How to decrease TCP connect() system call timeout?

Question

In command below I enable file /dev/tcp/10.10.10.1/80 both for reading and writing and associate it with file descriptor 3:

$ time exec 3<>/dev/tcp/10.10.10.1/80
bash: connect: Operation timed out
bash: /dev/tcp/10.10.10.1/80: Operation timed out

real    1m15.151s
user    0m0.000s
sys     0m0.000s

This automatically tries to perform TCP three-way handshake. If 10.10.10.1 is not reachable as in example above, then connect system call tries to connect for 75 seconds. Is this 75 second timeout determined by bash? Or is this system default? Last but not least, is there a way to decrease this timeout value?

Answer 1

It is determined by TCP. It can be decreased on a per-socket basis by application code.

NB The timeout only takes effect if there is no response at all. If there is a connection refusal, the error occurs immediately.

Answer 2

It's not possible in Bash without modifying the source as already mentioned, although here is the workaround by using timeout command, e.g.:

$ timeout 1 bash -c "</dev/tcp/stackoverflow.com/80" && echo Port open. || echo Port closed.
Port open.
$ timeout 1 bash -c "</dev/tcp/stackoverflow.com/81" && echo Port open. || echo Port closed.
Port closed.

Using this syntax, the timeout command will kill the process after the given time.

See: timeout --help for more options.

Answer 3

No: there is no way of changing timeout by using /dev/tcp/

Yes, you could change default timeout for TCP connection in any programming language.

But, bash is not a programming language!

You could have a look into source code (see: Bash Homepage), you may find lib/sh/netopen.c file where you could read in _netopen4 function:

s = socket(AF_INET, (typ == 't') ? SOCK_STREAM : SOCK_DGRAM, 0);

You could read this file carefully, there are no consideration of connection timeout.

Without patching bash sources, there is no way of changing connection timeout by a bash script.

Simple HTTP client using netcat (near pure bash)

There is a little sample HTTP client written in pure bash, but using netcat:

#!/bin/bash

tmpfile=$(mktemp -p $HOME .netbash-XXXXXX)
exec 7> >(nc -w 3 -q 0 stackoverflow.com 80 >$tmpfile)
exec 6<$tmpfile
rm $tmpfile

printf >&7 "GET %s HTTP/1.0\r\nHost: stackoverflow.com\r\n\r\n" \
    /questions/24317341/how-to-decrease-tcp-connect-system-call-timeout

timeout=100;
while ! read -t .001 -u 6 status ; do read -t .001 foo;done
echo STATUS: $status

[ "$status" ] && [ -z "${status//HTTP*200 OK*}" ] || exit 1

echo HEADER:
while read -u 6 -a head && [ "${head//$'\r'}" ]; do
    printf "%-20s : %s\n" ${head%:} "${head[*]:1}"
    done

echo TITLE:
sed '/<title>/s/<[^>]*>//gp;d' <&6

exec 7>&-
exec 6<&-

This could render:

STATUS: HTTP/1.1 200 OK
HEADER:
Cache-Control        : private
Content-Type         : text/html; charset=utf-8
X-Frame-Options      : SAMEORIGIN
X-Request-Guid       : 46d55dc9-f7fe-425f-a560-fc49d885a5e5
Content-Length       : 91642
Accept-Ranges        : bytes
Date                 : Wed, 19 Oct 2016 13:24:35 GMT
Via                  : 1.1 varnish
Age                  : 0
Connection           : close
X-Served-By          : cache-fra1243-FRA
X-Cache              : MISS
X-Cache-Hits         : 0
X-Timer              : S1476883475.343528,VS0,VE100
X-DNS-Prefetch-Control : off
Set-Cookie           : prov=ff1129e3-7de5-9375-58ee-5f739eb73449; domain=.stackoverflow.com; expires=Fri, 01-Jan-2055 00:00:00 GMT; path=/; HttpOnly
TITLE:
bash - How to decrease TCP connect() system call timeout? - Stack Overflow

Some explanations:

We create first a temporary file (under private directory for security reason), bind and delete before using them.

$ tmpfile=$(mktemp -p $HOME .netbash-XXXXXX)
$ exec 7> >(nc -w 3 -q 0 stackoverflow.com 80 >$tmpfile)
$ exec 6<$tmpfile
$ rm $tmpfile

$ ls $tmpfile
ls: cannot access /home/user/.netbash-rKvpZW: No such file or directory

$ ls -l /proc/self/fd
lrwx------ 1 user user 64 Oct 19 15:20 0 -> /dev/pts/1
lrwx------ 1 user user 64 Oct 19 15:20 1 -> /dev/pts/1
lrwx------ 1 user user 64 Oct 19 15:20 2 -> /dev/pts/1
lr-x------ 1 user user 64 Oct 19 15:20 3 -> /proc/30237/fd
lr-x------ 1 user user 64 Oct 19 15:20 6 -> /home/user/.netbash-rKvpZW (deleted)
l-wx------ 1 user user 64 Oct 19 15:20 7 -> pipe:[2097453]

$ echo GET / HTTP/1.0$'\r\n\r' >&7
$ read -u 6 foo
$ echo $foo
HTTP/1.1 500 Domain Not Found

$ exec 7>&-
$ exec 6>&-