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RE: App fails when rebuilt with newer library, but only when MTU is small
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From: Mark Fanara via curl-library <curl-library_at_lists.haxx.se>
Date: Wed, 5 Oct 2022 14:14:50 +0000
From: curl-library <curl-library-bounces_at_lists.haxx.se> On Behalf Of Dan Fandrich via curl-library
Sent: Thursday, September 29, 2022 5:09 PM
To: libcurl development <curl-library_at_lists.haxx.se>
Cc: Dan Fandrich <dan_at_coneharvesters.com>
Subject: Re: App fails when rebuilt with newer library, but only when MTU is small
On Thu, Sep 29, 2022 at 09: 58: 58PM +0000, Mark Fanara wrote: >> Some servers have an issue with 100-continue, and I don't recall which version >> of libcurl enabled it by default. You could try disabling it and see what >>
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>> When the MTU is larger, the TLS packet sequence continues
>This seems unlikely to me to be a curl issue. It sounds more likely to be
>something lower in the networking stack that's interfering. Try using Wireshark
>to sniff the traffic to see if the short application data packets are getting
>through, and if they are, if they're being acked.
The problem turned out to be the fact that an upstream device was sending ICMP ‘fragmentation needed’ back to me with a MTU next hop value < 576. My newer OS apparently discarded them because of the next hop value. I was able to change the OS path MTU behavior by setting /proc/sys/net/ipv4/ip_no_pmtu_disc to 1 (echo ‘1’ > /proc/sys/net/ipv4/ip_no_pmtu_disc) per https://www.kernel.org/doc/html/latest/networking/ip-sysctl.html.
Dan
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Date: Wed, 5 Oct 2022 14:14:50 +0000
From: curl-library <curl-library-bounces_at_lists.haxx.se> On Behalf Of Dan Fandrich via curl-library
Sent: Thursday, September 29, 2022 5:09 PM
To: libcurl development <curl-library_at_lists.haxx.se>
Cc: Dan Fandrich <dan_at_coneharvesters.com>
Subject: Re: App fails when rebuilt with newer library, but only when MTU is small
On Thu, Sep 29, 2022 at 09: 58: 58PM +0000, Mark Fanara wrote: >> Some servers have an issue with 100-continue, and I don't recall which version >> of libcurl enabled it by default. You could try disabling it and see what >>
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>> When the MTU is larger, the TLS packet sequence continues
>This seems unlikely to me to be a curl issue. It sounds more likely to be
>something lower in the networking stack that's interfering. Try using Wireshark
>to sniff the traffic to see if the short application data packets are getting
>through, and if they are, if they're being acked.
The problem turned out to be the fact that an upstream device was sending ICMP ‘fragmentation needed’ back to me with a MTU next hop value < 576. My newer OS apparently discarded them because of the next hop value. I was able to change the OS path MTU behavior by setting /proc/sys/net/ipv4/ip_no_pmtu_disc to 1 (echo ‘1’ > /proc/sys/net/ipv4/ip_no_pmtu_disc) per https://www.kernel.org/doc/html/latest/networking/ip-sysctl.html.
Dan
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