lib: introduce resolver library

This adds a simple, UDP-only DNS resolver library mimicking the operation of
the extended busybox nslookup applet.

Simply querying a domain name will perform A + AAAA resolving by default:

    # ucode -mresolv -Rs 'printf("%.J\n", resolv.query("example.com"))'
    {
        "example.com": {
            "A": [
                "93.184.216.34"
            ],
            "AAAA": [
                "2606:2800:220:1:248:1893:25c8:1946"
            ]
        }
    }

Passing IP addresses will automatically perform PTR requests:

    # ucode -mresolv -Rs 'printf("%.J\n", resolv.query("8.8.8.8"))'
    {
        "8.8.8.8.in-addr.arpa": {
            "PTR": [
                "dns.google"
            ]
        }
    }

    # ucode -mresolv -Rs 'printf("%.J\n", resolv.query("2001:4860:4860::8888"))'
    {
        "8.8.8.8.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.6.8.4.0.6.8.4.1.0.0.2.ip6.arpa": {
            "PTR": [
                "dns.google"
            ]
        }
    }

Additional options for query type and nameserver selection can be passed via
a second optional options dictionary:

    # ucode -mresolv -Rs 'printf("%.J\n",
        resolv.query([ "openwrt.org", "example.org", "doesnotexist.tld" ], {
            type: [ "A", "AAAA", "MX" ],
            nameserver: [ "1.1.1.1", "8.8.4.4" ],
            timeout: 5000,
            retries: 2,
            edns_maxsize: 4096
        }))'
    {
        "openwrt.org": {
            "A": [
                "139.59.209.225"
            ],
            "MX": [
                [
                    10,
                    "util-01.infra.openwrt.org"
                ]
            ],
            "AAAA": [
                "2a03:b0c0:3:d0::1af1:1"
            ]
        },
        "example.org": {
            "A": [
                "93.184.216.34"
            ],
            "AAAA": [
                "2606:2800:220:1:248:1893:25c8:1946"
            ],
            "MX": [
                [
                    0,
                    "."
                ]
            ]
        },
        "doesnotexist.tld": {
            "rcode": "NXDOMAIN"
        }
    }

Signed-off-by: Jo-Philipp Wich <jo@mein.io>

1 files changed, 15 insertions, 0 deletions

diff --git a/CMakeLists.txt b/CMakeLists.txt
index f993019..e5fb58a 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -19,6 +19,7 @@ OPTION(UBUS_SUPPORT "Ubus plugin support" ON)
 OPTION(UCI_SUPPORT "UCI plugin support" ON)
 OPTION(RTNL_SUPPORT "Route Netlink plugin support" ON)
 OPTION(NL80211_SUPPORT "Wireless Netlink plugin support" ON)
+OPTION(RESOLV_SUPPORT "NS resolve plugin support" ON)
 
 OPTION(LEGACY_SUPPORT "Support deprecated syntax features" ON)
 
@@ -136,6 +137,20 @@ IF(NL80211_SUPPORT)
   TARGET_LINK_LIBRARIES(nl80211_lib ${nl})
 ENDIF()
 
+IF(RESOLV_SUPPORT)
+  SET(LIBRARIES ${LIBRARIES} resolv_lib)
+  ADD_LIBRARY(resolv_lib MODULE lib/resolv.c)
+  SET_TARGET_PROPERTIES(resolv_lib PROPERTIES OUTPUT_NAME resolv PREFIX "")
+  CHECK_FUNCTION_EXISTS(res_mkquery RES_MKQUERY_FUNCTION_EXISTS)
+  CHECK_FUNCTION_EXISTS(clock_gettime CLOCK_GETTIME_FUNCTION_EXISTS)
+  IF (NOT RES_MKQUERY_FUNCTION_EXISTS)
+    TARGET_LINK_LIBRARIES(resolv_lib resolv)
+  ENDIF()
+  IF (NOT CLOCK_GETTIME_FUNCTION_EXISTS)
+    TARGET_LINK_LIBRARIES(resolv_lib rt)
+  ENDIF()
+ENDIF()
+
 IF(UNIT_TESTING)
   ENABLE_TESTING()
   ADD_DEFINITIONS(-DUNIT_TESTING)