Allowing ping for httpd CGI scripts

Problem

I use thttpd to run CGI scripts. One script uses the ping command to check the availability of another host.

Diagram showing how CGI handles web requests. By Pluke (2012). Public Domain.

Source: Wikimedia Commons

Unfortunately, SELinux does not allow raw ip packets to be created from a CGI script. Such scripts run in the httpd_sys_script_t security context.

You can see this when sending the output of the ping command to the remote host, e.g. from another box:

elinks http://your_server/your_script.cgi

(Or use curl or whatever you like instead of elinks.)

Then you will see something like ping: recvmsg: Permission denied. Basically, the ping answer (echo message) is blocked by SELinux.

After creating custom modules, the audit.log file shows no denied actions, so it is not clear how to solve this.

Tried without avail

I have tried a few things that would make sense.

# enables users to execute the ping command
setsebool user_ping on

You can see the denied stuff using, e.g.:

 tail -n 10 /var/log/audit/audit.log | grep denied

Even after using audit2allow to create and install custom policy modules, and getting rid of all denied lines from the audit.log, SELinux still gets in the way. The ping: recvmsg: Permission denied error still shows up.

So, all this just allows to use the ping command. The return packets are still blocked.

Solution

As a workaround, you can use a permissive domain, so SELinux is set to the permissive mode just for the httpd_sys_script_t security context.

Command to execute:

# use a permissive domain for cgi scripts
semanage permissive -a httpd_sys_script_t

If you still get an error like this:

type=AVC msg=audit(1433880351.642:270055): avc:  denied  { sigchld } for  pid=1337 comm="your_script.cgi" scontext=system_u:system_r:httpd_t:s0 tcontext=system_u:system_r:httpd_sys_script_t:s0 tclass=process

then you might try to wait for a while before trying again… Somehow after a night’s sleep, it suddenly worked for me 🙂

References

Deze blogpost werd in december 2022 overgezet van WordPress naar een methode gebaseerd op Markdown; het is mogelijk dat hierbij fouten of wijzigingen zijn ontstaan t.o.v. de originele blogpost.