How can I tell if a process (on Linux) that is opening TCP connections is setting the TCP_NODELAY flag

Question

I have an application that I'm testing/debugging I would like to know if the connections that it's opening have had the TCP_NODELAY set. Is there are a way to do this e.g. using netstat to look at the details of an open connection? Is something like strace or system tap my best option?

Answer 1

Sadly you can't after the socket has already been opened. The information linux kernel hands out lacks this information.

See http://lkml.indiana.edu/hypermail/linux/kernel/0409.1/2166.html

You can trace your code via strace and look for lines similar to this (taken from stracing mysql client connecting to a mysql server)

socket(PF_INET, SOCK_STREAM, IPPROTO_IP) = 3
fcntl(3, F_SETFL, O_RDONLY)             = 0
fcntl(3, F_GETFL)                       = 0x2 (flags O_RDWR)
connect(3, {sa_family=AF_INET, sin_port=htons(9000), sin_addr=inet_addr("127.0.0.1")}, 16) = 0
setsockopt(3, SOL_SOCKET, SO_RCVTIMEO, "\2003\341\1\0\0\0\0\0\0\0\0\0\0\0\0", 16) = 0
setsockopt(3, SOL_SOCKET, SO_SNDTIMEO, "\2003\341\1\0\0\0\0\0\0\0\0\0\0\0\0", 16) = 0
setsockopt(3, SOL_IP, IP_TOS, [8], 4)   = 0
setsockopt(3, SOL_TCP, TCP_NODELAY, [1], 4) = 0
setsockopt(3, SOL_SOCKET, SO_KEEPALIVE, [1], 4) = 0

If you're on a BSD or non-linux unixy system

lsof -a -p PID -i 4 -T f

could work ( See this http://stackoverflow.com/questions/1824115/display-socket-options similar question ).