{"id":106,"date":"2007-08-19T22:32:36","date_gmt":"2007-08-19T20:32:36","guid":{"rendered":"http:\/\/www.corelan.be:8800\/?p=85"},"modified":"2007-08-19T22:32:36","modified_gmt":"2007-08-19T20:32:36","slug":"running-snort-with-dynamic-ip-on-fedora","status":"publish","type":"post","link":"https:\/\/www.corelan.be\/index.php\/2007\/08\/19\/running-snort-with-dynamic-ip-on-fedora\/","title":{"rendered":"Running Snort with Dynamic IP on Fedora"},"content":{"rendered":"
\n

One of my Linux boxes has a direct cable connection to the internet. I've been using Snort in corporate environments for a long time now, but I never had to configure snort to look at interfaces that have a DHCP assigned IP address and actually use that IP address as its HOME_NET.<\/p>\n

There's an easy way (which may not work with an older version of Snort), and there's a somewhat 'harder' way (With a couple of scripts, this can be done as well).<\/p>\n

The easy way consists of using a Snort variable that refers to the Ethernet interface. Use the following line in your snort.conf file and you should be fine.
\nvar HOME_NET $eth1_ADDRESS<\/p>\n

If that doesn't work, you can use the scripts.<\/p>\n

Before going into the details, I need to make some assumptions :<\/p>\n

    \n
  1. eth1 is the internet facing interface, IP is DHCP assigned<\/li>\n
  2. I'm going to consider the IP address of that interface as my HOME_NET snort variable<\/li>\n
  3. I'm not going to use any other IP addresses or subnets in the HOME_NET variable (although this could be easily done by changing the scripts)<\/li>\n
  4. I've created two additional folders : \/root\/conf and \/root\/bin. Both are only accessible by root<\/li>\n<\/ol>\n

    First of all, you need to build the snort.conf file. Just leave out the HOME_NET setting at the top of the script. My snort installation sits under \/snortinternet. Put the (incomplete) snort.conf file (the file without the 'var HOME_NET=' entry somewhere else (under \/root\/conf for example)<\/p>\n

    Create a script called 'geteth1.sh<\/strong>' and put it under \/root\/bin. This script contains the following commands :<\/p>\n

    \n\n\n<\/col>\n<\/colgroup>\n\n\n
    #!\/bin\/bash
    \n\/sbin\/ifconfig eth1 | awk '\/inet\/ { print $2 }' | awk -F \":\" '{ print $2 }'<\/strong><\/span><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n

    (This script will output the IP address that is assigned to eth1)<\/p>\n

    Next, create another script called 'createsnortconf.sh<\/strong>' and save it under \/root\/bin :<\/p>\n

    \n\n\n<\/col>\n<\/colgroup>\n\n\n
    #!\/bin\/bash <\/strong><\/span><\/p>\n

    echo 'var HOME_NET ['`\/bin\/bash \/root\/bin\/geteth1.sh`'\/32]'>\/root\/conf\/snorttop.conf
    \ncat \/root\/conf\/snort.conf >>\/root\/conf\/snorttop.conf
    \ncp \/root\/conf\/snorttop.conf \/snortinternet\/snort.conf<\/strong><\/span><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n

    This script basically grabs the IP address of eth1, stores it into a temporary file, puts the temporary file and the (incomplete) snort.conf file from \/root\/bin together and overwrites the \/snortinternet\/snort.conf file with the new file.<\/p>\n

    If you keep you snort.conf file somewhere else, just edit the paths in the script and you'll be fine.<\/p>\n

    Next, make sure to run the script prior to starting snort. I usually create a 'runsnort.sh' script that will fire up snort with the necessary parameters, so that would be a good place to launch the script from.<\/p>\n

    Note : if you decide to make changes to your snort.conf, make sure to change the snort.conf file under \/root\/conf and then run the createsnortconf.sh script again to apply the changes to the \/snortinternet folder.<\/p><\/div>\n","protected":false},"excerpt":{"rendered":"

    One of my Linux boxes has a direct cable connection to the internet. I've been using Snort in corporate environments for a long time now, but I never had to configure snort to look at interfaces that have a DHCP assigned IP address and actually use that IP address as its HOME_NET. There's an easy … Continue reading \"Running Snort with Dynamic IP on Fedora\"<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"jetpack_post_was_ever_published":false,"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":false,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2}},"categories":[64,127],"tags":[133,131],"class_list":["post-106","post","type-post","status-publish","format-standard","hentry","category-linux","category-security","tag-snort","tag-linux"],"yoast_head":"\nRunning Snort with Dynamic IP on Fedora - Corelan | Exploit Development & Vulnerability Research<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/www.corelan.be\/index.php\/2007\/08\/19\/running-snort-with-dynamic-ip-on-fedora\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Running Snort with Dynamic IP on Fedora - Corelan | Exploit Development & Vulnerability Research\" \/>\n<meta property=\"og:description\" content=\"One of my Linux boxes has a direct cable connection to the internet. 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