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I am having trouble establishing some HTTPS connections from internal machines, even though these same connections work if initiated on the firewall itself.

The firewall machine is running Ubuntu 10.04.1 and shorewall 4.4.6. The internet connection is Bell PPPoE DSL (in Canada). I have tried various MTU settings, it doesn't seem to make any difference. Other protocols (HTTP, FTP, etc) generally work.

The problem seems to be limited to certain sites; this one never works from an internal machine, but always works from the firewall itself:

From internal machine:

$ wget
--2011-01-13 20:51:31--
Connecting to||:443... connected.

From firewall:

$ wget
--2011-01-13 20:58:28--
Connecting to||:443... connected.
HTTP request sent, awaiting response... 200 OK
Length: 840 [image/gif]
Saving to: `corp_logo.gif'
2011-01-13 20:58:28 (149 MB/s) - `corp_logo.gif' saved [840/840]

This URL always works from both internal and firewall:

Any troubleshooting tips would be greatly appreciated!

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While MTU changes on the firewall (PPPoE settings) never had any effect, reducing MTU on client machines solves the problem. 1500 -> 1454 and it works beautifully.

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Your firewall policy may be too restrictive and is preventing ICMP fragmentation messages from returning to the client. – mfarver Jan 14 '11 at 3:25

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