How to block large ip subset on the example of TOR

I wrote before how i blocked TOR exit nodes by iptables, disadvantages of method that i used before – big amount of rules (one per each ip). This solution easy and obvious, but had speed penalty. Today i want write about more effective solution that use ipset, let’s see what is ipset:

IP sets are a framework inside the Linux 2.4.x and 2.6.x kernel, which can be administered by the ipset utility. Depending on the type, currently an IP set may store IP addresses, (TCP/UDP) port numbers or IP addresses with MAC addresses in a way, which ensures lightning speed when matching an entry against a set. If you want to store multiple IP addresses or port numbers and match against the collection by iptables at one swoop; dynamically update iptables rules against IP addresses or ports without performance penalty; express complex IP address and ports based rulesets with one single iptables rule and benefit from the speed of IP sets then ipset may be the proper tool for you.

(c)

Because of exit nodes can exist on hosts that using dynamic IP i want to delete addresses from list after timeout (otherwise list will always growing and contain unused addresses or addresses without TOR exit nodes, larger list requires more memory and CPU time for processing). If addresses persist between list updates, timeout will be reseted.

To do this i used iptree set (you can learn more about set types in manual for ipset), because that type provide timeout for each address.

First i installed ipset:

# apt-get install xtables-addons-common

After that i modify scripts that i used before. In perl script i made a pair of minor bug fixes and in shell script i add new facility for loading rules into ipset.
Perl script:

#!/usr/bin/perl -w
use strict;
use LWP::Simple;
 
my $list = get("http://exitlist.torproject.org/exit-addresses");
my $i;
my @ips;
 
if( ! defined( $list ) ) {
    exit( 1 );
}
if( $#ARGV == -1 ) {
exit(0);
}
 
foreach $i (split( /\n/, $list )) {
    push( @ips, $1 ) if( $i =~ m/((?:\d{1,3}\.){3}\d{1,3})/);
}
 
if( $ARGV[0] eq "-ip" ) {
    print( join( "\n", @ips ) . "\n");
}

That script return exit code without arguments that signaled can this script fetch addresses or not, with parameter “-ip” they return list of addresses.
Next shell script:

#!/bin/bash
SOLVE="/root/bin/solve.pl"
IPSET="/usr/sbin/ipset"
 
case "$1" in
ipset)
        if ! /usr/bin/perl $SOLVE
        then
                echo "Can not fetch Tor exit nodes" 1>&2
                exit 1
        fi
        if ! $IPSET -L tor 2>&1 > /dev/null
        then
                $IPSET -N tor iptree --timeout 259200
        fi
        for i in `/usr/bin/perl $SOLVE -ip`
        do
                $IPSET -A tor $i 2> /dev/null
        done
 
;;
 
iptables)
        if /usr/bin/perl $SOLVE
        then
                /sbin/iptables -F TorExitnodes
                /sbin/iptables -I TorExitnodes -j RETURN
 
                for i in `/usr/bin/perl $SOLVE -ip`
                do
                 /sbin/iptables -I TorExitnodes -s $i -j TorBlockAndLog
                done
        else
                        echo "Can not fetch Tor exit nodes" 1>&2
        exit 1;
        fi
 
;;
 
*)
        echo "Usage ./$0 "
        exit 1
;;
esac
 
exit 0

This script can add rules into iptables (old variant) or into ipset. I added this script in cron and run every few hours. When ipset found that address all ready in list, they update timeout, if address will not observed in 72 hours, they will be automatically deleted.

Finishing touch – new rule in iptables:

# iptables -A INPUT -i eth+ -m set --match-set tor src -m comment --comment "Block TOR exit nodes thru IPSET" -j DROP

That’s all. Do not forget to place this rule before rules where you permit access to your server.

How to transfer data between hosts securely.

like a bossFrom time to time i faced with task how to transfer important data between servers securely (ie over ssl or something similar). I do not use passwords for remote access and do not have private keys on remote systems, so i can not use ssh for this purposes.

First i wanted to write about solution that i used few days ago (based on socat), but this solution is to complicated (later you will see why). I remembered that openssl can encrypt files with password and send result to STDOUT. While i was reading manual for openssl, i found that openssl can be used like netcat (s_server and s_client commands), unfortunately i did not found way how to use openssl for data transfer, because in session that you can establish openssl interprets some chars as commands (R for renegotiation for example), so if you want to use openssl client/server for data transfer, you need something like base64 encoding, but without control characters.

