OSEC

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Re: Not getting an SMTP 220 from the other side

From: Victor Duchovni (Victor.DuchovniMorganStanley.com)
Date: Fri Oct 05 2007 - 08:16:56 CDT

On Fri, Oct 05, 2007 at 03:01:50PM +0200, Per Jessen wrote:

>
> This is a tcpdump from a telnet session to <otherserver>:25 :
>
> 14:23:16.558163 IP 88.198.198.123.54285 > 212.203.120.99.25: S 555319065:555319065(0) win 5840 <mss 1440,sackOK,timestamp 3034849 0,nop,wscale 7>
> 14:23:16.578896 IP 212.203.120.99.25 > 88.198.198.123.54285: S 207923013:207923013(0) ack 555319066 win 16384 <mss 1460,nop,wscale 0,nop,nop,timestamp 0 0,nop,nop,sackOK>
> 14:23:16.579689 IP 88.198.198.123.54285 > 212.203.120.99.25: . ack 1 win 46 <nop,nop,timestamp 3034854 0>
>
> An example that DOES work looks like this:
>
> 14:26:13.607102 IP 88.198.198.123.51639 > 212.203.120.99.25: S 3658425386:3658425386(0) win 5840 <mss 1440,sackOK,timestamp 2613904048 0,nop,wscale 2>
> 14:26:13.630698 IP 212.203.120.99.25 > 88.198.198.123.51639: S 3354957398:3354957398(0) ack 3658425387 win 16384 <mss 1460,nop,wscale 0,nop,nop,timestamp 0 0,nop,nop,sackOK>
> 14:26:13.631337 IP 88.198.198.123.51639 > 212.203.120.99.25: . ack 1 win 1460 <nop,nop,timestamp 2613904055 0>
>
> I'm not much of a TCP expert as such, but I'm thinking that the window size of 46
> in the first example prevents the other side from sending the initial 220 ?

No, this window size is "scaled" by factor of 2^7, so it is realy
46*128 or 5888. Now if you have a firewall in the middle that does not
or incorrectly implements window scaling, you could have an issue.

--
        Viktor.

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