""Francisco Sedano/Inf-Pronet"" wrote in message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... > Hello! > > And there are some issues with several protocols, as Marc points out. In > FTP, for example, possibly the new packet will be larger than original one. > (IP Addresses are codified in the data payload as text). So, it's possible > the router has to fragment the packet. And that's not a simple header > rewrite.
I took a quick look at RFC's 1631 and 26something last night. 1631 is poorly written IMHO, and pretty sloppy in some of its language. Neither one talks specifically about what is supposed to happen in terms of which specific fields are replaced. However, both talk about issues with checksums, packet size changes, and both specifically mention FTP PORT ( and the other operation, which I can't recall ) > > Francisco Sedano > Informatica Pronet. > > > > > > "Marc Thach Xuan > Ky" > > cc: > Enviado por: Asunto: Re: NAT > [7:60784] > > [EMAIL PROTECTED] > > > 10/01/2003 > 12:08 > Por favor, responda > a > "Marc Thach Xuan > Ky" > > > > > > > Dwayne, > it's most likely that any NAT implementation would overwrite the header > data that it wishes to change, rather than rewrites the header in its > entirety. Of course the end result would look the same when you view > the packet, however you can recalculate the checksum from the old and > new IP addresses without reading the entire packet, so that's a gain for > not using the full header creation code. > Note though that some protocols which don't pass well through NAT are > handled by an ALG (Application Level Gateway), and these modules will > rewrite the IP data. Now if I were coding an ALG I'd certainly create > the entire header for scratch, and I might need to do the same with the > data. Think of an FTP ALG for example. Here the length of the data may > be changed, in particular it may grow. The buffer that is currently > allocated for the packet may not have room to grow, so in that case, > you'd need to copy the data into a larger buffer probably as you parse > and alter the data. > rgds > Marc > > > Dwayne Saunders wrote: > > > > Hi all, > > Was just wondering if any one could put me on to a good link in > > regards NAT and packet headers, simply what I am trying to find out is > the > > packet header total rewritten or just the ip address part of the header > and > > checksum, Or is a new header written to envelope the original header. > > > > Or does each application do it differently. > > > > Any help would be great. > > > > Regards > > > > D'Wayne Saunders Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=60818&t=60784 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
