path: root/net/dccp/ipv4.c
diff options
authorHannes Frederic Sowa <hannes@stressinduktion.org>2013-11-05 02:24:17 +0100
committerDavid S. Miller <davem@davemloft.net>2013-11-05 21:52:27 -0500
commit482fc6094afad572a4ea1fd722e7b11ca72022a0 (patch)
tree8ea3f61751718374f7d84b64e82ab46b93f89a57 /net/dccp/ipv4.c
parentb915550179d9df122fb781dc87d737ca92058b3c (diff)
Sockets marked with IP_PMTUDISC_INTERFACE won't do path mtu discovery, their sockets won't accept and install new path mtu information and they will always use the interface mtu for outgoing packets. It is guaranteed that the packet is not fragmented locally. But we won't set the DF-Flag on the outgoing frames. Florian Weimer had the idea to use this flag to ensure DNS servers are never generating outgoing fragments. They may well be fragmented on the path, but the server never stores or usees path mtu values, which could well be forged in an attack. (The root of the problem with path MTU discovery is that there is no reliable way to authenticate ICMP Fragmentation Needed But DF Set messages because they are sent from intermediate routers with their source addresses, and the IMCP payload will not always contain sufficient information to identify a flow.) Recent research in the DNS community showed that it is possible to implement an attack where DNS cache poisoning is feasible by spoofing fragments. This work was done by Amir Herzberg and Haya Shulman: <https://sites.google.com/site/hayashulman/files/fragmentation-poisoning.pdf> This issue was previously discussed among the DNS community, e.g. <http://www.ietf.org/mail-archive/web/dnsext/current/msg01204.html>, without leading to fixes. This patch depends on the patch "ipv4: fix DO and PROBE pmtu mode regarding local fragmentation with UFO/CORK" for the enforcement of the non-fragmentable checks. If other users than ip_append_page/data should use this semantic too, we have to add a new flag to IPCB(skb)->flags to suppress local fragmentation and check for this in ip_finish_output. Many thanks to Florian Weimer for the idea and feedback while implementing this patch. Cc: David S. Miller <davem@davemloft.net> Suggested-by: Florian Weimer <fweimer@redhat.com> Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org> Signed-off-by: David S. Miller <davem@davemloft.net>
Diffstat (limited to 'net/dccp/ipv4.c')
1 files changed, 1 insertions, 0 deletions
diff --git a/net/dccp/ipv4.c b/net/dccp/ipv4.c
index 720c36225ed9..d9f65fc66db5 100644
--- a/net/dccp/ipv4.c
+++ b/net/dccp/ipv4.c
@@ -174,6 +174,7 @@ static inline void dccp_do_pmtu_discovery(struct sock *sk,
mtu = dst_mtu(dst);
if (inet->pmtudisc != IP_PMTUDISC_DONT &&
+ ip_sk_accept_pmtu(sk) &&
inet_csk(sk)->icsk_pmtu_cookie > mtu) {
dccp_sync_mss(sk, mtu);

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