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>> Here's my layout: |
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>> |
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>> laptop+shorewall (MTU:1500) -> hotel router (MTU:?) -> internet -> |
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>> Westell modem/router (MTU:1492) -> desktop+shorewall (MTU:1500) |
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>> |
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>> Shouldn't PMTUD change the desktop's MTU to 1492? |
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> |
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> Your desktop's PMTUD will get an ICMP response from Westell as it tries to |
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> traverse through it and it will adjust the outgoing packet size accordingly. |
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> |
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> Ditto with your laptop, when it tries to establish a connection with your |
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> desktop. |
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> |
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> |
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>> Is the fact that it |
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>> doesn't due to a flaw in the Westell's operation? Should I manually |
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>> change the desktop's MTU to 1492 along with that of other systems on |
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>> its LAN? |
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> |
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> It won't harm if you do. However, we don't know for a *fact* that the Westell |
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> is not returning the appropriate ICMP packets (Type 3, Code 4) to your laptop, |
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> or your desktop. |
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> |
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> You can use tcpdump to see what's being sent back and forth. |
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|
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Ah, so PMTUD doesn't actually change the setting reported by ifconfig. |
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It's done on-the-fly. |
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|
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>> > If you are using Shorewall at your remote server I would expect it to |
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>> > behave properly and return the correct ICMP packet when it receives a |
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>> > DF. However, I am not familiar with the Shorewall properties and |
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>> > settings, so if you suspect this as the cause of your problem it would |
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>> > be better if you look into it properly. |
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>> |
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>> So I'm sure I understand, this doesn't apply if ICMP is dropped at the |
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>> Westell? |
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> |
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> ICMP echo request may be dropped by Westell's firewall, but ICMP Type 3, Code |
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> 4 could well be returned when a TCP connection is being initiated by your |
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> laptop. I don't know how ham-fisted Westell's firewall settings are. |
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> |
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> Meanwhile, stating the obvious, have you tried using the desktop squid proxy |
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> while you are inside your LAN with the same laptop to see if the problem |
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> remains? |
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|
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That's a fine idea and I will do so once I get back there. Thanks for |
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the education Mick. :) |
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|
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- Grant |