[Openvpn-devel,v4] Improve "recursive routing" warning message

Message ID 1540904039-2129-1-git-send-email-lstipakov@gmail.com
State New
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Series
  • [Openvpn-devel,v4] Improve "recursive routing" warning message
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Commit Message

Lev Stipakov Oct. 30, 2018, 12:53 p.m.
From: Lev Stipakov <lev@openvpn.net>

This patch provides additional information, such as
source address/port and destination address/port, to a
"recursive routing" warning message. It also mentiones
possible workaround.

Trac #843

Signed-off-by: Lev Stipakov <lev@openvpn.net>
---
v4:
 - remove unneeded format specifier for const string

v3:
 - factor out ports extraction code into own function
 to avoid code duplication
 - better commit message

v2:
 - print protocol, source/dest addresses and ports
 - mention "--allow-recursive-routing"
 - add possible usecase to manpage

 doc/openvpn.8         |  4 ++-
 src/openvpn/forward.c | 98 ++++++++++++++++++++++++++++++++++++++++++++++-----
 2 files changed, 93 insertions(+), 9 deletions(-)

Patch

diff --git a/doc/openvpn.8 b/doc/openvpn.8
index 94b5cc4..fb9eb84 100644
--- a/doc/openvpn.8
+++ b/doc/openvpn.8
@@ -4090,7 +4090,9 @@  notifications unless this option is enabled.
 .TP
 .B \-\-allow\-recursive\-routing
 When this option is set, OpenVPN will not drop incoming tun packets
-with same destination as host.
+with same destination as host. Could be useful when packets sent by openvpn
+itself are not subject to the routing tables that would move packets
+into the tunnel.
 .\"*********************************************************
 .SS Data Channel Encryption Options:
 These options are meaningful for both Static & TLS\-negotiated key modes
diff --git a/src/openvpn/forward.c b/src/openvpn/forward.c
index f8faa81..e96c546 100644
--- a/src/openvpn/forward.c
+++ b/src/openvpn/forward.c
@@ -1291,10 +1291,42 @@  read_incoming_tun(struct context *c)
     perf_pop();
 }
 
+/*
+ * Extracts sport and dport from packet, used
+ * in recursive routing warning.
+ */
+static void
+extract_sport_dport(const struct buffer *buf, uint8_t protocol, int offset_to_payload, int packet_len, uint16_t *sport, uint16_t *dport)
+{
+    /* whole packet? */
+    if (BLEN(buf) == packet_len)
+    {
+        const uint8_t *payload = BPTR(buf) + offset_to_payload;
+
+        if (protocol == OPENVPN_IPPROTO_UDP)
+        {
+            const struct openvpn_udphdr *udp = (const struct openvpn_udphdr *)payload;
+            *sport = ntohs(udp->source);
+            *dport = ntohs(udp->dest);
+        }
+        else if (protocol == OPENVPN_IPPROTO_TCP)
+        {
+            const struct openvpn_tcphdr *tcp = (const struct openvpn_tcphdr *)payload;
+            *sport = ntohs(tcp->source);
+            *dport = ntohs(tcp->dest);
+        }
+    }
+    else
+    {
+        *sport = 0;
+        *dport = 0;
+    }
+}
+
 /**
  * Drops UDP packets which OS decided to route via tun.
  *
- * On Windows and OS X when netwotk adapter is disabled or
+ * On Windows and OS X when network adapter is disabled or
  * disconnected, platform starts to use tun as external interface.
  * When packet is sent to tun, it comes to openvpn, encapsulated
  * and sent to routing table, which sends it again to tun.
@@ -1306,6 +1338,11 @@  drop_if_recursive_routing(struct context *c, struct buffer *buf)
     struct openvpn_sockaddr tun_sa;
     int ip_hdr_offset = 0;
 
+    uint32_t saddr, daddr;
+    struct in6_addr saddr6, daddr6;
+    uint16_t sport, dport;
+    uint8_t protocol;
+
     if (c->c2.to_link_addr == NULL) /* no remote addr known */
     {
         return;
@@ -1320,7 +1357,7 @@  drop_if_recursive_routing(struct context *c, struct buffer *buf)
         const struct openvpn_iphdr *pip;
 
