ipv4: yet another new IP_MTU_DISCOVER option IP_PMTUDISC_OMIT

IP_PMTUDISC_INTERFACE has a design error: because it does not allow the
generation of fragments if the interface mtu is exceeded, it is very
hard to make use of this option in already deployed name server software
for which I introduced this option.

This patch adds yet another new IP_MTU_DISCOVER option to not honor any
path mtu information and not accepting new icmp notifications destined for
the socket this option is enabled on. But we allow outgoing fragmentation
in case the packet size exceeds the outgoing interface mtu.

As such this new option can be used as a drop-in replacement for
IP_PMTUDISC_DONT, which is currently in use by most name server software
making the adoption of this option very smooth and easy.

The original advantage of IP_PMTUDISC_INTERFACE is still maintained:
ignoring incoming path MTU updates and not honoring discovered path MTUs
in the output path.

Fixes: 482fc6094afad5 ("ipv4: introduce new IP_MTU_DISCOVER mode IP_PMTUDISC_INTERFACE")
Cc: Florian Weimer <fweimer@redhat.com>
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/include/net/ip.h b/include/net/ip.h
index 4aa781b7f6091a655794786ab450cd25c22deeea..b885d75cede4d3ea4f71809fb8b93c9d8de7d7c0 100644 (file)
--- a/include/net/ip.h
+++ b/include/net/ip.h
@@ -266,7 +266,8 @@ int ip_dont_fragment(struct sock *sk, struct dst_entry *dst)
 
 static inline bool ip_sk_accept_pmtu(const struct sock *sk)
 {
-       return inet_sk(sk)->pmtudisc != IP_PMTUDISC_INTERFACE;
+       return inet_sk(sk)->pmtudisc != IP_PMTUDISC_INTERFACE &&
+              inet_sk(sk)->pmtudisc != IP_PMTUDISC_OMIT;
 }
 
 static inline bool ip_sk_use_pmtu(const struct sock *sk)
@@ -274,6 +275,12 @@ static inline bool ip_sk_use_pmtu(const struct sock *sk)
        return inet_sk(sk)->pmtudisc < IP_PMTUDISC_PROBE;
 }
 
+static inline bool ip_sk_local_df(const struct sock *sk)
+{
+       return inet_sk(sk)->pmtudisc < IP_PMTUDISC_DO ||
+              inet_sk(sk)->pmtudisc == IP_PMTUDISC_OMIT;
+}
+
 static inline unsigned int ip_dst_mtu_maybe_forward(const struct dst_entry *dst,
                                                    bool forwarding)
 {

diff --git a/include/uapi/linux/in.h b/include/uapi/linux/in.h
index 393c5de09d42c69393cb228e0d401189f072deb4..c33a65e3d62c85d104d38ab082d997df13cd4c0b 100644 (file)
--- a/include/uapi/linux/in.h
+++ b/include/uapi/linux/in.h
@@ -120,6 +120,10 @@ struct in_addr {
  * this socket to prevent accepting spoofed ones.
  */
 #define IP_PMTUDISC_INTERFACE          4
+/* weaker version of IP_PMTUDISC_INTERFACE, which allos packets to get
+ * fragmented if they exeed the interface mtu
+ */
+#define IP_PMTUDISC_OMIT               5
 
 #define IP_MULTICAST_IF                        32
 #define IP_MULTICAST_TTL               33

diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
index 22aa11971ed1e8d208e7bd62966209e049d3f379..e85445b2b10258b729ae37eda516b290a06b8cd4 100644 (file)
--- a/net/ipv4/ip_output.c
+++ b/net/ipv4/ip_output.c
@@ -824,8 +824,7 @@ static int __ip_append_data(struct sock *sk,
 
        fragheaderlen = sizeof(struct iphdr) + (opt ? opt->optlen : 0);
        maxfraglen = ((mtu - fragheaderlen) & ~7) + fragheaderlen;
-       maxnonfragsize = (inet->pmtudisc >= IP_PMTUDISC_DO) ?
-                        mtu : 0xFFFF;
+       maxnonfragsize = ip_sk_local_df(sk) ? 0xFFFF : mtu;
 
        if (cork->length + length > maxnonfragsize - fragheaderlen) {
                ip_local_error(sk, EMSGSIZE, fl4->daddr, inet->inet_dport,
@@ -1148,8 +1147,7 @@ ssize_t   ip_append_page(struct sock *sk, struct flowi4 *fl4, struct page *page,
 
        fragheaderlen = sizeof(struct iphdr) + (opt ? opt->optlen : 0);
        maxfraglen = ((mtu - fragheaderlen) & ~7) + fragheaderlen;
-       maxnonfragsize = (inet->pmtudisc >= IP_PMTUDISC_DO) ?
-                        mtu : 0xFFFF;
+       maxnonfragsize = ip_sk_local_df(sk) ? 0xFFFF : mtu;
 
        if (cork->length + size > maxnonfragsize - fragheaderlen) {
                ip_local_error(sk, EMSGSIZE, fl4->daddr, inet->inet_dport,
@@ -1310,8 +1308,7 @@ struct sk_buff *__ip_make_skb(struct sock *sk,
         * to fragment the frame generated here. No matter, what transforms
         * how transforms change size of the packet, it will come out.
         */
-       if (inet->pmtudisc < IP_PMTUDISC_DO)
-               skb->local_df = 1;
+       skb->local_df = ip_sk_local_df(sk);
 
        /* DF bit is set when we want to see DF on outgoing frames.
         * If local_df is set too, we still allow to fragment this frame

diff --git a/net/ipv4/ip_sockglue.c b/net/ipv4/ip_sockglue.c
index 0968b28c4cf3125d529d838557fac6e38b6a49fc..64741b9386329ba6a76b00ea660d78983d2efb8a 100644 (file)
--- a/net/ipv4/ip_sockglue.c
+++ b/net/ipv4/ip_sockglue.c
@@ -643,7 +643,7 @@ static int do_ip_setsockopt(struct sock *sk, int level,
                inet->nodefrag = val ? 1 : 0;
                break;
        case IP_MTU_DISCOVER:
-               if (val < IP_PMTUDISC_DONT || val > IP_PMTUDISC_INTERFACE)
+               if (val < IP_PMTUDISC_DONT || val > IP_PMTUDISC_OMIT)
                        goto e_inval;
                inet->pmtudisc = val;
                break;