On 27/11/15 14:05, Jonathan Ballet wrote:
- how long does it take to resolv the domain name to (at least) one of
its IP address
- against a specified name server or using the system configured servers
That is relatively straightforward.
- how many tries did it require
- if there were several tries, the timing of each ones
Typically, application-layer code doesn't retry a DNS lookup; rather the c or other runtime will handle this, for example getaddrinfo() in glibc, according to settings read from /etc/resolv.conf or compiled-in defaults.
So it depends on whether you want to emulate "typical" application code, a specific application stack that may or may not do it's own resolution (e.g. modern browsers) or something else.
- how long does it take to get the first bytes of the endpoint
- how long does it take to complete the TCP connection handshake
- the status of the packets exchanged (how many retries, how many
packets lost, etc.)
Some of this is available in platform-specific APIs e.g. SIOCGSTAMP and TCP_INFO socket options available on Linux.
In general, any timings you make based on return of control from kernel will include error relating to system/scheduling issues. If you're concerned about getting raw, on-the-wire timings, this is extremely difficult without being in-kernel, and even then various issues - TCP offload for example - can end up hiding data from you.
How far can I do this kind of things with Twisted? I know I can somewhat easily get the timings of the name resolution, the TCP connection handshake also and the time to first byte(s), but what about the packets? I haven't look at the code of Twisted Names yet, but if it's doing the DNS request by itself, I may be able to plug-in somewhere and have my request counter and the timers associated, but I'm not sure if the underlying details of the TCP protocol are exposed to the upper layer such as Twisted?
Only via platform-specific options.
To do this kind of thing "reliably", you'd need to reimplement TCP in user-space.
But the info above may be a helpful start.