don't allow netfilter --setmss to increase mss
Subsystems (3)
PATCH 1/1 don't allow netfilter --setmss to increase mss
Hi folks,
When terminating DSL connections for an assortment of random customers, I've found it necessary to use iptables to clamp the MSS used for connections to work around the various ICMP blackholes in the greater net. Unfortunately, the current behaviour in Linux is imperfect and actually make things worse, so I'm proposing the following: increasing the MSS in a packet can never be a good thing, so make --set-mss only lower the MSS in a packet.
Yes, I am aware of --clamp-mss-to-pmtu, but it doesn't work for outgoing connections from clients (ie web traffic), as it only looks at the PMTU on the destination route, not the source of the packet (the DSL interfaces in question have a 1442 byte MTU while the destination ethernet interface is 1500 -- there are problematic hosts which use a 1300 byte MTU). Reworking that is probably a good idea at some point, but it's more work than this is.
Thoughts? Would it be better to add a new flag?
-ben
net/netfilter/xt_TCPMSS.c +5 -2
@@ -88,8 +88,11 @@ tcpmss_mangle_packet(struct sk_buff **pskb, oldmss = (opt[i+2 ] <<8 ) | opt[i+3 ]; -if (info->mss ==XT_TCPMSS_CLAMP_PMTU && - oldmss <= newmss) +/* Never increase MSS, even when setting it, as +* doing so results in problems for hosts that rely +* on MSS being set correctly. +*/ +if (oldmss <= newmss)return 0 ; opt[i+2 ] = (newmss &0xff00 ) >>8 ;
Please send netfilter-related patches to netfilter-devel.
Quoted 3 lines · Benjamin LaHaise
When terminating DSL connections for an assortment of random customers, I've found it necessary to use iptables to clamp the MSS used for connections to work around the various ICMP blackholes in the greater net. Unfortunately, the current behaviour in Linux is imperfect and actually make things worse, so I'm proposing the following: increasing the MSS in a packet can never be a good thing, so make --set-mss only lower the MSS in a packet.
Yes, I am aware of --clamp-mss-to-pmtu, but it doesn't work for outgoing connections from clients (ie web traffic), as it only looks at the PMTU on the destination route, not the source of the packet (the DSL interfaces in question have a 1442 byte MTU while the destination ethernet interface is 1500 -- there are problematic hosts which use a 1300 byte MTU). Reworking that is probably a good idea at some point, but it's more work than this is.
Yes, this has always annoyed me too as it doesn't really work for me in a similar setup where traffic first goes through an IPsec tunnel, then through a MSS mangling gateway and then over a DSL line. I'll add a patch on top of yours to take the MSS in reverse direction into account.
Quoted 1 line
Thoughts? Would it be better to add a new flag?
No, this is obviously a good idea, I actually thought we'd already prevent increasing it in all cases. I've applied your patch, thanks.
Quoted 8 lines · Patrick McHardy
Quoted 1 line · Benjamin LaHaise
Yes, I am aware of --clamp-mss-to-pmtu, but it doesn't work for outgoing connections from clients (ie web traffic), as it only looks at the PMTU on the destination route, not the source of the packet (the DSL interfaces in question have a 1442 byte MTU while the destination ethernet interface is 1500 -- there are problematic hosts which use a 1300 byte MTU). Reworking that is probably a good idea at some point, but it's more work than this is.
Yes, this has always annoyed me too as it doesn't really work for me in a similar setup where traffic first goes through an IPsec tunnel, then through a MSS mangling gateway and then over a DSL line. I'll add a patch on top of yours to take the MSS in reverse direction into account.
This gets a bit too ugly because of rp-filters and actually might not properly work in setups with routing by fwmark, so I'll leave it as it is, its probably better to manually set the MSS in these cases.