         /* make sure we got whole IP header */
-        if (BLEN(buf) < ((int) sizeof(struct openvpn_iphdr) + ip_hdr_offset))
+        if (BLEN(buf) < ((int)sizeof(struct openvpn_iphdr) + ip_hdr_offset))
         {
             return;
         }
@@ -1331,12 +1368,21 @@  drop_if_recursive_routing(struct context *c, struct buffer *buf)
             return;
         }
 
-        pip = (struct openvpn_iphdr *) (BPTR(buf) + ip_hdr_offset);
+        pip = (const struct openvpn_iphdr *)(BPTR(buf) + ip_hdr_offset);
 
-        /* drop packets with same dest addr as gateway */
+        /* drop packets with the same dest addr as gateway */
         if (tun_sa.addr.in4.sin_addr.s_addr == pip->daddr)
         {
             drop = true;
+
+            /* collect information for warning message */
+
+            saddr = ntohl(pip->saddr);
+            daddr = ntohl(pip->daddr);
+            protocol = pip->protocol;
+
+            extract_sport_dport(buf, protocol, ip_hdr_offset + sizeof(struct openvpn_iphdr),
+                                ip_hdr_offset + (int)(ntohs(pip->tot_len)), &sport, &dport);
         }
     }
     else if (proto_ver == 6)
@@ -1344,7 +1390,7 @@  drop_if_recursive_routing(struct context *c, struct buffer *buf)
         const struct openvpn_ipv6hdr *pip6;
 
         /* make sure we got whole IPv6 header */
-        if (BLEN(buf) < ((int) sizeof(struct openvpn_ipv6hdr) + ip_hdr_offset))
+        if (BLEN(buf) < ((int)sizeof(struct openvpn_ipv6hdr) + ip_hdr_offset))
         {
             return;
         }
@@ -1355,22 +1401,58 @@  drop_if_recursive_routing(struct context *c, struct buffer *buf)
             return;
         }
 
-        /* drop packets with same dest addr as gateway */
+        /* drop packets with the same dest addr as gateway */
         pip6 = (struct openvpn_ipv6hdr *) (BPTR(buf) + ip_hdr_offset);
         if (IN6_ARE_ADDR_EQUAL(&tun_sa.addr.in6.sin6_addr, &pip6->daddr))
         {
             drop = true;
+
+            /* collect information for warning message */
+
+            saddr6 = pip6->saddr;
+            daddr6 = pip6->daddr;
+            protocol = pip6->nexthdr;
+
+            extract_sport_dport(buf, protocol, ip_hdr_offset + sizeof(struct openvpn_ipv6hdr),
+                                (int)sizeof(struct openvpn_ipv6hdr) + ip_hdr_offset + (int)ntohs(pip6->payload_len),
+                                &sport, &dport);
         }
     }
 
     if (drop)
     {
         struct gc_arena gc = gc_new();
+        struct buffer addrs_buf = alloc_buf_gc(128, &gc);
 
+        /* drop packet */
         c->c2.buf.len = 0;
 
-        msg(D_LOW, "Recursive routing detected, drop tun packet to %s",
-            print_link_socket_actual(c->c2.to_link_addr, &gc));
+        if (protocol == OPENVPN_IPPROTO_UDP)
+        {
+            buf_printf(&addrs_buf, "UDP");
+        }
+        else if (protocol == OPENVPN_IPPROTO_TCP)
+        {
+            buf_printf(&addrs_buf, "TCP");
+        }
+        else
+        {
+            buf_printf(&addrs_buf, "%d", protocol);
+        }
+
+        if (proto_ver == 4)
+        {
+            buf_printf(&addrs_buf, " %s:%d -> %s:%d",
+                       print_in_addr_t(saddr, 0, &gc), sport,
+                       print_in_addr_t(daddr, 0, &gc), dport);
+        } else if (proto_ver == 6)
+        {
+            buf_printf(&addrs_buf, " %s:%d -> %s:%d",
+                       print_in6_addr(saddr6, 0, &gc), sport,
+                       print_in6_addr(daddr6, 0, &gc), dport);
+        }
+
+        msg(D_LOW, "Recursive routing detected, drop packet %s. Fix your routing or consider using --allow-recursive-routing option.", BSTR(&addrs_buf));
         gc_free(&gc);
     }
 }