I'm having a hard time making Privoxy work. (I'm trying to get it working in conjunction with Tor, but I don't think this problem is tor related).

Privoxy has its own user and group. I've configured firefox to use 127.0.0.1:8118 as a http and https proxy server. I've uncommented the forwarding lines in the privoxy config file as per http://www.privoxy.org/user-manual/quickstart.html, including:
Yet I still get a http 503 for any browsing attempted using these proxy settings.

The error message:
What is a SOCKS4 negotiation, and does a write fail mean that a certain file doesn't have write permissions and should?

Since Privoxy returns the error page, have you uncommented and set the torrc ^Socks.* directives?

I haven't, but since I built tor from source, I don't yet have a torrc, just the included sample. As I said though, I don't believe this has anything to do with tor, because privoxy doesn't work when running on its own. If I disable the "Tor button" add-on, ensure there's nothing in the SOCKS proxy fields in the network settings (ie I remove tor's 127.0.0.1:9005 port info), and try to run firefox using just privoxy, I'm still getting the problem.
What does "write negotiation failed" mean - doesn't that mean the proxy connection to the TCP port 8118 on 127.0.0.1 was refused?

(Oh, and for the sake of it, I've copied the sample torrc file and uncommented these lines:
I take it those were the lines you referred to?

Wow, it's working now. Trouble is, I didn't really learn anything because it mysteriously started working after a stack of updates and a new kernel. Thanks anyway.

Yes.


I'm pretty sure the kernel has nothing to do with it. (Please at least fill in your distro information in your profile, TIA, it'll help you get nfo and instructions more geared towards the one you use.) Apart from commenting out the ^Socks.* lines to test that part, (and running both with full debug logging on) you could start by for instance running 'ldd' on the Privoxy and TOR binaries and see if any updates affected dependent libraries (but IIRC the only interesting one would be 'libevent' for TOR and you already said you compiled everything from source). Else there isn't much chance of finding out (since AFAIK both Privoxy and TOR don't have any "exotic" requirements wrt libs or networking) except for looking into what updates got installed and what other reconfiguration was done.