Solution with netcat and openssl:
First i created file for test (you can use data from STDIN, tar output for example, or transfer existing file):

client$ dd if=/dev/urandom of=/tmp/rand bs=1M count=10
10+0 records in
10+0 records out
10485760 bytes (10 MB) copied, 1.36697 seconds, 7.7 MB/s
client$ md5sum /tmp/rand
10fe36edbbd48cde844ad1a2a29a8e0f  /tmp/rand

Next, prepare server side:

server$ read pass
PasSwOrD
server$ nc -l -p 6667|openssl aes-256-cbc -d -k $pass > /tmp/rand

I used “read” to prevent save key into history file.
There “PasSwOrD” is your key, i use ssh to organize data transfer, so i did not worried that the traffic with key can be captured.
Next initiate transfer from client side:

client$ read pass
PasSwOrD
client$ cat /tmp/rand |openssl  aes-256-cbc -salt -k $pass|nc -w1 server.remote 6667

Check sum:

server$ md5sum /tmp/rand
10fe36edbbd48cde844ad1a2a29a8e0f  /tmp/rand

Yeah! Your see? I transfered mah file.
UPDATE: [2019-05-26] Nowaday openssl has broken backward compatibility, so when you try to decrypt file you cold get error message like `digital envelope routines:EVP_DecryptFinal_ex:bad decrypt:crypto/evp/evp_enc.c:535`, if so you need to add -md md5 or -md sha256 on both sides to openssl’s options.

Ok, next we will do it with socat.
First you need to generate client side and server side key and certificates, let’s do it on server:

server$ openssl genrsa -out server.key 2048
Generating RSA private key, 2048 bit long modulus
........+++
......................................+++
e is 65537 (0x10001)

Create certificate:

server$  openssl req -new -key server.key -x509 -days 108 -batch -out server.crt

Create pem file:

server$ cat server.* > ./server.pem

After that you will need to execute same commands on client side, but you will need to change filenames from “server” to “client”.
Next step is to exchange certificates between client and server (do it on both sides), they could be copy pasted:

server$ cat > ./client.crt << EOF
-----BEGIN CERTIFICATE-----
.... a lot of garbage ....
-----END CERTIFICATE-----
EOF

Now we ready to transfer file, prepare server:

server$ socat openssl-listen:4433,reuseaddr,cert=./server.pem,cafile=./client.crt STDIO > /tmp/rand

Client:

client$ socat STDIO openssl-connect:server.remote:4433,cert=./client.pem,cafile=./server.crt < /tmp/rand
client$ md5sum /tmp/rand
10fe36edbbd48cde844ad1a2a29a8e0f  /tmp/rand

Gotcha!

As you can see, socat with TLS not a easy solution if you need just a transfer file, so i will recommend to use first solution. Also, in debian, you can use snakeoil key and cert, but it is your homework.

UPDATE:
I found how to use openssl for data transfer, only one problem, they did not close socket after EOF, so you need to stop it by hands:
Prepare server (this time i used snakeoil cert):

server% sudo openssl s_server -quiet -accept 4343 -cert /etc/ssl/certs/ssl-cert-snakeoil.pem -key /etc/ssl/private/ssl-cert-snakeoil.key < /tmp/test

Run client:

client% md5sum /tmp/rnd
86246865b3932804979fdac48a99cebf  /tmp/rnd
client% openssl s_client -connect localhost:4343 -quiet > /tmp/rnd

After data transfered, hit ^C on server side and check:

^C
server% md5sum /tmp/test
86246865b3932804979fdac48a99cebf  /tmp/test

Tor blacklist

Few month ago i was interested in how to block incoming traffic from Tor network. Tor network have finitely numbers of exit nodes, so the solution is to block traffic from this nodes. I see two solutions how to block exit nodes.  First use TorDNSEL service, in brief you can check that connection comes from Tor exit node or not by querying special domain name. It is useful if you want to do this check in your php script, for example. But i do not know how to protect  host completely by this method. Another way is to use exitlist. By this way you can get list of addresses. Both ways have advantages and disadvantages. I prefer last solution because, it allow to protect my hosts completely.  Cons of this method – near 700-800 rules into firewall, that can slow down your host.

