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I am trying to modify the behaviour of the DNS lookup functions in Linux for my project by setting it to write a random string in a file as a test.

Linux use some DNS resolver functions, mainly gethostbyname. Looking forward, I found out that the resolver functions are within the glibc6 library. So, I downloaded it, compiled, generating the libresolv.so, libnss_dns.so dynamic libraries. Then, I replaced the existing ones on my system, at /usr/lib/x86_64-linux-gnu/.

Note: I found out these libraries are the ones that resolves queries by modifying gethostbyname code and compiling again. Then, I saw which dynamic libraries changed.

By creating a program that uses res_query directly (a resolver function) and compiling with -lresolv, it works (I used ldd command and it uses the resolver library I created). But, using gethostbyname directly from the code, using wget or browsing the web I can't get it right.

What am I doing wrong?

Lucas
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1 Answers1

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I discovered what should I do:

Firstly, the function called isn't gethostbyname. It is _nss_dns_gethostbyname3_r, defined in resolv/nss_dns/dns-host.c. Looking at the source I realized that this function called another one to resolve DNS names, __libc_res_nsearch, defined in resolv/res-query.c. So this is the function! I added some file writting commands there and it worked like a charm. I called it from C code using gethostbyname, used wget and Firefox, all worked. When you compile the code you should replace the libresolv.so of your system by the new one.

Note: my glibc version is 2.19.

I hope this helps someone.

Lucas
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