Starting in curl 7.26.0 (due to be released at the end of May 2012), we will shrink the User-agent: header that curl sends by default in HTTP(S) requests to something much shorter! I suspect that this will raise some eyebrows out there so even though I've emailed about it to the curl-users list before I thought I'd better write it up and elaborate.
A default 'curl localhost' on Debian Linux makes 170 bytes get sent in that single request:
GET / HTTP/1.1 User-Agent: curl/7.24.0 (i486-pc-linux-gnu) libcurl/7.24.0 OpenSSL/1.0.0g zlib/1.2.6 libidn/1.23 libssh2/1.2.8 librtmp/2.3 Host: localhost Accept: */*
As you can see, the user-agent description takes up a large portion of that request, and this for really no good reason at all. Without sacrificing any functionality I shrunk the same request down to 71 bytes:
GET / HTTP/1.1 User-Agent: curl/7.24.0 Host: localhost Accept: */*
That means we shrunk it down to 41% of the original size. I'll admit the example is a bit extreme and most other normal use cases will use longer host names and longer paths, but even for a URL like "http://daniel.haxx.se/docs/curl-vs-wget.html" we're down to 50% of the original request size (100 vs 199).
Can we shrink it even more? Sure, we could leave out the version number too. I left it in there now only to allow some kind of statistics to get extracted. We can't remove the entire header, we need to include a user-agent in requests since there are too many servers who won't function properly otherwise.
And before anyone asks: this change is only for the curl command line tool and not for libcurl, the library. libcurl does in fact not send any user-agent at all by default...