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authorChristoph Schulz <develop@kristov.de>2014-07-13 00:53:15 +0200
committerDavid S. Miller <davem@davemloft.net>2014-07-14 14:35:46 -0700
commita8a3e41c67d24eb12f9ab9680cbb85e24fcd9711 (patch)
tree8f4357fa6ddd7ce587adc64455497605eb0dd7a7 /drivers/net/ppp
parent9ecf07a1d8f70f72ec99a0f102c8aa24609d84f4 (diff)
net: pppoe: use correct channel MTU when using Multilink PPP
The PPP channel MTU is used with Multilink PPP when ppp_mp_explode() (see ppp_generic module) tries to determine how big a fragment might be. According to RFC 1661, the MTU excludes the 2-byte PPP protocol field, see the corresponding comment and code in ppp_mp_explode(): /* * hdrlen includes the 2-byte PPP protocol field, but the * MTU counts only the payload excluding the protocol field. * (RFC1661 Section 2) */ mtu = pch->chan->mtu - (hdrlen - 2); However, the pppoe module *does* include the PPP protocol field in the channel MTU, which is wrong as it causes the PPP payload to be 1-2 bytes too big under certain circumstances (one byte if PPP protocol compression is used, two otherwise), causing the generated Ethernet packets to be dropped. So the pppoe module has to subtract two bytes from the channel MTU. This error only manifests itself when using Multilink PPP, as otherwise the channel MTU is not used anywhere. In the following, I will describe how to reproduce this bug. We configure two pppd instances for multilink PPP over two PPPoE links, say eth2 and eth3, with a MTU of 1492 bytes for each link and a MRRU of 2976 bytes. (This MRRU is computed by adding the two link MTUs and subtracting the MP header twice, which is 4 bytes long.) The necessary pppd statements on both sides are "multilink mtu 1492 mru 1492 mrru 2976". On the client side, we additionally need "plugin rp-pppoe.so eth2" and "plugin rp-pppoe.so eth3", respectively; on the server side, we additionally need to start two pppoe-server instances to be able to establish two PPPoE sessions, one over eth2 and one over eth3. We set the MTU of the PPP network interface to the MRRU (2976) on both sides of the connection in order to make use of the higher bandwidth. (If we didn't do that, IP fragmentation would kick in, which we want to avoid.) Now we send a ICMPv4 echo request with a payload of 2948 bytes from client to server over the PPP link. This results in the following network packet: 2948 (echo payload) + 8 (ICMPv4 header) + 20 (IPv4 header) --------------------- 2976 (PPP payload) These 2976 bytes do not exceed the MTU of the PPP network interface, so the IP packet is not fragmented. Now the multilink PPP code in ppp_mp_explode() prepends one protocol byte (0x21 for IPv4), making the packet one byte bigger than the negotiated MRRU. So this packet would have to be divided in three fragments. But this does not happen as each link MTU is assumed to be two bytes larger. So this packet is diveded into two fragments only, one of size 1489 and one of size 1488. Now we have for that bigger fragment: 1489 (PPP payload) + 4 (MP header) + 2 (PPP protocol field for the MP payload (0x3d)) + 6 (PPPoE header) -------------------------- 1501 (Ethernet payload) This packet exceeds the link MTU and is discarded. If one configures the link MTU on the client side to 1501, one can see the discarded Ethernet frames with tcpdump running on the client. A ping -s 2948 -c 1 192.168.15.254 leads to the smaller fragment that is correctly received on the server side: (tcpdump -vvvne -i eth3 pppoes and ppp proto 0x3d) 52:54:00:ad:87:fd > 52:54:00:79:5c:d0, ethertype PPPoE S (0x8864), length 1514: PPPoE [ses 0x3] MLPPP (0x003d), length 1494: seq 0x000, Flags [end], length 1492 and to the bigger fragment that is not received on the server side: (tcpdump -vvvne -i eth2 pppoes and ppp proto 0x3d) 52:54:00:70:9e:89 > 52:54:00:5d:6f:b0, ethertype PPPoE S (0x8864), length 1515: PPPoE [ses 0x5] MLPPP (0x003d), length 1495: seq 0x000, Flags [begin], length 1493 With the patch below, we correctly obtain three fragments: 52:54:00:ad:87:fd > 52:54:00:79:5c:d0, ethertype PPPoE S (0x8864), length 1514: PPPoE [ses 0x1] MLPPP (0x003d), length 1494: seq 0x000, Flags [begin], length 1492 52:54:00:70:9e:89 > 52:54:00:5d:6f:b0, ethertype PPPoE S (0x8864), length 1514: PPPoE [ses 0x1] MLPPP (0x003d), length 1494: seq 0x000, Flags [none], length 1492 52:54:00:ad:87:fd > 52:54:00:79:5c:d0, ethertype PPPoE S (0x8864), length 27: PPPoE [ses 0x1] MLPPP (0x003d), length 7: seq 0x000, Flags [end], length 5 And the ICMPv4 echo request is successfully received at the server side: IP (tos 0x0, ttl 64, id 21925, offset 0, flags [DF], proto ICMP (1), length 2976) 192.168.222.2 > 192.168.15.254: ICMP echo request, id 30530, seq 0, length 2956 The bug was introduced in commit c9aa6895371b2a257401f59d3393c9f7ac5a8698 ("[PPPOE]: Advertise PPPoE MTU") from the very beginning. This patch applies to 3.10 upwards but the fix can be applied (with minor modifications) to kernels as old as 2.6.32. Signed-off-by: Christoph Schulz <develop@kristov.de> Signed-off-by: David S. Miller <davem@davemloft.net>
Diffstat (limited to 'drivers/net/ppp')
-rw-r--r--drivers/net/ppp/pppoe.c2
1 files changed, 1 insertions, 1 deletions
diff --git a/drivers/net/ppp/pppoe.c b/drivers/net/ppp/pppoe.c
index 2ea7efd11857..6c9c16d76935 100644
--- a/drivers/net/ppp/pppoe.c
+++ b/drivers/net/ppp/pppoe.c
@@ -675,7 +675,7 @@ static int pppoe_connect(struct socket *sock, struct sockaddr *uservaddr,
po->chan.hdrlen = (sizeof(struct pppoe_hdr) +
dev->hard_header_len);
- po->chan.mtu = dev->mtu - sizeof(struct pppoe_hdr);
+ po->chan.mtu = dev->mtu - sizeof(struct pppoe_hdr) - 2;
po->chan.private = sk;
po->chan.ops = &pppoe_chan_ops;