Anyway, first that i done – simply script that fetch list of IP and print them to standard output. When i started to writing script i wanted to expand features in future. I want to generate ready firewall rules for example, but now this script can only print list of IP’s. I do not want to integrate firewall rules application in this script, because i think this job must be done by another script.
So, now this script only fetch and print IP addresses or exit with error code if can not do it:

#!/usr/bin/perl -w
use strict;
use LWP::Simple;
 
my $list = get("http://exitlist.torproject.org/exit-addresses");
my $i;
my @ips;
 
if( ! defined( $list ) ) {
die( "Can not fetch addresses.\n" );
}
 
foreach $i (split( /\n/, $list )) {
push( @ips, $1 ) if( $i =~ m/((?:\d{1,3}\.){3}\d{1,3})/);
}
 
if( defined( $ARGV[0] ) &&  $ARGV[0]  eq "-ip" ) {
print( join( "\n", @ips ));
} else {
die( "Usage $0 -ip\n" );
}

Next that you need – apply rules on firewall. As i say before, when you apply one rule per IP you will get near 700-800 rules, it is reason why you need optimize sequence of your rules. It can be done in next manner:
1. Rule to accept packets flowing thru loopback.
2. Rule to accept packets of RELATED and ESTABLISHED connections.
3. Rule to drop packets from blacklisted sources.
4. Rule to accept packets from whitelisted sources.
5. Rule for fail2ban (optional).
6. Rule for Tor exit nodes.
7. Other rules (for example, accept connection on ssh, http, https ports).

This sequence allow  to make decision for most packets before they pass Tor filtration rules.
I organize Tor filtration rules in separate table called TorExitnodes, it allow to update only this table and do not touch other firewall rules. Also i create table TorBlockAndLog that contain LOG and DROP target, but it is optional, you can simply drop packets.

For updating firewall rules for exit nodes i made next bash scrip and place it in cront:

#!/bin/bash
SOLVE="/root/bin/solve.pl"
 
if /usr/bin/perl $SOLVE
then
/sbin/iptables -F TorExitnodes
/sbin/iptables -I TorExitnodes -j RETURN
 
for i in `/usr/bin/perl $SOLVE -ip`
do
/sbin/iptables -I TorExitnodes -s $i -j TorBlockAndLog
done
else
echo "Can not fetch Tor exit nodes" 1>&2
exit 1;
fi

Where is variable “SOLVE” store path to previous perl script, as i say before, if you do not need special action for incoming connections from Tor, you can change “TorBlockAndLog” on “DROP”.

Thats all.

PS

It is good idea to place whitelist before TorExitnodes, who can know, may be one fine day you found -s 0.0.0.0/0 -j DROP into this table. =)

Password protected access to “/wp-admin” on nginx

Here i found advice how to make wordpress more securely, idea is to protect access to “/wp-admin” by http auth, but “/wp-admin/admin-ajax.php” must stay available for everyone, same is true for some .css files.
I spend some time to research solution, it was a bit complicated:

location /wp-admin {
	location ~ /wp-admin/admin-ajax.php$ {
		# Php handler
	}
	location ~* /wp-admin/.*\.php$ {
		auth_basic            "You shall not pass!";
		auth_basic_user_file  $document_root/.htpasswd;
		# Php handler
	}
}

It is possible to add additional location to serve static content, but i am too lazy to do it.

PS
Also here i found receipts for apache and lighttpd if anybody interesting. Continue reading

Forbid access to php into “wp-content”.

I expected in few articles an advice to disable direct access to php scripts into “/wp-content/uploads” i done small research and observed, than .php placed into “/wp-content” and into subdirs was newer directly accessed thru web. So i just completely disabled direct access to php scripts placed into that dir and subdirs. Looks more securely.

PS
I made mistake. At least tiny-mce php scripts must be accessible thru web. So i just convert  this rules that touch ‘/wp-include’ for nginx:

location ~* /wp-includes {
	location ~* /wp-includes/[^/]+\.php$ {
		deny all;
	}
	location ~* /wp-includes/js/tinymce/langs/.+\.php$ {
		deny all;
	}
	location ~* /wp-includes/theme-compat {
		deny all;
	}
	location ~ /wp-includes/.+\.php$ {
		include php_wordpress_handler;
	}
}