TCP fragmentation caused by \x0a sent to socket

[drwetter]

As noted in the discussion of PR #1113 it seems that printf -- "$data" >&5 2>/dev/null does not do what it is intended. $data contains a ClientHello, 5 is the fd of a socket. "$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 spotted when using wireshark while running e.g.

testssl.sh --assume-http -p testssl.sh

Starting from the SSLv3 ClientHello the first reassembled packet ends with 0a.

This ticket is for continuing the discussion from #1113 (comment) on

[drwetter]

dd might be worth looking into it .I am afraid that it is a "feature" of bash

[drwetter]

It turned out not to be a feature of bash + sockets but of the internal printf:

prompt> strace -ewrite bash -c 'printf "one\ntwo\n"' >/dev/null
write(1, "one\n", 4)                    = 4
write(1, "two\n", 4)                    = 4
+++ exited with 0 +++
prompt> strace -etrace=write /usr/bin/printf "one\two\n" > /dev/null
write(1, "one\two\n", 7)                = 7
+++ exited with 0 +++

We could also use stdbuf or dd (latter either directly for writing into the socket or as a pipe after any printf). :

prompt> strace -etrace=write bash -c 'stdbuf -o1M printf "one\ntwo\n"  ' > /dev/null
write(1, "one\ntwo\n", 8)               = 8
+++ exited with 0 +++
prompt> bash -c 'printf "one\ntwo\n" | strace -e write,read -o/tmp/log.txt dd status=none bs=1M' > /dev/null
prompt> cat /tmp/log.txt
[..]
read(0, "one\ntwo\n", 1048576)          = 8
write(1, "one\ntwo\n", 8)               = 8
read(0, "", 1048576)                    = 0
+++ exited with 0 +++

[drwetter]

See commit. Solved

[drwetter]

see 305eefc and #1134, #1135

[drwetter]

BSDish systems are still open -- at least if they do a write flush after "\x0a" which I don't know but assume.

@dcooper16 : You mind checking? There's a small chance that OS X libc behaves different