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Managing iptables drop lists

Some material is very old and may be incorrect today

© May 2015 Anthony Lawrence

I've talked about my Logdropper script before. While making some changes to that script, I screwed up and accidentally blocked far more addresses than I wanted to. I could have just reset iptables entirely or rebooted, but I only wanted to get rid of the ones I just added.

Bash brace expansion helped me out (see Bash 3.00 brace expansion and Bash Brace Expansion). I can get a list of DROPS easily:

# /sbin/iptables --list -n --line-numbers | grep DROP 
5    DROP       all  --           
6    DROP       all  --           
7    DROP       all  --           
8    DROP       all  --           
9    DROP       all  --           
10   DROP       all  --           
11   DROP       all  --           
12   DROP       all  --           
13   DROP       all  --           
14   DROP       all  --           
15   DROP       all  --           
16   DROP       all  --           
17   DROP       all  --           
18   DROP       all  --           
19   DROP       all  --           
20   DROP       all  --     

I knew that the new rules started at line 8. You delete rules with "iptables -D INPUT linenumber" (assuming your DROPS are in the INPUT chain, see Understanding IPTABLES). Obviously it would be tedious to do that manually, so we will use brace expansion. However, we can't say "for i in {8..20}" because the lines will renumber after each command. Fortunately, Bash doesn't object to counting down:

for i in {20..8}; 
do iptables -D INPUT $i

Simple, right?

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