SemrushBot is an annoying web crawler that has proven to completely disregard the robots policies as well as hammering webservers hard by recursively following all the links on a website without delay and outright ignoring any repeating 403 Forbidden error messages.

Folklore claims that SemrushBot helps your site generate revenue from ads but the question is whether that revenue outweighs the money spent accommodating SemrushBot's rampant behaviour that yields a morbidly increased server load.

On the IP layer:

iptables -t mangle -A INPUT -p tcp --dport 80 -m string --string 'SemrushBot' -j DROP

Which is an awful solution to get rid of this pest without even hitting the application layer!

If are okay with your frontend being hammered by this total garbage, then the SemrushBot user agent can be blocked in Apache2.

Enable the rewrite module:

a2enmod rewrite

and include in virtual hosts:

       <IfModule mod_rewrite.c>
                RewriteEngine on
                RewriteCond %{HTTP_USER_AGENT} googlebot [NC,OR]
                RewriteCond %{HTTP_USER_AGENT} sosospider [NC,OR]
                RewriteCond %{HTTP_USER_AGENT} BaiduSpider [NC]
                # Allow access to robots.txt and forbidden message
                # at least 403 or else it will loop
                RewriteCond %{REQUEST_URI} !^/robots\.txt$
                RewriteCond %{REQUEST_URI} !^/403\.shtml$
                RewriteRule ^.* - [F,L]
       </IfModule>

which is a bad solution because Forbidden is meaningless to the greatness that is SemrushBot.

Perhaps blocking with Varnish may be a good compromise between having your Apache2 hammered and blocking the string SemrushBot on the IP layer:

sub vcl_recv {
    # Block user agents.
    if (req.http.User-Agent ~ "SemrushBot") {
        return (synth(403, "Forbidden"));
    }
 
    # ...
 
}

An even better method would be to use fail2ban to block SemrushBot by reading Varnish logs on the frontend or Apache2 log files on the backend which will prevent either of them to get hammered with requests.

For Varnish, copy /etc/fail2ban/filter.d/apache-badbots.conf to /etc/fail2ban/filter.d/varnish-badbots.conf thereby duplicating the Apache2 configuration (this works due to NCSA log format) and edit /etc/fail2ban/filter.d/varnish-badbots.conf to add SemrushBot to the list of custom bad bots:

badbotscustom = EmailCollector|WebEMailExtrac|TrackBack/1\.02|sogou music spider|SemrushBot

then correct the failregex line to:

failregex = ^<HOST> -.*(GET|POST|HEAD).*HTTP.*"(?:%(badbots)s|%(badbotscustom)s).*?$

and finally add the following to the jail configuration:

[varnish-badbots]
enabled  = true
port     = http,https
filter   = varnish-badbots
logpath  = /var/log/varnish/varnishncsa.log
maxretry = 1

and restart fail2ban.

To check that the bots are being banned, tail /var/log/syslog and look for:

fail2ban.jail[18168]: INFO Jail 'varnish-badbots' started

indicating that the varnish-badbots jail has started.

Hopefully followed by lines similar to:

NOTICE [varnish-badbots] Ban 46.229.168.68

Redirect SMTPs to custom port for SSLsniff:

iptables -t nat -A PREROUTING -p tcp --destination-port 995 -j REDIRECT --to-ports 4995
sslsniff -a -c /usr/share/sslsniff/certs/wildcard -s 4995 -w /dev/stdout

Example Session:

1385227016 INFO sslsniff : Added OCSP URL: ocsp.ipsca.com
1385227016 INFO sslsniff : Certificate Ready: *
sslsniff 0.8 by Moxie Marlinspike running...
1385227031 DEBUG sslsniff : Read from Server (mail.net.hu) :
+OK POP3 PROXY server ready <7575E80698581E88C26B60701C2C67717034A020@smtp.mail.net.hu>

1385227032 DEBUG sslsniff : Read from Client (mail.net.hu) :
USER harry

1385227032 DEBUG sslsniff : Read from Server (mail.net.hu) :
+OK Password required

1385227032 DEBUG sslsniff : Read from Client (mail.net.hu) :
PASS secretpassword

After successive updates and corrections, the password database might not get updated such that scanning /etc/passwd for users reveals multiple variants of the /usr/sbin/nologin shell such as:

systemd-coredump:x:998:998:systemd Core Dumper:/:/sbin/nologin
rslsync:x:999:999::/home/rslsync:/sbin/nologin
sshd:x:107:65534::/var/run/sshd:/usr/sbin/nologin

where the correct answer seems to be /usr/bin/nologin and as one might imagine, the other file paths do not even exist leaving an opportunity for an attacker to slide a shell into place.

A solution to batch-change the shell for all users that have no-login shells, correcting the path, would be the following:

for i in `cat /etc/passwd | grep nologin | awk -F ':' '{ print $1 }' | xargs`; do usermod -s /usr/sbin/nologin "$i"; done