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Re: bash sockets: printf \x0a does TCP fragmentation


From: Chet Ramey
Subject: Re: bash sockets: printf \x0a does TCP fragmentation
Date: Fri, 21 Sep 2018 19:34:52 -0400
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.13; rv:52.0) Gecko/20100101 Thunderbird/52.9.1

On 9/21/18 4:13 PM, address@hidden wrote:
> 
> Hello there,
> 
> we discovered a strange phenomenon in the project testssl.sh:
> 
> After opening a TCP socket with a fd (here: 5), when writing to it,
> it seems that
> 
> printf -- "$data" >&5 2>/dev/null
> 
> does not do what it is intended. "$data" is  a ClientHello like
> 
> '\x16\x03\x01\x2\x00\x01\x00\x1\xfc\x03\x03\x54\x51\x1e\x7a\xde\xad\xbe\xef\x31\x33\x07\x00\x00\x00\x00\x00\xcf\xbd\x39\x04\xcc\x16\x0a\...'
> 
> Each \x0a like the last one causes a new TCP fragment to begin which can be 
> easily
> spotted when using wireshark while running e.g.

Newline? It's probably that stdout is line-buffered and the newline causes
a flush, which results in a write(2).

> If there's a workaround, please let me know. (tried to add "%b" with no
> effect). Otherwise I believe it's a bug.

How? Does the emitted output not correspond to what's passed to printf
in some way?

-- 
``The lyf so short, the craft so long to lerne.'' - Chaucer
                 ``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, UTech, CWRU    address@hidden    http://tiswww.cwru.edu/~chet/



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