For the last few weeks I am trying to pinpoint the source of a network problem.
The short story: using the standard mtu of 1500 caused sending to stall (and fail).
This is seen when uploading a file (e.g. ftp) or when sending email.
With an mtu of 1456 everything works.
Everything worked unto 02:30 on 7/Nov. At that point some uploads started hanging.
I have cron scripts that upload small and larger files. The small ones get though, the larger ones hang.
There is enough space on the target.
The network is simple: my server connects to a modem (wireless internet over 4G).
BTW, is there a more suitable list (or forum) for discussing this issue?
I want to confirm where the problem lies. I did not change a thing on my server.
I suspect that the modem updated its fw overnight and the new fw has an issue.
However the ISP says no one else is complaining.
I did not find a way to downgrade the fw or install the old one.
I was given a new modem (with the old fw) but once connected to the 'net
the fw was updated, something I cannot control.
Below is an example of the issue.
$ sudo ifconfig eth1 mtu 1500 # this is the default, which now fails
$ ifconfig eth1
eth1: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
inet 192.168.2.7 netmask 255.255.255.0 broadcast 192.168.2.255
$ tracepath -4 -b -l 1500 -m 5 1.1.1.1
1: e5.eyal.emu.id.au (192.168.2.5) 1.109ms # this is the modem
2: no reply
3: no reply
4: no reply
5: no reply
Too many hops: pmtu 1500
Resume: pmtu 1500
## Does the above show that the server (.7) can talk to the modem (.5) but the modem
## cannot talk to the external net?
## Can it be an issue with the next hop (the 4G service)?
$ tracepath -4 -b -l 1456 -m 5 1.1.1.1
1: e5.eyal.emu.id.au (192.168.2.5) 0.760ms
2: 10.155.144.3 (10.155.144.3) 74.809ms
3: 10.246.20.29 (10.246.20.29) 39.707ms
4: 101.115.95.250 (101.115.95.250) 38.656ms
5: 203-220-16-149.tpgi.com.au (203.220.16.149) 35.127ms
Too many hops: pmtu 1456
Resume: pmtu 1456
$ sudo ifconfig eth1 mtu 1456 # with this everything works well
$ ifconfig eth1
eth1: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1456
inet 192.168.2.7 netmask 255.255.255.0 broadcast 192.168.2.255
$ tracepath -4 -b -l 1500 -m 5 1.1.1.1
1?: [LOCALHOST] pmtu 1456
1: e5.eyal.emu.id.au (192.168.2.5) 0.797ms
1: e5.eyal.emu.id.au (192.168.2.5) 0.614ms
2: 10.155.144.2 (10.155.144.2) 37.951ms
3: 10.246.20.29 (10.246.20.29) 36.881ms
4: 101.115.95.249 (101.115.95.249) 35.653ms
5: 203-220-16-145.tpgi.com.au (203.220.16.145) 36.673ms
Too many hops: pmtu 1456
Resume: pmtu 1456
$ tracepath -4 -b -l 1456 -m 5 1.1.1.1
1: e5.eyal.emu.id.au (192.168.2.5) 0.560ms
2: 10.155.144.2 (10.155.144.2) 33.008ms
3: 10.246.20.29 (10.246.20.29) 36.226ms
4: 101.115.95.250 (101.115.95.250) 36.278ms
5: 203-220-16-149.tpgi.com.au (203.220.16.149) 36.156ms
Too many hops: pmtu 1456
Resume: pmtu 1456
--
Eyal at Home (fedora@xxxxxxxxxxxxxx)
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