” >> >> To set the MTU for my eth0, as root: >> “echo 1300 > >> /sys/devices/pci0000:00/0000:00:03.0/0000:02:00.0/net/eth0/mtu” >> >> "ifconfig eth0" reports an MTU of 1300 now. >> >> >> And this works. But it is not my problem. Still not getting a TCP >> connection. Going to have to check with wireshark. Maybe it will >> help me... >> > Considering how much data you've provided us. :) I was told elsewhere that my ipv6 problem was an mtu path discovery problem. Try forcing my mtu so that it will get through whatever path (claim PPPoE link is the problem). So, all I was asking was how to set the mtu for a tcp connection. I asked here and on the list where I was testing the ipv6 app. There someone said, not be app, but by interface, and showed how he did it. So that is the story to now. But a 1300 mtu still does not work. So fire up wireshark. I think it is a firewall problem. SO I will go away again and look at the firewall.....

Robert Moskowitz wrote: > > On 03/27/2014 03:40 PM, Rick Stevens wrote: >> On 03/27/2014 09:50 AM, Robert Moskowitz issued this missive: >>> This is from a Fedora 20 client testing to a Centos 6.5 server. >>> >>> I can do a ping6 with no problems, but I cannot 'ssh -6' to the server >>> (ssh -4 works). I have reason to suspect the challenge is in my PPPoE >>> link with a restricted MTU size that v6's PMTU is not working right >>> with. >>> >>> So my question is: Is there a way to specify the MTU size for things >>> like ssh (and http) to use? >> >> The odds are that your PPoE stuff has problems with IPV6 addressing (as >> many things do). I really doubt the MTU has anything to do with it >> since MTU sets the maximum size of a packet and has nothing to do with >> the addressing mechanisms. You could have a router that doesn't like >> big packets, but that would affect both IPV4 and IPV6. > > ping6 works in both directions no problem. Addressing is right. I > actually > have my own /48 allocation here. > > It seems to be something with the firewall. > > You can set the myu using the route command (or ip, if that's what you know). The option is mss, used to set the payload size, when mtu discovery doesn't work due to firewall. See 'man route' mss section for some discussion.