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We have a mail server configured in Ubuntu 12.04 with postfix, dovecot and roundcube. It has two interfaces, one with real IP and one with private LAN IP. Incase of intranet we have branches and head office, head office is connected directly through layer 3 switch and different VLAN and different IP block, branches are connected through core router.

The mail server is working fine with the internet domain and in case of intranet we can browse webmail by the local IP from our head office. But the problem is that the webmail is not accessible from the branches.

ICMP reply from branches to server and from server to branch is okay. Trace route from branches to server is okay but from server to branch it can't trace after 2 hop. The webmail page shows broken from branches. Already tested using a simple index html file, it also shows broken page from branch.

The default gateway is towards the real IP. We have some more servers with other applications in this IP block which are working fine from the branches. As per my troubleshooting it might be the routing issue as the default gateway of the server is towards Internet instead of intranet.

Please help me how to resolve the routing.

HBruijn
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Tasbir
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2 Answers2

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You need to make sure that the routing table of your server is the same as on the servers that work. You probably have a lot more routes on the servers that work.

Law29
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  • Other servers are in Windows and routing table are same, in mail server there are not much routes. – Tasbir Jan 10 '16 at 04:44
  • @Tasbir I'm not sure I understand your comment, does it mean that all the Windows servers have identical routes and are working OK, and that the Ubuntu mail server that does not work has less routes? Then my answer was good: you need to make sure that the routing table of your Ubuntu mail server is the same as those on the Windows servers that work. – Law29 Jan 10 '16 at 10:39
  • the windows server have identical routes as ubuntu has. We have 2 blocks of IP 192.168.0.0/16 and 172.22.0.0/16. These 2 are routed in every machine. – Tasbir Jan 10 '16 at 11:19
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At last the issue resolved with tuning the branch router TCP ADJUST MSS of the tunnel. Go to the tunnel in config mode and execute tcp adjust mss 1250. the default is 1450. Then the webmail is working fine from branch.

Tasbir
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  • That does not explain why "Trace route from branches to server is okay but from server to branch it can't trace after 2 hop.", and in your original question you did not mention any tunnel. – Law29 Jan 10 '16 at 18:59