禁用Dropbear中DNS反向查询连接客户端地址补丁

Dropbear 是一套来自澳大利亚的轻量的sshd服务器，与OpenSSH相比，他更简洁，更小巧，运行起来占用的内存也更少，因此在嵌入式环境被广泛中使用.
Dropbear 与 OpenSSH在客户端连接到服务器时都有一个相同的动作:就是会向设备配置的DNS Server反向查询客户端地址. 如果设备配置的DNS服务器地址有问题或速度比较慢, 就会导致客户端连接时产生延时,在客户端产生停顿了感觉. 而OpenSSH 可以通过在sshd_config中配置"UseDNS No"来禁用此动作,而Dropbear却没有可配置的选项.

通过查看Dropbear代码, 发现Dropbear在每次客户端连接成功后都会调用getnameinfo去获取客户端连接信息(svr-main.c:main_noinetd/main_inetd->dbutils.c:getaddrstring/getaddrhostname). 所以只要将dbutils.c:getaddrstring/getaddrhostname两个函数代码中调用getnameinfo获取地址信息修改为获取简单的地址端口即可.

这里采用了kernel中的宏NIPQUAD来打印ipv4网络字节序地址:
#define NIPQUAD(addr) \
((unsigned char *)&addr)[0], \
((unsigned char *)&addr)[1], \
((unsigned char *)&addr)[2], \
((unsigned char *)&addr)[3]

以下是patch:

diff -Nur dropbear-0.52.orig/dbutil.c dropbear-0.52/dbutil.c
--- dropbear-0.52.orig/dbutil.c 2008-11-11 22:09:02.000000000 +0800
+++ dropbear-0.52/dbutil.c  2010-09-20 02:09:17.000000000 +0800
@@ -550,9 +550,25 @@
 #endif
 #endif

+// modified by kangzy
+#if 0
    ret = getnameinfo((struct sockaddr*)addr, len, hbuf, sizeof(hbuf),
            sbuf, sizeof(sbuf), NI_NUMERICSERV | NI_NUMERICHOST);
-
+   printf( "getnameinfo return %d.\n", ret );
+#else
+#define NIPQUAD(addr) \
+           ((unsigned char *)&addr)[0], \
+       ((unsigned char *)&addr)[1], \
+       ((unsigned char *)&addr)[2], \
+       ((unsigned char *)&addr)[3]
+
+   {
+       struct sockaddr_in *sa = (struct sockaddr_in *)addr;
+       snprintf(hbuf, NI_MAXHOST, "%.u.%u.%u.%u", NIPQUAD(sa->sin_addr));
+       snprintf(sbuf, NI_MAXSERV, "%d", sa->sin_port );
+   }
+   ret = 0;
+#endif
    if (ret != 0) {
        /* This is a fairly bad failure - it'll fallback to IP if it
         * just can't resolve */
@@ -599,10 +615,13 @@
 #endif
 #endif

-
+// modified by kangzy
+#if 0
    ret = getnameinfo((struct sockaddr*)addr, len, hbuf, sizeof(hbuf),
            sbuf, sizeof(sbuf), flags);
-
+#else
+   ret = EAI_NONAME;
+#endif
    if (ret != 0) {
        /* On some systems (Darwin does it) we get EINTR from getnameinfo
         * somehow. Eew. So we'll just return the IP, since that doesn't seem