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Date: Fri, 22 May 2015 16:32:49 +0200 From: Florian Westphal <fw@...len.de> To: <netdev@...r.kernel.org> Cc: hannes@...essinduktion.org Subject: [PATCH -next, V3 0/2] net: force refragmentation for DF reassembed skbs output path tests: if (skb->len > mtu) ip_fragment() This breaks connectivity in one corner case: If the skb was reassembled, but has the DF bit set and .. .. its reassembled size is <= outdev mtu .. .. we will forward a DF packet larger than what the sender transmitted on wire. If a router later in the path can't forward this packet, it will send an icmp error in response to an mtu that the original sender never exceeded. This changes ipv4 defrag/output path to a) force refragmentation for DF reassembled skbs and b) set DF bit on all fragments when refragmenting if it was set on original frags. tested via: #!/usr/bin/python from scapy.all import * dip="10.23.42.2" payload="A"*1400 packet=IP(dst=dip,id=12345,flags='DF')/UDP(sport=42,dport=42)/payload frags=fragment(packet,fragsize=1200) for fragment in frags: send(fragment) Without this patch, we generate fragments without df bit set based on the outgoing device mtu when fragmenting after forwarding, ie. IP (ttl 64, id 12345, offset 0, flags [+, DF], proto UDP (17), length 1204) 192.168.7.1.42 > 10.23.42.2.42: UDP, length 1400 IP (ttl 64, id 12345, offset 1184, flags [DF], proto UDP (17), length 244) 192.168.7.1 > 10.23.42.2: ip-proto-17 on ingress will either turn into IP (ttl 63, id 12345, offset 0, flags [+], proto UDP (17), length 1396) 192.168.7.1.42 > 10.23.42.2.42: UDP, length 1400 IP (ttl 63, id 12345, offset 1376, flags [none], proto UDP (17), length 52) (mtu 1400: We strip df and send larger fragment), or IP (ttl 63, id 12345, offset 0, flags [DF], proto UDP (17), length 1428) 192.168.7.1.42 > 10.23.42.2.42: [udp sum ok] UDP, length 1400 if mtu is 1500. And in this case things break; router with a smaller mtu will send icmp error, but original sender only sent packets <= 1204 byte. With patch, we keep intent of such fragments and will emit DF-fragments that won't exceed 1204 byte in size. Joint work with Hannes Frederic Sowa. Changes since v2: - split unrelated patches from series - rework changelog of patch #2 to better illustrate breakage -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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