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• How to prevent remote hosts from delivering mail to Postfix with spoofed From header?

  - by Hongli Lai

  I have a host, let's call it foo.com, on which I'm running Postfix on Debian. Postfix is currently configured to do these things: All mail with @foo.com as recipient is handled by this Postfix server. It forwards all such mail to my Gmail account. The firewall thus allows port 25. All mail with another domain as recipient is rejected. SPF records have been set up for the foo.com domain, saying that foo.com is the sole origin of all mail from @foo.com. Applications running on foo.com can connect to localhost:25 to deliver mail, with [email protected] as sender. However I recently noticed that some spammers are able to send spam to me while passing the SPF checks. Upon further inspection, it looks like they connect to my Postfix server and then say HELO bar.com MAIL FROM:<[email protected]> <---- this! RCPT TO:<[email protected]> DATA From: "Buy Viagra" <[email protected]> <--- and this! ... How do I prevent this? I only want applications running on localhost to be able to say MAIL FROM:<[email protected]>. Here's my current config (main.cf): https://gist.github.com/1283647

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• How to install/change locale on Debian?

  - by Hongli Lai

  I've written a web application for which the user interface is in Dutch. I use the system's date and time routines to format date strings in the application. However, the date strings that the system formats are in English but I want them in Dutch, so I need to set the system's locale. How do I do that on Debian? I tried setting LC_ALL=nl_NL but it doesn't seem to have any effect: $ date Sat Aug 15 14:31:31 UTC 2009 $ LC_ALL=nl_NL date Sat Aug 15 14:31:36 UTC 2009 I remember that setting LC_ALL on my Ubuntu desktop system works fine. Do I need to install extra packages to make this work, or am I doing it entirely wrong?

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• How to tune TCP TIME_WAIT timeout on Solaris?

  - by Hongli Lai

  I'm trying to change the TCP TIME_WAIT timeout on Solaris. According to some Google results I need to run this command: ndd -set /dev/tcp tcp_time_wait_interval 60000 However I get: operation failed: Not owner What am I doing wrong? I'm already running ndd as root. Is there another way to tune TIME_WAIT?

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• Where do these mysterious DNS lookups come from and why are they slow?

  - by Hongli

  I have recently obtained a new dedicated server which I'm now setting up. It's running on 64-bit Debian 6.0. I have cloned a fairly large git repository (177 MB including working files) onto this server. Switching to a different branch is very very slow. On my laptop it takes 1-2 seconds, on this server it can take half a minute. After some investigation it turns out to be some kind of DNS timeout. Here's an exhibit from strace -s 128 git checkout release: stat("/etc/resolv.conf", {st_mode=S_IFREG|0644, st_size=132, ...}) = 0 socket(PF_INET, SOCK_DGRAM|SOCK_NONBLOCK, IPPROTO_IP) = 5 connect(5, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("213.133.99.99")}, 16) = 0 poll([{fd=5, events=POLLOUT}], 1, 0) = 1 ([{fd=5, revents=POLLOUT}]) sendto(5, "\235\333\1\0\0\1\0\0\0\0\0\0\35Debian-60-squeeze-64-minimal\n\17happyponies\3com\0\0\1\0\1", 67, MSG_NOSIGNAL, NULL, 0) = 67 poll([{fd=5, events=POLLIN}], 1, 5000) = 0 (Timeout) This snippet repeats several times per 'git checkout' call. My server's hostname was originally Debian-60-squeeze-64-minimal. I had changed it to shell.happyponies.com by running hostname shell.happyponies.com, editing /etc/hostname and rebooting the server. I don't understand the DNS protocol, but it looks like Git is trying to lookup the IP for Debian-60-squeeze-64-minimal as well as for happyponies.com. Why does Debian-60-squeeze-64-minimal come back even though I've already changed the host name? Why does Git perform DNS lookups at all? Why are these lookups so slow? I've already verified that all DNS servers in /etc/resolv.conf are up and responding slowly, yet Git's own lookups time out. Changing the host name back to Debian-60-squeeze-64-minimal seems to fix the slowness. Basically I just want to fix whatever DNS issues my server has because I'm sure they will cause more problems that just slowing down git checkout. But I'm not sure sure what the problem exactly is and what these symptoms mean.

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• Is there a lightweight multipart/form-data parser in C or C++?

  - by Hongli

  I'm looking at integrating multipart form-data parsing in a web server module so that I can relieve backend web applications (often written in dynamic languages) from parsing the multipart data themselves. The multipart grammar (RFC 2046) looks non-trivial and if I implement it by hand a lot of things can go wrong. Is there already a good, lightweight multipart/form-data parser written in C or C++? I'm looking for one with no external dependencies other than the C or C++ standard library. I don't need email attachment handling or buffered I/O classes or a portability runtime or whatever, just multipart/form-data parsing. Things that I've considered: GMime - depends on glib, so no go. libapreq - too large, depends on APR, badly documented, no unit tests. I've also looked at writing a parser with Ragel, but I can't figure out how to do it because the grammar is not static: the boundary can change arbitrarily.

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• What does ECONNRESET mean in the context of an AF_LOCAL socket?

  - by Hongli

  I understand that for TCP sockets ECONNRESET has got something to do with RST packets. But I've seen ECONNRESET errors for AF_LOCAL sockets too, on read() and write() calls. What does this mean? How is ECONNRESET different from read() returning 0 or write() throwing EPIPE?

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• What can cause a spontaneous EPIPE error without either end calling close() or crashing?

  - by Hongli

  I have an application that consists of two processes (let's call them A and B), connected to each other through Unix domain sockets. Most of the time it works fine, but some users report the following behavior: A sends a request to B. This works. A now starts reading the reply from B. B sends a reply to A. The corresponding write() call returns an EPIPE error, and as a result B close() the socket. However, A did not close() the socket, nor did it crash. A's read() call returns 0, indicating end-of-file. A thinks that B prematurely closed the connection. Users have also reported variations of this behavior, e.g.: A sends a request to B. This works partially, but before the entire request is sent A's write() call returns EPIPE, and as a result A close() the socket. However B did not close() the socket, nor did it crash. B reads a partial request and then suddenly gets an EOF. The problem is I cannot reproduce this behavior locally at all. I've tried OS X and Linux. The users are on a variety of systems, mostly OS X and Linux. Things that I've already tried and considered: Double close() bugs (close() is called twice on the same file descriptor): probably not as that would result in EBADF errors, but I haven't seen them. Increasing the maximum file descriptor limit. One user reported that this worked for him, the rest reported that it did not. What else can possibly cause behavior like this? I know for certain that neither A nor B close() the socket prematurely, and I know for certain that neither of them have crashed because both A and B were able to report the error. It is as if the kernel suddenly decided to pull the plug from the socket for some reason.

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• C++: is it safe to read an integer variable that's being concurrently modified without locking?

  - by Hongli

  Suppose that I have an integer variable in a class, and this variable may be concurrently modified by other threads. Writes are protected by a mutex. Do I need to protect reads too? I've heard that there are some hardware architectures on which, if one thread modifies a variable, and another thread reads it, then the read result will be garbage; in this case I do need to protect reads. I've never seen such architectures though. This question assumes that a single transaction only consists of updating a single integer variable so I'm not worried about the states of any other variables that might also be involved in a transaction.

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