Re: End System PMTUD behavior question

n, Jeffrey H." writes:
> Mark,
> 
> Could you elaborate on what you mean by "DNS servers just tell the kernel t=
> o fragment at network mtu for UDP/IPv6 and ensure that DF is off for UDP/IP=
> v4."  What is the "network MTU?" Also, to which implementations of DNS serv=
> er are you referring?

	Cut-and-pasted from BIND's bsd socket implementation.

	Mark

#ifdef IPV6_USE_MIN_MTU        /* RFC 3542, not too common yet*/
                /* use minimum MTU */
                if (sock->pf == AF_INET6) {
                        (void)setsockopt(sock->fd, IPPROTO_IPV6,
                                         IPV6_USE_MIN_MTU,
                                         (void *)&on, sizeof(on));
                }
#endif

#if defined(IP_MTU_DISCOVER) && defined(IP_PMTUDISC_DONT)
                /*
                 * Turn off Path MTU discovery on IPv4/UDP sockets.
                 */
                if (sock->pf == AF_INET) {
                        int action = IP_PMTUDISC_DONT;
                        (void)setsockopt(sock->fd, IPPROTO_IP, IP_MTU_DISCOVER,
                                         &action, sizeof(action));
                }
#endif
#if defined(IP_DONTFRAG)
                /*
                 * Turn off Path MTU discovery on IPv4/UDP sockets.
                 */
                if (sock->pf == AF_INET) {
                        int off = 0;
                        (void)setsockopt(sock->fd, IPPROTO_IP, IP_DONTFRAG,
                                         &off, sizeof(off));
                }
#endif

 
> Best Regards,=20
> =A0=20
> Jeffrey Dunn=20
> Info Systems Eng., Lead=20
> MITRE Corporation.
> (301) 448-6965 (mobile)
-- 
Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742                 INTERNET: Mark_Andrews@isc.org
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