Messages in this thread Patch in this message | | | Subject | Logical bug in ipv4 (and ipv6?) PMTU handling? | From | (Harald Nordgård-Hansen) | Date | Thu, 12 Jun 2003 22:11:15 +0200 |
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I was helping a collegue here trying to get some udp packets through a link, and stumbled upon what seems to be a bug in the ipv4 code of current kernels (checked 2.4.20, 2.4.21-pre8 and 2.5.70).
According to 'man 7 ip', the IP_PMTU_DISCOVER flag i controlled by the ip_no_pmtu_disc sysctl for SOCK_STREAM, and off for all other socket types. But in net/ipv4/af_inet.c, the flag is set based on ip_no_pmtu_disc, regardless of socket type.
This causes all udp packets to be send out with the ip don't fragment flag set, and unless the application happens to handle the resulting error messages generated, the packets will not get through on lower-mtu path segments. A quick dump of some udp packets confirms that the don't fragment flag is set on all the packets.
The attached one-liner will limit setting IP_PMTU_DISCOVER on sockets to only those of the SOCK_STREAM type, which is what the documentation says should happen (as would be logical, as other sockets need the application to explicitly handle the size adjustments). It should apply cleanly to 2.4.20 and 2.4.21-pre8, and shouldn't be hard to adapt to 2.5.x.
I don't know enough about ipv6 to understand if this behaviour should be the same there, but net/ipv6/af_inet6.c contains mostly the same logic, and should probably be fixed as well.
-Harald -- Harald Nordgård-Hansen, Linpro AS <>< http://harald.nordgard-hansen.net/ Pancoveien 7, NO-1624 Gressvik, Norway ><> Phone/Fax: +47 6935 2424/25 --- net/ipv4/af_inet.c.orig 2003-06-12 21:18:05.000000000 +0200 +++ net/ipv4/af_inet.c 2003-06-12 21:18:41.000000000 +0200 @@ -369,7 +369,7 @@ sk->protinfo.af_inet.hdrincl = 1; } - if (ipv4_config.no_pmtu_disc) + if (SOCK_STREAM != sock->type || ipv4_config.no_pmtu_disc) sk->protinfo.af_inet.pmtudisc = IP_PMTUDISC_DONT; else sk->protinfo.af_inet.pmtudisc = IP_PMTUDISC_WANT; | |