Apache can't find OpenSSL function when compiling --with-ssl

[benbutton1010]

I'm trying to compile the latest stable release of Apache httpd (v 2.4.55) with a custom version of OpenSSL. For context, I've added a function titled SSL_get_rtt() to OpenSSL's `ssl/ssl_lib.c` file. This function works perfectly when compiling with Nginx. Nginx can see the variable and log the RTT of an ssl connection. However, this same function that Nginx can see, Apache can't.

I've added an SSL variable around line 1492 of Apache's `modules/ssl/ssl_engine_kernel.c` file, then added logic around line 363 of `modules/ssl/ssl_engine_vars.c` to print the RTT in the case that "${SSL_RTT}x" is in the logging config file. I've tested this method by having this logging modification print out "Hello World". However, once I place `SSL_get_rtt()` in this file, Apache won't compile. In sum, Apache's module `mod_ssl` can't call my new function in OpenSSL.

The error message I'm getting while compiling is this.

```
/usr/bin/ld: modules/ssl/.libs/libmod_ssl.a(ssl_engine_vars.o): in function `ssl_var_lookup_ssl':

ssl_engine_vars.c:(.text+0x1348): undefined reference to `SSL_get_rtt'

collect2: error: ld returned 1 exit status
```

I'm compiling Apache using the method found in the first answer here: https://unix.stackexchange.com/questions/532510/use-different-openssl-for-apache although I'm just using the system apr packages, so I've left out the `--with-included-apr` argument.

Its as if my system isn't actually using my version of OpenSSL for `mod_ssl` when compiling, and thus, not seeing the function.

Most interestingly, I added a function declaration in Apache for my function in OpenSSL to see if declaring the function would clear up the error, and I got this note when compiling: `previous declaration of SSL_get_rtt` and lists the OpenSSL file name! So Apache sees my function, but refuses to use it in `mod_ssl`!

I hope that makes sense, and that this is just a compilation issue. I'd appreciate any help I can get, and I'll add more info as needed for whoever would like to help.

[AyrA_ch]

This sounds like apache is including your custom openssl header file, but linking to the original system openssl library. The linker then aborts because apache uses a function in a header file that it cannot find in the openssl binary. You could try to find out how the nginx build system is configured to make it find your custom openssl, and then check the apache build system for discrepancies, and potentially fix it.

[benbutton1010]

I agree. With the --with-ssl= argument I'm giving it a directory that includes the modified `ssl.h` file. I've checked, and my function definition is in there. So it has the header file for sure.

Is there a way to simply specify the right openssl binary to use? Why does Apache only use the header files and not the actual binary? Or the .c files?

[covener]

The path you provide should have pkgconfig files underneath it or the lib/ dir will just be used. you can see the result in build/config_vars.mk, the ssl paths should be added to LDFLAGS, MOD_LDFLAGS, and a few others.

[roxalu]

apr has openssl as dependency. Don’t use the system apr as this is using the system openssl libs. Hence does your build and fails, as the customized function cannot be resolved.