CURL and DNS resolver

# CURL and DNS resolver The common question in curl is why my browser, dig and wget work except curl. the error message looks like this ``` $ curl https://github.com curl: (6) Could not resolve host: github.com ``` ## Possilbe file and function curl_resolver_getaddrinfo https://github.com/curl/curl/blob/fe7b64228d553274f780ebc98a05a9ae266df828/lib/asyn-thread.c ## Build curl from source Use autoconf, then got an error ``` # apt install autoconf curl# autoconf configure.ac:22: error: possibly undefined macro: dnl If this token and others are legitimate, please use m4_pattern_allow. See the Autoconf documentation. configure.ac:41: error: possibly undefined macro: AM_MAINTAINER_MODE configure.ac:93: error: possibly undefined macro: AC_MSG_RESULT configure.ac:316: error: possibly undefined macro: AM_CONDITIONAL configure.ac:522: error: possibly undefined macro: AC_DEFINE ``` intall automake libtool reference: * https://unix.stackexchange.com/questions/18673/some-m4-macros-dont-seem-to-be-defined/18680?ssl_ ``` apt install automake libtool $ autoconf # pass $ ls configure configure ``` After configuring, I got following errors: ``` # ./configure ./configure: line 2604: XC_OVR_ZZ50: command not found ./configure: line 2605: XC_OVR_ZZ60: command not found ./configure: line 2606: CURL_OVERRIDE_AUTOCONF: command not found ./configure: line 2614: AM_MAINTAINER_MODE: command not found ./configure: line 2617: CURL_CHECK_OPTION_DEBUG: command not found ./configure: line 2618: CURL_CHECK_OPTION_OPTIMIZE: command not found ./configure: line 2619: CURL_CHECK_OPTION_WARNINGS: command not found ./configure: line 2620: CURL_CHECK_OPTION_WERROR: command not found ./configure: line 2621: CURL_CHECK_OPTION_CURLDEBUG: command not found ./configure: line 2622: CURL_CHECK_OPTION_SYMBOL_HIDING: command not found ./configure: line 2623: CURL_CHECK_OPTION_ARES: command not found ./configure: line 2624: CURL_CHECK_OPTION_RT: command not found ./configure: line 2625: CURL_CHECK_OPTION_ECH: command not found ./configure: line 2627: XC_CHECK_PATH_SEPARATOR: command not found checking for sed... /usr/bin/sed checking for grep... /usr/bin/grep checking for egrep... /usr/bin/grep -E checking for ar... /usr/bin/ar ./configure: line 2900: XC_CHECK_PROG_CC: command not found ./configure: line 2902: CURL_COVERAGE: command not found ./configure: line 2904: XC_AUTOMAKE: command not found checking curl version... 7.80.0-DEV configure: error: select TLS backend(s) or disable TLS with --without-ssl. ``` run configure with --without-ssl, then got another error ``` # ./configure --without-ssl ./configure: line 2604: XC_OVR_ZZ50: command not found ./configure: line 2605: XC_OVR_ZZ60: command not found ./configure: line 2606: CURL_OVERRIDE_AUTOCONF: command not found ./configure: line 2614: AM_MAINTAINER_MODE: command not found ./configure: line 2617: CURL_CHECK_OPTION_DEBUG: command not found ./configure: line 2618: CURL_CHECK_OPTION_OPTIMIZE: command not found ./configure: line 2619: CURL_CHECK_OPTION_WARNINGS: command not found ./configure: line 2620: CURL_CHECK_OPTION_WERROR: command not found ./configure: line 2621: CURL_CHECK_OPTION_CURLDEBUG: command not found ./configure: line 2622: CURL_CHECK_OPTION_SYMBOL_HIDING: command not found ./configure: line 2623: CURL_CHECK_OPTION_ARES: command not found ./configure: line 2624: CURL_CHECK_OPTION_RT: command not found ./configure: line 2625: CURL_CHECK_OPTION_ECH: command not found ./configure: line 2627: XC_CHECK_PATH_SEPARATOR: command not found checking for sed... /usr/bin/sed checking for grep... /usr/bin/grep checking for egrep... /usr/bin/grep -E checking for ar... /usr/bin/ar ./configure: line 2900: XC_CHECK_PROG_CC: command not found ./configure: line 2902: CURL_COVERAGE: command not found ./configure: line 2904: XC_AUTOMAKE: command not found checking curl version... 7.80.0-DEV configure: error: cannot find install-sh, install.sh, or shtool in "." "./.." "./../.." ``` Found answer here https://askubuntu.com/questions/27677/cannot-find-install-sh-install-sh-or-shtool-in-ac-aux?ssl_ let autoreconf install package for you ``` $ autoconf -i $ configure ... ... processing $ make ``` re-run with debug symbol ``` ./configure --without-ssl --enable-debug make make install ``` run gdb with curl ``` ~/curl# gdb -q /usr/local/bin/curl Reading symbols from /usr/local/bin/curl... (gdb) ``` curl loads lib after running it, so set the first breakpoint on curl_easy_init. After running it, you're able to set breakpoint in any function in curl . ``` (gdb) info breakpoint Num Type Disp Enb Address What 1 breakpoint keep y 0x00007ffff7f03fa8 in curl_easy_init at easy.c:290 breakpoint already hit 4 times 2 breakpoint keep y 0x00007ffff7ef1a92 in Curl_resolver_is_resolved at asyn-thread.c:569 3 breakpoint keep y 0x00007ffff7f69702 in resolve_server at url.c:3327 breakpoint already hit 1 time ``` resovler breakpoint ``` Breakpoint 3, resolve_server (data=0x7ffff7f56d23 <Curl_speedinit+44>, conn=0x693c48, async=0x7fff00035757) at url.c:3327 ``` this might be the place where curl resolve the url https://github.com/curl/curl/blob/fe7b64228d553274f780ebc98a05a9ae266df828/lib/url.c#L3305 from here we can see `Curl_resolv_timeout` is the function to handle resolver https://github.com/curl/curl/blob/fe7b64228d553274f780ebc98a05a9ae266df828/lib/url.c#L3378 ``` resolve_server | ---> Curl_resolv_timeout | ---> Curl_resolv | ---> Curl_getaddrinfo | ---> Curl_resolver_getaddrinfo https://github.com/curl/curl/blob/fe7b64228d553274f780ebc98a05a9ae266df828/lib/asyn-thread.c#L691 | --> init_resolve_thread https://github.com/curl/curl/blob/fe7b64228d553274f780ebc98a05a9ae266df828/lib/asyn-thread.c#L430 | --> getaddrinfo_thread (Run this one) | https://github.com/curl/curl/blob/fe7b64228d553274f780ebc98a05a9ae266df828/lib/asyn-thread.c#L463 | -> Curl_getaddrinfo_ex | -> getaddrinfo(glibc lib) | --> gethostbyname_thread ``` `getaddrinfo` returned -2 then curl return host cannot be resovled.