Category: Linux

Sometimes it might be beneficial to redistribute unicast UDP traffic to multiple other clients. I tried to accomplish this using iptables but it doesn’t seem to be able to resend the packets to the local broadcast address.

This program provides a solution to this problem. It listens on a specified UDP port on all local addresses and resends those packets to the local broadcast address. To avoid creating a loop the incoming and outgoing ports must be different. The application can also ensure that only packets coming from a specified source are being retransmitted. While the latter doesn’t provide any real security since it is trivial to spoof the source IP address, it does provide some immunity from other applications.

The source code for the application (uc2bc) is included in the gzipped archive below. Just compile with “gcc -o uc2bc uc2bc.c”. A sample init script is also included. This would go in the /etc/init.d folder.

Thanks to Andreas who got me (re)started with socket programming on *NIX.

Attachments

• uc2bc.tar.gz

I use Microsoft Virtual Server 2005 R2 to host a couple of server. I have become accustomed to having template virtual machines so I don’t have to start from scratch for every installation. When I did that to a newly installed Ubuntu 7.10 I immediately noticed that I lost all networking.

It turns out that Ubuntu 7.10 has moved some settings for MAC address mapping for interfaces to the file /etc/udev/rules.d/70-persistent-net.rules. Edit that file and make sure that the MAC address matches the MAC address given by the virtual host.

I was trying to install Ubuntu 7.10 on Virtual Server 2005 R2 and was continuously met by an error saying “your installation CD-ROM couldn’t be mounted”.

It seems that every time I try to install Ubuntu, I run into problems. Last time it was caused by lack of hardware support in the CPU on my VIA motherboard for some compiled-in feature. That was solved by using “alternate” build. In this case it seems to be a mismatch between my version of Virtual Server and Ubuntu. What worked this time was to use a minimal installation ISO which pulls down the rest of the installation over the network. This still doesn’t make the CD work but I have no use for the CD in this specific installation so that is fine by me.Sidenote: The same version worked flawlessly in VMWare Fusion. Lessons learned?

RFC822 from way back in 1982 stipulates that any SMTP mail server system must accept mail for postmaster@<domain> so that someone may send email to the administrator of the email system without knowing any personal email addresses.

When setting up my postfix server I set up DNS Blacklists (DNSBL) to block email being sent from known open relays to minimise the number of spam. This was a great success and I now very rarely receive any spam to my personal address. However, a year or so back I noticed that email were coming in to the postmaster address, past any spam checks. I quickly learned that postfix by default disables any blocks for the postmaster address and when I searched the Internet it looked as if a recompilation of postfix was necessary.

Then, eventually, I found that if the value address_verify_sender is set to something else than postmaster@<domain> DNSBL could again be enabled for the postmaster alias. The value address_verify_sender is used when the mail server verifies the sender. It can be set to null but that will probably lead to problems since many mailservers will not accept it. The solution is simple – I created another alias in /etc/aliases and set the same alias to address_verify_sender in /etc/postfix/main.cf.

Now DNSBL is in place for postmaster@<domain> but it is disabled for the address used for verification. However, that address is not as easily guessed and doesn’t get any spam. Legitimate email to postmaster@<domain> still gets through so I don’t feel that this is in violation to RFC822.

I used to run this blog on Drupal 4.7.3 and really hadn’t upgraded the software in more than a year. For some reason, the whole blog felt very slow and it took ages to get into the administration pages. It was clearly time for change. The question was whether I should upgrade to Drupal 5 or switch platform altogether. To be honest, I really didn’t actually use any of the more CMS-ish features of Drupal and I have a couple of other sites on WordPress so I thought long and hard about switching to WordPress.

But what about all the previous content? Sure, I hadn’t written that much but there was a little and it is always nice not having to start from scratch. Unfortunately there didn’t seem to be any recommended solution for migrating the content. Some other bloggers had posted MySQL scripts that could take the data from one format to another but it was supposedly very uncertain if it was going to work with my combination of versions. Now, I have no problem whatsoever in writing SQL scripts and do so ever so often in my job – I just felt that there had to be a simpler way. And there was.

It turns out that WordPress has a very nice RSS import feature and I had set up Drupal to provide an RSS feed. I just switched on the setting in Drupal that makes the RSS feed include all the text and not just the teaser but then I had one small issue – Drupal doesn’t let you show more than 30 posts in the RSS feed.

It was time to get dirty and jump into the PHP code. 43% down in the file /modules/system.module there is an array with the number of posts to show. Just stick “, 9999” (or some other large number) at the end of the list and go back to the setting page in Drupal and pick it. Then just save the RSS feed and import it into WordPress. Ta da!

Some ISPs that I use block outgoing SMTP traffic on port 25. As an example, my mobile operator  Telenor blocks outgoing traffic from my phone on TCP port 25 which means that I couldn’t connect my laptop through their network to send email using SSL-encrypted SMTP via my company’s SMTP server. I can understand their intention but they make it very difficult for people like myself to use external mail servers for outgoing mail. The standard solution to this is to use the submission port (TCP 587) and if there is a fully configurable firewall in front of the mail server it is normally straightforward to make the firewall send traffic on two external ports (25 & 587) to the same internal port.

However, in my case it wasn’t so easy. The firewall could route traffic to internal IP addresses but it couldn’t change the destination port.

After briefly consulting Google, many people talked about having Postfix listen to multiple ports but others warned about the risk of ending up with an open relay. Naturally I didn’t want that so I looked for an easier solution. And there is. Just use iptables to rewrite the port on incoming packets:

iptables -t nat -A PREROUTING -p tcp --dport 587 -j REDIRECT --to-ports 25

To make this stick when the server is rebooted, use iptables-save or similar and apply the rules when the network is activated. The nice thing with this setup is that Postfix is not touched and still only listens to port 25.