IP fragmentation (DF) and ip_no_pmtu_disc in 2.2 vs 2.4
In 2.2.17 when /proc/sys/net/ipv4/ip_no_pmtu_disc is 0/false we're seeing outbound udp packets with the IP DF (don't fragment) bit clear. With 2.4.0-test11, when ip_no_pmtu_disc is still 0/false we're seeing outbound udp packets with the IP DF bit set. Is this change in default behavior a fix or a break?
[start non expert thinking]ip_no_pmtu_disc = 0/false means that we DO want MTU discovery.ip_no_pmtu_disc = 1/true means that we DON't want MTU discovery.
to do MTU discovery you want DF set, so if fragmenting is necessary to reach your target you get an unreachable error and can try smaller MTUs.
So, it appears that 2.4 fixed a problem with 2.2, correct? [stop non expert thinking]
The problem that led us to notice this behavior was:
Intel PXE uses tftp to download boot images and discards IP packets with the DF bit set; so a tftpd server on 2.4 with the default ip_no_pmtu_disc set to 0/false can't serve tftp to PXE. Changing ip_no_pmtu_disc to 1/true "fixes it". One problem is that we'd rather have our tftpd server w/ 2.4 configured for mtu discovery.
We've tried to setsockopt(sock, SOL_IP, IP_MTU_DISCOVER, ...) with the IP_PMTUDISC_DONT option but can't make it work. How does one change MTU discovery and/or the don't fragment bit on a single socket?
JP Navarro
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John-Paul Navarro (630) 252-1233 Mathematics & Computer Science Division Argonne National Laboratory [email protected] Argonne, IL 60439 http://www.mcs.anl.gov/~navarro - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] Please read the FAQ at http://www.tux.org/lkml/
Quoted 3 lines
With 2.4.0-test11, when
ip_no_pmtu_discis still 0/false we're seeing outbound udp packets with the IP DF bit set. Is this change in default behavior a fix or a break?
Its a change in behaviour
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So, it appears that 2.4 fixed a problem with 2.2, correct? [stop non expert thinking]
2.2 only supports it for TCP
Quoted 2 lines
Intel PXE uses tftp to download boot images and discards IP packets with the DF bit set; so a tftpd server on 2.4 with the default
Then Intel PXE is buggy and you should go spank whoever provided it as well as doing the workarounds. Supporting received frames with DF set is mandatory.
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Quoted 7 lines · Alan Cox
Quoted 2 lines
Intel PXE uses tftp to download boot images and discards IP packets with the DF bit set; so a tftpd server on 2.4 with the default
Then Intel PXE is buggy and you should go spank whoever provided it as well as doing the workarounds. Supporting received frames with DF set is mandatory.
SGI Indy PROM also has this behaviour (for bootp).
Could we perhaps make ip_no_pmtu_disc a per interface option? the machine i use as bootp/tftp/nfs server is also a dial-up internet gateway, and it's a shame to have to lose pmtu discovery across the board.
regards,
--paulj
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Followup to: <[email protected]> By author: Paul Jakma <[email protected]> In newsgroup: linux.dev.kernel
Quoted 11 lines · Alan Cox
Quoted 8 lines
Quoted 3 lines
Intel PXE uses tftp to download boot images and discards IP packets with the DF bit set; so a tftpd server on 2.4 with the default
Then Intel PXE is buggy and you should go spank whoever provided it as well as doing the workarounds. Supporting received frames with DF set is mandatory.
Intel PXE is buggier than hell. Don't even get me started.
-hpa
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<[email protected]> at work, <[email protected]> in private! "Unix gives you enough rope to shoot yourself in the foot." http://www.zytor.com/~hpa/puzzle.txt - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] Please read the FAQ at http://www.tux.org/lkml/