From: Yu Watanabe <watanabe.yu+github@gmail.com>
Date: Mon, 18 Feb 2019 14:41:43 +0900
Subject: sysctl-util: add sysctl_read_ip_property()

(cherry picked from commit a6b3b0aace152b77682d68d99b3e41580c955efb)
---
 src/shared/sysctl-util.c | 22 ++++++++++++++++++++++
 src/shared/sysctl-util.h |  1 +
 2 files changed, 23 insertions(+)

diff --git a/src/shared/sysctl-util.c b/src/shared/sysctl-util.c
index 480e6c3..ba89489 100644
--- a/src/shared/sysctl-util.c
+++ b/src/shared/sysctl-util.c
@@ -69,3 +69,25 @@ int sysctl_read(const char *property, char **content) {
         p = strjoina("/proc/sys/", property);
         return read_full_file(p, content, NULL);
 }
+
+int sysctl_read_ip_property(int af, const char *ifname, const char *property, char **ret) {
+        _cleanup_free_ char *value = NULL;
+        const char *p;
+        int r;
+
+        assert(IN_SET(af, AF_INET, AF_INET6));
+        assert(property);
+
+        p = strjoina("/proc/sys/net/ipv", af == AF_INET ? "4" : "6",
+                     ifname ? "/conf/" : "", strempty(ifname),
+                     property[0] == '/' ? "" : "/", property);
+
+        r = read_one_line_file(p, &value);
+        if (r < 0)
+                return r;
+
+        if (ret)
+                *ret = TAKE_PTR(value);
+
+        return r;
+}
diff --git a/src/shared/sysctl-util.h b/src/shared/sysctl-util.h
index fd7c78b..22f52f8 100644
--- a/src/shared/sysctl-util.h
+++ b/src/shared/sysctl-util.h
@@ -5,3 +5,4 @@ char *sysctl_normalize(char *s);
 int sysctl_read(const char *property, char **value);
 int sysctl_write(const char *property, const char *value);
 
+int sysctl_read_ip_property(int af, const char *ifname, const char *property, char **ret);