Sometimes it's convenient to be able to execute a command directly on
the stream, whether we're connecting or accepting an incoming connection.
New command 'X' makes this possible. It simply calls execvp() on the
next arguments and branches stdin/stdout/stderr on the socket. Optionally
it's possible to limit the passed FDs to any combination of them by
appending 'i', 'o', 'e' after the X. In any case the program ends just
after executing this command.
Examples :
- chargen server
tcploop 8001 L A Xo cat /dev/zero
- telnet server
tcploop 8001 L W N A X /usr/sbin/in.telnetd
NULL is Linux-centric and we're not focused on performance here but
portability and reproducibility. Don't use NULL and use the trash
instead. It may lead to multiple recv() calls for large blocks but
as a benefit it will be possible to see the contents with strace.
Just got this while cross-compiling :
tcploop.c: In function 'tcp_recv':
tcploop.c:444:48: error: 'INT_MAX' undeclared (first use in this function)
tcploop.c:444:48: note: each undeclared identifier is reported only once for each function it appears in
This one jumps back to the oldest post-fork and post-accept action,
so it allows to recv(), pause() and send() in loops after a fork()
and an accept() for example. This is handy for bugs that reproduce
once in a while or to keep idle connections working.
By passing "S:<string>" instead of S<size> it's possible to send
a pre-defined string, which is convenient to write HTTP requests or
responses.
Example : produce two responses, one in keep-alive, one not for ab :
./tcploop 8001 L W N2 A R S:"HTTP/1.0 200 OK\r\nConnection: keep-alive\r\nContent-length: 50\r\n\r\n0123456789.123456789.123456789.123456789.123456789" R S:"HTTP/1.0 200 OK\r\nContent-length: 50\r\n\r\n0123456789.123456789.123456789.123456789.123456789"
With 20 such keep-alive responses and 10 parallel processes, ab achieves
350kreq/s, so it should be possible to get precise timings.
This is helpful to show what state we're dealing with. The pid is
written, optionally followed by the time in 3 different formats
(relative/absolute) depending on the command line option (-t, -tt, -ttt).
Fork is a very convenient way to deal with independant yet properly
timed connections. It's particularly useful here for accept(), and
ensures that any accepted FD will automatically be released. The
principle is that when we hit a fork command, the parent restarts
evaluating the actions from the beginning and the child continues
to evaluate the next actions. Listen and connect are skipped if the
connection is already established. Fork() is amazingly cheap on
Linux, 21k forked connections per second are handled on a single
core, and 38k on two cores.
For now it's not possible to have two different code paths so in order
to have both a listener and a connector, two distinct commands are
still needed.
netcat, nc6 and socat are only partially convenient as reproducers for
state machine bugs, but when it comes to adding delays, forcing resets,
waiting for data to be acked, they become useless.
The purpose of this utility is to be able to easily script some TCP
operations such as connect, accept, send, receive, shutdown and of
course pauses.
