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Re: Fw: [NTBUGTRAQ] Win 2003 DNS requests makes replies over 512 byte PIX limit
From: Valdis.Kletnieks () vt edu
Date: Thu, 08 May 2003 17:13:17 -0400
On Thu, 08 May 2003 22:36:16 +0200, Mathias Gerber <mathias () intergga ch> said:
AFAIK the DNS uses TCP for larger replys.
Back when the maximum usable MTU in the Arpanet was 584, the DNS protocol basically said "Send the query as UDP, if reply is over 512 bytes long server sends back 'too big', and retry the query as TCP". RFC2671 specifies an extension mechanism for DNS (EDNS0), and even if you don't use any other extensions provides a convenient way of saying "Use UDP if the packet is under 1280 (or 4K, or whatever you specify)". This allows the (hopeful) savings of a 3 packet handshake to set up a TCP session and another several packets at FIN time. However, just as with older firewalls that break RFC3168 ECN (explicit congestion notification) because they don't like the use of previously "reserved" bits in the TCP SYN packet, some gear doesn't like seeing the RFC2671-format DNS queries and drop them on the floor... -- Valdis Kletnieks Computer Systems Senior Engineer Virginia Tech
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Current thread:
- Fw: [NTBUGTRAQ] Win 2003 DNS requests makes replies over 512 byte PIX limit hggdh (May 08)
- Re: Fw: [NTBUGTRAQ] Win 2003 DNS requests makes replies over 512 byte PIX limit Mathias Gerber (May 08)
- Re: Fw: [NTBUGTRAQ] Win 2003 DNS requests makes replies over 512 byte PIX limit Valdis . Kletnieks (May 08)
- Re: Fw: [NTBUGTRAQ] Win 2003 DNS requests makes replies over 512 byte PIX limit Derek Atkins (May 08)
- Re: Fw: [NTBUGTRAQ] Win 2003 DNS requests makes replies over 512 byte PIX limit hggdh (May 08)
- Re: Fw: [NTBUGTRAQ] Win 2003 DNS requests makes replies over 512 byte PIX limit Mathias Gerber (May 08)