How can I close a netcat connection after a certain character is returned in the response? - linux

Question

We have a very simple tcp messaging script that cats some text to a server port which returns and displays a response.
The part of the script we care about looks something like this:
cat someFile | netcat somehost 1234
The response the server returns is 'complete' once we get a certain character code (specifically &001C) returned.
How can I close the connection when I receive this special character?
(Note: The server won't close the connection for me. While I currently just CTRL+C the script when I can tell it's done, I wish to be able to send many of these messages, one after the other.)
(Note: netcat -w x isn't good enough because I wish to push these messages through as fast as possible)

Answer 1

Create a bash script called client.sh:
#!/bin/bash
cat someFile
while read FOO; do
echo $FOO >&3
if [[ $FOO =~ `printf ".*\x00\x1c.*"` ]]; then
break
fi
done
Then invoke netcat from your main script like so:
3>&1 nc -c ./client.sh somehost 1234
(You'll need bash version 3 for the regexp matching).
This assumes that the server is sending data in lines - if not you'll have to tweak client.sh so that it reads and echoes a character at a time.

Answer 2

How about this?
Client side:
awk -v RS=$'\x1c' 'NR==1;{exit 0;}' < /dev/tcp/host-ip/port
Testing:
# server side test script
while true; do ascii -hd; done | { netcat -l 12345; echo closed...;}
# Generate 'some' data for testing & pipe to netcat.
# After netcat connection closes, echo will print 'closed...'
# Client side:
awk -v RS=J 'NR==1; {exit;}' < /dev/tcp/localhost/12345
# Changed end character to 'J' for testing.
# Didn't wish to write a server side script to generate 0x1C.
Client side produces:
0 NUL 16 DLE 32 48 0 64 # 80 P 96 ` 112 p
1 SOH 17 DC1 33 ! 49 1 65 A 81 Q 97 a 113 q
2 STX 18 DC2 34 " 50 2 66 B 82 R 98 b 114 r
3 ETX 19 DC3 35 # 51 3 67 C 83 S 99 c 115 s
4 EOT 20 DC4 36 $ 52 4 68 D 84 T 100 d 116 t
5 ENQ 21 NAK 37 % 53 5 69 E 85 U 101 e 117 u
6 ACK 22 SYN 38 & 54 6 70 F 86 V 102 f 118 v
7 BEL 23 ETB 39 ' 55 7 71 G 87 W 103 g 119 w
8 BS 24 CAN 40 ( 56 8 72 H 88 X 104 h 120 x
9 HT 25 EM 41 ) 57 9 73 I 89 Y 105 i 121 y
10 LF 26 SUB 42 * 58 : 74
After 'J' appears, server side closes & prints 'closed...', ensuring that the connection has indeed closed.

Answer 3

Try:
(cat somefile; sleep $timeout) | nc somehost 1234 | sed -e '{s/\x01.*//;T skip;q;:skip}'
This requires GNU sed.
How it works:
{
s/\x01.*//; # search for \x01, if we find it, kill it and the rest of the line
T skip; # goto label skip if the last s/// failed
q; # quit, printing current pattern buffer
:skip # label skip
}
Note that this assumes there'll be a newline after \x01 - sed won't see it otherwise, as sed operates line-by-line.

Answer 4

Maybe have a look at Ncat as well:
"Ncat is the culmination of many key features from various Netcat incarnations such as Netcat 1.x, Netcat6, SOcat, Cryptcat, GNU Netcat, etc. Ncat has a host of new features such as "Connection Brokering", TCP/UDP Redirection, SOCKS4 client and server supprt, ability to "Chain" Ncat processes, HTTP CONNECT proxying (and proxy chaining), SSL connect/listen support, IP address/connection filtering, plus much more."
http://nmap-ncat.sourceforge.net

Answer 5

This worked best for me. Just read the output with a while loop and then check for "0x1c" using an if statement.
while read i; do
if [ "$i" = "0x1c" ] ; then # Read until "0x1c". Then exit
break
fi
echo $i;
done < <(cat someFile | netcat somehost 1234)