Acontece que foi um problema com o datacenter Linode.
traceroute
apontou que os pacotes estavam sendo descartados na chegada ao datacenter da Linode Atlanta.
Observação: o IP público do servidor e o nome do host foram intencionalmente omitidos
$ sudo traceroute -p 3128 HOSTNAME --tcp
traceroute to HOSTNAME (PUBLIC_IP), 30 hops max, 60 byte packets
1 192.168.0.254 (192.168.0.254) 0.922 ms 1.337 ms 2.016 ms
2 200.150.94.3 (200.150.94.3) 11.970 ms 12.337 ms 12.699 ms
3 trunk11-src1km3a-src1cos.copel.net (200.150.92.107) 11.980 ms 12.928 ms 12.962 ms
4 177.84.164.33 (177.84.164.33) 14.230 ms 15.943 ms 16.306 ms
5 149.3.181.42 (149.3.181.42) 17.045 ms 17.889 ms 18.127 ms
6 xe-7-2-1.ashburn2.ash.seabone.net (195.22.199.187) 171.647 ms xe-0-1-2.ashburn2.ash.seabone.net (89.221.40.3) 163.118 ms 161.458 ms
7 xe-7-0-1.ashburn2.ash.seabone.net (195.22.199.189) 163.412 ms xe-2-2-2.ashburn2.ash.seabone.net (195.22.199.183) 159.763 ms xe-7-0-1.ashburn2.ash.seabone.net (195.22.199.189) 162.821 ms
8 ae13.er2.iad10.us.zip.zayo.com (64.125.12.1) 161.005 ms 160.879 ms 161.481 ms
9 ae7.er1.iad10.us.zip.zayo.com (64.125.25.49) 146.142 ms 144.668 ms 146.361 ms
10 ae6.cr1.dca2.us.zip.zayo.com (64.125.20.117) 147.399 ms 147.831 ms 148.792 ms
11 ae2.mpr4.atl6.us.zip.zayo.com (64.125.25.46) 150.200 ms ae4.mpr3.atl6.us.zip.zayo.com (64.125.31.197) 144.751 ms ae2.mpr4.atl6.us.zip.zayo.com (64.125.25.46) 146.808 ms
12 ae4.mpr3.atl6.us.zip.zayo.com (64.125.31.197) 143.534 ms 143.451 ms 146.218 ms
13 * * *
...
30 * * *
Como mencionado anteriormente, na porta 53128 ele passou.
$ sudo traceroute -p 53128 HOSTNAME --tcp
traceroute to HOSTNAME (PUBLIC_IP), 30 hops max, 60 byte packets
1 192.168.0.254 (192.168.0.254) 0.931 ms 1.458 ms 1.968 ms
2 200.150.94.3 (200.150.94.3) 12.079 ms 12.222 ms 12.362 ms
3 trunk11-src1km3a-src1cos.copel.net (200.150.92.107) 51.764 ms 52.274 ms 52.466 ms
4 177.84.164.33 (177.84.164.33) 14.533 ms 14.910 ms 15.449 ms
5 149.3.181.42 (149.3.181.42) 16.408 ms 17.265 ms 17.720 ms
6 xe-2-2-2.ashburn2.ash.seabone.net (195.22.199.183) 171.688 ms xe-7-2-1.ashburn2.ash.seabone.net (195.22.199.187) 161.815 ms xe-0-1-2.ashburn2.ash.seabone.net (89.221.40.3) 162.661 ms
7 xe-2-3-1.ashburn2.ash.seabone.net (195.22.199.181) 163.044 ms xe-2-2-2.ashburn2.ash.seabone.net (195.22.199.183) 160.476 ms xe-7-0-2.ashburn2.ash.seabone.net (195.22.199.185) 161.247 ms
8 ae13.er2.iad10.us.zip.zayo.com (64.125.12.1) 162.917 ms 162.569 ms 164.899 ms
9 ae7.er1.iad10.us.zip.zayo.com (64.125.25.49) 149.290 ms 147.938 ms ae6.cr1.dca2.us.zip.zayo.com (64.125.20.117) 150.054 ms
10 ae2.mpr4.atl6.us.zip.zayo.com (64.125.25.46) 150.416 ms ae6.cr1.dca2.us.zip.zayo.com (64.125.20.117) 145.685 ms 146.062 ms
11 ae4.mpr3.atl6.us.zip.zayo.com (64.125.31.197) 145.271 ms ae2.mpr4.atl6.us.zip.zayo.com (64.125.25.46) 146.286 ms ae4.mpr3.atl6.us.zip.zayo.com (64.125.31.197) 143.456 ms
12 128.177.104.134.IPYX-092136-ZYO.zip.zayo.com (128.177.104.134) 143.646 ms ae4.mpr3.atl6.us.zip.zayo.com (64.125.31.197) 145.377 ms 143.066 ms
13 128.177.104.134.IPYX-092136-ZYO.zip.zayo.com (128.177.104.134) 147.494 ms 148.962 ms 148.619 ms
14 router1-atl.linode.com (64.22.106.10) 148.807 ms 146.101 ms PRIVATE_HOSTNAME.members.linode.com (PUBLIC_IP) 145.706 ms
Com isso, consegui abrir um ticket de suporte e obtive minha resposta definitiva. Fico feliz por não ter sido um problema com o meu servidor.
Hello there,
We've investigated further and it turns out this is actually an ongoing issue with our Atlanta data center, so we apologize we couldn't get this answer to you sooner. Currently we are working on getting this port un-blocked but we wouldn't be able to give a time estimate for that.
In the mean time, will that different port work for you? I could also migrate you to a different datacenter if you want with a working 3128 port -- perhaps Fremont, CA, or Newark, NJ?:
https://www.linode.com/speedtest
Let us know.
Regards, Roland Linode Support Team